Exploring Continental Margin Carbon Fluxes on a Global Scale
K.-K. Liu1, L. Atkinson2, C.T.A. Chen3, S. Gao4, J. Hall5, R.W. Macdonald6, L. Talaue
McManus7, R. Quiñones8
1. National Center for Ocean Research, Taipei, Taiwan (Email: kkliu@ccms.ntu.edu.tw)
2. Center for Coastal Physical Oceanography, Old Dominion University
3. Institute of Marine Geology and Chemistry, National Sun Yat-sen University,
Kaohsiung, Taiwan
4. Institute of Oceanology, Chinese Academy of Sciences, Qingdao, China
5. National Institute of Water and Atmospheric Research, Hamilton, New Zealand
6. Institute of Ocean Sciences, Sidney, Canada
7. Marine Science Institute, University of the Philippines,
8. Departamento de Oceanografia, Universidad de Concepcion, Chile
Eos, Transactions, American Geophysical Union (Volume 81 Number 52)
December 26, 2000
How fast anthropogenic CO2 is being absorbed by the ocean remains one of the
most critical and yet elusive fluxes sought by climate researchers. Since the ocean is a
major sink of anthropogenic CO2, an accurate estimate of the present oceanic uptake rate
of anthropogenic CO2 is essential for reliable prediction of the future CO2 level in the
atmosphere. This number is difficult to pin down because the net air-to-sea CO2 flux is a
small difference (about 2 Pg C yr-1, 1 Pg = 1015 g) between two huge fluxes - namely, the
uptake and the release of CO2 at sea surface (each about 90 Pg C yr-1). A global ocean
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carbon budget helps to constrain the estimate of anthropogenic CO2 flux. To construct a
complete carbon budget, we need to know not only the air-sea exchange but also the
carbon fluxes from the continental margins [Liu et al., 2000]. For brevity, we use CO2
source to indicate release of CO2 from the ocean to the atmosphere and sink to indicate
absorption of atmospheric CO2 by the ocean.
The riverine carbon fluxes from land sum up to about 0.8 Pg C yr-1. Although
dwarfed by the air-sea CO2 exchange, this total flux is on the same order of magnitude as
the net air-to-sea CO2 transfer. Continental margins that receive riverine carbon fluxes
are not merely reservoirs, which store all the discharged carbon, nor conduits, which pass
everything to the interior ocean. They are sites of active physical and biogeochemical
processes that transform, transport or bury carbon. Many on-going and recently
completed regional studies (Fig. 1) indicate that active cross-shelf transport and
biogeochemical processes in the margins influence the carbon cycle of the ocean as a
whole. The findings strongly suggest that continental margins constitute important, but
often neglected, components in the global carbon cycle. The intriguing role of margins in
global biogeochemical cycles has been the topic of exciting new results presented and
discussed at recent meetings, including the AGU/ASLO 2000 Ocean Sciences Meeting in
San Antonio, Texas and the JGOFS Open Science Conference in Bergen, Norway.
Here, we highlight some of the exciting new findings and issues, and report on
planned activities for data collection and synthesis. An attempt to synthesize the
available information on continental margin carbon fluxes, co-sponsored by the Joint
Global Ocean Flux Study (JGOFS) and the Land-Ocean Interaction in the Coastal Zone
(LOICZ), two core projects of the International Geosphere-Biosphere Program (IGBP), is
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being conducted by the Continental Margins Task Team (CMTT). The goal is to
establish a global estimate of continental margin carbon fluxes. Considering the diversity
of continental margins and the different ways they may respond to global change, we
seek contributions from all continental margin studies in this global synthesis effort.
(Background information, reference materials, rationale and strategy for the CMTT
synthesis plan, and means to participate are posted at the web site
http://www.ncor.ntu.edu.tw/cmtt/ or its mirror sites which may be found at
http://ads.smr.uib.no/jgofs/Science/cmtt.htm or http://www.nioz.nl/loicz/.).
