Diffuse spectral reflectance as a proxy for percent carbonate Joseph Ortiz
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PALEOCEANOGRAPHY, VOL. 14, 14, NO. NO.2, APRIL 1999 PALEOCEANOGRAPHY, VOL. 2, PAGES 171-186, 171-186, APRIL 1999 Diffuse spectral spectral reflectance reflectance as as aa proxy proxy for for percent percent carbonate carbonate Diffuse content in content in North North Atlantic Atlantic sediments sediments Joseph Ortiz Joseph Ortiz Lamont-Doherty Earth of University, Palisades, New Lamont-Doherty EarthObservatory Observatory ofColumbia Columbia University, Palisades, NewYork York Alan Mix and Sara Harris Harris'• Alan Mix College Sciences, Oregon Corvallis Collegeof of Oceanic Oceanicand andAtmospheric Atmospheric Sciences, OregonState StateUniversity, University,Corvallis Suzanne Suzanne O'Connell O'Connell Department of Sciences, Wesleyan University, Middletown, Connecticut Department of Earth Earthand andEnvironmental Environmental Sciences, Wesleyan University, Middletown, Connecticut Abstract. Diffuse reflectance records from sediment percent percent carbonate. carbonate. A A high-resoluhigh-resoluAbstract. Diffuse reflectance records fromFern FeniDrift Driftin inthe theNorth NorthAtlantic Atlanticfaithfully faithfullyrecord recordsediment tion, reflectance-based age model for these sediments derived from an orbitally tuned age model for western equatorial Atlantic, tion, reflectance-based agemodelfor thesesediments derivedfrom an orbitallytunedagemodelfor westernequatorialAtlantic,Ceara Ceara Rise sediments was by frequency mapping. of record indicate indicate statistically significant subRise sediments wasgenerated generated byspectral spectral frequency mapping.Power Powerspectra spectra of the theFern Feni Drift Drift record statistically significant subMilankovitch infer that that these these -8 -8 and -5 kyr aalinkage North Milankovitchcyclicity cyclicityat at 7.6-8.4 7.6-8.4and and4.8-6.1 4.8-6.1kyr. kyr. We We infer and-5 kyrcycles cyclesdocument document linkagebetween between Northand andequaequatorial Atlantic cycles influence influence Atlantic Atlantic basin basin carbonate carbonate prior prior to to the the torial Atlanticclimate climategiven givenour ourability abilityto to correlate correlatethese theserecords. records.These Theseclimate climatecycles intensification of Northern Hemisphere glaciation and thus must arise from some portion of the climate system other than the dynamics intensification of NorthernHemisphereglaciationandthusmustarisefrom someportionof the climatesystemotherthanthe dynamics of of clipped precession, precession, implies implies aa possible possible non-linear non-linear of large largeice ice sheets. sheets.The The presence presence of these thesepeaks, peaks,which whichcould couldbe be related relatedto to equatorial equatorialclipped response to to Milankovitch response Milankovitchforcing. forcing. part records with temporal resopartso sothat thathigh-quality high-quality records withmuch muchgreater greater temporal reso- 1. Introduction 1. Introduction lution are needed to observe it. North or lution North Atlantic Atlantic Heinrich Heinrich events or the (D-O) of theDansgaard-Oscheger Dansgaard-Oscheger (D-O) events eventsprovide provideexamples examples ofthese these processes. Likewise, no no overriding central theory theory to to explain processes. Likewise, overridingcentral explainthe the While paleoceanographic study of cause or causes of of millennial millennial or or sub-Milankovitch-scale sub-Milankovitch-scale variabilvariabilWhile paleoceanographic study of Earth's Earth'sMilankovitch-scale Milankovitch-sca!e cause climate significantly in the band climate(20-100 (20-100 kyr kyr cycles) cycles)has hasprogressed progressed significantlyin the ity ity has hasbeen beenagreed agreedupon. upon.Climate Climatecycles cyclesin inthis thisfrequency frequency band past little has response to pastdecades, decades, littleemphasis emphasis hasbeen beenplaced placedon onthe thestudy studyof ofsubsub- have havebeen beenattributed attributedto to aaforced forcedharmonic harmonic response tovariability variability Milankovitch-scale variability until until recently. recently. The The reasons reasons for in berg et et al., al., 1994] or to to speMilankovitch-scale variability for aa in the theMilankovitch Milankovitchbands bands[e.g. [e.g.Hage! Hagelberg 1994]or spelack are pragmatic. Records Records cific mechanisms arising the lackof of focus focuson onhigher higherfrequencies frequencies arelargely largelypragmatic. cific mechanisms arisingfrom from unforced unforcedinternal internaloscillations oscillationsin the of sufficiently high resolution and quality are only recently availclimate system [e.g. Ghil and LeTreut, 1981; Yiou etal., 1994]. of sufficiently highresolution andqualityareonlyrecentlyavail- climate system[e.g. Ghil and LeTreut, 1981; lqouet al., 1994].ItIt able. research able. Second, Second,much muchof ofthe thepaleoceanographic paleoceanographic researchof of the the has has been beenpostulated, postulated,for for example, example,that thatHeinrich Heinrichevents, events,which which 1970's in occur 1970'sand and1980's 1980'swas wasdriven drivenby byinterest interest intesting testingthe theveracity veracityof of occur with with aa quasi-periodicity quasi-periodicityof of 7-10 7-10 kyr kyr arise arisefrom fromunforced unforced the which due to undertheMilankovitch-Croll Milankovitch-Crollhypothesis, hypothesis, whichstipulates stipulatesthat thatclimate climate oscillations oscillationsdue to the theinteractions interactionsof of large largeice ice sheets sheetswith with undervariability is driven in bedrock, a mechanism referred to as the "binge-purge variabilityis drivenin in large largepart partby bychanges changes inEarth's Earth'sorbital orbital lying lying bedrock, a mechanismreferred to as the "binge-purge geometry [MacAyea!, geometry[Hays [Hayset et al., al.,1976; 1976;Imbrie lmbrieet etaL, al.,1984]. 1984].ItIt isisnow nowfairly fairly hypothesis" hypothesis" [MacAyeal,l993a 1993ab; b;Alley Alleyand andMacAyeal, MacAyeal,1994]. 1994]. well accepted accepted that that variations in the the Earth's Diffuse spectral well variationsin Earth'sorbit orbitexert exertaa profound profound Diffuse spectralreflectance reflectanceprovides providesaa rapid, rapid,high-resolution, high-resolution, influence on 20-100 tool suitable high-quality influenceon 20-100 kyr kyr climate climatecyclicity cyclicity[Imbrie [lmbrieet et al., al.,1992, 1992, noninvasive noninvasive tool suitablefor for generating generating high-qualityrecords recordsnecnec1993]. essary for study of sub-Milankovitch climate. Here we 19931. essaryfor studyof sub-Milankovitchclimate.Here we use usereflecreflecIn contrast, variability, tance data In contrast, sub-Milankovitch-scale sub-Milankovitch-scale variability, which which is is tance datacollected collectedon onOcean OceanDrilling DrillingProgram Programcores cores(Table (Table1) 1) defined rate sites definedhere hereas asclimate climatecycles cyclesranging rangingfrom from11 to to 20 20 kyr, kyr, generally generally from from high high sedimentation sedimentation rate sitesin in the theNorth Northand andequatorial equatorial exhibits exhibits much less less variance variance than than its itsMilankovitch-band Milankovitch-band countercounterAtlantic of cliAtlantic(Figure (Figure1) 1) to to test testfor forthe thepresence presence ofmillennial-scale millennial-scale climate variability in marine sediments prior to the intensification of matevariabilityin marinesediments priorto the intensification of Northern glaciation. The proxy NorthernHemisphere Hemisphere glaciation. Thereflectance-based reflectance-based proxyperpercent carbonate records will will allow us to that cent carbonaterecords allow us to test testthe thehypothesis hypothesis that Education Association (SEA), Woods Hole, MassachuI Now Nowat atSea Sea Education Association (SEA), Woods Hole, Massachu- Heinrich-like, millennial-scale climate variability with aaperiodicHeinrich-like, millennial-scale climate variability with periodicsetts. setts. ity prior of ity of of 7-10 7-10 kyr kyrwas wasnot notpresent present priortotothe theintensification intensification of 1.1. The 1.1. TheSearch Searchfor forSub-Milankovitch Sub-MilankovitchVariability Variability Northern Negation of of this Northern Hemisphere Hemisphereglaciation. glaciation. Negation thishypothesis hypothesis Copyright Union. Copyright1999 1999by bythe theAmerican AmericanGeophysical Geophysical Union. would that millennial-scale variability wouldsuggest suggest thatthe theobserved observed millennial-scale variabilitymust must arise arisefrom from some someportion portionof of the theclimate climatesystem systemother otherthan thanthe the dynamics dynamicsof of large largeice ice sheets. sheets. Paper number number 1998PA900021. Paper 1998PA900021. 0883-8305199/1998PA90002 0883-8305/99/1998PA900021 l$l2.00 $12.00 171 171 ORTIZ ET ET AL.: AL.: NORTh ORTIZ NORTH ATLANTIC ATLANTIC CARBONATE CARBONATE 172 Table Data Carbonate Time Time Series Series Table1. 1. Locations, Locations, DataTypes, Types,and andSources Sourcesfor for the theCarbonate Carbonate Carbonate Method Method Location Location SCAT Feni SCAT Feni Drift reflectance reflectance Drift ODP Site Site Latitude -Latitude Longitude Longitude 980980- 55°28.700'N 55o28.700, N Depth Depth frequency mapping frequency mapping 2171 m 981 981 14°40.592'W 14ø40.592'W 2171 m Rockall SCAT SCAT Rockall 982 Bank reflectance reflectanceBank 982 57°30.760'N 57ø30.760'N 15°51.251'W 15ø51.25 I'W Rockall Rockall Coulometry CoulometryBank Bank 57°30.760'N 57o30.760,N 982 Gardar SCAT SCAT Gardar Drift reflectance reflectance Drift 983 982 15°51.25l'W 15ø51.25 I'W 1134 m 1134 m 1134 m 1134 m 60°24.210'N 60ø24.210'N 1983 m 23°38.440'W 983 23o38.440, W 1983 m Gardar Gardar Age Age Model Model Water Water to site 926 to site 926 983 Bjorn Bjorn reflectance reflectance Drift Drift 984 Cearra SCAT SCAT Cearra reflectance reflectanceRise Rise 926 SCAT SCAT 984 926 this paper paper this this this paper; paper; Leg 162 162 shipboard Party [1996b] Leg magnetostratigraphy shipboardParty[1996b] biostratigraphy and biostratigraphy and magnetostratigraphy n/a n/a Venzetetal. al. [1999] [ 1999] this biostratigraphy and thispaper; paper; biostratigraphy and Leg shipboard Party 996c] magnetostratigraphy Leg162 162shipboard Party[I[ 1996c] magnetostratigraphy 60°24.210'N 60ø24.210'N n/a Channel! et el al., [1997] Channell 61°25.519'N 61ø25.519'N m 24°04.945'W 24o04.945,W1648 1648 m biostratigraphy and biostratigraphy and this paper; paper; this magnetostratigraphy Leg 162 shipboard Party [I 996d] 03°43.141'N 03ø43.141' N 3598 m 42°54.498'W 42o54.498,W 3598 m Orbitally tuned Orbitally tuned magnetic suceptibility Harris etal. Bickeri et etal. Harris et al.[19971; [ 1997]' Bickert al. m 23°38.440'W 983 23o38.440, W 1983 1983 m Drift Coulometry Coulometry Drift Data Data Source Source Leg162shipboard Party[ 1996d] magnetostratigraphy [1997]; fledeniann and Franz [1997] [ 1997]' Tiedemann and Franz[ 1997] magnetic suceptibility ODP, Ocean Drilling SCAT, split-core analysis track. Average site and are on for atat ODP,Ocean DrillingProgram; Program; SCAT,split-core analysis track. Average sitelocation location anddepth depth arebased based onvalues values forall allholes holes each site. 1.2. A 1.2. A North North Atlantic-Equatorial Atlantic-Equatorial Atlantic Atlantic Climate Climate Connection? Connection? Why search equatorial Why searchfor for aalinkage linkagebetween between equatorialAtlantic Atlanticand and North Atlantic carbonate records? Such a linkage may North Atlanticcarbonaterecords?Sucha linkagemaynot notbe besursurprising. reported a alinkage onon decadal totocenprising.Hughen Hughenetal. et al.