Particulate organic matter and ballast fluxes measured using Time-Series and
Settling Velocity sediment traps in the northwestern Mediterranean Sea
Cindy Lee1*, Michael L. Peterson2†, Stuart G. Wakeham3, Robert A. Armstrong1, J.
Kirk Cochran1, Juan Carlos Miquel4, Scott W. Fowler1,4, David Hirschberg1,
Aaron Beck1, and Jianhong Xue1
1
Marine Sciences Research Center, Stony Brook University, Stony Brook, NY
11794-5000, USA
2
School of Oceanography, University of Washington, Box 357940, Seattle, WA
98195-7940, USA
3
Skidaway Institute of Oceanography, 10 Ocean Science Circle, Savannah, GA
31411, USA
4
International Atomic Energy Agency, Marine Environment Laboratories, 4 Quai
Antoine 1er, MC98000 Monaco
†
Present Address: Ocean Science Consulting and Research, 9433 Olympus Beach Road
NE, Bainbridge Island, WA 98110
*Corresponding Author: 631-632-8741 (Tel); 631-632-8820 (Fax);
cindy.lee@sunysb.edu
1
Abstract
Prompted by recent data analyses suggesting that the flux of particulate organic
carbon sinking into deep waters is determined by fluxes of mineral ballasts, we undertook
a study of the relationships among organic matter (OM), calcium carbonate, opal,
lithogenic material, and excess aluminum fluxes as part of the MedFlux project. We
measured fluxes of particulate components during Spring and Summer of 2003, and
Spring of 2005, using a swimmer-excluding sediment trap design capable of measuring
fluxes both in a time-series (TS) mode and in a configuration for obtaining particle
settling velocity (SV) profiles. On the basis of these studies, we suggest that distinct
OM-ballast associations observed in particles sinking at a depth of ~200 m imply that the
mechanistic basis of the organic matter - ballast association is set in the upper water
column above the Twilight Zone, and that the importance of different ballast types
follows the seasonal succession of phytoplankton. As in other studies, carbonate appears
to enhance the flux of organic matter over opal. Particles must be at least half organic
matter before their settling velocity is affected by ballast concentration. This lack of
change in ballast composition with SV in particles with <40% OM content suggests that
particle SV reaches a maximum because of the increasing importance of inertial drag.
Relative amounts of OM and opal decrease with depth due to decomposition and
dissolution; carbonates and lithogenic material contribute about the same amount to total
mass, or increase slightly, throughout the water column. The high proportion of excess
Al cannot be explained by its incorporation into diatom opal or reverse weathering, so Al
is most likely adsorbed to particulate oxides. On shorter time scales, dust appears to
2
increase particle flux through its role in aggregation rather than by nutrient inputs
enhancing productivity. We suggest that the role of dust as a catalyst in particle
formation may be a central mechanism in flux formation in this region, particularly when
zooplankton fecal pellet production is low.
Key words: MedFlux, organic carbon flux, opal flux, carbonate flux, ballast ratio
hypothesis, Mediterranean Sea biogeochemistry, excess aluminum
Running title: Particulate organic matter and ballast fluxes in the Mediterranean Sea
3
1. Introduction
Most (>90%) particulate organic carbon (POC) that is biosynthesized in the upper
ocean is returned to inorganic form and redistributed in the water column as particles
sink. Much of this remineralization occurs in the 100-1000 m depth, the mesopelagic or
“Twilight” zone, an area that is significantly understudied. Observations made during the
Joint Global Ocean Flux Study (JGOFS) indicate that 50-80% of the vertical flux of
carbon through the mesopelagic zone and into the deep ocean occurs by gravitational
sinking of particles (e.g., Gardner, 2000; Baliño et al., 2001; Fasham et al., 2001). This
redistribution determines the depth profile of dissolved CO2, including its concentration
in the surface mixed layer, and hence the rate at which the ocean can absorb CO2 from
the atmosphere and sequester it in the deep ocean. The depth-scale of OM
remineralization also determines the depth profile of nutrient regeneration, which in turn
determines the time scale for return of mineral nutrients to the photic zone. A
quantitative and mechanistic understanding of water column POC remineralization is thus
critical to predicting the response of the global carbon cycle to environmental change.
Collection of sinking material in a manner that enables identification of underlying
mechanistic controls on settling and decomposition will lead to better estimates of
vertical fluxes and remineralization depth scales.
Particles sinking out of the ocean surface contain both organic matter (OM) and
minerals. Minerals (opal, CaCO3, and aluminosilicates) typically constitute more than
half the mass of sinking particles (Honjo et al., 1982; Buat-Menard et al., 1989; Ittekkot
and Haake, 1990; Honjo, 1996), and this fraction increases dramatically with depth
4
(Armstrong et al., 2002, Klaas and Archer, 2002). Minerals are important for making
low density OM sink, and may also protect OM from degradation (Hedges and Oades,
1997; Nelson et al., 1999), allowing it to penetrate deeper into the ocean. OM
preservation in sediments is thought to occur through sorptive protection with OM in
mineral mesopores or between mineral interstices (Mayer, 1994; Ransom et al., 1997;
Bock and Mayer, 2000; Arnarson and Keil, 2005); and/or through encapsulation of OM
into a resistant organic matrix (Knicker et al., 1996) or biomineral matrix (Ingalls et al.,
2003). Protective interactions between OM and minerals in sinking particles remain to be
elucidated. Nonetheless, Armstrong et al. (2002) demonstrated that ratios of POC to
mineral ballast converge to a nearly constant value (~3-7 wt% POC) at depths >1800 m,
and Klaas and Archer (2002) showed that variability in the OM flux data might largely be
explained (r2 = 85-90%) by the chemical composition of the ballast (i.e., proportions of
opal, carbonate, and aluminosilicate).
To understand better the mechanisms that control particle sedimentation and
decomposition, and thus remineralization depth scale, we developed a sediment trap that
sorts particles into discrete classes in-situ as a function of their settling velocity, thereby
providing samples of particulate matter for chemical analysis with minimal handling
(Peterson et al., 2005). To our knowledge there have been no prior in-situ fractionations
of sinking particles on the basis of their settling velocity followed by chemical
characterization of the individual classes. Using this settling velocity (SV) trap we
observed during the MedFlux program that ~60% of particulate mass in the northwestern
Mediterranean Sea sinks with velocities of 200-500 m d-1 (Peterson et al., 2005, 2008;
5
Armstrong et al. 2008). Estimates of average settling velocities have been made from
observations in the laboratory of phytoplankton and fecal pellet settling, in the field by
SCUBA or submersible observation (Komar et al., 1981; Shanks and Trent, 1980; Asper,
1987; Alldredge and Gotschalk, 1989; Pilskaln et al., 1998), and by using marker events
in trap fluxes measured at different depths (Honjo and Manganini, 1993; Honjo et al.,
1999; Berelson 2002). Most of these measurements or estimates suggest that particles
settle at rates of 70-330 m d-1 in the upper 500 m. Recent statistical evaluation of highresolution time-series fluxes from U.S.JGOFS sites (i.e., ASPS data with cup rotation
times of ≤8.5 days) and from MedFlux suggests an average sinking velocity of 205 m d-1
(Xue and Armstrong, 2008). Armstrong et al. (2008) obtain an average settling velocity
of 242 m d-1 using an independent statistical approach on the MedFlux SV trap data.
In this paper we describe compositions of particles collected during the 2003 and
2005 MedFlux field programs. We compare the chemical nature of particles sorted by
settling velocity using our IRS-SV sediment traps (Peterson et al., 2005; 2008) with
material collected during the same deployment using the conventional time-series (TS)
mode to evaluate relationships between physical and biological forcing and particle
dynamics.
2. Methods
2.1. Time-series samples
MedFlux sampling was carried out at the French JGOFS DYFAMED (Dynamics
of Atmospheric Fluxes in the Mediterranean) site in the Ligurian Sea (northwestern
Mediterranean), ~52 km off Nice (43°20”N, 7°40”E) in ~2300 m water depth. This
6
location was chosen because its physics, biology, and chemistry have been extensively
studied for over a decade (Marty, 2002), providing a wealth of background information
with which to design our field program and interpret its results. The region is described
further in the Preface to this volume (Lee et al., 2008).
In 2003, a mooring with sediment trap arrays was deployed 6 March (Day of Year
65) and recovered 6 May (DOY 126); this trap deployment will be referred to as Period 1
(P1). The array was redeployed a week later on 14 May (DOY 134) and recovered again
on 30 June (DOY 181); this trap deployment will be referred to as Period 2 (P2).
Indented-rotating sphere (IRS) valve traps (Peterson et al., 1993) were fitted with TS
carousels to determine temporal variability of particulate matter flux. TS traps were
fitted with “dimpled” spheres (see Peterson et al., 2005, 2008). Vertical flux at ~200 m
depth is considered to be equivalent to new or export production (Miquel et al., 1994),
and traps sampled at 238 and 117 m during P1 and P2, respectively. We also collected
TS material at 711 m during P1 and at 1918 m during P2. Upon recovery, samples were
split using a McLane™ WSD splitter to allow multiple chemical analyses. Here we
report 2003 data on TS particulate mass, and the contributions of organic carbon (OC),
opal, lithogenic material and calcium carbonate to mass. Particulate radionuclide
(Stewart et al., 2007; Cochran et al., 2008; Szlosek et al., 2008) and organic
compositional (Wakeham et al., 2008) data are presented elsewhere.