The fate of riverine carbon fluxes
Prior to the Industrial Revolution, the ocean carbon cycle was presumably close to
steady state [Siegenthaler and Sarmiento, 1993]. If there were no carbon transfer
between land and ocean, the exchange fluxes of CO2 between the atmosphere and the
ocean would have matched each other. However, as pointed out by Smith and Mackenzie
[1987], the global ocean must consume more organic carbon than it produces due to the
input of organic carbon from land. The current estimate of the total riverine flux is about
0.8 PgC yr-1 (the dashed arrow under “Rivers” in Fig. 2), of which 45% is organic and the
rest inorganic. For a balanced carbon budget before the Industrial Revolution, the CO2
release to the atmosphere must have exceeded the absorption flux by an amount equal to
the riverine carbon influx minus the small burial flux of carbon to sediments. Following
the Industrial Revolution, atmospheric CO2 has increased from a total amount of 600 Tg
C to more than 750 Tg C at present due to fossil fuel burning and deforestation
[Siegenthaler and Sarmiento, 1993]. The increased CO2 air concentration has resulted in
surface seawater undersaturation on average, which renders the ocean a net CO2 sink,
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with the consequence that the carbon content of the ocean is presently increasing with
time. Despite the increase in the atmospheric CO2, human perturbation to land-ocean
carbon transfer is probably limited and, therefore, the estimate of current total riverine
carbon flux is assumed applicable also to the pre-Industrial era.
The oceanic uptake rate of anthropogenic CO2, which is defined as the net carbon
influx to the ocean, equals the total increase rate of the oceanic carbon pool (Fig. 2),
which is 0.4 PgC yr-1 for the surface ocean plus 1.6 PgC yr-1 for the intermediate and deep
waters. Because the burial rate of carbon in marine sediments is estimated at 0.2 PgC yr-1
(the dashed arrow marked as “Sedimentation” in Fig. 2) the remainder of the river input
(0.8 PgC yr-1), about 0.6 PgC yr-1 (the dashed arrow from ocean to atmosphere in Fig. 2),
must have been outgassed by the ocean before the Industrial Revolution [Siegenthaler
and Sarmiento, 1993]. This outgassing flux, which is presumed still operating as before,
offsets the downward flux of CO2 and makes the net air-to-sea transfer smaller than the
anthropogenic CO2 invasion rate. Therefore, the oceanic uptake rate of anthropogenic
CO2 equals the net air-to-sea flux of CO2 plus the 0.6 Pg C yr-1 from continental margins.
This explains the rather modest value of the net air-to-sea CO2 flux of 1.4 Pg C yr-1 based
on earlier observations compared to the model-predicted oceanic uptake of anthropogenic
CO2 at 2 Pg C yr-1 [Siegenthaler and Sarmiento, 1993]. The latter estimate was accepted
by the Intergovernmental Panel on Climate Change (IPCC) 1995 assessment of the global
carbon budget (Fig. 2). Recently the revised air-to-sea CO2 fluxes based on newly
obtained data of differences in CO2 partial pressure PCO2) across surface seawater
from WOCE-JGOFS joint effort has been increased to as high as 2.2 Pg C yr-1 [Takahashi
et al., 1999]. If amended by the terrestrial input, the uptake rate of anthropogenic CO2
would be as high as 2.8 Pg C yr-1, which is significantly higher than the model prediction.
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One explanation offered for this discrepancy between models and observations is
that the coarsely gridded PCO2 data set misses most, if not all, of the continental
margins, and, therefore, it represents the air-sea CO2 exchange appropriate only to the
interior ocean. If the river-discharged carbon fluxes were mostly outgassed in continental
margins and little carbon got transported across the shelf, the margins would have little
influence on the carbon budget in the interior ocean. Furthermore, continental margins
would be a net CO2 source (0.6 Pg C yr-1) which, for a total area of 36x1012 m2 [Liu et al.,
