GEOPHYSICAL RESEARCH LETTERS, VOL. 36, L20601, doi:10.1029/2009GL040046, 2009
Out-gassing of CO2 from Siberian Shelf seas by terrestrial organic
matter decomposition
L. G. Anderson,1 S. Jutterström,1 S. Hjalmarsson,1 I. Wåhlström,1 and I. P. Semiletov2,3
Received 13 July 2009; revised 3 September 2009; accepted 14 September 2009; published 16 October 2009.
[1] The Siberian shelf seas cover large shallow areas that
receive substantial amounts of river discharge. The river
runoff contributes nutrients that promote marine primary
production, but also dissolved and particulate organic
matter. The coastal regions are built up of organic matter
in permafrost that thaws and result in coastal erosion and
addition of organic matter to the sea. Hence there are
multiple sources of organic matter that through microbial
decomposition result in high partial pressures of CO2 in the
shelf seas. By evaluating data collected from the Laptev and
East Siberian Seas in the summer of 2008 we compute an
excess of DIC equal to 10 1012 g C that is expected to be
outgassed to the atmosphere and suggest that this excess
mainly is caused by terrestrial organic matter decomposition.
Citation: Anderson, L. G., S. Jutterström, S. Hjalmarsson,
I. Wåhlström, and I. P. Semiletov (2009), Out-gassing of CO2 from
Siberian Shelf seas by terrestrial organic matter decomposition,
Geophys. Res. Lett., 36, L20601, doi:10.1029/2009GL040046.
the transport to the deep central Arctic Ocean as well as the
atmospheric exchange. However the substantial input of
both dissolved and particulate organic matter to the shelf
seas [Macdonald et al., 2008], especially during the ice free
summer season, hampers light penetration and results in
moderate marine primary production outside the river
mouths of the great Siberian rivers. Hence, terrestrial
organic matter dominates over marine produced organic
matter as a source of CO2 to the atmosphere in this area.
[4] Here we show that significant microbial decay of
terrestrial organic matter occurs in the Siberian Shelf Seas,
which results in a substantial flux of CO2 to the atmosphere.
Our findings suggest that the release of CO2 from decaying
terrestrial organic matter within the Siberian Shelf Seas
could be an important and so far underestimated source of
CO2 to the atmosphere. This source of CO2 will likely
increase, as predicted results of future warming include
increased permafrost thawing, river discharge and coastal
erosion.
1. Introduction
[2] The Arctic permafrost constitutes a globally significant carbon pool [e.g.. Zimov et al., 2006] that has been
considered stable during the last thousand years. With
climate change, permafrost degrades [e.g., Lawrence and
Slater, 2005; Smith et al., 2005; Lawrence et al., 2008;
Tarnocai et al., 2009] leading to elevated flux of carbon
dioxide and methane from the tundra to the atmosphere
[e.g., Frey and Smith, 2005; Schuur et al., 2009; Shakhova
et al., 2005; Semiletov, 1999]. Furthermore, terrestrial
carbon is released into the Arctic Ocean through a flux of
particulate and dissolved organic carbon (DOC) by river
discharge [e.g., Guo et al., 2007; Neff et al., 2006], a
discharge that increases with climate warming resulting in
amplified DOC export [Spencer et al., 2009]. Climate
change also enhances particulate organic carbon input to
the Arctic Shelf Seas through elevated coastal erosion rates
[e.g., Rachold et al., 2004; Mars and Houseknecht, 2007;
Jones et al., 2009], caused by both the thawing permafrost
and a lengthening of the ice-free season that will result in
less coastal sea ice coverage in the fall when heavy storms
hit the coast.
[3] The Siberian Shelf Seas is a highly dynamic region
where significant carbon transformation occur that impact
1
Department of Chemistry, University of Gothenburg, Gothenburg,
Sweden.
2
Pacific Oceanological Institute, Far Eastern Branch of the Russian
Academy of Sciences, Vladivostok, Russia.
3
Also at International Arctic Research Center, University of Alaska
Fairbanks, Fairbanks, Alaska, USA.