[1996] [ 1996] reported linkage decadal centennial by of tennialtimescales timescales by correlating correlatingthe thefine finestructure structure ofgrayscale grayscale records percent content) records(a (a proxy proxyfor forsediment sediment percentcarbonate carbonate content)from fromthe the Cariaco Cariaco basin basin in in the thewestern westernAtlantic Atlanticwith with the theair airtemperature temperature record recordfrom from the theGreenland GreenlandIce Ice Core CoreProject Project(GRIP) (GRIP) ice icecore. core.FurFurthermore, thermore,they they cite cite aa number numberof of references referencesthat thatdocument documentaa correcorre- lation lation between betweenthe theair-temperature air-temperaturevariability variability observed observedin in the the On linkages between equatorial winds Onlonger longertimescales, timescales, linkages between equatorial windsand and North McIntyre NorthAtlantic Atlanticclimate climatehave havealso alsobeen beenreported. reported. Mcintyreand and Molfino [1996] observed observed variability variability in in the Molfino[1996] thedepth depthof ofthe theequatorial equatorial nutricline with a periodicity of 7.6 kyr which is presumably driven nutricline witha periodicity of 7.6 kyr whichis presumably driven by changes in equatorial winds. Mcintyre and Molfino [1996] by changes in equatorial winds.Mcintyreand Molfino[1996] speculate that arises from speculate thatthe theorigin originof ofthis thisperiodicity periodicity arises fromaaharmonic harmonic interaction of and forcing interaction of precession precession andeccentricity eccentricity forcingduring duringtimes timesof of weak eccentricity forcing of anomalously short period. Their weakeccentricity forcingof anomalously shortperiod.Theirevievidence are of dencefor forthese theseclimatic climaticchanges changes areshifts shiftsin inthe theabundance abundance of the the nutricline dwelling Florisphaera profunda. The nutricline dwellingcoccolithophorid coccolithophorid Florisphaera profunda. The relative of within the relativeimportance importance of this thisspecies species within thecoccolithophorid coccolithophorid community increases when equatorial winds relax, community increases whenequatorial windsrelax,which whichdeepens deepens the equatorial nutricline and vice versa [Molfino and the equatorial nutricline andviceversa[MolfinoandMcintyre, Mcintyre, 1990]. Mcintyre McIntyre and and Mo!fino aacorrespondence 1990]. Molfino[1996] [1996]document document correspondence between the the timing timing of of peaks record from between peaksin in the theF F.profunda profunda record fromRC24RC2408 and and the theHeinrich Heinrich events eventsas asrecorded recordedin in the theice-rafted ice-rafteddebris debris GRIP GRIP record recordand andaavariety varietyof ofNorth NorthAtlantic Atlanticclimate climateproxy proxyrecords records [Dansgaard etetal., and [Dansgaard al.,1989; 1989;Lehman Lehman andKeigwin, Keigwin,1992; 1992;Koc-Karpuz Koc-Karpuz and 1992]. of from circuandJansen, Jansen, 1992].On Onthe thebasis basis ofinferences inferences fromgeneral general circulation et al.. lationmodel model(GCM) (GCM) sensitivity sensitivitystudies studies[Rind [Rind et al., 1986, 1986,OverOverpeck in pecket et al., al., 19891 1989]they theyconclude concludethat thatdecreases decreases in North NorthAtlantic Atlantic (IRD) (IRD) record [Bond and and Lotti, Lotti, 1995]. In contrast contrast to to recordfrom fromV23-8 V23-811 [Bond 1995].In sea (SST) in trade seasurface surfacetemperature temperature (SST)drive drivevariations variations inequatorial equatorial trade Hughen et and [1996] Hughen etal. al.[1996], [1996],Mcintyre Mclntyre andMo!fino Molfino [1996]infer inferthat thatthe the wind prowind strength, strength,which whichin in turn, turn,cause causevariations variationsin in carbonate carbonate pro- equatorial heat through its on wind equatorial heatengine, engine, through itsinfluence influence onlow-latitude low-latitude wind duction duction within within the the Cariaco Cariaco Basin. Basin. These These carbonate carbonate shifts shifts are are strength, drives variations in high-latitude climate. Use of reflecstrength, drives variations in high-latitude climate. Use of reflecrecorded The recordedin in the thegrayscale grayscalevalues valuesof of the theunderlying underlyingsediments. sediments.The tance data on Ocean Drilling tance datagenerated generated ontwo twoseparate separate Ocean DrillingProgram Program correlations that are correlations thatHughen Hughenet etal. al. [1996] [ 1996]observed observed arestriking. striking.HowHow(ODP) linkages between (ODP)legs legswill will allow allowus usto toexplore explorethe thepotential potential linkages between ever, given the the uncertainties uncertainties in decadal time time ever,given in their theirage agemodel modelon on the thedecadal the equatorial Atlantic and and the North Atlantic on aa variety of the equatorial Atlantic the North Atlantic on variety of scale lead-lag scaleand andthe theinability inabilityto todocument document lead-lagphase phaserelationships relationships timescales and in sediments sedimentsmuch much older older than thancan canbe berecovered recovered between concluding that in betweenthe thetwo tworecords, records, concluding thatchanges changes inNorth NorthAtlanAtlan- with standard piston coring methods. with standard pistoncoringmethods. tic may tic SST SST are arethe thedriving drivingfactor factorin inthis thisrelationship relationship maybe bepremapremature. ture. Additional Additional coupled-ocean coupled-oceanatmosphere atmosphereGCM GCM sensitivity sensitivity studies studiesin in which whichequatorial equatorialtrade tradewind wind strength strengthare are varied variedand and 2. 2. Methods Methods North North Atlantic Atlantic SST SST response responseis ismonitored monitoredare arewarranted. warranted.This This would the hypothesis, Sediment was at would provide provide aa means meansof oftesting testing thealternative alternative hypothesis, Sedimentdiffuse diffusereflectance reflectance wasmeasured measured atsea seaduring duringODP ODP namely that shifts Leg analnamely thatvariations variationsin in equatorial equatorialwind windpatterns patternsmay maydrive driveshifts Leg 162 162using usingthe theOregon OregonState StateUniversity University(OSU), (OSU),split-core split-core analin North North Atlantic ysis of in Atlantic SST. SST ysistrack track(SCAT). (SCAT). The The Leg Leg162 162configuration configuration ofthe theSCAT SCAT(Fig(Fig- 173 173 ORTIZ ET AL.: ATLANTIC CARBONATE ORTIZET AL.:NORTh NORTH ATLANTIC CARBONATE 70øN .,......•.• ... • ',• /' </•"•---F•ite 983 _? •,;< .x ...•. • /-•"J site982 60øN ;' "•' ' •-•'?S•• , •,,p' ...,,....•.,-" .¾---I .,, .,.• ' ! ..,Site9•0-1 "$•: • / 50øN 40øN • 30ON 20øN 10øN site 926 ," Eq. . 10øS 60øW I 40øW "1 30°W 30øW Longitude (degrees) Longitude (degrees) , I' 20øW Figure 1. of in the the North North Atlantic Atlantic and and site site 926 926 on on the the Figure 1.The Thelocations locations ofOcean OceanDrilling DrillingProgram Program(ODP) (ODP) sites sites980-984 980-984 in Ceara Rise Ceara Risein in the thewestern westernequatorial equatorialAtlantic. Atlantic. ure used during operations on ure2) 2) was wasfirst firstused duringpostcruise postcruise operations onODP ODP Leg Leg154 154 [Harris with [Harris er etal., al., 1997]. 1997].Reflectance Reflectancemeasurements measurements withthis thisversion version of ratio that that is is an an order order of of of the the instrument instrumenthave haveaa signal-to-noise signal-to-noise ratio magnitude greater than the prototype instrument deployed during magnitudegreaterthanthe prototypeinstrumentdeployedduring ODP Leg 138 [Harris et et aL, al., 1997; Mix et etal., etal., ODP Leg 138 [Harris 1997;Mix al., 1992]. 1992].Ortiz Ortiz et al., [1999] provide aa more [1999] provide moredetailed detaileddescription descriptionof of the theinstrument's instrument's operation. During consecutive 10 operation. DuringODP ODPcoring coringoperations, operations, consecutive 10m mlong long sediment cores are raised from the deep sea and cut on deck sedimentcoresare raisedfrom the deepseaand cut on deckinto into six or or seven whole-round sections six sevenwhole-round sectionsup up to to 1.5 1.5 m m in in length. length.The The secsec- Lions are allowed allowed to to reach reach ambient then tionsare ambienttemperature, temperature, thenare aresplit splitin in half for description, analysis, and halflengthwise lengthwise forimmediate immediate description, analysis, andsampling sampling in SCAT measurements were in the theship's ship'scorelab. corelab. SCATreflectance reflectance measurements wereconconducted by the shipboard sedimentologists on the lightly ductedby the shipboard sedimentologists on thelightlyscraped scraped archive archivehalf half of of each eachwet wetsediment sedimentsection sectionimmediately immediatelyafter aftersplitsplitting process. The tingand andduring duringthe thecore coredescription description process. Thesplit splitcore coresecsections with tionswere werenot notcovered covered withGladwrapTM Glad-wrapTMduring duringmeasurement measurement [e.g., Scientific Party, 1995; 1995; Balsam Balsam et et al., al., [e.g.,Leg Leg154 154Shipboard Shipboard Scientific Party, 1997]. While While this this precaution precaution is to the 1997]. isnecessary necessary toprotect protect theintegrating integrating ORT!Z ORTIZ ET ET AL.: AL.' NORTH NORTH ATLANTIC ATLANTIC CARBONATE CARBONATE 174 VsbIe-iR UV Light Source 2x 650MB disks, Light Source 508 tape S.- V Ethernet Ethernel Connection Connection t CS 4 Fiber Optics To To Sample Sample Part Port Focusing Focusing Mirrors Mirrors To To Reference Reference Pod Beam Port Steerer II II(%'"'::-.. ::_..._...•!::"n::'""x........ •:-.'-• .... ::.::::::• I -s: Spectroif !sl•, radiometer •' Integrating Integrating Sphere Spectroradiometer Sphere • i -% I I V Entrance Slit i \1 '*4 Cooled [ CCD Array Cooled CCD Array Entra• Diffraction Diffraction To Detector Grating Grating •Lander Core secrion - - Core Track Core Track Figure illustration ofofthe of State split-core analysis track Figure2.2.AAschematic schematic illustration themajor majorcomponents components ofthe theOregon Oregon StateUniversity University split-core analysis track (OSU SCAT). Reprinted from of (OSU SCAT). Reprinted fromOrtiz Ortizet et al. al. [1999] [ 1999]with withpermission permission ofthe theOcean OceanDrilling DrillingProgram. Program. resolution) percent percent reflectance reflectance data data set. set. The sphere The 10 10nm nmresolution resolution sphereof of the thehand-held hand-heldMinolta MinoltaCM-2002 CM-2002spectrophotometer spectrophotometer resolution) reflectance data data were firstbecause it is reflectance were used usedto tocalculate calculateaacenter-weighted, center-weighted, firstbecauseof of its its measurement measurementgeometry, geometry,it is not notneeded neededwith with the the derivative spectra, for for each each reflectance automated automated SCAT. SCAT. derivativespectra reflectancesample sampleconsisting consistingof of 68 68 first-derivative channels. The The first-derivative of the first-derivative channels. first-derivative of the reflectance reflectance The were The SCAT SCAT measurements measurements were generally generallytaken takenat at 88 cm cminterinterthe "spectral vals, although although higher-resolution higher-resolution measurements measurements (4 (4 cm cm resolution) resolution) spectra spectra(aRIa?) (3R/3)0 defines definesthe "spectralshape" shape"of of the thesample. sample.It It is is vals, were taken taken when time permitted. ensure that were when time permitted.To To ensure that composite compositereflecreflec- tance tance splices splicescould could be be generated, generated,cores coresfrom from additional additionalholes holes drilled drilled at at each eachsite sitewere wereselected selectedfor for analysis analysison onthe thebasis basisof of conconcurrent gamma ray attenuation porosity estimator (GRAPE) current gamma ray attenuationporosity estimator(GRAPE) or or magnetic susceptibility susceptibility data. data. Because Because of of time time constraints, constraints, the the sedsedmagnetic iments composite depth imentsfrom from 00 to to -90 --90meters meters composite depth(MCD) (MCD) atatsite site981 981 useful of usefulfor for assessment assessment of trace traceminerals mineralssuch suchas ashematite hematiteand andgoetgoethite and Deaton, hite [Balsam [Balsam and Deaton, 1991; 1991; Deaton Deaton and and Balsam, Balsam,1991]. 