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The North Atlantic Carbon Cycle: New Perspectives from
·JGOFS·and WOCE
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Scott Doney
National Center for Atmospheric Research
Doug Wallace
Brookhaven National Laboratory
Hugh Ducklow
Virginia Institute of Marine Science
Nick Bates
Bermuda Biological Station for Research
lCES Annual Science Conference
Reykjavik, lCeland
(C M 1996/0:15>'
September 27-0ctober 4, 1996
Abstract
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The rationale for studying the North Atlantic C02 system revolves around the high'
latitude formation of deep and intermediate water masses and the subsequent important
role of the North Atlantic in the natural earbon cycle and uptake of anthropocentric C02.
Doth the vitality of the debate and present levCI of uncerti1inty are highlighted by aSeries
of papers presented at the Royal Society North AtIantic Carbon ,Cycle meeting in thc
Fall of 1994. Thc major terms for a North Atlantic earbon budget include the net CO 2
air-sea ßux, the meridional ßuxes of dissolved inorganie and organic (DIC and DOC)
associated with the overturning cireulation, vertical redistribution through particle and
DOC export arid subsequent remineralization, riverine input and sediment burial, and
temporal changes in the DIC inventory. We review eurrent model and observational
estimates, including potential errors, for eaeh component both for the modern and preindustrial ocean. At present, the earbon budget for the North Atlantic eannot be closed to
within 0.2-0.3 Gt C/yr, and significant disagreements exist over the major term balances.
Future research directions for addressing the North Atlantic earbon budget are also
discussed.
.
A central objective of the Joint Global Ocean Flux Study (JGOFS) is to improvc our. estimatcs
and understanding of the flux of carbon both within thc ocean and across thc oeean-atmosphere
and oeean-sediment interfaces. Onc rationale for studying the North Atlantic CO2 system revolves
around the high latitude formation' of deep and intermediate water masses and thc subsequent
important role of thc North Atlantic in thc natural earbon eycle and uptakc of anthropoeentrie
C02 (Sarmiento ct al., 1995). A seeond, equally compelling, argument rests on using the diverse
range of biogeographical and hydrographie regimes (Longhurst, 1995) across the basin to explorc the
interaetion among air-sea exchange, circulation and biological processes that determine the surfaec
distribution of dissolvcd inorganic earbon.
Thc first major JGOFS field cffort focuscd on the spring bloom in thc eastern North Atlantic
(North Atlantic Bloom Experiment, NABE; Ducklow and Rarris, 1993), while a long-term biogeochemical timc-seri~s was initiated in the Sargasso Sea in 1988 (Dcrmuda Atlantie Timc-Series Study,
North Atlantic CMbon Cyc1e
2
DATSi Michaels and Knap, 1996). The large-sealedistribution of oeean earbon parameters.cor the
basin are also being reevaluated as part of the JGOFS global C02 survey in eonjunetion with the
World Oeean Cireulation Experiment (WOCE) hydrographie programme. Together with a wealth
of historical datasets (e.g. TTO; Drewer et al., 1985; Takahashi et al.; 1995), the data collected over
the last deeade in the North Atlantie provide perhaps our best opportunity to eonstrain the earbon
budget for an oeean basin.
The marine earbon system is a ehallenging subject, requiring a detailed knowledge of the interplay between oeean biogeoChemistry and physics. The oeean earbon eycle is also perturbed by
climate variability on timescales ranging from the interannual ENSO and North Atlantie Oseillation
to millenia and glacialjinterglacial transitions. The oeeanic uptake of anthropogenie CO 2 adds further complications. Traditional time-series data 01- oeeanographic cruise data are limited in spatial
and temporal cxtent, arid a variety of approaehes incorporating ship of opportunity, moorings aßd
satellite data, data-assimilation models, and novel measurement systems are needed to address such
issues on the regional or basin scale.·
•
~"",.