2000], implies an average outgassing flux per unit area of 17 gC m-2 yr-1. Recent
observations clearly contradict both of these implications.
Recent observations of continental margin carbon fluxes
In the past ten years continental margin studies have been conducted all over the
world (Fig. 1), some of them focussing on carbon fluxes, whereas others have
emphasized biology, physics or sedimentology while still shedding light on the carbon
cycle. PCO2 has been surveyed on continental margins in the North Sea, the Baltic Sea
and the East China Sea (ECS), all of which have a significant terrestrial input of carbon.
Contrary to conventional wisdom which suggests these margins to be releasing CO2 to
the atmosphere, they have all been found to be net CO2 sinks. The ECS is an instructive
example. It receives an enormous amount of carbon from some of the world’s largest
rivers, such as Changjiang (the Yangtze River) and Huanghe (the Yellow River). The
riverine carbon fluxes comprise 12 Tg C yr-1 in organic form and 20 Tg C yr-1 in
dissolved inorganic form [Chen and Wang, 1999]. If half of this carbon flux were
outgassed evenly over the ECS shelf (0.9x1012 m2), the average efflux per unit area
would be 23 gC m-2 yr-1. Instead, the ECS absorbs atmospheric CO2 with an estimated
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mean flux per unit area as large as 35 gC m-2 yr-1 [Tsunogai et al., 1999]. Similarly, in the
Baltic Sea, the mean absorption flux of CO2 per unit area was 11 gC m-2 yr-1 for an entire
annual cycle [Thomas and Schneider, 1999].
Continental shelf waters have limited capacity to store absorbed CO2 and riverdischarged carbon: all but a small fraction of the received carbon must either be exported
to the open sea or be buried in sediments. The Shelf Edge Exchange Processes (SEEP)
projects in the Mid Atlantic Bight were conducted precisely to look for the export flux of
POC from the shelf. Although SEEP was prompted by the notion that a major fraction of
the primary production on the shelf could be exported, the observed export fluxes were
disappointingly small, and less than 5% of primary production in total [cited in Liu et al.,
2000]. The observations during SEEP projects may, however, have missed important
export processes and, like the ECS and Baltic Sea, the Mid Atlantic Bight appears also to
be a net sink for CO2 as indicated by recent observations by M. DeGrandpre and
colleagues.
The phenomenon of the shelf sea serving as a CO2 sink has been called the
“continental shelf pump” [Tsunogai et al., 1999]. The ECS shelf pump’s estimated
capacity of 20-30 Mt C yr-1 is facilitated by the active biological carbon uptake in
summer and high solubility of CO2 in winter. The estimated organic carbon burial rate in
the ECS shelf sediments is no more than 10 Mt C yr-1 [Chen and Wang, 1999; S. Gao,
unpublished data). It is apparent that carbon burial in sediments cannot even store all of
the riverine input let alone the absorbed CO2. A large fraction must be exported from the
shelf as particulate organic carbon (POC), dissolved organic carbon (DOC) and/or
dissolved inorganic carbon (DIC). Advective export of POC was observed in association
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with a cyclonic eddy at the shelf edge northeast of Taiwan [Liu et al., 2000] during the
Kuroshio Edge Exchange Processes project. Similarly strong export was also found near
Cape Hatteras on the eastern margin of the North Atlantic. On the western margin of the
Atlantic, observations by the Ocean Margin Exchange project showed a 15% export from
the shelf primary production in the northern Gulf of Biscay [R. Wollast and L. Chou,
personal communication, 2000). The shelf pump is effected by physical and
biogeochemical processes which are not yet fully understood, and warrant further study.
Very effective but entirely different export processes take place on polar margins
and tropical coasts. In the Arctic, ice formation produces an algal habitat and alternately
stablizes or destablizes the water column when it melts or forms. The coupling of the