In 2005, traps were deployed as described above for 55 d during a single period
from 4 March (DOY 63) to 1 May (DOY 121). TS traps were fitted with “dimpled”
spheres (Peterson et al., 2008). TS particulate matter was collected from 313 m and
7
924 m. Here we report 2005 data on TS particulate mass, and on the contributions of
organic carbon (OC), opal, lithogenic material, and calcium carbonate to mass.
2.2. Settling velocity samples
In-situ settling-velocity separation of particles was accomplished by rotating the
IRS valve once daily and then rotating the sample carousel through all 11 sample
chambers at appropriate time intervals over the next 24 h as described in Peterson et al.
(2005). SV traps were fitted with “saddle and ridge” spheres in 2003 and with “deeper
grooved” spheres in 2005 (see Fig. 3B and C in Peterson et al., 2008). Because each cup
of the SV trap collects material daily over the entire deployment, fluxes measured in
individual cups of the SV traps are not directly comparable to the TS trap samples, which
are collected in sequential and discrete time intervals. Thus, we will report both total
mass flux for the entire deployment period, and discrete time-integrated mass flux
densities (IMFD) for each collection cup. IMFD as we use it is the mass of any
constituent per square meter of trap area integrated over the length of the deployment
time and divided by the width of the SV interval, which is defined here as the
dimensionless log10 of the ratio of the highest and lowest settling velocities in each SV
interval, or log10(SVmax) – log10(SVmin) = log10(SVmax/SVmin) for that interval. IMFD has
the same relationship to mass flux that probability has to probability density: if IMFD is
plotted as a histogram, the area of each bar of the histogram is mass flux per square meter
in that SV class, while the height of each bar is the IMFD (Armstrong et al., 2008). This
normalization procedure allows us to compare samples collected using different SV
intervals as we did in 2003 and 2005.
8
In 2003, the sample carousel was advanced at 1, 1, 3, 5, 10, 25, 45, 90, 180, 360
and 720 min intervals after each IRS rotation. The free-fall distance between the IRS
valve and the sample carousel was 0.68 m, so that the sequence of settling velocities was
0.68-1.4, 1.4-2.7, 2.7-5.4, 5.4-11, 11-22, 22-49, 49-98, 98-196, 196-490, 490-980, and
>980 m d-1, respectively. Duplicate SV samples were recovered at 238 m during P1;
samples from duplicate 117-m traps during P2 were combined into a single set of SV
samples due to their small size.
In 2005, we increased resolution in high-SV regions of the settling velocity
spectrum by splitting 2003 SV class 98-196 into 98-140 and 140-196, and 2003 class
196-490 into 196-326 and 326-490 m d-1. The 2005 carousel rotation scheme also
combined the 3 slowest 2003 classes into a 0.68-5.4 m d-1 interval. In 2005, SV samples
were recovered from 313 m, 524 m (duplicates), and 1918 m (duplicates). After
recovery, samples were split and analyzed for multiple constituents as per the TS traps.
For both 2003 and 2005, we report particulate mass and the contributions of
organic (OC), opal, lithogenic material, and calcium carbonate to that mass. Particulate
radionuclide (Szlosek et al., 2008), organic compositional data (Wakeham et al., 2008),
and further analysis of mass flux data (Armstrong et al., 2008) for SV traps are presented
elsewhere in this volume.
2.3. Elemental analyses
Mass was determined by filtering 30-40% (in 2003) or 20% (in 2005) of the
sample onto pre-weighed 0.4 µm Nuclepore filters, gently washing collected material
with 5 ml DI water, drying it overnight at 60°C, and weighing it using a semi-micro
9
balance (10-5 g capability). These samples were later used for radionuclide and ballast
mineral analyses. In 2003, percentages of OC and total nitrogen (TN) in PM were
determined using a procedure modified from that described by Hedges and Stern (1984);
this procedure is described in detail in Peterson et al. (2005). Samples (usually 20% of
the total) on 25 mm GFF filters were subsampled using a punch so that 0.8 or 1.1% of the
total sample was analyzed. Punches were exposed to HCl vapor to remove carbonate and
analyzed using a Carlo Erba 1106 Elemental Analyzer. This analysis also provides a
replicate total nitrogen determination (TNa). Because analytical precision associated with
small and inhomogeneous 2003 P2 samples gave unusually variable CN results (Peterson
et al., 2005), samples from this series were rerun using a CE Instruments NC2500
elemental analyzer. We report the average of the two 2003 results here.
Carbon and nitrogen contents for samples from 2005 were run in duplicate at
SUNY using a Carlo Erba model 1602 CNS analyzer. We used 7-mm punches from a
10% split of the total sample; 1.2% of the total sample was analyzed. Inorganic carbon
(IC) was removed using 10% HCl, and carbonate content was generally less than 5% of
total carbon. For this analyzer, precision for N is ±5%, and for C is ±2%.
Sediment trap samples were analyzed for elements representative of dust (Ti, Al)
and biogenic minerals (CaCO3 and opal). For 2003 samples, a small amount of material
was removed from the mass filters and digested following trace-metal-clean protocols
(Eggiman and Betzer, 1976; Brügmann and Kuss, 1999; Newman and Zhao, 2005).
Samples were sequentially leached with base and acid to separate biological and
10
lithogenic silica (Brzezinski and Nelson, 1989; Ragueneau and Tréguer, 1993). Al and Ti
concentrations were measured using Inductively Coupled Plasma Mass Spectrometry
(ICP-MS) on a ThermoFinnigan Element II ICP-MS. Silicon concentration was
determined by Graphite Furnace atomic absorption spectrometry (GF-AAS), and Ca
concentration by Flame-AAS using a Perkin-Elmer AAnalyst 800 spectrometer. In 2003,
sample sizes were extremely small, 0.1-0.5 mg in low flux times, so that analytical error
was subsequently high (50-75%). Even in high flux times, sample size was only 1-4 mg.
For 2005 samples, a 20% split of each sample was digested in 750 µL HCl, 250 µl HNO3,
and 50 µl HF (all trace-metal clean) for one week and diluted to 10 mL with MilliQ
water. Total Si, Ca, Al, and Ti were measured using atomic absorption spectrometry.
Elemental concentration data are archived on the Medflux web site
(www.msrc.sunysb.edu/MedFlux).
Elemental data are presented as masses of CaCO3, opal, OM, lithogenic material
and Alxs. Calcium carbonate was determined from measured IC values, assuming that all
carbonate was present as CaCO3. This estimate using IC values was always higher than
estimates using Ca concentrations, suggesting the presence of other carbonates as well.
In 2003, opal was determined from Si measured in the base leachate; we assumed that
opal is present as SiO2.H2O. In 2005, Si in the biogenic fraction was determined from
measured total Si minus lithogenic Si, which was calculated as three times lithogenic Al
(Klaas and Archer, 2002). Lithogenic material (in mg) was calculated as 11.9 times the
lithogenic Al concentration (in mg), i.e., the measured total Al minus Alxs, where Alxs
was determined as the difference between measured total Al and 15.4 times the measured
11
total Ti (Murray and Leinen, 1996). The Al/Ti ratio in Saharan dust is the same as in
average crustal rock (Formenti et al., 2003). Aluminum is seawater is found as 76.2%
Al(OH)4- and 23.7% Al(OH)3 (Van den Burg et al., 1994), so we report excess Al as ‘Alxs
hydroxide’ using a molecular weight of 81. OM was determined as 2.199 times OC
concentrations (Klaas and Archer, 2002). When plotting ballast and OM compositions
below, we determined their proportion of the total calculated mass (i.e., sum of the
individual OM, CaCO3, opal, and lithogenic material masses) rather than the measured
mass. Total calculated masses averaged 66% of the measured mass in spring 2003, 87%
in spring 2005, and 104% in summer 2003. We exclude ‘Al hydroxides’ from the
composition plots for comparison with past work.
Dissolved organic carbon (DOC) was measured in all 2003 TS and SV trap cups
using a high temperature combustion MQ-1001 analyzer after acidification (Peterson et
al., 2003). DOC in excess of background seawater values made up 5-10% of the total
organic carbon (POC + DOC). This OC undoubtedly was lost from the particles through
leaching. Dissolved opal and carbonate were not measured.