Copyright 2009 by the American Geophysical Union.
0094-8276/09/2009GL040046
2. Methods
[5] In the summer of 2008 (August 15 to September 26)
the International Siberian Shelf Study was conducted with
the objective to investigate the flux and transformation of
carbon from land over the shelf seas and into the deep
central basins of the Arctic Ocean. An extensive sampling
program was undertaken on board the Russian vessel Yacob
Smirnitskyi in the waters of the Laptev Sea (LS), East
Siberian Sea (ESS) and Chukchi Sea (CS) (Figure 1). At
96 stations, depth profiles of nutrients, oxygen, pH, total
alkalinity (TA) and dissolved inorganic carbon (DIC) were
collected and determined on board using state of the art
analytical techniques. The data are archived at the PANGEA
information system under the EU project European Project
on Ocean Acidification (EPOCA).
[6] DIC was determined by coulometric detection of CO2
extracted from a known mass of acidified seawater [Johnson
et al., 1987], with the precision as determined by duplicate
samples typically being ±1 mmol kg 1. The accuracy was
set by analysis of Certified Reference Material (CRM),
supplied by A. Dickson, Scripps Institution of Oceanography (USA) and should hence be about twice that of
precision. Total alkalinity was determined by potentiometric
titration [Haraldsson et al., 1997], precision 1 mmol kg 1,
the accuracy set with CRM. pH was determined by a
spectrophotometric method [Clayton and Byrne, 1993;
Lee and Millero, 1995] with a precision 0.003 pH units.
The partial pressure of CO2 (pCO2) was computed from pH
and total alkalinity using the software CO2SYS [Lewis and
Wallace, 1998]. The carbonate dissociation constants (K1
and K2) used were those of Roy et al. [1993] as they show
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Figure 1. Distribution of surface water pCO2 in the study area with all hydrography station positions noted. The locations
of the sections presented in Figure 2 are shown.
the best internal consistency in the low temperature waters
of the Arctic Ocean when using any two of pH, DIC or TA
as input parameters. Oxygen was determined using an
automatic Winkler titration system, giving a precision of
1 mmol kg 1. The [O2]saturation was calculated using the
equation of Weiss [1970]. The nutrients were determined on
board using an automatic spectrophotometric system
(SmartChem from Westco). The samples were filtered
before measured and evaluated by a 6 to 8-points calibration
curve, precision being about 1%.
[7] The excess DIC in mmol kg 1 was computed as the
difference between the DIC of a water in equilibrium with
an atmospheric pCO2 of 385 matm, all other properties
being as observed, and the measured DIC. This excess was
then integrated throughout the water column to get the unit
gC m 2.
3. Results and Discussion
[8] Most of the surface waters of the LS were oversaturated relative to atmospheric CO2 levels (>385 matm,
green in Figure 1). This was also the situation for the waters
of the western ESS, while the ones in the eastern part were
under-saturated (Figure 1). The latter waters had relatively
high salinities (>25 psu) that are typical for Arctic shelf
surface waters strongly impacted by inflow from the Pacific
Ocean through Bering Sea.
[9] The oxygen concentration was oversaturated in the
surface water of the eastern ESS where pCO2 was undersaturated, illustrated by the Apparent Oxygen Utilization
[O2]measured) concentration being
(AOU = [O2]saturated
negative (Figure 2). The phosphate concentration was high,
around 1 mmol kg 1, while the nitrate concentration was
low, below 0.5 mmol kg 1. The nutrient concentrations were
about the same in the surface water of western ESS, while
the AOU was close to zero (oxygen being saturated) and
pCO2 were oversaturated. In the LS the phosphate concentration was below 0.1 mmol kg 1 in the surface water but
the nitrate concentrations were about the same as in the
ESS. In all sections there was a signature of mineralization
of organic matter in the bottom waters, with high values of
nutrients, AOU and pCO2, even if this was strongest in the
ESS.