biological cycle with the ice cycle provides an effective carbon pathway to the deeper
interior ocean, one that is particularly vulnerable to change. The Mackenzie River and
shelf serves as one example for the polar margin. Its coastal zone is ice free and stratified
in summer; productivity is high, and organic carbon is deposited on the shelf. In winter,
sea ice formation produces brine which may sink from the surface down the continental
slope. Dense, cold water not only carries anthropogenic CO2 but provides a mechanism to
transport re-suspended organic matter and regeneration products to the deep basin. The
strength of this process remains uninvestigated but the total flow, estimated at 1 Sv
(32,000 km3 yr-1; Melling, 1993], suggests it ought not to be neglected. By virtue of brine
export and sinking of algal mats from the ice, the Arctic contains potentially some of the
most efficient shelf export regions in the world. In contrast, many tropical watersheds
discharge a high volume of water and sediments to the ocean due to frequent floods.
Indeed, more than half of the total runoff and land-derived sediments are discharged to
the oceans from tropical coasts, especially the Indo-Pacific Archipelago [Nittrouer et al.,
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1995]. The narrow shelves and weak Coriolis force favor cross shelf transport of
sediments, which carry a significant amount of carbon, to the deep ocean.
In contrast to the margins mentioned above, some others are indeed CO2 sources.
Recent results from Biogeochemical Budgeting Modeling Project of LOICZ led by
Stephen Smith and Fred Wulff indicate that many coastal embayments and lagoons
export more phosphate (an essential nutrient for marine primary producers) than they
receive, suggesting these systems to be net heterotrophic or respiring CO2. Apparently, a
fraction of the riverine carbon does get remobilized and released to the atmosphere in
nearshore environments. It seems that systems with sluggish water exchange tend to be
CO2 sources. Eastern boundary current systems form another type of coastal CO2 source.
For example, observations on the western coast of Iberian Peninsula indicate it to be a
weak net CO2 source supported by the upwelling of the CO2 laden Eastern North Atlantic
Central Water rather than the input of terrestrial carbon. Strong outgassing of CO2 was
also observed in the coastal zones off Oregon and Chile during strong upwelling events.
Although coastal upwelling stimulates phytoplankton growth, which consumes CO2, the
high partial pressure of CO2 in the upwelled water often overwhelms the consumption of
CO2 and results in a net release of CO2 to the atmosphere. A confounding phenomenon
within these systems is the juxtaposition of strong CO2 sources and sinks which are
respectively associated with poorly- and well-developed phytoplankton communities.
Recent findings suggest that iron supply from shelf sediments may play a role in
regulating the growth of phytoplankton.
To summarize, riverine carbon fluxes do not necessarily make an adjacent
continental margin a CO2 source. Instead, the margin may serve as a net CO2 sink by
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exporting a significant fraction of locally produced or river discharged organic carbon to
the interior ocean. The shelf biological pump is driven by both the riverine nutrients, and,
more importantly, the upwelled nutrients. Shelf released iron is also an important, but
poorly quantified, micro-nutrient controlling air-sea CO2 exchange. The export of shelf
primary production accounts for 7-27% of the global biological pump [Liu et al., 2000],
which is defined as the carbon transfer from the surface water to the intermediate and
deep waters mediated by biological activities. To illustrate how continental margin
carbon fluxes may affect the global ocean carbon budget, we have constructed one likely
scenario based on the following assumptions: (See Fig. 3 for fluxes in discussion that are
noted below.)