2.4. Ancillary Data
Dust deposition was examined using the Dust REgional Atmospheric Model
(DREAM) of the Barcelona Supercomputing Center for the periods of trap deployment
(http://bscct01.bsc.es/DREAM/index.psp). This model forecasts deposition of Saharan
dust to the Mediterranean Sea and adjacent areas. Based on 6 h forecasts, major
(50-300 mg m-2) deposition events during P1 in 2003 occurred March 1-3 and 29-31, and
April 29-30, at times reaching 750-1500 mg m-2. Dust events are indicated by shading on
12
Figs. 1 and 2 with higher intensity events in darker shading. Dust deposition continued
throughout May with a very large (1500-4500 mg m-2) event May 7-8 that was verified
by personal observations during the trap turn-around cruise. During P2, dust deposition
continued throughout May and into early June. Bartoli et al. (2005) report dust events
during these same times based on recordings from Cape Ferrat near Nice. Their 6-y
record between 1986-2003 shows major interannual variation in both timing and
magnitude of dust events at this location. During the 2005 trap deployment, the DREAM
model forecast major dust deposition events for March 26-27 reaching 300-750 mg m-2,
and April 10-18 at times reaching 1500-4500 mg m-2, generally higher than during 2003
P1. In a detailed study of dust deposition at DYFAMED from 1983-1994, Moulin et al.
(1997) found that two-thirds of the dust was deposited in summer, that there was an
average of 16 events per year, and that deposition was highly variable from year to year.
Time-series chlorophyll-a and sea surface temperature data were obtained from
Jay O’Reilly and Teresa Ducas, NOAA/NMFS in Narragansett, RI, USA, who used
SeaWiFS data for Chl and AVHRR, TERRA, and AQUA datasets for SST. We
compared 2003 chlorophyll concentrations and 2003/2005 integrated primary
productivity measurements available from J. Chiaverini as part of the DYFAMED project
(http://www.obsvlfr.fr/sodyf/Main Frameset_en.htm) with the satellite Chl data and
found good agreement in the patterns over time where the samples overlapped.
13
3. Results
3.1. TS and SV trap mass flux comparison
Replication of sediment trap data is rare because it generally means
compromising the number of depths sampled in what are generally instrument-limited
experiments. However, we felt it more important to test the reproducibility of the traps
than to obtain more depth information. Replication produced reasonable results. To
compare average fluxes in 2003, we summed mass collected in all cups in each TS and
SV trap and calculated an average flux over the entire deployment time (Table 1). At
238 m during P1, TS trap mass flux averaged 346 mg m-2d-1 over the 61-d deployment
period, ranging from 82-910 mg m-2d-1. The average SV trap flux at 238 m
(298 mg m-2d-1) was about 15% lower. Duplicate SV trap fluxes at 238 m agreed within
10%. The TS trap flux at 771 m averaged 260 mg m-2d-1 over the entire P1 deployment
period, about one-third lower than mass fluxes at 238 m. During P2, TS trap flux at
117 m averaged 46 mg m-2d-1, while the SV average trap flux (samples from duplicate
traps were composited before analysis due to their small size) was 35 mg m-2d-1, about
25% less. At 1918 m, TS mass flux averaged 72 mg m-2d-1, about twice the surface mass
flux.
In 2005, TS trap fluxes averaged 426 mg m-2d-1 at 313 m and 322 mg m-2d-1 at
924 m, higher than in 2003 at similar depths and time (Table 1). Maximum mass fluxes
were similar in the two years, 1020 mg m-2d-1 in 2003 and 906 mg m-2d-1 in 2005. SV
trap samples were recovered at 313 m (475 mg m-2d-1), and in duplicate at 524
(498 mg m-2d-1) and 1918 m (438 mg m-2d-1). The SV flux over the entire deployment
14
time at 313 m was 11% higher than the total TS flux at this depth. Average mass fluxes
from duplicate traps agreed well, within 6% at 524 m, and within 1% at 1918 m.
3.2. Time-series fluxes
3.2.1. 2003 time-series
Elevated surface mass and OC fluxes occurred in early spring (Fig. 1A, B) when
phytoplankton, mostly diatoms, normally bloom in the northwestern Mediterranean
(Marty and Chiavérini, 2002); the 2003 spring bloom period was confirmed by satellite
Chl (Fig. 1C), although there was no correlation between Chl concentration and mass
flux. In addition to the high flux at the start of P1, there are two much smaller peaks in
surface OC flux between Days 100 and 140 (Fig. 1B), also seen in mass. Satellite Chl
showed a peak during the first flux event, but not the second. This second peak
(extrapolated peak since part of it was during the trap redeployment) was just after a dust
deposition event. Surface mass flux continued to decrease further until the end of the
deployment. Although mass and OC fluxes at 771 m at the start of P1 were much lower
than those nearer the surface, they quickly increased to surface values, suggesting that the
surface water phytoplankton bloom quickly reached 771 m and that little particulate mass
was lost between these depths. OC fluxes were slightly lower at 771 m than at 238 m,
however, suggesting some loss in OC with depth. During P2, higher mass fluxes at 1918
m than at 117 m might be due to the deeper trap collecting material produced earlier in
surface waters or to resuspension (see SV elemental compositions below).
Maximum fluxes of particulate mass and OC at 238 m during P1 in 2003 (Fig. 1A
and B) were about 3 times higher than maximum 200-m fluxes (335 mg m-2d-1mass and
15
39 mg m-2d-1 OC) measured previously at DYFAMED in 1987-1988 (Miquel et al., 1994,
when DYFAMED was slightly closer to Corsica), and in 1993-1995 (Stemmann et al.,
2002). In 2005, the MedFlux mass flux was slightly higher, but the OC flux lower, than
at DYFAMED. The decadal flux record of Miquel and La Rosa (1999) shows
considerable annual variation in flux. The lower mass (150-200 mg m-2d-1) and OC (1520 mg m-2d-1) fluxes we measured after the bloom period in 2003 were comparable to
previous flux measurements in the post-bloom summer oligotrophic conditions common
at this site. We did not observe the 4-fold decrease in mass and OC flux with increasing
trap depth that was observed by Miquel et al. (1994) between 200 and 1000 m (see Figs.
1 and 2).
Calcium carbonate, opal, lithogenic aluminosilicates, and Alxs were measured to
evaluate relationships between OM and ballast minerals (Table 2). Carbonate and
aluminosilicate fluxes measured nearby during 1987-1988 as part of the DYFAMED
project, ~25-150 mg m-2d-1 at 200 m and <10 mg m-2d-1 at 1000 m (Miquel et al., 1994),
were slightly lower than MedFlux fluxes.
3.2.2. 2005 time-series
Fluxes of bulk particulate matter (mass) and OC in 2005 (Fig. 2) did not exhibit
the same pattern observed in 2003 (Fig. 1). Data collected before 60 DOY would have
been useful in making the comparison. However, in general, one might consider that
both years are characterized by a succession of mass flux peaks in the spring, each
successive one smaller than the previous; a possible explanation for this will be discussed
later. Satellite data from 2005 show that Chl was generally lower throughout the spring
16
than in 2003; Chl peaked in April, 2005, two weeks after the Chl record indicated the
2003 bloom period had already ceased (Figs. 1C and 2C); as in 2003, there was no clear
correlation between surface mass flux and satellite Chl in 2005. However, mass flux
peaks in March and April 2005 followed dust events; peaks in 2003 also followed dust
events, although they were not as closely correlated as in 2005 due to the larger number
of small events in the later year. Calcium carbonate fluxes were similar in magnitude to
those during the same period in 2003, but lithogenic and opal fluxes were higher (Table
3). Alxs fluxes were much higher, and OM fluxes much lower than in 2003.
3.3. Time-series sample compositions
OC concentrations increased with time in both 2003 and 2005. At both 238 m
and 771 m during the 2003 P1 deployment %OC increased from ~6% during the high
mass flux period to ~15% when mass fluxes fell at the end of the bloom period (Fig. 1D).
During the second deployment period, low particle fluxes even at 117 m made accurate
measurements of %OC difficult to achieve (see Peterson et al., 2005, for details).
Nonetheless, %OC at 117 m was higher (~20-30%) than during the bloom period.
During the second deployment, %OC at 1918 m was ~5-10% until the very last samples
when it rose, approaching values for material collected in the 117 m trap, possibly
suggesting that high OC, surface-derived material was reaching the deep trap at that time.
OC:TNatomic (%TN data not shown, but see Peterson et al., 2005) at 238 m decreased
from a high of 12-13 during Days 60-80 of P1 to a low of ~8 after the bloom/high flux
period, indicating higher amounts of fresh planktonic OM. During P2, OC:TNatomic at
117 m was lower, ranging from ~6-9, although small sample size made this analysis
17
uncertain. Organic C and N compositions for MedFlux were consistent with DYFAMED
measurements in 1987-88 reported by Miquel et al. (1994), where surface %OC increased
from spring (8-10%) to summer (20-25%), but with little change (~5%) at 1000 m.
Lower OC:TNatomic values (5-6) were associated with the bloom compared to both preand post-bloom (10-12).
In 2005, OC concentrations (Fig. 2D) increased during the deployment with
similar values at 313 and 924 m until the last collection period. Values were initially
~3% at both depths, lower than in 2003, and increased to 13% at 313 m and 8% at 924 m.