Preliminary cfforts for a basin-wide synthesis of thc North Atlantic carbon budget are undcrway, and both the vitality of the debate and the prcscnt level of uncertainty are highlighted by a
scries of papers presented at the Royal Society North Atlantic Carbon Cycle meeting thc Fall oC
1994 (Eglinton et al., 1995). Here, we present one view of the problem, in particular stressing the
eomponents of the earbon budget that require further work.
in
The major terms of a North Atlantie carbon budget, shown sehematically in Figure 1, include:
the net air-sea CO2 Hux, the meridional ßuxes of dissolved inorganic"and organic (DIC and DOC) at
the northern and southern boundaries, veitic8J. redistribution through particle and DOC export and
subsequent remineralization, riverine input arid sediment burial, and temporal changes in the DIe
inventory related to uptake of anthropogenie e02. The magnitudes of these ßuxes can be estimated
to varying levels of eertainty.
.
Two quantities are needed to compute thc air-sea C02 Hux-the ocean-atniosphere partial pressure difference (A pC0 2) and the gas exchange rate, which is generally parameterized as a function of
. wind speed. Surfaee water pC02 increases with DIC and temperature arid deereases with alkalinity
and less strongly with salinity. Biological DIC uptake for the production of organie matter typically
dominates over biological ealcification, and the so-called "biological" and "solubility" pumps (Volk
and Hoffert, 1985) orten are out of phase over the seasonal eyde, with temperature domintated,
summer outgassing (positive ß pC0 2s) in oligotrophie regions (e.g. Dates et al., 1996), and biology
dominated, summer drawdown (negative pC02s) at the more produetive, higher latitudes (Chipman
et al., 1993;
Takahashi
et al., 1993). .
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.
.
Although a fairly extensive historical surfaee pC02 data set exists for the North Atlantic (e.g.
Takahashi et al., 1995), the coverage is insufficient to adequately represent the large seasonal and
spatial variability in the surfaee water ßpC02 (-120 uatm to +100 uatm), and some form of spacetime extrapolation must be applied to the data (e.g. Takahashi et al., 1995; Lefevre, 1995). Advances
have also been m3.de in eonstraining gas exchange rates using dual-tracer release experiments, but
questions remain regarding how. to scale such results to C02 (Wanninkhof, 1992; Watson et al.,
1995); at present, air-sea gas eXchange rates are known to no better than a factar of two.
Dased on a historicai reconstruction of the seasonal pC02 field, Takahashi et al. (1995) estimate
that annual mean, pC02 difference with the atmosphere is about -8.6 uatm over the North Atlantic
basin (18S-78N). This rcsults in a net acean 002 uptake of bctwccn 0.2 ta 0.5 Gt O/yr, dcpcnding
on the choice of gas exchange parameterization. Most of the uptake occurs in the subpolar gyre
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,
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North Atl:intic C:irbon Cyc1e .
3
north of 42N t with a moderate nct subtropical sink nearly balanced by ~ tropical Atlantic sourc~..
FUrther refinement of thc net North Atiantie air-sea flrix is diffieult considering thc spatial and
tEmiporal data requiremcnts and size of the signal in pC02. A 2.5 uatni change in mean pC0 2
aecounts for a fiux of about 0.1 Gt C/yr over the basin eompared with current measurcmcnt uncertainties at about 1.0 uatm. Barring a dramatic iricrease in the amount of availabie pC02 data t
one approach for improving basin-scale pC02 maps would bc take advantage of correlations pC02 '
with properties such a.S SST, nutdents and ocean color (Watson et al., 1991j Tak3.I1ashi et al., 1993j'
Stephens et al., 1995) that ean bc measured either by remotc sensing or estimated from the broader'
hydrographie databases.
•
On the basin scaie, th~ nct meridional transports of DIC due tö thcrmohaline overturning, Winddriven gyres, and surfaee Ekman fiow become increasingly important. Thc northward inBow of warm
surface watcr and export of cold deep water sets up a natural CO2 solubility pump with net airsca. uptake at high latitudes and southward DIC outBow at depth. The thermohaline eomponent is
closely linked to the overturning transport (10-20 X 106 m 3 s-l) and northward ocean heat transport,
and a. number of diagnostic calculations give a range of preindustrial carbon ßuxes out of the North
Atlantie: 0.6 Gt C/yr (Broecker and Peng, 1992)j 0.5±0.1 Gt C/YI: (Watson et al.; 1995)j 0.33±0.15
Gt C/yr (Kceling and Peng, 1995).