The continental margins as a whole is a weak CO2 sink (0.1 PgC yr-1 as indicated by
the net downward flux between atmosphere and margins).

The storage of anthropogenic CO2 in margins (0.05 PgC yr-1 in parenthesis) is
proportional to their seawater volume.

Three quarters of the carbon burial occurs in margins (dashed arrow of 0.15 PgC yr-1)

Half of the remobilizable carbon from the river runoff is released to the atmosphere
from margins (dashed arrow of 0.3 PgC yr-1 to the atmosphere).

The export of shelf primary production accounts for 20% of the global biological
pump (2 PgC yr-1 from margins to intermediate and deep waters), which is
compensated by upwelling of dissolved inorganic carbon of equal strength.
Given these assumptions, the net air-to-sea CO2 flux in the interior ocean would
be 1.3 Pg C yr-1 (net downward flux between atmosphere and the surface water of interior
ocean as shown in Fig. 3). For this we also assume the total oceanic uptake of
anthropogenic CO2 and the total physical and biological pumps remain the same as those
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in Fig. 2. However, if the net air-to-sea CO2 flux in the interior ocean is as large as the
current estimate (2.2 Pg C yr-1), the actual uptake rate of anthropogenic CO2 would be as
large as 2.9 Pg C yr-1 due to the net input of terrestrial carbon (0.6 Pg C yr-1) and net CO2
uptake in the margins (0.1 Pg C yr-1).
Global synthesis efforts and ongoing regional studies
The new findings highlighted above clearly demonstrate that continental margins
are complicated systems with sufficiently active physical and biogeochemical processes
to affect the global carbon cycle. Within the world oceans, these margins are almost
certainly the most vulnerable to perturbations from human activities. Continental margins
are much more heterogeneous than the interior ocean so that continental margin carbon
fluxes – and perturbations to them – cannot be adequately represented by the coarsely
gridded maps of global ocean carbon fluxes we have today. The CMTT is keen to
synthesize the currently available data on as many different margins as possible to obtain
a global estimate.
The overall goal of the team is to estimate the contribution of continental margins
and seas to CO2 sequestration and horizontal fluxes of carbon, nitrogen and phosphorus
across the ocean-continental margin boundary. Specifically, the team will:

Develop a conceptual framework to integrate continental margin carbon, nitrogen and
phosphorus fluxes and to assess anthropogenic influence on the fluxes;

Identify relevant and appropriate data sets from continental margin studies and
investigate their availability to IGBP projects;

Attempt to quantify vertical and horizontal carbon, nitrogen and phosphorus fluxes in
different types of continental margins, such as:
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a. Eastern boundary currents
b. Western boundary currents
c. Marginal seas
d. Tropical coasts
e. Polar margins

Produce an overall synthesis and assessment of carbon, nitrogen and phosphorus
fluxes on and across continental margins to feed into the IGBP program;

Identify major gaps and uncertainties in the current understanding of continental
margin carbon, nitrogen and phosphorus fluxes and recommend priority needs for
observational and modeling efforts.
The importance and difficulties of the task and the various approaches to reach the
goals have been discussed by Liu et al. [2000]. For the first attempt on global synthesis,
C.T.A. Chen, formerly a co-chair of CMTT, has taken the lead to write a chapter on
Continental Margin Exchanges for the JGOFS synthesis book that is due out in 2001. A
meeting of the CMTT was held in conjunction with the IGBP Congress in Japan in May
1999 to produce a synthesis plan which is available at CMTT homepage. There will be a
series of products leading to the publication of a book whose aim is to synthesize the
current knowledge on C, N, P fluxes in the continental margins. These products,
including the book, are as follows:

Web pages: The CMTT homepage has already been established at
http://www.ncor.ntu.edu.tw/cmtt/, which serves as an archive for storage of
documents and data and for on-line retrieval and provides a forum for discussion.

List of continental margins projects, PI’s and contact addresses
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
Bibliography of continental margins research

Regional synthesis reports (case studies) produced from workshops

Synthesis book, which includes the following chapters: Introduction, Background,
Typology Synthesis, Global Summary, Human Impacts, Global Change Impacts, and
Conclusions.
To synthesize the available information and data, we will organize several
workshops focused on one or two of the continental margin types. Organized by Larry
Atkinson and Renato Quiñones, the first meeting will be the Western and Eastern
Boundary Current Workshop, which was held from 27th through 29th November, 2000 in
Norfolk, Virginia. (More information may be found at
http://www.ccpo.odu.edu/~atkinson/bcworkshop/.) The second, for marginal seas and
tropical coasts (contact: kkliu@ccms.ntu.edu.tw), will be held in Taipei, Taiwan in
September 2001. The third, for polar margins (contact: MacDonaldRob@pac.dfompo.gc.ca), will be announced on the CMTT homepage.
While synthesis and modeling for continental margin carbon fluxes is under way,
new data are being generated in a few on-going regional studies, such as:

Carbon Retention In a Colored Ocean (CARIACO) , which is a study of upwelling
system of Cariaco Basin off Venezuela. Contact: F. Müller-Karger (University of
Southern Florida), http://paria.marine.usf.edu/.