Satellite Chl (Fig. 2C) suggested the beginning of a bloom just as our deployment period
ended, but the OC flux does not show any general increase. OC:TNatomic (data not shown)
at 313 m was slightly higher than in 2003, varying between 10 and 16, with the lowest
numbers when flux was highest.
Relative contributions of calcium carbonate, opal, and lithogenic material
changed over the course of the 2003 time series (Fig. 3). CaCO3 and organic matter
became dominant as time progressed from P1 to P2, while lithogenic material and opal
decreased in importance. Lithogenic material became more important in the deep trap
during P2, probably because this material does not dissolve easily. During 2005, relative
contributions of calcium carbonate, opal, and lithogenic material in shallow traps differed
greatly from those in 2003 (Fig. 4). CaCO3 made up less of the flux in 2005, and
lithogenic material much more of the material, resulting in an almost equal distribution of
the three ballast types in surface material. Proportions of both CaCO3 and lithogenic
18
material increased with depth, while opal almost disappeared. Average compositions at
all depths for TS traps in 2003 and 2005 are shown in Table 4.
Alxs constituted most of the Al in our 2003 TS samples, and its proportion
increased slightly with depth, 90% in the 238-m samples in May and virtually all the Al
at 771 m; in July Alxs was 77% of total Al in the 117-m sample, and 89% at 1980 m. In
2005, Alxs made up 67% of the total Al at 313 m and 66% at 524 m. Dissolved aluminum
concentrations in the Mediterranean are the highest in the World’s oceans, 200 times
higher than in the Pacific Ocean and 4 times higher than in the Atlantic Ocean (Chou and
Wollast, 1997; Alibo et al. 1999; Guerzoni et al., 1999); these high concentrations likely
explain why sorbed Al is so high. In sediments of the equatorial Pacific and Western
Antarctic Peninsula, Kryc et al. (2003) and Murray and Leinen (1996) found Alxs to be
40-50% of the total Al, considerably less than we found in the Mediterranean.
To investigate the relation between dust and mass flux, we plotted 2003 lithogenic
Al concentrations versus mass fluxes (Fig. 5). There was a clear relation seen that was
not observed when lithogenic and biogenic Ca or Si were regressed against mass flux
(latter data not shown). In 2005 when the concentration of lithogenic Al was much
higher and a shorter time series was collected (Spring only), this relation was not evident.
To investigate the relationship between OM and each ballast type over the course
of the experiment, we plotted ratios of CaCO3, opal, lithogenic material, and Alxs
hydroxide to total ballast against the ratio of OM to mass in all shallow TS trap samples
from both years (Fig. 6). The ballast types were divided by total ballast mass instead of
total particle mass to remove the influence of OM. This plot shows that CaCO3
19
dominates the ballast types at all OM concentrations and becomes more important as OM
content increases. Opal and lithogenic material are equally important at low OM
contents, but as OM content increases, the importance of opal slightly decreases and the
importance of the lithogenic fraction increases.
3.4. Settling velocity flux densities
In both 2003 and 2005 time-integrated mass flux densities calculated from SV
sediment traps exhibited a large peak at high settling velocities (200-500 m d-1) and a
much smaller tailing peak at the lower end of the SV spectrum measured (Fig. 7). SV
spectra remained the same regardless of differences in phytoplankton bloom development
in 2003 (P1 vs. P2), annual differences between 2003 and 2005, and differences in depth
in 2005 (Fig. 8). About 60-70% of total flux was found in the rapidly settling class at all
times and depths. This peak was better resolved in 2005 than in 2003 because we
changed the carousel rotation scheme to sample more frequently in that SV range (200500 m d-1). In a companion paper, Armstrong et al. (2008) describe a model of these SV
spectra that calculates average and modal particle settling rates; possible causes of the
smaller secondary peak at the lowest settling velocities are also discussed.
Profiles of organic matter and ballast flux densities showed the same general shape
as IMFD in 2003 (Fig. 9 A, B) and 2005 (Fig. 10). Since each SV cup collects material
over the entire deployment time, the composition of these samples will on average reflect
the contribution of material falling during the time period when fluxes were highest. In
2003, calcium carbonate flux densities in SV traps were highest during P1, and CaCO3
fluxes were also highest in TS traps at this time. In P2, when TS mass fluxes were
20
generally lower, SV OM and CaCO3 flux densities were similar. In 2005, lithogenic
material and opal flux densities were highest. Ballast flux densities were similar at 313
and 524 m, and increased somewhat at 1918 m, suggesting a source from lateral input or
sediment resuspension. The agreement in OM and ballast flux densities between
duplicate traps at 524 m and 1918 m can be seen in Figure 10. SV flux density spectra
were generally similar, but there were major differences in composition.
3.5. Settling velocity sample compositions
Surface SV trap compositions were generally similar to those in the corresponding
TS trap samples. Indeed, the higher %OC in P2 than P1 observed in TS samples in 2003,
is clearly also seen in SV samples (Fig. 8B). There was a very slight downward trend in
%OC with higher settling velocities in P1 of 2003 (Fig. 8B), but this trend was seen in
2005 (Fig. 8D) only if the first and last points were excluded. Values of %OC in 2005
generally decreased with depth, being highest at 313 m and lowest at 1918 m.
Relative contributions of OM (from calculated rather than measured mass as
described earlier) and ballast materials in surface traps in P1 of 2003 showed no clear
trend with SV (Fig. 9 C); OM and CaCO3 dominated all SV classes. The slight
downward trend seen in %OC in P1 was not seen on a relative basis for OM. In P2, OM
was generally a larger fraction of the material in slower settling fractions, while ballast
made a greater contribution to faster settling particles (Fig. 9D). As in TS traps, opal and
lithogenic material were much less important in P2. Alxs constituted over half of the total
Al in 2005 samples: 66% in the 313-m samples, 73% at 524 m, and 66% at 924 m and
21
1918 m. These percentages were somewhat less than in 2003, and no clear increase with
depth was observed.
As in the high flux P1 of 2003, there was very little trend in composition with
settling velocity in 2005 (Fig. 11), and compositions were generally similar to those of
the TS traps (Fig. 4). However, surface sample compositions were very different
between the two years; organic matter and carbonate were more important in 2003, while
the ballast minerals were equally important in 2005 at the same time. The 2005 313-m
SV trap showed relatively equal contributions from the three ballast types as did the TS
trap at this depth. SV trap compositions at 524 m were similar although lithogenic
material contributed more to these traps than at 313 m. Traps at 1918 m had less opal
and more carbonate than more shallow traps. Comparison of duplicate SV traps at
1918 m shows reasonable agreement for the various parameters, while there is more
variation between the traps at 524 m. Opal was particularly lower in SV2 at 524 m.
Average compositions at all depths for SV traps in 2003 and 2005 are shown in Table 4.
4. Discussion
4.1. Relation between organic matter and mineral ballast fluxes
A major goal of MedFlux was to elucidate the relationship between mineral
ballast and the vertical transport and remineralization of POM. We focused on opal,
carbonate, and aluminosilicate, generally indicative of biogenic silica in diatoms,
calcareous shells of coccolithophores and foraminifera, and lithogenic inputs from
riverine and atmospheric sources, respectively. Analyses of sinking particle fluxes
previously indicated that the rain of mineral ballast is an excellent predictor of POC flux
22
in the deep ocean (Armstrong et al., 2002). Klaas and Archer (2002) further examined
published data on POC flux below 1000 m from 52 locations in the world ocean and
concluded that POC flux could largely be explained by chemical composition of the
ballast. They suggested that different ballast types transport different amounts of OC,
with opal being less efficient than carbonate. Since transport efficiencies of different
ballast types did not vary significantly with depth below 1000 m, the data analyses of
Armstrong et al. (2002) and Klaas and Archer (2002) suggested that the mechanistic basis
of these patterns resides largely in the upper 1000 m. Data presented here from surface
traps show a correlation between OM and mineral ballast types (Fig. 6; see discussion
below), and further suggest that the mechanistic basis of the OM-ballast association
occurs even above ~200 m.
Data from the MedFlux study in both 2003 and 2005 demonstrate temporal and
depth-related trends in OM and ballast content. In the 2003 shallow traps (238 and 117
m) where we have the longer time series, ballast minerals comprised on average 65-70%
of the particle mass during the high flux P1, decreasing to 55-60% in P2 when mass flux
was lower (Table 4). This can also be seen in more temporal detail in TS measurements
of carbonate and opal fluxes showing higher relative contributions of organic matter and
reduced contributions of mineral ballast in surface traps during the lower flux period
(Fig. 3). CaCO3 was consistently the dominant mineral phase in sinking particles, but its
relative abundance was lower during the first deployment period when the diatom-bloomdriven opal flux was strong. Lithogenic minerals were not a major component of the
overall ballast flux in 2003. These observations are consistent with the seasonal
23
phytoplankton succession (spring diatom bloom, followed by haptophyte and
chrysophyte nanoflagellates, and eventually picoplankton) as stratification and
oligotrophic conditions build in the NW Mediterranean summer (Marty and Chiavérini,
2002).