,
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. . .,
, " , ' .
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Meridional COi transports can aIternatively be calculated by combining zonal DIC scction data
.With hydrographie scetion velocity estimatcs t preferably derivcd from an invcrse proccdurc. Thc
resulting C02 ßuxes include the thermohalinc componerit as well as ßuxcs due to ageostrophie
surface Ekman ßow and wind-driven gyre cireulation t and the ealculations are eomplieatcd by thc
urieeii.ainty in the barotropie velocity field and surface wind stress and by the poor CO 2 coverage
in space and time. Computed C02 southward net transports.for thc preindustrial pefiod are 0.4 Gt
C/yr (25 N, Brcwer ct al., 1989),0.4-;-0.8 Gt C/yr (20-60 N, Martel arid Wunsch, 1993), and 0.6 Gt
C/yr (11-30 S, Holfort et al., 1996). The present day ßui estimates are lower by about 0.2 Gt C/yr
because of the increase from anthropogenie earbon of thc DIC in inflowing surface water.. .
•
An additional and often neglccted component of thc meridional C02 transport arises bccausc
there is a nct southward mass flux in thc basin rcquircd to balance the net atmospherie water vapor
fluX from thc Atlantie to thc Pacific (e.g. Wijffies et al., 1992). The mass flux, which ean be
estimated from flow through thc BeringStrait modified by the net freshwater input ovcr thc Arctie
and North Atlantic, can be thought of as' a zonal average barotropie flux and contributes another
approximate 0.6 Gt C/yr to thc southward transport (Holfort et al., 1996). Some confusion has
arisen in the literature beeause this throughflow is typically not inchided in all section estimates nor
. many numerical models.
The strcngth of the biologieal cacbon pump in the North Atlantie is dÜficult to access directly
from obserVations due to our limited undcrstanding of the controIs of cxport fiux from thc surface.
Using CZCS occan .color composites and primary production algorithms, Longhurst ct al. (1995)
estimatethat thc total primary productivity for the basin is about 6.3 Gt C/yr. Using their suggested
basin-wide f-ratio of 0.1, the ratio of export production to total, yields a ncw production of 0.63
GtC yr-I. For comparison, the Princeton GCM ecosystem model (Sarmiento et al., 1993) produces
0.9-1.3 Gt C/yr of new production for the North Atlantic, with an average f-ratio of 0.42. The
model f-ratio is likely too high, related to excessive nutrient upwelling, and a rcasonable ·first ordcr
cstimate is in the range of 0.6-0.9 Gt C/yf. Based on data from BATS and a fcw othcr sites where
good DOC data exist, roughly about a. 1/3 of this export production niaybe leaving the euphotic
zone as serni-Iabile DOC rather thiui particulate maiter (DuckIow ct al., 1995).
4
North Atlantic CMbon Cyc1e
A number of other, smaller components (riverine input, sediment burial, net DOC remincralization) must bc incIuded to balance tbc basin carbon budget at thc 0.1-0.2 Gt/yr level. Often the
riverine input is taken to approximately balance thc sediment carbon fluX over the basin. The magnitude and Variability of the meridional DOC fluxes and net DOC convergence/divergence for the
basin are still almost entirely unknown but may be addresscd in the near future with the devclopmcnt
of improved DOC an3Jytical techniques (e.g. Ducklow et al., 1995).
, Model caleulations suggest that the ocean takes up at present abotit 40% of the fossil fuel carbon
rclcascd to the atnlOsphere. The rcsulting anthropogenie signal in the aurfaee water (50 umol/kiü
dcereases rapidly with depth and is small relative to the natural variations in backgrouml DIC
eoneentrations (1900-2200 urriol/kg). The high qualityDIC time serics collected at Hydrostation S
and DATS show an average inerease of about 1.7 umol/kg/yr in surface waters for thc last deeade
(Dates et al., 1996), elose to the expected equilibrium rcsponse.