1991]. We We employed center-weightedfirst-derivatives, first-derivatives,the theaverage average R/? employed center-weighted ofof 3R/3)• for for the the reflectance reflectancebands bandsabove aboveand andbelow belowthe the wavelength wavelengthband bandof of interest because they they are more stable stable [Press [Press et et al., interestbecause are numerically numericallymore al., 1992] derivatives. The The processed 1992] than than first-difference first-difference derivatives. processed10 10 nm nm data are on reflectanceand and first-derivative first-derivativedata are archived archivedon on CD-ROM CD-ROM by by on the the Fern Feni Drift Drift were werenot notanalyzed analyzedbecause becausethis thissediment sediment reflectance Ortiz etal., [1999]. sequence represents a lower sedimentation rate repeat of the shalOrtiz et al., [ 1999]. sequencerepresentsa lower sedimentation rate repeatof the shalUsing the the 10 data, we we gengenlow Using 10 nm nm reflectance reflectanceand andfirst-derivative first-derivative data, low sediments sedimentsat at site site980, 980, another anothercoring coringsite sitedrilled drilledon onthe theFeni Feni to Drift [Leg 162 Shipboard Shipboard Scientific Party, 1996a]. The reflectance eratedaa stepwise, stepwise,multiple multiplelinear linearregression regression toestimate estimateproxy proxyperperDrift' [Leg162 Scientific Party, 1996a]. The reflectance erated cent from percent carbonate [Leg records from the the two centcarbonate carbonate frompaired pairedshipboard shipboard percent carbonate [Leg162 I62 records from two Feni Feni Drift Drift sites sites (site (site 980 980 and and 981) 981) can can be be Shipboard Party, l996a,b,c,d] and measureeasily composite record. record. ShipboardScientific ScientificParty, 1996a,b,c,d] andreflectance reflectance measureeasilyspliced splicedin in the theage agedomain domainto to yield yield aa composite ments. Following Following Mix Mixet et al. al. [1995] [1995] and and Harris Harris et ments. et al. al. [1997], [1997], we we also use the square of the 10 rim reflectance bands as potential also use the square of the 10 nm reflectance bands as potential 3. 3. Results Results and and Discussion Discussion input variables variables for for the the regression. While empirical, this this approach approach input regression. While empirical, has been used successfully by Mix et al. [1995] and Harris et al. al. hasbeenusedsuccessfully by Mix et al. [ 1995] andHarris et 3.1. Proxy Percent Percent Carbonate Carbonate 3.1. Reflectance-Based Reflectance-BasedProxy [[1997] 1997] because becauseit it allows allowsthe the reflectance reflectanceequation equationto to incorporate incorporateaa curvilinear response response to to variations in percent carbonate. Shipboard Shipboard Postcruise data data processing processing of of the the SCAT data set set consisted consisted of of curvilinear variationsin percentcarbonate. Postcruise SCAT data percent carbonate measurements were were matched matched to to the quality carbonatemeasurements the closest closest quality control control to to assure assurethat thatthere therewere wereno nomeasurement measurementoffsets offsets percent reflectance measurements measurements within cm. Percent carbonate samples samples and data into into aa 70-channel reflectance within 5 5 cm. Percentcarbonate and averaging averagingthe the 1024-channel 1024-channeldata 70-channel(10 (10 nm nm 175 17 5 ORTIZ ORTIZ ET AL.: NORTH NORTH ATLANTIC ATLANTIC CARBONATE CARBONATE 100 80 Q60 V w 40 '5 E 0 20 LU 0 0 20 10 i 30 i i ß . 90 90 80 80 70 50 60 40 Measured Measured % % Carbonate Carbonate 100 100 40 b 20 Cs '5 •'' : -20 .............. 0'..... 0 D: 0 0 0 .................. a : .... 0ß O0 10 10 20 . ' -40 -40 .............................. 0 :. I 30 . ' : . O. 0 . .0..... ß....... 0 .'......... I 50 40 60 Measured % Measured % Carbonate Canonate _. : ß......... 0 O:............... I 70 70 80 80 O: 0...: ......... I 90 90 100 100 Figure 3. measured by estimated by by diffuse Figure 3. (a) (a)Percent Percentcarbonate carbonate measured bycoulometry coulometryversus versuspercent percentcarbonate carbonateestimated diffuse spectral spectral reflectance for for ODP ODP sites carbonate estimates estimates as as aa reflectance sites in in the the North North Atlantic. Atlantic. (b) (b) Residual Residualerrors errorsin in reflectance-based reflectance-basedcarbonate function of measured by function of carbonate carbonate measured by coulometry. coulometry. with measurements that that were were farther farther than than 5 cm were with reflectance reflectance measurements were not The equation notincluded includedin in the theanalysis. analysis. Themultiple multipleregression regression equationis is based carbonate pairs basedon on384 384reflectance-percent reflectance-percent carbonate pairsfrom fromthe theNorth North Atlantic sites sites drilled drilled during during Leg Leg 162 Atlantic 162 (sites (sites980-984). 980-984). It It has hasthe the form form tance and at tance andpercent percentcarbonate carbonatemeasurements measurements at site site980, 980, where whereperpercent centcarbonate carbonatevariations variationswith with depth depthare areabrupt. abrupt.The The reflectancereflectancebased carbonate carbonate estimates estimates are are given givenin inAppendix AppendixA1.1 Al.' based The records The character characterof of the theresulting resultingcarbonate carbonate recordsvaries variesdramatidramatically to cally from from southeast southeast to northwest northwestwhen whenplotted plottedusing usingaa prelimiprelimi- nary age biostratigraphic nary age model model based basedonon biostratigraphicand and magnetomagnetostratigraphic datums determined stratigraphicdatums determinedduring duringthe the research researchcruise cruise(Fig(Figure from ure4). 4). Measurements Measurements fromFern FeniDrift Drift in in the theRockall RockallTrough Trough(sites (sites ++2.91R925 + 0.03(R465)2 0.06(R555)2 9.23 2.91R925 + 0.03(R465) 2- 0.06(R555) 2-9.23 980 980 and and981) 981) and andon onthe theRockall RockallBank Bank(site (site982) 982) exhibit exhibitaa typical typical North Atlantic carbonate pattern, similar to that observed at where R represents the 10 nm reflectance band centered on wave'atsite site whereR•,represents the10nmreflectance bandcentered onwave- North Atlanticcarbonatepattern,similarto that observed 607 or other ODP sites further to the south [Ruddiman and 607 or other ODP sites further to the south [Ruddiman and length ?. None of the first-derivative terms were significant in the lengthX,.Noneof thefirst-derivative termsweresignificant in the Raymo, Raymo et er al., final multiple Several of the Raymo, 1988; 1988; Raymo al., 19891. 1989]. In In contrast, contrast,carbonate carbonatemeameafinalstepwise stepwise multiplelinear linearregression. regression. Severalof theterms termsin in the have toward the surementsfrom from the the Gardar Gardar(site (site983) 983) and andBjorn Bjorn(site (site984) 984) Drifts Drifts theregression regression havewavelengths wavelengths toward theblue blueend endof ofthe thevisible visible surements Ridge variations that spectrum (465, exhibits alongthe theReykjanes Reykjanes Ridgeexhibit exhibitcarbonate carbonate variations thatwhile while spectrum (465, 525, 525, and and555 555nm) nm)where wherecarbonate carbonate exhibitsits its along similar to each other, are radically different from the Rockall sites maximum diffuse reflectance [Mix er al., 1995] or in the near similar to each other, are radically different from the Rockall sites maximum diffuse reflectance[Mix et al., 1995] or in the near due to carbonate dilution by fine grained terrigenous sediment. infrared (815 and 925 nm), indicating the potential importance of due to carbonate dilution by fine grained terrigenous sediment. infrared(815 and925 nm), indicatingthepotentialimportance of We verify verify the the calibration equaclay terms We calibrationof of the theproxy proxypercent percentcarbonate carbonate equaclayminerals mineralsor oroxides. oxides.The Theregression regression termsare arethus thusphysically physically tion by comparison to two extensive carbonate data sets (Figure meaningful as these sediments represent in large part a mixture meaningfulas thesesediments represent in largepart a mixture tion by comparisonto two extensivecarbonatedata sets(Figure from between 5) totaling totaling1900 1900observations observations from Rockall RockallBank Bank[Venz [Venz et et al., al., betweencarbonate carbonateand and lithogenic lithogenicmaterial materialof of both bothmarine marineand and 5) 1999] and and Gardar continental origin. origin. The The unforced equation is statistiGardar Drift Drift [Channel! [Channell et et al., al., 1997] 1997]sediments. sediments.Data Data continental unforcedregression regression equationis statisti- 1999] + -- 2.60R815 proxy% CaCO3 = -3.50R proxy% CaCO3 = -3.50R465 + 6.65R525 6.65R525 2.60R815 cally significant (p<<O.00I)and and accounts accountsfor for 94.2% 94.2% of of the catty significant (p<<0.001) the observed percent carbonate variance with square error observed percent carbonate variance withaaroot rootmean mean square error from two span the of carbonate and fromthese these twosites sites span thefull fullrange range ofpercent percent carbonate and percent reflectance variability observed during Leg 162. Quantitapercent reflectance variability observed during Leg162.Quantita- (RMSE) of measured per(RMSE) of of 8.0%. 8.0%.Comparison Comparison ofcoulometrically coulometrically measured percent carbonate cent carbonate with with the thereflectance-based reflectance-basedcarbonate carbonateestimates estimates demonstrates little bias Al electronically atat World Data Center-A for demonstrates little statistical statistical bias(Figure (Figure3). 3).Outliers Outliersthat thatare areevievi1Appendix Appendix A1isisavailable available electronically World Data Center-A for dent vs. (Figure Paleoclimatology, NOAAINGDL, Boulder, dentin in the themeasured measured vs.estimated estimated (Figure3a) 3a)and andresidual residualplots plots Paleoclimatology, NOAA/NGDL, 325 325Broadway, Broadway, Boulder,Colorado Colorado(e(email: paleo@mail.ngdl.noaa.gov; paleo@mail.ngdl.noaa.gov; URL: (Figure mismatches between reflecURL: http://www.ngdl.noaa.gov/paleo). http://www.ngdl.noaa. gov/paleo). (Figure3b) 3b)likely likelyresult resultfrom fromdepth depth mismatches between reflec- mail' 176 ORTIZ ORTIZ ET AL.: AL.: NORTH NORTH ATLANTIC ATLANTIC CARBONATE CARBONATE lOOSite 980 :100 I o I I I I o 50 50 I I C I Il DVII 2 a- I 0 o o.o 0.0 i I I 0.5 0.5 1.0 1.0 1.5 2.0 2.0 2.5 3.0 3.5 3.5 40 4.0 I I 0.5 0.5 1.0 1.5 1.5 2.0 2.0 2.5 2.5 3.0 3.0 3.5 3.5 4.0 4.0 4.5 4.5 I I 4.5 4.5 50 5.0 100 100 Site . 982 C 0 o 50 5o x 0 a- o 0 00 o.o G) 60 ß ___,e 60 ite 983 m I 50 5.0 I C 0 o •240 40 (U 0 ,20 o-.e 20 x o a.0o ..J1u.g 0.0 o.o 60 • 60 ite9984 site I I I I I 0.5 0.5 1.0 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.0 4.5 4.5 • • • • i I I I I 50 5.0 C 0 o 240 (U 0 40 ø"P' 20 o 00 o 0.0 0.0 iL 0.5 0.5 LIItA 1.0 1.0 1.5 1.5 ..... . 