In spring 2005, lithogenic material made up a quarter of the mass flux, and Alxs
was a significant fraction (5-6%). The contribution of biogenic opal was similar to the
2003 spring data, while OM and CaCO3 contributed less to the total mass than in 2003.
The dust records, while incomplete, suggest that dust input was more intense in 2005 than
in 2003. The observation that both large and small mass peaks were usually preceded by
a dust event is consistent with a role for dust in catalyzing particle-sinking events. What
appears to be a succession of smaller flux peaks could be due to a succession of dust
events; that the flux peaks become smaller with time could be because previous dust
events had scavenged from the water column most of the biogenic particles produced
earlier in the spring. In a similar vein, Guerzoni et al. (1999) concluded that atmospheric
nutrient inputs were negligible during the most intense part of the upwelling period at
DYFAMED, but that during stratified periods of summer and early fall, atmospheric
inputs might trigger “small but detectable phytoplankton blooms”.
Dust can play two roles in particle flux. It can release nutrients that result in algal
growth (possibly a bloom), although in the Mediterranean these blooms do not appear to
be as large as those due to seasonal convective overturn, in spite of the relatively large
dust inputs (Guerzoni et al., 1999; Herut et al., 2002; Dulac et al., 2004). Dust is also a
strong catalyst of aggregation of suspended particles that results in sinking (Hamm,
24
2002). Although there can be problems interpreting satellite data in the presence of dust
(Schollaert et al., 2003), the lack of correlation between satellite color data and flux (Figs.
1 and 2), and the suggestion of a correlation between dust and flux (Fig. 5), indicates that
dust may be behaving more as a particle aggregator than as a source of nutrients that
stimulates algal growth.
The relationship between ballast type and organic matter follows an interesting
pattern that may be related to seasonal phytoplankton succession during the change from
a mesotrophic to an oligotrophic ecosystem at this site (Fig. 6). In shallow sinking
particles collected in 2003 and 2005, the proportion of the ballast made up by carbonate
generally increases as %OM increases, while the proportion made up by opal is relatively
independent of %OM, and the proportion of lithogenic material decreases. Alxs
hydroxide also decreases with increasing %OM, but always makes up a small percent (112%) of the total ballast. Sinking particles from the lower Chl period early in 2005 make
up most of the data set for particles with <20% OM (or ~10%OC). These low-OM
particles were the only samples in which lithogenic material made up a significant
portion of the ballast. Samples with %OM between 20-40% are mostly from 2003 P1
and early P2 samples. Samples with %OM greater than 40% are from 2003 P2 and were
clearly dominated by carbonate as the ballast material. If this pattern holds more widely,
it suggests that there are distinct OM-ballast associations in particles sinking at 100300 m depth that depend on the state of phytoplankton succession. We will discuss this
again later in terms of particle flux mechanisms.
25
One caveat in the discussion of MedFlux ballast data is that masses calculated by
summing ballasts (including ‘Al hydroxide’) averaged 66% of the measured mass in
spring 2003, 87% in spring 2005, and 104% in summer 2003. Calculated mass deficits
and excesses for MedFlux and other samples are treated in detail in Xue (2008). The
lower mass deficit in the spring than during the summer is interesting and warrants
further investigation into other metal oxides that might serve as additional ballast at that
time.
4.2. Effect of ballast composition on settling velocity
Our ability to sample a settling velocity profile allowed us to investigate further the
depth at which the association between organic matter and ballast minerals was initiated.
Although Armstrong et al. (2002) investigated this relationship primarily below 2000 m,
we wanted to determine whether differences in density caused by differences in ballast
composition might affect the settling rate of particles in the upper water column. Stokes
Law suggests that particle SV is proportional to the particle radius squared and the
particle’s excess density relative to seawater (McCave, 1975); minerals normally provide
a large part of the density differential needed to allow particles to sink (Smayda, 1970;
McCave, 1975; Ittekkot and Haake, 1990; Honjo, 1996). Since OM is less dense than
mineral ballast, the density of the same size particle would decrease as the OM content
increases, and the particle should sink more slowly. However, Khelifa and Hill (2006)
suggest that settling velocity reaches a maximum because of the increasing importance of
inertial drag on large flocs. Since the Reynolds number for large flocs is around unity,
Stokes Law no longer applies.
26
In fact, ballast composition appeared to have little or no effect on particle settling
velocity in MedFlux SV samples in the SV range measured, but this may also depend on
the state of the phytoplankton community that produces the particles. Before and during
blooms in 2003 P1 and 2005 when OC was < 20% (and OM <40%), there was very little
relation between ballast composition and settling rate of traps >200 m (Figs. 9C and 11).
Although there were differences in composition between the two years, the basic pattern
of OM and ballast type did not change with SV. However, during 2003 P2 when OM
was greater than 40% and most of the mineral ballast was CaCO3, there was a definite
increase in %OM as SV decreased (Fig. 9D). This suggests that particles must be at least
half organic matter before their settling velocity is affected by ballast concentration. A
particle that is half OM (and thus lower in ballast) may form larger and fluffier
aggregates that sink more slowly (Passow and De la Rocha, 2006; Engel et al., 2008a; see
later discussion). It should be noted that although we had planned to have spring and
summer traps at the same 200-m depth, they were in fact 100 m apart, so part of the
differences observed could be due to this depth difference.
An interesting realization is that for particles of all settling velocities to have the
same composition as in spring 2003 and 2005, the particles must be homogeneous and
fragment to different sizes, or they must undergo considerable exchange with each other.
At times when the composition varies with SV such as in summer 2003 P2, these
constraints are not necessary. Since the spring flux peaks usually originate from
phytoplankton bloom-zooplankton interactions throughout a deeper (20-30 m) mixed
layer, and the summer material usually originates at the deep Chl max at or below the
27
base of the 5-10 m mixed layer (Faugeras et al., 2003; D’Ortenzio et al., 2005), the
material sinking at the two times is likely to be very different as suggested by the ballast
composition in Fig. 9 and the organic matter composition in Wakeham et al. (2008).
4.3. Change in composition with depth
Changes in average composition of particles with depth were generally in line with
expectations from previous trap studies (Table 4). The proportion of OM decreased with
depth during all three sampling periods. During 2003 P1 and in 2005, both of which
were during spring (March-May), opal was relatively high in surface waters (20-30%)
and decreased with depth. The higher proportions of opal and lithogenic material in the
deepest samples suggest that these traps may in fact contain some resuspended material
that had been transported laterally from the continental margin. However, it has
generally been thought that advection of riverine particles or resuspended sediments to
this site is minor, since no major rivers flow into the basin, and the Ligurian current
decreases the offshore transport of suspended coastal material (Copin-Montegut, 1988;
Durrieu de Madron et al., 1990) and coastal plankton (Stemman et al., 2002).
Temperature-salinity diagrams from our cruises (data available on the U.S. Ocean Carbon
and Biogeochemistry Data System, http://ocb.whoi.edu/jg/dir/OCB/MedFlux/) show little
change near the bottom and reflect the presence of Western Mediterranean Deep Water
throughout 2003 and 2005 (Ahumada and Cruzado, 2007). There was no sign of
influence from the Eastern Mediterranean transient (Schröder et al., 2006) at the time of
sampling.
28
The proportion of CaCO3 generally remained constant or increased slightly with
depth suggesting that it did not dissolve quickly. Production of foraminifera is likely to
account for much of the CaCO3 production in the surface waters (Schiebel, 2002), but
would not cause an increase with depth. During the 2003 summer low-flux P2, %CaCO3
was higher (41-54%) than in spring samples, partly due to the lower opal content at that
time. Lithogenic material was 2-10 times higher in 2005 than in 2003. We do not have a
complete dust record for the two years, but what we have suggests major dust inputs in
both years, with 2005 inputs being more intense (Figs. 1 and 2). In addition, Ti
concentrations measured in MedFlux samples averaged twice as high in 2005 as in 2003.
Lithogenic material increased slightly with depth in 2003 P2 and 2005, but high
contributions of lithogenic material at 1918 m may be due to lateral input or sediment
resuspension as mentioned above. Previous sediment trap experiments near the present
DYFAMED site have shown that particulate Al derived from an atmospheric pulse of
dust could be removed from the surface layers and exported below ~200 m in less than
two weeks primarily through the repackaging of dust-containing particles into rapidly
sinking zooplankton fecal pellets (Buat-Menard et al., 1989). Zooplankton sampling
during both the MedFlux cruises suggested a greater abundance of zooplankton during
spring 2005 than was noted in 2003 (data not shown).