Coastal Ocean Processes program. Contact: R.A. Jahnke (Skidway Institute of
Oceanography), http://www.skio.peachnet.edu/coop/coop.html.

Circulation and Physical-Biological Interactions in the Humboldt Current System
(HCS) and their Impact upon Regional Biogeochemical Cycling (FONDAP12
HUMBOLDT Program, CONICYT, Chile). Contact: Victor A. Gallardo (University
of Concepción, Chile), Email: vagallar@udec.cl.

Long-Term Observation and Research of the East China Sea (LORECS). Contact: G.C. Gong (National Taiwan Ocean University), Email: gcgong@mail.ntou.edu.tw.

MBARI Time-series, which is a continuous observation of upper ocean
biogeochemistry undertaken by the Monterey Bay Aquarium Research Institute.
Contact: F. Chavez, http://www.mbari.org/bog.

Ocean Margin Exchange II project, which is a study of the northern Iberian shelf.
Contact: Rolland Wollast and Lei Chou (Universite Libre de Bruxelles),
http://www.pol.ac.uk/bodc/omex.html.

Shelf-Basin Interactions (SBI) in the Arctic. Contact: J. Grebmeier (University of
Tennesee), Email: jgreb@utkux.utk.edu.
Several interesting new research directions are under development. For example,
monitoring of PCO2 by means of ship-borne automatic instrumentation has been
proposed for regional studies in the North Sea and the ECS. Moored PCO2 monitoring
system, which has been successfully deployed at Mid Atlantic Bight with exciting new
results, is expected to be more widely used. The Global Ocean Observing System
(GOOS) may also support biogeochemical monitoring in the coastal ocean.
Because of insufficient observations and the lack of appropriate modeling tools, it
is unlikely that the desired level of sophistication can be achieved for the global synthesis
of continental margin carbon fluxes within the next few years. When new global ocean
biogeochemistry programs are being planned, the following issues should definitely be
considered:
13

The fate of riverine carbon fluxes in the ocean.

The spatial and temporal variations of air-sea exchange of CO2 and other important
gases in continental margins.

The margin-interior exchange of carbon species, especially, the cross shelf export of
POC and DOC.

The fluxes of shelf released iron and their effects on the ocean carbon cycle.