2.0 2.0 2.5 2.5 Age Age (Ma) (Ma) i I I 3.0 3.0 3.5 3.5 4.0 4.0 4.5 4.5 50 5.0 Figure 4.4.North carbonate estimates derived from reflectance using function described Figure NorthAtlantic Atlantic carbonate estimates derived frompercent percent reflectance usingthe thetransfer transfer function described in the as of age on and in thetext. text.Data Dataare areplotted plotted asaafunction function ofage ageusing usingaapreliminary preliminary agemodel modelbased based onbiostratigraphy biostratigraphy andmagnemagnetostratigraphy. tostratigraphy. Live comparison of of these these percent data tive comparison percentcarbonate carbonate datawith withthe theproxy proxy percent carbonate predictions for the same depth interval reveal percentcarbonate predictions for thesamedepthinterval revealan an RMSE 982 While RMSEof of 8.0 8.0and and5.6% 5.6%atatsites sites 982and and983, 983,respectively. respectively. While the RMSE RMSE at the the Gardar Drift is is smaller smaller than than for the Rockall Bank record, the proxy percent carbonate regression perfonns better record,theproxypercentcarbonate regression performs betterin in the brighter sediments at the Rockall Bank (Figure 5). The reason thebrightersediments at theRockallBank(Figure5). Thereason for this this is carbonate content becomes low, for is simple: simple:when whenpercent percent carbonate content becomes low,it it exerts less less influence influence on brightness. While perexerts onsediment sediment brightness. Whilefine-scale fine-scale percent features by centcarbonate carbonate featuresat at the theGardar GardarDrift Driftare arerecorded recorded bythe the proxy percent carbonate record, there is a tendency to overestiproxypercentcarbonaterecord,thereis a tendencyto overestimate carbonate content carbonate content drops mate carbonate contentwhen whenpercent percent carbonate content drops below of we our below-20%. -20%.In Inthe theremainder remainder ofthis thispaper paper wefocus focus ourattention attention on of carbonate record ontime timeseries seriesanalysis analysis ofthe theproxy proxypercent percent carbonate recordfrom from the rates the brighter brightersediments sedimentsat at Feni FeniDrift Drift where wheresedimentation sedimentation ratesare are high (-10 cm kyf1), and a continuous record back to 4.5 Ma high(-10 cmkyr'l),anda continuous record backto4.5Mais is available. available. 3.2. A A High High Resolution Resolution Reflectance-Based 3.2. Reflectance-BasedTimescale Timescale A prerequisite prerequisite to A to time timeseries seriesanalysis analysisof of the thereflectance-derived reflectance-derived proxy proxycarbonate carbonaterecord recordat atFern FeniDrift Drift isisaareliable, reliable,high-resolution high-resolution age of apparent temagemodel. model.Careful Carefulinspection inspection ofFigure Figure44indicates indicates apparent temporal "offsets" between clearly synchronous carbonate events poral "offsets"betweenclearly synchronous carbonateeventsat at sites most in sites980-I 980-1and and982. 982.This Thisisisperhaps perhaps mostclearly clearlyillustrated illustrated inthe the time between 2.5 2.5 and time interval intervalbetween and 33 Ma. Ma. The The most mostlikely likely cause causefor for these rate theseoffsets offsetsare are changes changesin in sedimentation sedimentation ratebetween betweenthe the two two sites, age sites,pointing pointing out out the the need needfor foraahigh-resolution high-resolution agemodel. model. Developing such by generating an an Developing suchaa model modelcould couldbe beaccomplished accomplished by generating oxygen isotope stratigraphy for sediments from Feni Drift or by oxygenisotopestratigraphy for sediments fromFeni Drift or by applying an age site applying anexisting existing agemodel modelfrom fromanother another siteto tothe theproxy proxyperpercent centcarbonate carbonaterecord recordat at Feni FeniDrift. Drift. Because Becausethe thegeneration generationof of oxygen for entails oxygenisotope isotopechronologies chronologies forthe theLeg Leg162 162sediments sediments entailsonongoing at laboratories, aa composite oxygen goingresearch research atseveral severalisotope isotope laboratories, composite oxygen ORTIZ ET ET AL.: ORTIZ AL.: NORTH NORTH ATLANTIC ATLANTIC CARBONATE CARBONATE 100 100 I I I I 177 177 I I I a: site 982 !, !: "..• 80 8o 260 w ,, ;: '..1 40 40 , 20 20 ....Coulometry -- 0 5 0 100 100 10 10 I 15 I 20 25 meters meters composite compositedepth depth I I b: site site 983 983 b: 80 80 SCAT :: ij Reflectance I I I 30 30 35 35 40 40 I 45 45 I .......Coulometry Coulometry - -• SCAT SCATReflectance Reflectance 0 •0 6060 (5 2 o • 4o -- o 0 0 I 5 5 I 10 10 I I 20 meters composite meters compositedepth depth 15 15 I 25 I 30 35 Figure of carbonate transfer using coulometric coulometric carbonate carbonate data data from Figure5.5.Validation Validation ofthe thereflectance-derived reflectance-derived carbonate transferfunction functionusing from(a) (a) the the Rockall Rockall Bank Bank (site (site982) 982) and and(b) (b) the theGardar GardarDrift Drift (site (site983). 983). The The55 cm cmresolution resolutioncoulometric coulometriccarbonate carbonatemeasuremeasurements were smoothed with running mean to the 4-8 4-8 cm ofthe reflectance meaments were smoothed withaatwo twopoint point running mean toapproximate approximate the cmresolution resolution o the reflectance measurements. Coulometric Coulometric carbonate carbonate data data from from Venz eral. aL[1999] [1999](site (site982 982on onthe theRockall RockallBank) Bank)and and Channell Channel!et et al. al. surements. Venzet (site 983 983 on of the the data data used used to to construct construct the the transfer transfer function function presented presented in in [[1997] 1997] (site on the the Gardar GardarDrift) Drift) are are independent independent of the text. text. isotope for Feni isotopetimescale timescalefor Feni Drift Drift is is not notyet yetavailable availablefor forour ouruse. use. Accordingly, Accordingly,we we have haveadopted adoptedthe thelatter latterstrategy. strategy. Our is frequency-mapping method Our approach approach isto touse usethe theinverse, inverse, frequency-mapping method of the of Martinson Martinsonet et al. al. [1987] [1987]totodetermine determine theoptimal optimalcorrelation correlation between the proxy between the reflectance-based reflectance-based proxy percent percentcarbonate carbonaterecord record from site 926 on the Ceara Rise (ODP Leg 154) from site 926 on the Ceara Rise (ODP Leg 154)and andthe thereflecreflectance-based proxy percent tance-based proxy percentcarbonate carbonaterecord recordfrom from Feni Feni Drift Drift gengenerated erated as as part partof ofthis thisstudy. study.AAsimilar similarmapping mappingstrategy strategywas was employed by Harris Harris et the employedby etal. al.[1997] [1997]totocorrelate correlate thereflectance reflectance records us recordsfrom from the the ODP ODP Leg Leg 154 154sites. sites.This Thismethodology methodologyallows allowsus to carbonate record to map mapthe theproxy proxypercent percent carbonate recordfrom fromFeni FeniDrift Drift onto onto the theorbitally orbitallytuned tunedtimescale timescalefor for site site926 926 on onthe theCeara CearaRise. Rise.The The basic is basicassumption assumptionembedded embeddedin in this thisprocedure procedure isthat thatclimatically climatically driven, driven, carbonate carbonatedilution dilution or or dissolution dissolutionevents eventsin the the North North Atlantic Atlantic tic at at Feni Feni Drift Drift and andwestern westernequatorial equatorial Atlanticon onthe theCeara CearaRise Rise vary varyin in phase. phase. We selected selected the the reflectance record from from site site 926 926 as We reflectancerecord as aa target target curve for several reasons. First, the orbitally tuned magnetic curvefor severalreasons.First, the orbitallytunedmagneticsussusceptibility was ceptibilityage agemodel modelfor for the theLeg Leg154 154Ceara CearaRise Risesediments sediments was derived from derivedon on sediments sediments fromsite site926. 926.Orbital Orbitaltuning tuningof ofthe themagnetic magnetic susceptibility records at at the susceptibility records the Ceara CearaRise Risesites sitesis isaaviable viablestrategy strategy because of the continuous nature of the magnetic becauseof the continuousnature of the magneticsusceptibility susceptibility records in are recordsand andthe thefact factthat thatvariations variations inmagnetic magneticsusceptibility susceptibility are driven by changes of namely drivenby changesin in the thebulk bulkproperties properties of the thesediments, sediments, namely the to the dilution dilutionof of carbonate carbonateby by terrigenous terdgenousinput. input.It It is is reasonable reasonable to infer that this process will have an orbital response. The timescale infer thatthisprocesswill havean orbitalresponse. The timescale for for site site926 926 was wasdevised devisedby byBicker: Bickertet etal., al.,[1997] [ 1997]and andTiedemann Tiedemann and andFranz Franz[1997], [1997], who whoorbitally orbitallytuned tunedthe theCeara CearaRise Risemagnetic magnetic susceptibility record to to the the La93(1,1) La93(l,l) orbital et susceptibility record orbitalsolution solutionof of Lasker Laskeret al., [1993]. The La93(1,1) orbital solution is a generalized extenal., [1993]. The La93(1,1) orbital solutionis a generalizedextension sion of of the the La90(0,1) La90(0,1) orbital orbital solution solution[Lasker [Lasker et et al., al., 1990]. 1990]. It It difdiffers for fers from from the the La90(0,l) La90(0,1)by byaccounting accounting for lunar lunartidal tidaleffects effectson on ORTIZ ET ET AL.' AL.: NORTh ORTIZ NORTH ATLANTIC ATLANTIC CARBONATE CARBONATE 178 17g Earth's orbit Earth's orbitin in addition additionto to"dynamical "dynamicalvariability" variability"in in the theecceneccentricity records tricity of of the theEarth's Earth'sorbit. orbit.Second, Second,the thereflectance reflectance recordsfor forsite site 926 926 and and the the other othersites siteson onthe theCeara CearaRise Risewere weregenerated generatedby by the the OSU so that OSU SCAT SCAT so that the thedata dataare aredirectly directlycomparable comparableto to our ourdata data from from Feni Feni Drift. Drift. Finally, Finally, as asboth bothsite site926 926 on onthe theCeara CearaRise Rise(3598 (3598 m) and site 980 on the Feni Drift (.-2171 m) are bathed m) and site 980 on the Feni Drift (-2171 m) are bathedby byNorth North Atlantic Atlantic Deep Deep Water Water(NADW) (NADW) during duringinterglacial interglacialperiods, periods,we we expect expectthat thatthey theyshould shouldexhibit exhibitsimilarities similaritiesin in their theirpercent percentcarboncarbon- 1.0 a a) 0) a) cO5 0 0 ß (0 0a) a) ate records. records. The of could in The intercorrelation intercorrelation ofthese theserecords records couldbe beaccomplished accomplished in either the wave (time) domain or in the frequency domain. Mapeitherthe wave (time) domainor in the frequencydomain.Map- 0.0 0.0 ping ping in in the thewave wavedomain domainconsists consistsof ofestablishing establishingdiscrete discrete"tie "tie •,, 2.0 2.0 points" points"between betweenaa reference referenceor ortarget targetcurve curveand andthe therecord recordbeing being operated operatedon. on. In In contrast, contrast,the theinverse inversemethod methodof of Martinson Martinsonet etal. al. [1987] operates largely in the frequency domain, although an [1987] operateslargely in the frequencydomain,althoughaniniinitial To tial age agemodel model(tie (tiepoints) points)must mustbe besupplied. supplied. Totest testthe thesensitivity sensitivity of of the the orbital orbital solution solution to to this this initial initial condition, condition, we we ran ran simulations simulations in which in which we we added addednoise noisein in the theform formof of"erroneous" "erroneous"age-depth age-depth pairs. In all cases this spurious information pairs. In all casesthis spuriousinformationwas wasrejected rejectedduring during the iterative iterative solution, the solution,resulting resultingin in aarelatively relativelystable stableresult. result.WorkWorking in in the the frequency frequency domain of determining determining the the ing domainhas hasthe the advantage advantageof offsets between between the the two of offsets two records recordsbased basedon oncomparison comparison of aa multimultivariate statistical statistical measure measure of of the variate the two two time time series: series: their their Fourier Fourier transform, rather transform, rather than than aa univariate univariate statistical statisticalmeasure, measure,the thecorrelacorrelation coefficient. method tion coefficient.The The frequency-mapping frequency-mapping methodgenerally generallyleads leads to more to more continuous continuous sedimentation sedimentation rates rates and and removes removes much much of of the the arbitrariness inherent arbitrariness inherentin in correlating correlatingtwo twotime timeseries seriesby bythe theaddition addition of individual tie points. of individualtie points.The The analogous analogousconcept conceptto to the the tie tie point pointin in the inverse the inversemapping mappingmethod methodis is the thenumber numberof of Fourier Fouriercoefficients coefficients needed to to map needed map(or (or relate) relate)the theoperating operatingsignal signalonto ontothe thereference reference signal. signal. To map map the To theCeara CearaRise Riseage agemodel model[Bickert [Bickertetal., et al.,1997; 1997;TiedeTiedemann and and Franz record, mann Franz1997] 1997]onto ontothe theFeni FeniDrift Drift proxy proxycarbonate carbonaterecord, we placed the age we placed the data datafrom fromthe theFeni FeniDrift Drift cores coresonto ontopreliminary preliminaryage and magnetostratigraphic magnetostratigraphic models based models basedon onbiostratigraphic biostratigraphicand datums [Leg datums [Leg 162 162 Shipboard ShipboardScientific ScientificParty, Party,1996a]. 