Fluxes of Alxs, or that not accounted for by aluminosilicates, were also higher in
2005 than 2003, mirroring the lithogenic material. Alxs could come from several sources,
e.g., incorporation into diatom opal, diagenetic exchange, or adsorption onto the surface
of particles. Incorporation into the structure of diatom opal appears to result in Al:Si
29
ratios that are no more than 0.16 even in Al-rich coastal waters (Van Bennekom et al.,
1989; Van Cappellen et al., 2002). In MedFlux surface particles, Alxs: biogenic Si ratios
averaged 0.4 in 2003 and 0.8 in 2005, suggesting that Alxs is added to the biogenicallyderived particles post-mortem. Higher values in 2005 than 2003 (and in P1 than P2)
likely reflect the greater dust inputs in P1 and in 2005. Michalopoulos et al. (2000) found
that diatom opal was converted to aluminosilicates as part of diagenetic alteration in
sediments; but in laboratory diatom cultures, they found that this reverse weathering
process takes 20-23 months, months longer than the residence time of large particles in
the water column. Therefore the most likely source for the Alxs is surface complexation
of dissolved Al in the water column onto oxyhydroxides as suggested by Kryc et al.
(2003). As mentioned earlier, dissolved Al concentrations are 4-200 times higher in the
Mediterranean than in the Pacific and Atlantic Oceans (Alibo et al. 1999), and would thus
provide a ready source of Al for surface sorption. In their leaching studies, Kryc et al.
(2003) found that Alxs was associated with oxides rather than OM; our results also show
little association between Alxs and OM relative to other components of the particulate
fraction (Fig. 6). Alxs flux does not increase with depth, however, suggesting that if
sorption is indeed the major source, scavenging is complete in the surface waters.
4.4. Implications for particle flux mechanisms
Armstrong et al. (2002) suggested that at least two mechanisms might determine
the asymptotic POC:ballast ratios observed below 2000 m. One was that OM was
protected in some unknown way by the mineral matrix, perhaps through surface sorption
(Mayer, 1994) or entombment (Lowenstam and Weiner, 1989; Knicker et al., 1996). The
30
second was that OM might serve as a glue to bind particles together. If the OM “glue” is
too low in concentration, either inherently as particles are first formed or as OM
degrades, particles could disintegrate. Klaas and Archer (2002) and François et al. (2002)
further suggested that carbonate was the most important carrier of POC, implying a
biogenic source of OM from coccolithophores and foraminifera.
A significant modification to these mechanisms was recently suggested by
Passow (2004). She pointed out, as had Klaas and Archer (2002), that OM degradation
rates in the deep sea are not fast enough to account for the reduction in organic content of
surface particles to 5%, so that protection from degradation by ballast minerals was moot.
Given that we now find particle settling velocities to be at least twice as fast as previously
assumed (Xue and Armstrong, 2008; Armstrong et al., 2008), this argument is even more
cogent. However, in addition to decomposition, we must also consider rapid dissolution
of soluble OM from particles being exported from the euphotic zone, a process that is
always part of early mass loss after cell death. Dissolution is likely to be much faster
than degradation, especially if there is considerable disaggregation and reaggregation of
particles (Wakeham and Canuel, 1988; Hill, 1998).
Passow (2004) extended our “glue hypothesis” to suggest that the 3-8% POC:
ballast relationship was more likely due to the physical scavenging of minerals by sinking
POC, as Deuser et al. (1983) suggested in the early days of open ocean sediment traps.
Passow argues that if the carrying capacity of POC for inorganic particles were constant,
then the observed POC:ballast ratios could result. This would require the presence of an
excess of mineral particles too small to sediment on their own that the sinking POC could
31
interact with. Incorporation of inorganic minerals might increase the sinking rates of the
POC-rich marine snow material, but would not initially ballast the particles sufficiently to
cause them to sink. In this regard, Hamm (2002) and Passow and de la Rocha (2006)
both show that diatom aggregates could scavenge mineral particles in laboratory
experiments; Passow and de la Rocha found a carrying capacity in their experiments
large enough to account for a 2-3% POC:ballast ratio. Correlation does not imply
causation, however.
Passow and de la Rocha (2006) further observed an interesting result in that
additional incoporation of minerals caused medium-sized diatom aggregates to fragment
into many tiny, dense aggregates. Engel et al. (2008 a, b) describe experiments that make
similar observations with aggregates made from coccolithophores. In experiments
comparing aggregation of the naked form and the CaCO3-producing forms of Emiliania
huxleyi, the CaCO3-producing form made many small aggregates compared to the large
fluffy aggregates formed by the naked form. Mineral-bearing coccolithophoride
aggregates removed more of the suspended cells as aggregates, suggesting that a greater
proportion of mineral-bearing organisms might be exported from the euphotic zone
compared to non-mineral-bearing organisms. Thus, the minerals would play a major role
in enhancing initial aggregation.
The role of lithogenic material will depend on its relative contribution and source.
Passow’s (2004) results were obtained in an area of high lithogenic flux, although from
fluvial inputs rather than dust. Even though lithogenic material does not seem to be a
major carrier of OM (Klaas and Archer, 2002), in the MedFlux experiments there was a
32
peak in mass flux after almost every major spring dust event for which we have data,
although in 2003 the dust contribution to total mass flux was lower than in 2005. But
there was little correlation between size of a dust event and flux, since dust inputs
generally intensified whereas flux generally decreased over time. One could argue that
this supports Passow (2004): as POC production decreases after the spring bloom, it may
become limiting over time even when enough dust is present to be scavenged. However,
the summer low productivity time is when %OC in sinking particles is highest.
We propose a new scenario for discussion. Why is flux so decoupled from
satellite Chl values (which are well correlated with in-situ measurements)? We think it is
because particle aggregation requires a catalyst. At the MedFlux site, that catalyst
appears to be either ballasted plankton (likely cycled through zooplankton) or dust.
During the major spring flux period in both 2003 and 2005, the %OC is relatively
constant at 3-5% (Fig. 1D and 2D). Satellite Chl data suggests that phytoplankton were
present in the surface waters before the large flux peaks between DOY 80 and 95, more
so in 2003. There were dust events prior to both of these flux events that could have
catalyzed aggregation; the possibility that dust is critical is supported by Fig. 5, which
shows the correlation between lithogenic Al concentration and mass flux that was not
seen between Ca and Si concentrations and mass flux.
On the other hand, ballasted plankton (like diatoms) could have bloomed just
prior to the large mass flux peak; organic geochemical evidence suggests that the large
peak occurred soon after the diatom bloom at a time when zooplankton production was at
a maximum (Wakeham et al., 2008). Just after the large flux peak in both years, the
33
%OC begins to increase (to 20-30% in 2003 and 15% in 2005), possibly because
ballasted plankton and dust have been scavenged from the surface waters during the first
large dust/productivity event. Subsequent small flux peaks follow dust events. In
summer when dust inputs are frequent and intense, there is not enough plankton biomass,
particularly of the ballasted type, in the water column to drive large flux events. Annual
average fluxes such as we analyzed in Armstrong et al. (2002) would reflect the average
%OC of both periods. However, because most of the particle flux is in the earlier part of
the year (and is poorly represented in annual averages), and the average %OC would be
low, the 5-8% OC observed would result. More surface trap data is needed to assess
these ideas.
Boyd and Trull (2007) criticized regression approaches as being overly simplified
but generally acknowledge the need to find ways to parameterize export as suggested in
Sarmiento and Armstrong (1997). We continue to search for a mechanistic basis for
parameterizing export that could be applied to the entire ocean, since determining “b”
values (á la Martin et al., 1987) for every region of the World’s oceans cannot be as
easily extrapolated to the future ocean. Unfortunately there are very few
surface/epipelagic sediment trap data available, and even fewer that include OM and
ballast compositional components. However, our MedFlux results make us optimistic
about continuing the searching for a mechanistic basis for OM-ballast interactions to
provide hypotheses for future testing in the field.
34
Acknowledgements: MedFlux is supported by the National Science Foundation
Chemical Oceanography Program and the IAEA in Monaco. The International Atomic
Energy Agency is grateful for the support provided to its Marine Environment
Laboratories by the Government of the Principality of Monaco. The captains and crews
of the RV Seward Johnson II, Endeavor, and Tethys were exceedingly helpful in
deploying and recovering the sediments traps used in this study. We thank NASA IDS
fellow team member J.E. O’Reilly for providing satellite data for the DYFAMED station,
and fellow MedFlux program members, particularly B. Gasser, J. Szlosek, and Z. Liu, for
help with sample collection and handling. We also, once more, thank J.I. Hedges who
was there at the beginning and in spirit throughout. This is MedFlux contribution No.
XX and MSRC contribution No. XXXX.
35
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Table 1. Average mass fluxes (mg m-2d-1) over the entire deployment period for
time-series (TS) and settling velocity (SV) sediment traps and ranges of TS fluxes
over distinct time intervals. TS and SV flux records were obtained for 3.5 months
in 2003, excluding the 1-week redeployment interval, and 2 months in 2005. The
complete sediment trap data set is available at the MedFlux web site
(http://msrc.sunysb.edu/MedFlux).