•
MeaSurement errors for DIC have, until the recent introduction of refCrE!llCe DIC standards (Dickson and Goyet, 1994), limitcd efforts to directly estimate anthropogenie CO 2 uptakeby monitöring
the change in DIC coneentrations over time. A number of more indireCt methods have been introdueed, each with its own weaknesses, but togetherthey provide relatively consistcnt pieture for
thc removal of eXcess C02 by the North Atlantic (Wallaee, 1995).
a
One approach, originally introduccd by Brcwer (1978) and Cben (1993), involves computing
tllEi pre-industrial DIC distributio'n from preformed nutrients, moorig relationships, and Redfield
ratios. Using the GEOSECS and TTO datasets, Gruber et al. (1996) ealculate a North Atlantic
anthropogenie CO2 burden of 20±4 GT C in the early 1980's.
'
Anthropogenie CO2 uptake ia 'primarily limited by ocean transport, and 3-D general cireulation
model (GCM) predicted uptake rates for,thc North Atlaritic 18.4 GT C (or about 0.4 GT C/yr in
the 1980's; Sarmiento et al., 1995) are similar in magnitüde to that found by Gruber et al. (1996)
but with a different spatial distribution. Tbe current generation of numerieal models contains a
number of known deficiencics, particularly with the formation and outflow of North Atlantic Deep
Water. Preliminary efforts to use the measured distributions of transient tracers such aB tritium and
thc CFCs to estimate C02 uptake, particularly in the maln thermocline, show good promise.
" Although earbon budgets ean be cOIllltructed for the North Atlantie that balance to about 0.1
. Gt C/yr, a doser cxamination of the terms suggest that our present uncertainties Me no better than
about 0.2-0.3 Gt C/yr, or about at the edgc of saJing whether the pre-industrial North Atlantie was
an important net earbon sink. For example, Sarmiento ct al. (1995) find essentially no net C02
, uptake in pre-industrial timcs and a modern anthropogenie carbon storage of 0.4 Gt C/yr balanced
by 0.3 Gt C/yr net air-sea flux and a small meridional DOC convergcnee. For contrast, Holfori et
al. (1996) report tbat nearly al1 of the excess CO2 iIiventory in the North Atlantic aetually entered
tbc oeean in the south and was advected northward; they also auggest that the present net air-sea
flux (0.5 Gt C/yr in their calculation) has changed liÜle since the pre-industrial.
Solving the question of the North Atlantie C02 sink will require a variety of new and novel
approaches. Three areas that show promise are automated Chemical measurement systems, next
generation biogeoChc~ieal data-assimilation models, and atmospherie inversion techniques.
At the root of mueh of the prcscnt uneertainty in the North Atlantie carbon budget is the sparsity
of data, particularly at high latitudes during the wintet. Thc desired temporal and spatial data seales
are incompatible with traditional occanographie shipboard saInpling, and alternativcs based on tbe
rcmote mcasurcmcnt of chemical spccies from voluntecr observing ships (VOS), moorings, drifters
•
5
North Atlantic Ca.rbou Cycle
and Boats are required. For cxample, Fricderich et al. (1995) demonstrated a capability'from tests
on a mooring in Monterey Bay for measuring the air-sea pC02 difference with 1-2 uatm accuracy
houdy for up to several months.
On-going improvements in three dimensional numerical models offer an alternative and complementary approach, and recent results from the Princeton carbon cyde model (Sarmiento et al.,
1995), which incIudes a complete carbonate system and simple phosphate based biological model, are
generally consistcnt with botb tbe net air-sea and meridional COz flux estimates. Future modeling
directions will incIude more emphasis on data assimilation, with the ability to 'directly incorporate
near real-time, basin-scale satellite datasets such wind speed, sea surface bcigbt, SST and ocean
color.
•
Finally, cfforts are undcrway to back-out surfacc COz llux patterns by inverting atmospheric
, data from tbe extensive NOAA/CMDL CO 2 monitor program using atmospheric transport codes
(Tans et al., 1996).
'
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