1996a].We We then thenrere- Squared Squared Shipboard Site 926 Age Age Shipboard Site926 Age Age Model, Model, Ma Ma Number of Numberof Coefficients Coefficients Model, Model, Ma Ma Correlation, Correlation, r2 10 1.0 Initial Initialage age model model(Ma) (Ma) b a) 0) •151.5 0 ß C Ce 0 a) a) 1.0 1.0 ' 1.5 20 1.5 2.0 Initial Initialage age model model(Ma) (Ma) 10 1.0 .-. 3.0 C ß 2.5 rr' 2.0 20 2.0 2.5 2.5 3.0 3.0 Initial Initialage age model model(Ma) (Ma) 4.0 ,-.4.0 d a) 0') • 3.5 3.5 C Table for Mapping Table2. 2. Statistics Statistics forFrequency Frequency Mappingthe theCeara CearaRise RiseAge Age Model Onto Onto Feni Model Feni Drift Drift Sediments Sediments ' 0.5 0.5 00 0.0 , Ce C.) a) a) r,r-3.0 3.0/ 30 3.0 .-.5.0 .5.0 3.5 4.0 3.5 4.0 Initial age age model Initial model(Ma) (Ma) e Site Site 980 980 0.0007-0.9905 0.0007-0.9905 0.0000-1.0000 0.0000-1.0000 40 0.704 0.704 0.9905-1.6627 0.9905-1.6627 10000-1.6620 1.0000-1.6620 40 0.847 1.6612-2.6490 1.6612-2.6490 1.6620-2.6260 1.6620-2.6260 40 0.604 0.604 2.6490-3.7117 2.6490-3.7117 2.6260-3.7170 2.6260-3.7170 48 48 0.657 0.657 3.7117-4.6615 3.7117-4.6615 3.7170-4.6590 3.7170-4.6590 40 0.470 0.470 • 4.5 Site 98! Site 981 Site 981 Mapped to Reflectance Site 981Mapped toSite Site980 980Where Where Reflectance 1.5148-1.6620 1.5148-1.6620 16 4.5 5.0 4.5 5.0 Initial age model (Ma) Initialage model (Ma) Figure 6. mapping function for Figure 6.Feni FeniDrift Driftage-age age-age mapping function for(a) (a)00 -- 1, 1,(b) (b)11-2, The 2, (c) (c) 22 -- 3, 3, (d) (d) 33 -- 4, 4, and and(e) (e)44 -- 55 Ma Ma time timeintervals. intervals. Themapping mapping function is is derived function derived by by inverse inversecorrelation correlationof of the thereflectancereflectance- Signals Overlap Overlap Signals 1.4961-1.6612 1.4961-1.6612 4.0 4.0 4.0 4.0 0.934 0.934 derived carbonate carbonate records recordsfrom from the the sites sites on on Feni Feni Drift Drift and and the the derived Ceara Rise Rise as as explained explained in Ceara in the thetext. text. 179 179 ORTIZ ET AL.: NORTH ATLANTIC ATLANTIC CARBONATE Age datums datums and and reflectance age models: models: Leg Leg 162, 162, Sites980-981 Sites 980-981 Age reflectanceage I I I I I I I I - FAD ..... FADE. E.hux. hux. 0/ LAD P.lac. '•/' LAD P.lac. xx 50 50 ß '• [:] ß U B/M B/M JT •). [:][:] 00JT JB LAD P. lac. LAD P.lac.'• [:]JB B/M 100 lOO _ - JT B/M jT•-T...• _ JB OT OB JB R2T LAD Gephyro. Gephyro. A/B A/B LAD a) V o 150 15o SITE 980 980 SITE • M/G (deep) C') 0 0C) a) a) SITE 981 SITE 981 M/G (deep) E 200 200 250 250 -- 0 n 300 -300 '•D P. lac. 0 980 O Site Site 980age agedatums datums FAD P. lac. Site Site 981 981 age age datums datums ...... Site Site 980 980 reflectance reflectanceage age model model 0 -• SiteSite 981981 reflectance age reflectance agemodel model 350 350 00 0.0 • 0.5 0.5 • 1.0 1.0 • 1.5 LAD LAD A. A. primus primus • 2.0 2.0 • 2.5 2.5 Age Age (Ma) (Ma) • 3.0 3.0 I I I 3.5 3.5 4.0 4.0 4.5 4.5 50 5.0 Figure plot from ODP Leg Leg 162. 162. Open Open circles circles and and stars stars refer refer Figure7.7.Depth-age Depth-age plotfor forFern FeniDrift Driftrecords records fromsite site980 980and andsite site981, 981, ODP to biostratigraphic and paleomagnetic datums determined for sediments from sites 980 and 981, respectively [Leg to biostratigraphic and paleomagnetic datumsdeterminedfor sediments from sites980 and 981, respectively[Leg 162 Shipboard Scientific Party, 1996a]. Solid lines lines represent represent the the high-resolution high-resolution age age models models developed developed by by frequency frequency 162 Shipboard ScientificParty, 1996a].Solid mapping carbonate carbonate record record mappingthe thereflectance-based reflectance-based carbonaterecord recordfrom from sites sites980 980 and and981 981 onto ontothe the reflectance-based reflectance-based carbonate from Ceara Ceara Rise. events, depth, while while LAD LAD refers from Rise.For Forbiostratigraphic biostratigraphic events,FAD FAD refers refersto to first-appearance first-appearance depth, refersto to last-appearlast-appearance depth. for marker marker species species are are E. E. hux., P. lac, lac, P. P lacunosa; ance depth.Abbreviations Abbreviationsfor hux., Emiliania Emiliania huxleyi; huxleyi;P. lacunosa;Gephyro GephyroA/B, A/B, Gephyrocapsa spp. spp. A/B, A. Magnetostratigraphic events Gephyrocapsa A/B, and andA. A.primus, primus, A.primus. primus. Magnetostratigraphic eventsare areBIM, B/M,BrunheslMatuyama Brunhes/Matuyama boundary; boundary;iT, JT,top topof ofJaramillo; Jaramillo;JB, JB,bottom bottomof ofJaramillo; Jaramillo;OT, OT, top topof of Olduvai; Olduvai;OB, OB, bottom bottomof of Olduvai; Olduvai;R2T, R2T, top top of of Reunion 2; boundary Reunion 2; and andM/G M/G (Deep), (Deep),Matayama/Gauss Matayama/Gauss boundary(deeper (deeperpick). pick). sampled at sampledall all three threerecords records at11kyr, kyr,which whichis isgreater greaterthan thanor orequal equalto to the nominal sample spacing of the Fern Drift records but the nominal samplespacingof the Feni Drift recordsbutwhich which oversamples the factor of of ~2. -2. This oversamples theCeara CearaRise Riserecord recordby by aa factor Thiscomprocompro- (?, ofofcommon variance) exceeds 0.6 as (t2,aameasure measure common variance) exceeds 0.6with withvalues values as high as high as0.85, 0.85, indicating indicatingthe thehigh highquality qualityof ofthe thefit fit between betweenthe thetwo two records (Table 2). Lower correlation between 2.7 and 3.3 Ma records(Table2). Lowercorrelationbetween2.7 and3.3 Ma may may mise mise allowed allowed us us to to retain retain the the additional additional information information within within the the result from Despite result from aa mismatch mismatchin in age agebetween betweenthe thetwo tworecords. records. Despite Fern signal the correlation Feni Drift Drift record, record,rather ratherthan thanstarting startingwith withaadegraded degraded signalfor for this thiscaveat caveatthe correlationbetween betweenthe thetwo tworecords recordsis is striking. striking. correlation. The record that correlation. The record was then divided divided into into six subsections subsections that This age This frequency-mapped frequency-mapped agemodel modelfor for Feni FeniDrift Driftwas wasdeveldevelwere independently mapped (Table 2). This This step were independentlymapped (Table 2). stepwas wasnecessary necessary oped opedwithout withoutthe the direct directuse useof of oxygen oxygenisotope isotopedata, data,although althoughthe the because but of magnetic becauseof of limitations limitationsin in available availablecomputer computerresources, resources, but also also method methoddepends dependson onan anassumption assumption ofconstant constant magneticsusceptisusceptiallows us us to to evaluate evaluate the the fit fit between between the the two two records records as as aa function function bility allows bility phasing phasingrelative relativeto toorbital orbitalforcing forcingas aspart partof ofthe theorbital-tunorbital-tunof time. The final results were independent of the way in which ing process [Bickert et al., 1997; liedemann and of time. The final resultswere independentof the way in which ing process[Bickert et al., 1997; TiedemannandFranz, Franz,1997]. 1997]. the record record was to The The powerful described here of the wassubdivided subdividedand andimply imply only onlyminor minormodifications modifications to powerfulapproach approach described herehas hasthe theadvantages advantages ofbeing being the ages at nondestructive to core and the shipboard-derived shipboard-derived ages at Feni Feni Drift, Drift, especially especiallyfrom from 00 to to33 nondestructive to corematerial materialat atall all steps, steps,inexpensive, inexpensive, andless less Ma Ma (Figure (Figure6). 6). We We qualitatively qualitativelyevaluate evaluatethe thereliability reliabilityof of the themapmap- time time consuming consumingthan thangenerating generatingan anoxygen oxygenisotope isotopestratigraphy. stratigraphy. ping operntion on the the basis plot estimate that that the in ping operationon basisof of the thedepth-age depth-age plotfor forFeni FeniDrift. Drift. We We conservatively conservatively estimate theage agemodel modelgenerated generated in this this The age model is consistent with the of resolution of The resulting resultinghigh-resolution high-resolution age modelis consistentwith the way way has hasuncertainties uncertainties of -4 ~4kyr kyrgiven giventhe thesampling sampling resolution of observed shipboard and age kyr for for the records 1-2 kyr the proxy proxypercent percentcarbonate carbonate recordsat at Feni FeniDrift Drift and and observed shipboardbiostratigraphic biostratigraphic andpaleomagnetic paleomagnetic age 1-2 datums [Leg 162 Shipboard Shipboard Scientific Party, 1996a] 1996a] for for these these two two Ceara provided are datums [Leg162 ScientificParty, CearaRise, Rise,respectively, respectively, providedthat thatchanges changesin in carbonate carbonate are sites (Figure 7). This gives us confidence in the quality of the high the primary features of variance. The disadvantage of this sites(Figure7). Thisgivesusconfidence in thequalityof thehigh the primary features of variance. The disadvantageof this resolution reflectance reflectance based based age age model. model. In In all all but but the is us phase resolution the oldest oldestsecsec- approach approach is that thatitit precludes precludes usfrom fromassessing assessing phaserelationships relationships tion of of the correlation coefficient record tion therecord record(3.7-4.7 (3.7-4.7Ma) Ma) the thesquared squared correlation coefficient of of the theFeni FeniDrift Drift proxy proxypercent percentcarbonate carbonate recordrelative relativeto to the the 180 ORTIZ ET ET AL.' AL.: NORTH NORTh ATLANTIC ORTIZ ATLANTIC CARBONATE CARBONATE Ages Proxy % Carbonate Carbonate at at Sites Proxy% Sites 980/981 980/981 Mapped Mappedonto ontoSite Site 926 926 Ages 100 lOO I I I I I I I I I 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 I I I .... I I I I 50 o o lOO I I I I I I I I i I I 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3 • • • • • • I 3.2 3.2 o 100 50 0 100• 0• 33 100 100. 3.1 3.1 • • • • 3.3 3.3 ,,, r,.,, . • . I • 3.4 3.4 • iv-'--,•.,•.v.•/-•',•r"'--'""•T ....'• • 3.6 3.6 3.5 3.5 .r• •' 3.7 3.7 •, • I • [J 3.8 3.8 3.9 3.9 I I 4 50 0 44 4.1 4.1 4.2 4.2 4.3 4.3 4.4 4.4 4.5 4.5 4.6 4.6 4.7 4.7 I I 4.8 4.8 4.9 4.9 5 Age Age (Ma) (Ma) Figure carbonate for site Figure 8. 