The development of coupled physical-biogeochemical models for different types of
continental margins
Acknowledgments
The authors who are present and past members of the Continental Margins Task
Team acknowledge supports from the JGOFS and LOICZ projects of IGBP. The authors
are grateful for valuable information provided by F. Chavez, L. Chou, G. Daneri, M.
DeGrandpre, G.-C. Gong, F. Müller-Karger, and R. Wollast. This report benefits from
comments and suggestions provided by Kathy Lo and S.V. Smith. Preparation of this
report was supported by a grant (NSC 89-2611-M-002-004-OP1) and by the National
Center for Ocean Research (NCOR), Taipei, Taiwan. This is NCOR Contribution No. 32.
References
Chen, C.T.A. and S.-L. Wang, Carbon dioxide and related parameters in the East China
Sea, J. Geophys. Res., 104, 20675-20686, 1999.
Harris, P.T., E.K. Baker, A.R. Cole, and S.A. Short, A preliminary study of
sedimentation in the tidally dominated fly River delta, Gulf of Papua, Cont. Shelf
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Res., 13, 441-472, 1993.
Liu, K.K., K. Iseki and S.Y. Chao, Continental margin carbon fluxes. In: The Changing
Ocean Carbon Cycle, R.B. Hanson, H.W. Ducklow, & J.G. Field (Editors), pp.
187-239. Cambridge: Cambridge University Press, 2000.
Melling, H, The formation of a haline shelf front in wintertime in an ice-covered Arctic
sea, Cont. Shelf Res., 13, 1123-1147, 1993.
Nittrouer, C.A., G.J. Brunskill, A.G. Figueiredo, Importance of tropical coastal
environments, Geo-Marine Letters, 15, 121-126, 1995
Siegenthaler, U. and J.L. Sarmiento, Atmospheric carbon dixoide and the ocean, Nature,
365, 119-125, 1993.
Smith, S.V. and F. T. Mackenzie, The ocean as a net heterotrophic system: Implications
from the carbon biogeochemical cycle, Global Biogeochemical Cycles, 1, 187-198,
1987.
Takahashi, T., R. H. Wanninkhof, R. A. Feely, R. Weiss, D. W. Chipman, N. Bates, J.
Olafson, C. Sabine, and S. C. Sutherland, Net sea-air CO2 flux over the global
oceans: an improved estimate based on the sea-air pCO2 difference. Proceeding of
the 2nd Internat. Symp. CO2 in the Oceans, Tsukuba, Jan. 1999, pp. 9-15, 1999.
Thomas, H. and B. Schneider, The seasonal cycle of carbon dioxide in Baltic Sea surface
waters. J. Mar. Systems 22, 53-67, 1999.
Tsunogai, S., S. Watanabe, and T. Sato, Is there a "continental shelf pump" for the
absorption of atmospheric CO2? Tellus, 51B, 701-712, 1999.
Figure captions
Fig. 1. Examples of continental margin biogeochemical studies in recent years. The
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margins are classified into five types: Western Boundary Current Systems
(indicated by arrows in green), Eastern BCS (red), marginal seas (brown), tropical
coasts (yellow), and polar margins. For each area, the type of image is explained.
Clockwise starting from upper left corner: 1. Mid Atlantic Bight: CZCS pigment
image; 2. Bay of Biscay & Iberian Coast: Bathymetry; 3. Baltic Sea: distribution of
CO2 uptake [Thomas and Schneider, 1999]; 4. East China Sea: Chlorophyll
distribution in summer with CZCS image as the background; 5. Lingayen Gulf,
Philippines: a study site (LOICZ Newsletter No. 10); 6. Gulf of Papua: bathymetry
with mud patch of the Fly Delta [Harris et al., 1993]; 7. West coast of South Africa:
SeaWiFS image; 8. Chilean coast at 23oS: PCO2 flux map for July 1997 [N.
Lefevre et al., pers. comm., 2000]; 9. Cariaco Basin area: CZCS image; 10.
California Current system: SeaWiFS images during strong and weak upwelling
conditions.
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Fig. 2. Global carbon cycle 1980-1989 [Siegenthaler and Sarmiento, 1993]. The carbon
fluxes are in units of Pg C yr-1 (1015 g C yr-1). The oceanic uptake of anthropogenic
CO2 is 2 Pg C yr-1, but the net air-to-sea CO2 transfer is offset by 0.6 Pg C yr-1,
which represents outgassing of the riverine carbon (indicated by dashed arrows).
However, if all of the remobilized riverine carbon is outgassed in continental
margins, then there will be no offset for the CO2 flux in the interior ocean.
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Fig. 3. A modified global carbon cycle with consideration of continental margin carbon
fluxes. The net exchanges between the ocean and other reservoirs remain the same
as those in Fig. 2. It is assumed that continental margins are a net CO2 sink (0.1 Pg
C yr-1), the shelf export accounts for 20% of oceanic biological pump which is
compensated by upwelling of dissolved inorganic carbon of the same strength, and
about half of the total riverine carbon flux gets exported from the shelf. (See text
for detail.)
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