Depth
Total AVG
(m)
Mass Flux
2003-Period 1: 6 March-6 May
TS
346
SV1
238
284
SV2
312
TS
771
2003-Period 2: 14 May-30 June
TS
117
SV1/2
TS
1918
2005: 4 March - 1 May
TS
313
SV
Flux Range
82-910
-
260
46-1024
46
35
3-164
-
72
4-124
426
475
27-906
-
SV1
SV2
524
470
525
-
TS
924
322
39-621
SV1
SV2
1918
438
439
-
45
Table 2. Time-series fluxes (mg m-2d-1) for 2003 of organic matter, calcium
carbonate, opal, lithogenic material and Alxs as defined in text. Near-surface fluxes of
carbonate, opal, and lithogenic material were high during the March bloom period but
decreased to only a few mg m-2d-1 at the end of June. Patterns over time of calcium
carbonate, lithogenic material, OM, and to some extent opal flux closely followed
that of total mass.
Cup
OM
CaCO3
Opal
Litho
Alxs
hydroxide
Cup
OM
Period 1
110
156
117
99.5
26.4
25.3
26.3
46.7
43.2
28.4
34.0
37.3
38.0
60.7
127
19.9
17.3
20.6
25.0
26.2
10.8
28.3
Litho
Alxs
hydroxide
11.3
22.3
2.66
0.77
0.52
0.53
0.51
0.46
0.08
ns
1.09
7.21
3.37
1.09
0.34
0.42
0.35
0.29
0.18
0.03
ns
0.19
7.52
12.1
c
6.51
15.8
11.7
7.84
9.73
4.69
ns
1.89
9.90
14.9
c
7.06
10.9
13.3
12.2
14.7
6.05
ns
2.70
TS 117 m
175
148
217
74.6
57.6
21.1
43.1
7.42
37.9
22.7
41.7
47.8
96.8
63.7
80.7
58.1
23.0
13.1
32.2
26.2
10.3
19.8
86.9
74.7
109
27.1
19.0
3.99
4.91
4.88
6.32
1.14
8.87
61.1
49.3
59.0
23.5
13.0
3.16
1.67
5.12
4.56
0.98
5.38
2
3
4
5
6
7
8
9
10
11
12
73.4
70.9
39.5
30.4
11.2
11.8
7.46
6.12
7.48
11.0
5.39
TS 771 m
2
3
4
5
6
7
8
9
10
11
12
Opal
Period 2
TS 238 m
2
3
4
5
6
7
8
9
10
11
12
CaCO3
93.3
123
237
93.8
31.0
22.6
10.2
19.3
24.7
6.82
26.4
19.5
43.8
22.4
49.8
22.4
15.8
10.3
10.5
9.61
3.72
4.34
52.3
86.8
32.3
13.3
7.29
18.9
5.66
34.6
5.07
nd
16.42
12.49
12.79
6.42
6.64
1.96
1.48
2.08
1.41
0.60
ns
2.06
TS 1918 m
18.9
13.7
93.7
21.0
6.51
5.53
4.77
4.62
4.26
0.88
0.77
12.6
10.3
76.8
17.0
4.53
4.30
3.46
4.06
0.50
0.58
1.96
2
3/4
5
6
7
8
9
10
11
12
24.3
18.6
c
12.6
9.20
11.8
11.6
18.0
5.18
3.55
2.45
24.2
38.5
c
17.2
24.5
44.8
11.9
31.0
7.27
4.29
1.00
4.52
6.12
c
5.93
5.96
5.68
4.73
5.34
2.92
ns
0.09
nd, not detected due to small sample size; c, the sample was combined with the
previous cup and averaged; ns, no sample
46
Table 3. Time-series fluxes (mg m-2d-1) for 2005 of organic matter, calcium carbonate,
opal, lithogenic material and Alxs as defined in text. Patterns with time of calcium
carbonate, lithogenic material, and opal flux closely followed that of total mass.
Cup
OM
CaCO3
Opal
Lithogenic
Alxs hydroxide
57.4
57.8
20.4
37.7
32.8
54.2
36.0
14.2
39.9
29.9
7.8
186
226
68.6
86.8
114
238
86.4
39.2
41.4
38.6
23.2
193
188
44.6
54.0
41.4
236
56.3
nd
113
34.7
nd
214
191
94.2
106
104
158
76.9
29.0
30.5
19.2
11.7
69.0
83.7
32.0
41.5
38.9
99.4
38.4
4.65
13.6
7.96
7.14
nd
19.0
10.1
nd
nd
38.4
nd
4.7
16.3
11.0
nd
126
142
143
119
190
223
152
14.0
6.3
4.7
25.2
42.1
41.3
39.1
33.6
47.2
51.5
57.5
12.7
9.34
3.88
4.83
TS 313 m
2
3
4
5
6
7
8
9
10
11
12
TS 924 m
2
35.5
78.4
3
25.4
89.1
4
26.5
101
5
26.5
105
6
33.7
132
7
33.5
54.7
8
29.6
116
9
17.6
20.6
10
13.9
14.9
11
11.4
22.2
12
16.9
9.9
nd, not detected due to small sample size
47
Table 4. Percent of each component in SV and TS sediment traps averaged over
the collection period and relative to calculated mass.
Trap
2003 Period 1
TS
SV 1&2 (avg)
Depth
(m)
OM
(%)
CaCO3
(%)
Opal
(%)
Alxs
Lithogenic
hydroxide
(%)
(%)
238
32
34
32
32
22
21
9.5
8.5
5.0
4.3
771
32
35
19
8.8
4.8
117
42
49
46
47
7.8
1.7
3.7
2.4
1.1
0.48
1918
30
39
8.9
TS
SV
313
12
17
28
20
SV 1&2 (avg)
524
12
TS
924
14
TS
2003 Period 2
TS
SV
TS
15
7.0
25
28
26
24
8.6
11
21
18
37
12
31
9
35
11
34
14
33
2005
SV 1&2 (avg)
1918
8.2
48
9.7
Figures
Fig. 1. Mass flux (A), POC flux (B), Satellite Chl and sea surface
temperature (C), and percent OC (D) in 2003 TS sediment traps. Vertical bands
show dust inputs, with darker bands being heavier inputs. Surface fluxes of bulk
particulate matter (mass) and OC were highest (up to ~1000 mg m-2d-1 for mass
and 70 mg m-2d-1 for OC) during the first two weeks of the first deployment
period and decreased by about an order of magnitude thereafter. There was no
obvious correlation between Chl and flux. Surface mass flux continued to
decrease further until the end of the deployment on June 30. At 771 m, mass and
OC fluxes at the start of 2003 P1 were about 4-fold lower than at 238 m, but
quickly increased till fluxes at 771 m were essentially the same as at 238 m. OC
fluxes peaked at slightly different times at 238 and 771 m. After the initial two
weeks of P1, mass fluxes at 238 and 771 m were generally similar. OC fluxes
were slightly lower at 771 m than at 238 m. During P2, mass fluxes at 117 m
were very low, and mass fluxes at 1918 m were often higher. OC fluxes at the
two depths showed a pattern generally similar to mass flux.
Fig. 2. Mass flux (A), POC flux (B), satellite Chl and sea surface
temperature (C), and percent OC (D) in 2005 TS sediment traps. Vertical bands
show dust inputs, with darker bands being heavier inputs. The high point shown
in Chl is a single point. Mass fluxes at 313 and 924 m in 2005 were almost as
high as surface fluxes in 2003, up to ~900 mg m-2d-1 mass. However, OC fluxes
never reached the higher 2003 levels, peaking at 25 mg m-2d-1. Mass fluxes at
313 m were higher than at 924 m, but not greatly so, suggesting again that
49
material from the surface phytoplankton bloom quickly reached 924 m, and that
little particulate material was lost between these depths. However, OC fluxes at
313 m were higher on average than 924 m fluxes, again suggesting some loss of
OC with depth.
Fig. 3. Contribution of organic matter, calcium carbonate, opal, and
lithogenic material in 2003 TS sediment traps during P1 at 238 m (A) and 771 m
(B) and P2 at 117 m (C) and 1918 m (D). CaCO3 was almost always the largest
fraction of surface ballast flux during P1, and became even more dominant during
P2 later in the summer and in deeper water. Lithogenic material was most
important in both surface and mid-depth waters early in P1, decreasing later and
becoming very low in surface waters during P2. However, lithogenic material
was again important in the deep 1918 m traps during P2. Opal was usually 3050% of CaCO3 values during P1 and became even less important during P2.
Fig. 4. Contribution of organic matter, calcium carbonate, opal, and
lithogenic material in 2005 TS sediment traps at 313 (A) and 524 m (B). In the
near-surface trap, CaCO3, opal, and lithogenic material were almost equal (~30%)
in importance at the beginning of the deployment period. In the deeper trap, opal
was much less important and lithogenic material was the dominant component.
The lithogenic component decreased with time.
Fig. 5. Relationship between lithogenic Al concentration (mg g-1) and
mass flux (m-2d-1) for 2003 P1 and P2 samples (m=21.9; r2=0.71); the correlation
was significant (n=22) at the 95% confidence level. There was no correlation
between mass flux and biogenic Si (r2=0.02) or Ca (r2=0.03) concentration.