8. Reflectance-derived Reflectance-derived carbonaterecords recordsfor site 926 on on the the Ceara Ceara Rise Rise and and sites sites980 and and 981 on on the theFeni Feni Drift Rise mapping function as part part of of this this study. study. Drift plotted plottedon onthe theCeara C•eara Riseage agemodel modelusing usingthe theage-age age-age mapping functionderived derivedas Ceara mapping CearaRise Riseproxy proxypercent percentcarbonate carbonate mappingtarget. target.This Thisshortshortcoming by for comingcan canbe beovercome overcome byusing usingoxygen oxygenisotopes isotopes forage ageconconstraints when that data become available. available. The The transition transition at at -.2.5 -2.5 Ma Ma is is believed believed to to record record the the intensificaintensification of glaciation to tion of Northern NorthernHemisphere Hemisphere glaciationin in response response to the thegrowth growth of larger of largerice ice sheets. sheets.This Thistransition transitionis is clearly clearlyevident evidentin in long long benthic •5180 I8O records records from from the Atlantic [e.g. Clemens and Tiedebenthic the Atlantic [e.g. Clemens and 7•'ede3.3. 3.3. Cyclicity Cyclicityof of the the Feni Feni Drift Drift Proxy Proxy Percent Percent mann, mann,1997]. 1997]. Records Recordsof of other otherclimate climateproxies proxiessuch suchas aspercent percent Carbonate Carbonate Record Record carbonate carbonate or from from other other ocean ocean basins basinssometimes sometimes demonstrate demonstrate an an earlier transition, closer to 2.75 Ma [e.g., .Jansen and Sjoholm, Spectral aameans of the Spectralanalysis analysisprovides provides means ofquantifying quantifying theperiodicperiodic- earlier transition,closerto 2.75 Ma [e.g., Jansenand Sjoholm, 1991; Haug etal., 1995; Maslin etal., 1995]. The proxy carbonate ities in the Feni carbonate record ities in the FeniDrift Drift proxy proxypercent percent carbonate recordand andstudying studying 1991;Hauget al., 1995;Maslinet al., 1995].Theproxycarbonate record from first sustained record from Feni Feni Drift Drift exhibits exhibits its first sustained carbonate carbonate how variance in the climate system changes with time. An inherhow variancein the climatesystemchangeswith time. An inherdecreases at -2.8 Ma as well as a near step-function decrease in ent assumption of the method is that the time series studied is stadecreases at -2.8 Ma as well as a near step-function decrease in ent assumptionof the methodis that the time seriesstudiedis stacarbonate content content at at -2.55 the tionary. However, However, prior prior studies studies of -2.55Ma Ma(Figure (Figure8). 8).This Thissuggests suggests thepossipossitionary. of the theclimate climatesystem systemdemonstrate demonstrate carbonate bility that differences in shifts in in the at -2.5 -2.5 (or bility thatthe theobserved observed differences in timing timingof of this thisevent eventmay may shifts thedominant dominantfrequency frequencyof of climate climatevariability variabilityat (or arise from comparison of multiple events rather than a single time2.75) and at 0.95 Ma. The causes of these transitions are not well 2.75) causesof these transitionsare not well arisefromcomparison of multipleeventsratherthana singletimetransgressive process. process. To To deal deal with in the understood. To To accommodate accommodate these these changes changes in in the the proxy with this thisnonstationarity nonstationarity in the understood. proxypercent percent transgressive record, we will exclude the portion of the records from 2.4 to carbonate record, we divided the proxy percent carbonate time record, we will exclude the portion of the records from 2.4 to2.8 2.8 carbonaterecord,we dividedthe proxy percentcarbonatetime series for spectral analysis: 0-0.95, Ma from seriesinto intothree threesubrecords subrecords forseparate separate spectral analysis: 0-0.95, Ma from our ourspectral spectralanalysis. analysis.The The transition transitionat at 0.95 0.95 Ma Ma (some(some0.95-2.4, in times referred referred to Revolution") denotes the 0.95-2.4, and and 2.8-4.5 2.8-4.5 Ma. Ma. Because Becausethe thenon-stationarity non-stationarity in the the times to as asthe the"Mid-Pleistocene "Mid-Pleistocene Revolution") denotes the proxy data greater isotope transition from from a a climate by the proxycarbonate carbonate dataappears appears greaterthan thanin inthe theoxygen oxygen isotope transition climateregime regimedominated dominated by the41 41 kyr kyrobliqobliqdata, uity-related cycle Sepdata,we wedecided decidedagainst againstthe theuse useof ofevolutive evolutivespectral spectralmethods. methods. uity-related cycleto tothe the100 100kyr kyrcycle cycleof ofthe thelate latePleistocene. Pleistocene. Sep- 181 181 AL.: NORTH ATLANTIC ORTIZ ET AL.' ORTIZ ATLANTIC CARBONATE I 107 i 0-0.95 Ma 0-0.95 Ma 103.4 103.4 99%Cl 99% CI 2.8-4.5 Ma 0.95-2.4 Ma 0.95-2.4 Ma io7 99% 99% Cl CI 95% Cl 95% CI 95% CI Cl 95% 90% CI Cl 90% -99%CI 1o7 - 99% CI 95%CI 95% CI -........... 90% 90%CI CI -. -. Red ..... Rednoise noise 90% 9O% CI CI 41.7 41.7 Red Red noise noise -: - Red Red noise noise 40.8 22.1 22.1 0 41.7 1066 10 106 19.1 21.9 (6 C 16.4 13.4 14.1 10.6 12.4 9.8 10 11.5 7.8 10.4 6.0 10 105 0 a a 104 I 0.00 0.00 0.05 0.05 b b I I I 0.10 0.15 0.20 0.10 0.15 0.20 frequency (kyr-1) (ky(1) frequency 0.25 0.25 0.00 0.00 0.05 0.05 c C I I 0.10 0.15 0.20 0.20 0.10 0.15 frequency (ky(1) frequency (kyr-1) 0.25 0.25 0.00 0.00 II IL II I 0.05 0.05 0.10 0.10 0.15 0.15 0.20 0.20 l 0.25 0.25 frequency(kyr-1) (kyr') frequency Figure 9. power spectra for percent carbonate at for time time interval interval from from (a) (a) 0 0 to to 0.95, 0.95, (b) (b) Figure 9.Multitaper Multitaper power spectra forproxy proxy percent carbonate atFeni FeniDrift Driftfor 0.95 to are with resolution of 0.95 to 2.4, 2.4,and and(c) (c)2.8 2.8to to4.5 4.5Ma. Ma.All Allspectra spectra arecalculated calculated withaatime-bandwidth time-bandwidth resolution of four fourand andseven seven tapers. Significance levels at from intervals relative to median, tapers. Significance levels at90%, 90%,95%, 95%,and and99% 99%are arederived derived fromconfidence confidence intervals relative toaarobust, robust, median, red noise background [Mann and Lees, 1996]. Significant spectral peaks are labeled by period in kyr. rednoisebackground [MannandLees,1996].Significant spectral peaks arelabeled byperiodin kyr. arating the record sufficient aratingthe recordat at 0.95 0.95Ma Ma appears appears sufficientto to deal dealwith withthis this transition transition in the Feni Feni Drift Drift carbonate record. Spectral analysis analysis was was conducted using Spectral conducted usingthe theThompson Thompsonmultimultitaper 1982]. Given the the uncertainty uncertainty of of the the age age tapermethod method[Thompson, [Thompson, 1982].Given model, we we present the spectra from kyrT', model, present thefrequency frequency spectra from00toto0.25 0.25 kyr'1, rather than than plotting to frequency (0.5 kyf1). Highrather plotting tothe theNyquest Nyquest frequency (0.5kyr-]). Highresolution spectra spectra were using tapers and and a a timeresolution werecalculated calculated usingseven seventapers timebandwidth bandwidthresolution resolutionof of four four (Figure (Figure9). 9). These Theseparameters parameterswere were used to of (subused to provide providebetter betterresolution resolution ofthe thehigh-frequency high-frequency (subin the variability in Milankovitch) variability Milankovitch) therecord. record.A A trade-off trade-offof of this this approach is that we expect some smearing of variance in the approach is thatwe expectsomesmearing of variance in thepriprimary bands. testing of peaks maryMilankovitch Milankovitch bands.Significance Significance testing of spectral spectral peaks (Figure by of (Figure99 and andTable Table3) 3)was wasaccomplished accomplished bycomparison comparison ofthe the power of median, red backpowerspectra spectra of the thedata datawith withaarobust, robust, median, rednoise noise background Markov groundbased basedon onaafirst-order first-order Markovmodel model[Mann [Mannand andLees, Lees, 19961. This model has the same white noise variance and 1996].This modelhasthe samewhitenoisevariance andfirstfirstorder as the orderautocovariance autocovariance as thedata datafor foreach eachtime timeperiod periodanalyzed. analyzed. Several interesting interesting characteristics characteristics are in the the data. data. Several areseen seenin Significant spectral peaks peaks in band Significantspectral in the theMilankovitch Milankovitch bandwere were observed with periods ranging from observed withnominal nominal periods ranging from100 100to to19 19kyr kyr(Figure (Figure 99 and observed [Ruddiman and andTable Table3). 3).As Ashas hasbeen beenpreviously previously observed [Ruddiman and Mcintyre, 1984], isisdominant from Mcintyre, 1984],the the100 100kyr kyrcycle cycle dominant from00to to0.95 0.95Ma, Ma, while variability) is dominant in the while41 41 kyr kyr(obliquity-related (obliquity-related variability)is dominant in the older sections of this high-latitude record. The stochastic variabiloldersections of thishigh-latitude record.Thestochastic variability system, characterized by red ityin inthe theclimate climate system, characterized bythe themedian, median, rednoise noise background, and at frequencies as background, andmeasured measured atthe thesame same frequencies asthe thespectral spectral peaks, increases increases with variability peaks, withtime time(Table (Table3). 3).Sub-Milankovitch Sub-Milankovitch variability with nominal periodicities ranging from 16 to 5 kyr with nominalperiodicities rangingfrom 16 to 5 kyr was wasalso also observed (Figure 9 and Table 3). The longer of the sub-Milankoobserved (Figure9 andTable3). Thelongerof thesub-Milankovitch those ranging ranging from from -16 -16 to to -11 vitchperiodicities, periodicities, those ~11kyr, kyr,were werenot not present as features throughout the (Figure 99and present assignificant significant features throughout therecord record (Figure and Table This was cycles Table3). This wasnot notthe thecase casefor forthe thehigh-frequency high-frequency cyclesat at -10, -8, and -5 kyr. Because of the increase in background sto-10, ~8, and~5 kyr.Because of theincrease in background stochastic variance at the in chastic variance atall allfrequencies, frequencies, thevariance variance inthe thesub-Milanksub-Milankovitch peaks peaks tends tends to with the variance variance in in ovitch toincrease increase withtime. time.However, However, the these bands bands relative relative to to the the red red noise background does does not not indicate indicate these noisebackground 7.6 xlO5 40.8(>90%) 22.5 (>95%) lOOkyr 41 kyr 23 kyr o• o• A O z 9.5 xlO4 No value indicates no significant peak. 5.1 (>99%) 3.3 x104 2.9x 2.3x 5.2 (>99%) 6.1 (>99%) 7.5 (>99°,'o) 5.3 xlO4 6.1 xlO4 1.0 xlO5 1.6 xlO5 2.2 xlO4 3.0 xlO4 4.3 xlO4 8.0 x104 9.7 xlO4 2.4x 2.Ox 2.3x 2.0X 2.Ox 3.5x 2.6 x105 21.9 (>99%) 8.6xl04 l2.4(>90%) 4.8 (>99%) 6.0 (>99%) 8.2 (>99%) 9.0 (>99%) 1.6 x104 2.2 x104 4.3 x104 5.3 xlO4 7.2x104 14.1 (>90%) o'• A 5 kyr 7.0 xlO4 10.4 (>95%) 1.9 x105 l.3xl05 5.Sxl& 4l.7(>99%) o'• A 6 kyr A 1.6 x105 l.8x 11.5 (>95%) 4.6x105 13.4(>99%) 2.3x 2.7x 4.8x 3.lx Spectral Density, log units o'• A 7.8 (>99) 1.0 xlO5 l.6x 4.3xl& 16.4(>99%) l.9xl05 2.5xl05 3.3 x105 9.7xl05 Relative Spectral Density 6.1 xlO3 9.3 x103 1.6 xlO4 1.9 xlO4 3.5x104 4.4x104 9.5 xl0 2.4x105 Red Noise Density, log units 2.8 -4.5 Ma Spectrum Observed Period and Significance Level, kyr (%) A 1.8 xI& 1.2 x io5 6.7x105 1.6 xlO6 3.OxlO6 Sub-Milankovitch Bands 19.1 (>99%) 22.1 (>99%) 41.7(>99%) Milankovitch Bands Spectral Density, log units Red Noise Density, log units ,•, 8 kyr 9.8 (>90%) c• A 10 kyr A 10.6 (>90%) 2.Ox l.4x 9.9x Relative Spectral Density Observed Period and Significance Level, kyr (%) A 11 kyr 3.8 xlO5 7.Oxl& I.OxlO6 Red Noise Density, log units 0.95 -2.4 Ma Spectrum ,•, l3kyr l4kyr l6kyr 1.9 xlO5 l.OxlO6 103.4(>99%) A o• l9kyr 9.9xl06 A '•' Nominal Period Spectral Density, log units Observed Penod and Significance Level, kyr (%) 0- 0.95 Ma Spectrum Table 3. Power Spectral Characteristics of the Feni Drift Proxy Percent Carbonate Record 2.6x 2.4x 2.7x 2.8x 2.5x l.6x 2.7x 2.3x Relative Spectral Density 182 ORTIZET AL.: NORTHATLANTICCARBONATE A 183 183 ORTIZ ET AL.: AL.' NORTH NORTHATLANTIC ATLANTIC CARBONATE CARBONATE Laskerg3(1 .1) Precession Precession and and rectified Precession Lasker93(1,1) rectifiedPrecession -0.02 -0.02 CD V I I U -O.04 -0.04 -0.06 -0.06 fl I ' ' E ''''''' MI ihi -_ r-.-I. UI Lu .11 . r. r _fl!1_ . I r. I Ill,. ,ll ,, II Ill .].., ,., iI II• 4[• ,,.. Ill, _[, U lIii!III 1II --Ittllilllt• I IlII ii '" Fr,•iHIHII, "'""" ""'""""":l •l-milml '"'"'I•l iiilli,iiitai'r] • I• •1.-[l[iik'hlilE ,,,.,,,,,,. ,,,, ,,,,,11 r.tlllllHit .•. '&! IIIt:I,i•iltf[ihdtllll--#Hi 1 _ I 0.0 100 100 I I I I I I I 1.5 1.5 2.0 2.5 2.5 Age (Ma) Age (Ma) 3.0 3.0 3.5 3.5 4.0 4.0 4.5 4.5 0.5 1.0 1.0 • • [ 23-19 kyr1 23-19 kyr -1 b [ , [ [ , [ , 0.16 0.16 0.18 0.18 0.20 0.20 ] , 5.0 5.0 16.4 kyr CD C) C 54 kyr1 54kyr -1 CD CD 36ky( > >1 (•10_ O_ $ 0.0 100 100 oe C I III kyr1 ^ 14.7 14.7kyr -1 12.9 kyr IAA 0.02 0.04 0.04 0.06 0.08 [ ] [ • 434kyr -1 -23-19 kyr -1 CD C) ] 11.6-9.6 kyr -1 ,.b•/'• (• > io-5 •10 ,,. •.•:• .• ' '•'"••' '• ;• CD CD 0.0 0.14 0.10 0.12 Frequency (ky(1) Frequency (kyr-1) 0.02 0.04 0.06 0.08 [ , 8.6-6.7kyr -1 .. ,• 0.14 0.12 0.10 0.12 0.14 Frequency (ky(1) Frequency (kyC1) 0.16 0.16 5.8-5.2•r -1 /""•" 0.18 0.18 0.20 0.20 Horizontal lines represent rectification et Figure 10. record for 0 0 -- 55Ma Lasker Figure 10.(a) (a)Precession Precession record for Mafrom from Lasker etal. al.[1993]. [1993]. Horizontal lines represent rectification plotted in Figure lOa, thresholds used for the curve. (b) spectra of precession record thresholds used forclipping clipping theprecession precession curve. (b)Power Power spectra ofthe the precession record plotted inFigure 10a, (c) Power spectra for precession record rectified at the levels indicated in Figure lOa. (c)Power spectra forprecession record rectified atthelevels indicated inFigure 10a. the physical mechanism suggests the for thelack lackof ofaaplausible plausible physical mechanism suggests theneed need for caution when interpreting these potential cycles. Additional work cautionwheninterpreting thesepotential cycles. Additional work is needed needed to this in records and/or analnoise appear to than the red is toreplicate replicate thisobservation observation inother other records and/or analappear todo doso soat ataarate ratethat thatisisgreater greater than thestochastic stochastic rednoise ysis with higher-order statistical methods such as bispectral analyanalybackground. This suggests that variance within the sub-Milankoysis with higher-order statistical methods such as bispectral background. Thissuggests thatvariance withinthesub-Milankosis can sisbefore beforedefinitive definitivestatements statements canbe be made maderegarding regardingthe the nature nature vitch may insensitive to changes changes induced induced vitchfrequencies frequencies maybe berelatively relatively insensitive to of this longer-frequency suborbital variability. by the of Northern Hemisphere glaciation. of thislonger-frequency suborbital variability. by theintensification intensification ofNorthern Hemisphere glaciation. cycles, aahigher-order Spectral peaksat peaks at periods similar to of Theshorter shorter cycles,from from11 11to to55kyr, kyr,require require higher-order Spectral periods similar tothe thelonger longer ofthe theobserved observed The response to orbital forcing (i.e., harmonics) or aanonlinear origin sub-Milankovit9h. cycles (-16, -14, and -13 kyr) are present in response to orbital forcing (i.e., harmonics) or nonlinear origin sub-Milankovit/,9h •cycles (- 16,- 14,and- 13kyr)arepresent in within the response of the climate system itself. Of potentially the precessi&i bandhiorcing, although they are significantly less within the response of the climate system itself. Of potentially theprecessi•;n '•d'd•orcing, although they aresignificantly less great significance is the identification of concentrations of varienergetic than the the primary primary doubletSin douhlelSt 23 19 lOb is theidentification of concentrations of varienergetic than 23and and 19kyr kyr(Figure (Figure 10b greatsignificance centered on and kyr. and Table Table 3). 3). While While the the peak (Figanceat atperiods periods centered on7.5-8.2 7.5-8.2 and4.8-6.1 4.8-6.1 kyr.The Thevariation variation and peakat at16 16kyr kyrin inthe thedata dataspectrum spectrum (Fig- ance in the period of of these these two two peaks peaks is is most 14 kyr, kyr, it it ure 9c) likely independent of at in theestimateçl estimated period mostlikely likelydue dueto to ure 9c)is ismost mostlikely independent ofthe thepeaks peaks at13 13and and14 uncertainties in to at in the theage agemodel, model,which whichare areuntuned untuned tovariability variability at is unclear ifif the represent independent concentrations of is unclear thelatter latter represent independent concentrations of uncertainties these frequencies. Could sub-Milankovitch peaks in the spectral variance or a a single age Couldsub-Milankovitch peaksin thespectral variance or singlepeak peakat at13 13or or14 14kyr kyrsplit splitby bypotential potential age thesefrequencies. we arise artifacts resulting from the the model errors. Power Power near near 13-14 kyr could could arise arise as as a a harmonic harmonic of of ranges ranges weobserve observe ariseas asnumerical numerical artifacts resulting from modelerrors. 13-14kyr nonlinearity inherent in percent data? IfIf we consider aasimple syset the primary orbital frequencies in a number of ways [e.g., Yiou nonlinearity inherent in percent data? we consider simple systheprimary orbital frequencies in a number ofways[e.g.,Yiouet tem (e.g., carbonate and al., it be to (1) of temwith withtwo twocomponents components (e.g.,deep-sea deep-sea carbonate andterrigeterrigeal.,1994]. 1994].For Forexample, example, it could could berelated related to (1)an aninteraction interaction of nous input) that vary as independent linear responses Yi to the doublet and cycle linearresponses Yland andY2 Y2to the19 19kyr kyrprecession precession doublet andthe the41 41kyr kyrobliquity obliquity cycle(1I (1/ nousinput)thatvaryasindependent orbital forcing x, (e.g. Yt = m1x+b1; Y2 = m2x+b2) we see that 19+1/41 = 1/12.98), (2) an interaction of the precession doublets we seethat 19+1/41 1/12.98),(2) an interaction of theprecession doublets orbitalforcingx, (e.g.Yl = rn•x+bi;Y2= rn2x+b2) expressing y] Yi as as a a percentage percentage of of (Yl (y ++Y2) results in in a a non-linear 1/ at the obliquity cycle ([1/19+1/231-1/41 == 1/ expressing Y2)results non-linear at19 19and and23 23kyr kyrwith with the obliquity cycle ([1/19+1/23]-1/41 %y ==100(m1x+b1)/(m1x+b1 ++m2x+b2). Despite this this we 13.94), or or (3) (3) the of (3/41 measure: %Yi 100(rnix+bi)/(rn•x+bi rn2x+b2). Despite we 13.94), thethird thirdharmonic harmonic ofobliquity obliquity (3/41== 1/13.67). 1/13.67). measure: do not believe that this is the underlying cause of the spectral do not believe that this is the underlying cause of the spectral Unfortunately, none of possibilities provides an match Unfortunately, none ofthese these possibilities provides anexact exact match we because the peaks arising in in to variance, nor of these peaks weobserve observe because thespurious spurious peaks arising inthis thisway way inperiod period tothe theobserved observed variance, norare arewe weaware aware ofhow how these peaks should not be band-limited to discrete frequency ranges. In should not be band-limited to discrete frequency ranges. In addiaddiorbital interactions may be linked to specific physical mechanisms orbitalinteractions maybelinkedtospecific physical mechanisms tion, sub-Milankovitch variability variability has has been been observed by other observed by other of change. While between the theoretiofclimate climate change. Whilethe thediscrepancies discrepancies between the theoreti- tion, sub-Milankovitch al., [Bond et studies using grayscale and reflectance data cal frequencies could errors, usinggrayscale andreflectance data[Bondet al.,1993; 1993; caland andobserved observed frequencies couldarise arisefrom fromage agemodel model errors, studies sub-Milankovitch vanaa clear or cleartrend trendof ofincreasing increasing ordecreasing decreasing sub-Milankovitch variability. Thus, while increases with not ability. Thus, whiletheir theirvariance variance increases withtime, time,ititdoes does not 184 184 ORTIZ ORTIZ ET ET AL.: AL.: NORTh NORTH ATLANTIC ATLANTIC CARBONATE CARBONATE susceptibility Hagelberg et er al., Hagelberg al., 1994; 1994;Boyle, Boyle,1997], 1997],magnetic magnetic susceptibility [Moms et al., 1997], and isotope data [Yiou etal., 1994; et [Moroset al., 1997],andisotopedata[¾iouet al., 1994;Raymo Raymoet al., 1998]. This implies that its origin cannot be accounted for as al., 1998].Thisimpliesthatits origincannotbe accounted for asaa simple data. simpleartifact artifactof of percentage percentage data. To asses To asseswhether whetheraa nonlinear nonlinearprocess processsuch suchas asequatorial equatorialclipclipping ("rectification") could give rise to spectral peaks ping("rectification")couldgiveriseto spectralpeaksnear near7.5-8.2 7.5-8.2 and 4.8-6.1 kyr, we an experiment similar to to that that of of and 4.8-6.1 kyr, we performed performedan experimentsimilar Hagelberg et et al., al., [1994] record Hagelberg [1994] using usingaa 55Myr Myrlong longprecession precession record derived from from the the La93(l,l) derived La93(1,1) solution. solution.Similar Similar results resultswere were also also obtained using using the obtained theBerger Bergerand andLoutre Loutre[19911 [1991] orbital orbitalsolution solution(not (not shown). The precession record was subdivided into shown).The precession recordwassubdivided intononoverlapnonoverlapping 1 Myr segments and windowed with a 1 Myr Hanning winWe believe believe the the 7.5-8.2 7.5-8.2 kyr kyr peak peakmay maybe berelated relatedto toHeinrich-like Heinrich-like ping 1 Myr segmentsand windowedwith a 1 Myr Hanningwindow prior dow prior to to estimating estimatingpower power spectra spectrafor for the the full full amplitude amplitude variability, which exhibits a similar quasi-periodicity [e.g., Mcinvariability,whichexhibitsa similarquasi-periodicity [e.g.,Mcinprecession record and for clipped records with rectification threshprecession record and for clipped records with rectification threshtyre and Molfino, 19961. The presence of a 7.5-8.2 kyr peak tyre and Molfino, 1996]. The presenceof a 7.5-8.2 kyr peak olds of 0.1, 0.2, and 0.3 precession units (Figure 1 Oa). This test throughout the carbonate record throughout theproxy proxypercent percent carbonate recordat atFern FeniDrift Drift propro- olds of 0.1, 0.2, and 0.3 precessionunits(Figure 10a). This test differs from that of !