50
Fig. 6. Relation between the proportion of each ballast and percent
organic matter. Explained variances (r2) for the proportion of calcium carbonate
(), opal (∇), lithogenic material (), and Alxs hydroxide (◊) in total ballast versus
%OM are 0.31, 0.004, 0.50, and 0.36, respectively. Correlations of calcium
carbonate, lithogenic material, and Alxs hydroxide with organic matter are all
significant at the 95% confidence level. The correlation between opal and organic
mater is not significant. Data are for all 2003 and 2005 samples from nearsurface traps (~200m).
Fig. 7. Time-integrated Mass flux density (IMFD) at 238 m in P1 (A) and
117 m in P2 (B) in 2003, and 313 m (C), 524 m (D), and 1918 m (E) in 2005 SV
sediment traps. SV1 and SV2 are duplicate traps on the same array and are shown
by dashed and solid lines. The area represented by the bars is total mass flux.
Although total mass fluxes determined in duplicate SV traps for the entire
collection period agreed within 10-25% in 2003 and 1-6% in 2005 (Table 1), the
mass collected in individual cups in duplicate traps did not agree well. At the
highest IMFD, agreement between duplicate traps in 2003 was within 38% at
238 m, and in 2005 within 24% at 524 m and 27% at 1918 m.
Fig. 8. Time-integrated mass flux density (A) and percent OC (B) during
Periods 1 and 2 in 2003, and average IMFD (C) and %OC (D) at all three depths
sampled in 2005. For easier comparison of patterns, mass flux density is
represented by a single point at the SV interval midpoint. All depths are shown in
each figure. Variability in 2003 P2 %OC values may reflect error due to low
sample size rather than actual changes as discussed in the text.
51
Fig. 9. Time-integrated organic matter and ballast flux densities in 2003
surface SV sediment traps during Periods 1 (A) and 2 (B). Contribution of
organic matter, calcium carbonate, opal, and lithogenic material in 2003 surface
SV sediment traps during Periods 1 (C) and 2 (D). Compositional data calculated
in a relative sense gives a slightly different story than mass flux densities as it
normalizes the data to the sum of OM, CaCO3, opal and lithogenic material.
Also, samples where one of these components is missing cannot be considered
(blank spaces in figures).
Fig. 10. Time-integrated organic matter and ballast flux densities in 2005
SV sediment traps at 313 m, 524 m, and 1918 m. Flux density is represented by a
single point at the SV interval midpoint. Duplicates are shown at 524 and
1918 m.
Fig. 11. Contribution of organic matter, calcium carbonate, opal, and
lithogenic material in 2005 SV sediment traps at 313 (A), 524 (B and C) and
1918 m (D and E). Duplicates are shown at 524 and 1918 m. Blank spaces occur
where not all four components were measured.
52
1000
80
A
238 m
OC flux (mg m-2 d-1)
Mass flux (mg m-2 d-1)
1200
117 m
771 m
800
1918 m
600
400
200
0
30
C
SST
25
Chl-a
1.5
20
1.0
15
0.5
0.0
40
20
50
70
90
110
130
150
170
10
190
35
30
D
25
OC (%)
Chl-a (mg m-2)
2.0
60
0
Sea Surface Temperature (C)
2.5
B
Day of Year 2003
20
15
10
5
0
50
70
90
110
130
150
Day of Year 2003
Figure 1
170
190
1000
30
A
313 m
924 m
800
OC flux (mg m-2 d-1)
Mass flux (mg m-2 d-1)
1200
600
400
200
B
20
10
0
C
25
1.5
Chl-a
SST
1.0
15
0.5
0.0
50
20
70
90
110
Day of Year 2005
130
10
150
16
D
12
OC (%)
Chl-a (mg m-2)
2.0
30
(?)
Sea Surface Temperature (C)
4.0
0
8
4
0
Figure 2
50
70
90
110
Day of Year 2005
130
150
MedFlux 2003 Time Series Traps
A
238 m P1
B
117 m P2
C
771 m P1
D
1918 m P2
Composition (%)
100
80
60
40
20
0
80
60
40
20
16
8.5
17
3.5
17
8.5
13
6
14
0
14
4
14
8
15
2
15
6
16
0
16
4
10
5
11
1
11
7
12
3
99
93
0
67
.5
72
.5
77
.5
82
.5
87
.5
Composition (%)
100
Day of Year 2003 (mid-point)
Carbonate
Opal
Day of Year 2003 (mid-point)
Lithogenic
Figure 3
Organic matter
2005 Time Series Traps
A
313
B
924 m
Composition (%)
100
80
60
40
20
0
80
60
40
20
96
10
1
10
6
11
1
11
6
86
91
76
81
0
66
71
Composition (%)
100
Day of Year 2005 (mid-point)
Carbonate
Opal
Lithogenic
Organic matter
Figure 4
1000
Mass flux (mg m-2 d-1)
800
600
400
200
0
0
10
20
30
Al (mg g-1)
Figure 5
40
50
Ballast type / total ballast (g g-1)
1.0
CaCO3
0.8
0.6
0.4
Opal
0.2
Litho
Alex
0.0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
Organic matter / total mass (g g-1)
Figure 6
0.7
0.8
24
20
A
238 m P1
SV-1
SV-2
24
16
12
12
8
8
4
4
0
0
1.6
1.4
28
B
24
SV-2
1.0
16
0.8
12
0.6
D
524 m
SV-1
SV-2
20
1.2
C
313 m
20
117 m P2
SV-1+SV-2
2005
28
16
1.8
IMFD (g m-2 SVI-1)
2003
8
0.4
0.2
4
0
0
1
10
100
Settling velocity (m
1000
28
d-1)
IMFD (g m-2 SVI-1)
IMFD (g m-2 SVI-1)
28
Settling Velocity Traps
24
SV-1
SV-2
20
16
12
8
4
0
Figure 7
E
1918 m
1
10
100
1000
Settling velocity (m d-1)
2003 SV traps
25
1.5
50
1.2
40
SV-1 P1 238 m
SV-2 P1 238 m
SV1+2 P2 117 m
15
0.9
10
0.6
5
0.3
0
0
OC (%)
20
IMFD P2 (g m-2 SVI-1)
IMFD P1 (g m-2 SVI-1)
A
B
(?)
30
20
10
0
2005 SV traps
12
C
20
313 m
524 m
1918 m
15
10
10
6
4
5
0
D
8
OC (%)
IMFD (g m-2 SVI-1)
25
2
1
0
10
100
1000
Settling velocity (m d-1)
1
10
100
1000
Settling velocity (m d-1)
Figure 8
2003 Settling Velocity Traps
6
IMFD (g m-2 SVI-1)
5
0.7
A
0.6
Organic matter
CaCO3
Lithogenic
Opal
4
3
B 117 m P2
0.5
0.4
0.3
2
0.2
1
0.1
0
0
1
C
238 m P1
10
100
Settling velocity (m d-1)
1000
1
D
238 m P1
10
100
1000
-1
Settling velocity (m d )
117 m P2
80
60
40
20
-1
.4
42
2. .7
75.
5. 4
411
11
-2
2
22
-4
9
49
-9
98 8
-1
19 96
64
49 90
098
>9 0
80
1.
0.
68
-1
1. .4
42
2. .7
75.
5. 4
411
11
-2
2
22
-4
9
49
-9
98 8
-1
19 96
64
49 90
098
>9 0
80
0
0.
68
Composition (%)
100
Settling velocity range (m d-1)
Carbonate
Opal
Settling velocity range (m d-1)
Lithogenic
Figure 9
Organic matter
IMFD (g m-2 SVI-1)
7.5
A
2005 Settling Velocity Traps
SV-2 313 m
Organic matter
5.0
CaCO3
Lithogenic
Opal
2.5
0
IMFD (g m-2 SVI-1)
7.5
B
C
SV-1 524 m
SV-2 524 m
D
E
5.0
2.5
0
IMFD (g m-2 SVI-1)
7.5
SV-1 1918 m
SV-2 1918 m
5.0
2.5
(?)
0
1
10
100
(?)
1000
1
Settling velocity (m d-1)
10
100
1000
Settling velocity (m d-1)
Figure 10
2005 Settling Velocity Traps
A
SV-2 313 m
100
Composition (%)
80
60
40
20
0
B
C
SV-1 524 m
SV-2 524 m
Composition (%)
100
80
60
40
20
0
D
E
SV-1 1918 m
SV-2 1918 m
80
60
40
20
Opal
0
98
0
>9
80
49
0-
6
49
32
6-
6
32
19
6-
19
40
14
0-
.9
97
.9
-1
.0
-9
7
49
.0
-4
9
.8
-2
1
21
.9
10
Settling velocity range (m d-1)
Settling velocity range (m d-1)
Carbonate
.8
.9
4
10
4-
-5
.
5.
68
0.
98
0
>9
80
0
49
49
0-
6
32
6-
6
32
19
6-
19
40
0-
14
-1
.9
97
.9
-9
7
.0
.0
49
21
.8
-4
9
.8
.9
-2
1
10
.9
10
45.
68
-5
.
4
0
0.
Composition (%)
100
Lithogenic
Figure 11
Organic matter