-iagelberg et al. [1994], who clipped only at differs from that of Hagelberg et al. [1994], who clipped only at vides several into the vides severalpotentially potentiallyimportant importantinsights insightsinto theoperation operationof of zero amplitude precession and and used used aa shorter 400 kyr precession zero amplitude precession shorter 400 kyr precession the climate system. First, the Heinrich-like periodicity observed the climatesystem.First, the Heinrich-likeperiodicityobserved record derived derived from from the record thesolution solutionof of Berger Bergerand andLoutre Loutre[1988]. [ 1988].We We here cannot dynamics here cannotarise arisesolely solelyfrom fromice icesheet-bedrock sheet-bedrock dynamicsas as formulated this test to simulate a climatic response with formulatedthis test to simulatea climaticresponsewith aa gain gain hypothesized by model ofofMacAyeal [I[ 1993a,b] 993a,b] hypothesized bythe the"binge-purge" "binge-purge" model MacAyeal sensitive to the magnitude of the external precessional forcing. forcing. and Alley Alley and and MacAyeal MacAyeal [1994]. [1994]. If If that and thatwere werethe thecase, case,one oneshould should sensitiveto the magnitudeof the externalprecessional The results indicate aa translation of variance from the 19-23 kyr The results indicate translation of variance from the 19-23 kyr not observe 7.5-8.2 kyr variability in the record before 2.5 or 2.75 notobserve7.5-8.2 kyr variabilityin therecordbefore2.5 or 2.75 doublets (Figure (Figure lOb) to both longerand shorter-frequency doublets 10b) to both longerand shorter-frequency Ma, prior to the intensification of Northern Hemisphere glaciaMa, prior to the intensification of NorthernHemisphere glaciaresponses at at 434, 434, 122, responses 122,11.6 11.6-- 9.6, 9.6, 8.6 8.6 -- 6.7, 6.7, and and5.8 5.8 -- 5.2 5.2 kyr kyr (Fig(Figtion. Second, of percent tion. Second,the thepresence presence of 7.5-8.2 7.5-8.2kyr kyr variability variabilityin in the thepercent ure lOc). The higher frequency bands may represent the second, ure 10c). The higher frequency bands may represent the second, carbonate record that carbonate recordprior priorto to2.75 2.75Ma Masuggests suggests thatdilution dilutionof ofthe theperperthird and and fourth doublets at at cent signal fourth harmonics harmonicsof of the the primary primaryprecession precession doublets centcarbonate carbonate signalby byice-rafted ice-rafteddebris debriscannot cannotbe bethe thesole solecause cause third 23 and 19 kyr. Variance transfers of this type were postulated by 23 and 19 kyr. Variance transfers of this type were postulated by of the observed variability in the proxy percent carbonate records. of the observedvariabilityin the proxypercentcarbonaterecords. Short et et al. al. [1991] observed by and Mix Short [ 1991] and andindependently independently observed by Pokras Pokrasand Mix Alternative mechanisms mechanisms for for the Alternative the cause causeof of this thisvariability variabilitycould couldbe be [1987], Park et al. [1993], Hagelberg et al. [1994], and Clemens [ 1987], Park et al. [ 1993], Hagelberget al. [ 1994], and Clemens shifts with locashiftsin in carbonate carbonateproduction productionassociated associated with changes changesin in the the locaand Tiedemann of and Tiedemann[1997] [ 1997]for for various variouscomponents components of the theclimate climatesyssystion of carbonate dissolution dissolution tion of the the polar polarfrontal frontalzone zoneor or variations variationsin in carbonate tem. The presence of peaks at 6.7-8.6 and 5.2-5.8 kyr in the tem. The presence of peaks at 6.7-8.6 and 5.2-5.8 kyr in the associated with changes in the mixing ratio of northern compoassociatedwith changesin the mixing ratio of northerncompoclipped precession precession record record suggests suggests that that at at least of the the varileastsome someof varinent (NADW-like) (Antarctic nent (NADW-like) and andsouthern southerncomponent component (AntarcticBottom Bottom clipped ance in these bands in in the ance observed observedin thesefrequency frequencybands the Feni Feni Drift Drift proxy proxy Water (AABW) (AABW) -like) waters Water watersat at Feni Feni Drift. Drift. Given Given the thedata dataat athand, hand, percent carbonate record may arise from a nonlinear response to percent carbonate record may arise from a nonlinear responseto we cannot distinguish between these two possibilities. we cannotdistinguishbetweenthesetwo possibilities. precessional forcing. Hagelberg et al. [1994] estimated as much precessional forcing. Hagelberg et al. [1994] estimated as much The spectral peak at 4.8-6.1 kyr is intriguing because it proThe spectralpeak at 4.8-6.1 kyr is intriguingbecauseit proas 30% of the variance in the 10-12 kyr band may be externally vides of videsfurther furtherinsight insightinto intopotential potentialmechanisms mechanisms of millennial-scale millennial-scale as 30% of the variancein the 10-12 kyr band may be externally forced. Indeed, the presence of sub-Milankovitch peaks peaksthroughthroughclimate. Hagelberg et of climate.Hagelberg et al., al., [1994] [1994]report reportthe thepresence presence ofspectral spectral forced.Indeed,the presenceof sub-Milankovitch out the Feni Drift record suggest a "pacing" that may arise from from out the Feni Drift record suggest a "pacing" that may arise peaks at 5 and 10-11 kyr in a 400 kyr rectified version of the prepeaksat 5 and 10-11 kyr in a 400 kyr rectifiedversionof the preorbital forcing. These results suggest the need for more climateorbital forcing. These results suggest the need for more climatecession record of Berger and Loutre [1988]. The reader should cessionrecordof Berger and Loutre [1988]. The readershould modeling studies studies designed designed to to test test the the climate climate syssysmodeling the sensitivity sensitivitythe note by Berger notethat thatfurther furtherdetails detailsof of the theorbital orbitalsolution solutiondescribed described by Berger tem to external forcing at sub-Milankovitch frequencies. tem to externalforcingat sub-Milankovitchfrequencies. and by andLoutre Loutre[1988] [1988]were werepublished published byBerger Bergerand andLoutre Loutre[1991], [1991], the more the morefamiliar familiar source sourcefor for this thisorbital orbitalsolution. solution.One One potential potential physical 4. Conclusions physicallink link between betweenthe therectified rectifiedprecession precessionrecord recordand andclimate climate 4. Conclusions is referred et is aa mechanism mechanism referredto to as asequatorial equatorial"clipping" "clipping"[e.g., [e.g.,Short Shortet The ofofdiffuse reflectance measurements al., that The relative relativebrightness brightness diffuse reflectance measurements al., 1991]. 1991]. ItIt has hasbeen beenpostulated postulated thatthe theclimate climatesystem systemshould shouldbe be from ODP sites 980-984 are controlled largely by variation in very sensitive to precessional maxima (minima) at low latitudes largelyby variation insedsedvery sensitiveto precessionalmaxima (minima) at low latitudes fromODP sites980-984arecontrolled iment CaCO3 content. Accordingly, a highly significant transfer because the out of phase response of the Northern and Southern iment CaCO 3 content. Accordingly, a highly significant transfer becausethe out of phaseresponseof the Northernand Southern Hemispheres to function that that accounts accounts for for 94% with an 94% of the carbonate carbonate variance with Hemispheres to orbital orbitalvariations variationsmeans meansthat thatlow-latitude low-latitudeequatoequato- function rial twice maxima RMSE of of 8.0% 8.0% can be be devised devised from from the the reflectance reflectance data. This rial regions regionsare areinfluenced influenced twicefor foreach eachprecessional precessional maxima RMSE (minima). This This phenomenon should to allows us us to continuous records (minima). phenomenon shouldresult resultin in increases increases toequatoequato- relationship relationship allows to generate generate continuous recordsof of proxy proxy rial seasonality and intensification of monsoonal forcing. percent for rial seasonalityand intensificationof monsoonalforcing.While While percentcarbonate carbonate forthe thepast past4.5 4.5Myr Myr at atthese thesesites. sites.Strong StrongcorrecorreHagelberg lations records Hagelberg et et al. al. [1994] [1994] point pointout outthat thatthese theseharmonics harmonicsare arenot not lationsbetween betweenproxy proxypercent percentcarbonate carbonate recordsfrom fromCeara CearaRise Rise directly forcing, in in the the equatorial equatorialAtlantic Atlantic and and Fern Feni Drift Drift in in the theNorth NorthAtlantic Atlantic directlypresent presentin in the theastronomical astronomical forcing,they theyalso alsonote notethat thatthe the presence allow us to to transfer transfer the presenceof of such suchharmonics harmonicsin in climate climaterecords recordscould couldresult resultfrom from allowus theorbitally orbitallytuned tunedage agemodel modelfrom fromCeara CearaRise Rise either of of careither(1) (1) aanonlinear nonlinearresponse response of the theclimate climatesystem systemto toexternal external to to Feni FeniDrift. Drift.Power Powerspectra spectra ofthe theFeni FeniDrift Driftproxy proxypercent percent carforcing ininthe mechanism. bonate time time series forcingor or(2) (2) aanonlinearity nonlinearity theclimate climaterecording recording mechanism. bonate seriesfor for time timeintervals intervalsranging rangingfrom from4.5-2.8, 4.5-2.8, 2.42.4In to an 0.95, and of variance in the In the thefirst firstcase casethe theclimate climatesystem systemresponds respondsnonlinearly nonlinearlyto an 0.95, and0.95-0 0.95-0 Myr Myr document documentthe theevolution evolutionof variancein the external by Milankovitch eccentricity, obliquity, bands. externalforcing forcingwhich whichis isthen thenlinearly linearlyrecorded recorded bysome someclimate climate Milankovitch eccentricity, obliquity,and andprecession precession bands.SignifSignifproxy. Amplification Amplification of of monsoonal in response response to to preicant sub-Milankovitch climate with proxy. monsoonalvariability variabilityin pre- icant sub-Milankovitch climatevariability variabilityis is observed observed withperipericessional provides one one such [e.g., Short Short et et ods centered near 7.6-8.4, and 4.8-6.1 kyr. The climate cessionalrectification rectificationprovides suchexample example [e.g., odscenterednear7.6-8.4, and4.8-6.1 kyr. The climatecycle cycleof of al., eretaL, 7.6-8.4 kyr kyr duration duration is throughout the al., 1991; 1991;Park Parket etal., al.,1993; 1993;Hagelberg Hagelberg al.,1994]. 1994].In In the thesecond second 7.6-8.4 is present present throughout therecord recordand andmay maybe be scenario, the linearly related to to Heinrich-like The presence presence of of 7.6-8.4 7.6-8.4 kyr kyr scenario, theclimate climatesystem systemmay mayrespond respond linearlyto toexternal externalforcforcrelated Heinrich-likeperiodicity. periodicity.The ing, but the proxy recording system may be nonlinear in cyclicity in in the the percent percent carbonate record ing, but the proxyrecordingsystemmaybe nonlinear inthe theway wayitit cyclicity carbonate recordprior priorto tothe theintensification intensification "remembers" the This may of Northern glaciation this "remembers" theclimatic climaticresponse. response. Thisscenario scenario maybe beapplicaapplica- of NorthernHemisphere Hemisphere glaciationsuggests suggests thisHeinrich-like Heinrich-like ble level records records [Crowley [Cmwleyetal., cyclicity must must arise arise from from some some portion portion of the climate climate system system other other ble to to lake lake level et al.,1992]. 1992]. cyclicity of the 3.4. Clipping 3.4. Heinrich-Like Heinrich-LikeCydlicity Cyclicityand andPrecessional Precessional Clipping 185 185 ORTIZ ORTIZ ET AL.: NORTh NORTH ATLANTIC ATLANTIC CARBONATE CARBONATE than the the dynamics than dynamicsof of large largeice ice sheets. sheets.The The presence presenceof of variance variance peaks record peaksat at 8.6 8.6 -- 6.7 6.7 and and5.8 5.8 -- 5.2 5.2kyr kyrin in the theclipped clippedprecession precession record suggests that at least some of the variance observed in these suggests that at leastsomeof the varianceobservedin thesefrefrequency bands bands in in the quency the Feni Feni Drift Drift proxy proxypercent percentcarbonate carbonaterecord record may arise to forcing may arisefrom fromaa nonlinear nonlinearresponse response toprecessional precessional forcingdriven driven by equatorial equatorial clipping. by clipping. Acknowledgments. of the the R/V RN Acknowledgments.We Wethank thankthe thecaptain captainand andcrew crew of JOIDES Resolution, Resolution, the the Leg JOIDES Leg 162 162 co-chief co-chiefscientists, scientists,M. M. Raymo Raymo and at andE. E. Jansen, Jansen,for for their theirassistance assistance at sea seaand andthe theLeg Leg 162 162 sedimensedimentologists the tologistswho whohelped helpedoperate operate theSCAT. SCAT.W. W. Rugh Rughprovided providedassisassistance tancesetting settingup upthe theSCAT SCATsystem systemon onthe theRJV R/V JOIDES JOIDESResolution. Resolution. Samples analysis Samplesfor for shipboard shipboard analysiswere wereprovided providedby bythe theOcean OceanDrillDrilling ing Program. Program.D. D. Hodell Hodelland andK. K.Venz Venzkindly kindlyprovided providedunpublished unpublished carbonate carbonatedata. data.M. M. Lyle Lyleand andP. P.Delaney Delaneyprovided providedhelpful helpfulcritical critical and editorial suggestions. 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