[10] The surface water pattern of (1) low nutrient concentration, oxygen at saturation and high pCO2 in the LS,
(2) high nutrient concentration, oxygen at saturation and
high pCO2 in the western ESS, and (3) high nutrient
concentration, oxygen over-saturation and low pCO2 in
the eastern ESS, strongly indicate that more than marine
primary productivity and the corresponding decay of the
produced organic matter was active, at least in the first two
regions. However, this does not mean that marine primary
production does not occur in all regions, only that the
typical chemical signatures during the productive season
has been modified by yet another process.
[11] The organic matter that is mineralized at the sediment surface results in high values of nutrients, AOU and
pCO2 that can be of both terrestrial and marine origin.
However, since CO2 is oversaturated in the surface waters
of the LS and in the western ESS, either the decay of
organic matter exceeds that of marine primary production,
or this signal is a result of vertical mixing of oversaturated
sub-surface water. Either way, a corresponding nutrient
supply to the surface would be expected. As this is not
observed, the only plausible explanation is decay of terrestrial organic matter with a nutrient content comparatively
low relative to marine organic matter.
[12] In the eastern ESS, it is clear that marine primary
productivity is the main process behind the surface water
signature. The front between over- and under-saturation of
pCO2, marked as a solid line in Figure 1, agrees with the
position of the d13Corg isoline of 24.5 % (Figure 3). This
line represents the boundary between the ‘‘typical terrestrial’’ d13Corg values (lighter than 24.5 %) in the western
ESS and the ‘‘typical marine’’ values in the eastern ESS
[Naidu et al., 2000; Semiletov et al., 2005]. As the western
ESS and the LS is the area of high surface sediment content
of terrigenous particulate material [Semiletov et al., 2005]
and low surface water salinity, we argue that the observed
pCO2 signature is dominated by decay of terrestrial organic
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Figure 2. Sections of phosphate, nitrate, pCO2 and Apparent Oxygen Utilization (AOU = [O2]saturated [O2]measured) in
(top) the Laptev Sea, (middle) the western East Siberian Sea, and (bottom) the eastern East Siberian Sea.
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Figure 3. Distribution of the organic carbon (d13Corg) isotope ratio in the upper 0 – 5 cm layer of bottom sediments in the
East Siberian Sea (modified from Semiletov et al. [2005]).
matter added both by runoff and by coastal erosion. Decay
of terrestrial organic matter has earlier been suggested to
cause high pCO2 levels [Semiletov et al., 2007], but then
primarily in the bottom waters of the LS with values up to
more than 2000 matm.
[13] In order to quantify the carbon released from decaying
organic matter, the excess of DIC was computed (Figure 4).
The result shows extensive excess in the western ESS and in
the LS but a deficit in the south-eastern ESS and southern
CS (above zero in Figure 4). These signals also to a great
extent reflect the observations of the surface layer. However,
in the northern CS and north-eastern ESS the extreme
excess is dominated by deep and bottom waters, which
illustrates the export of dissolved carbon to the deep central
basin. No sampling of the slopes of the western ESS or the
LS was done and hence it cannot be ruled out that export
occurs also in these regions.
[14] If integrated, the excess of DIC that is observed in
waters shallower than 50 m, i.e., waters that are largely
homogenized in the autumn due to wind mixing and brine
release from sea ice formation gives an excesses for half of
the ESS of 5 1012 g C and about the same for the whole
LS. Hence a potential CO2 out-gassing of 10 1012 g C to
the atmosphere could occur within a year if the water mixes
and open water persist during the summer season. This
number is of the same order as the reported input of total
organic carbon to the LS and ESS 10 1012 g C yr 1 from
runoff and 4 1012 g C yr 1 from coastal erosion
[Rachold et al., 2004], to which marine produced organic
carbon in the order 45 1012 g C yr 1 is added [Sakshaug,
2004]. However, the latter number is highly uncertain as it
for the ESS is calculated by extrapolation of data from the
neighboring seas.
4. Conclusions
[15] We have shown that in the summer of 2008 there
was an excess of DIC equal to 10 1012 g C in the LS and
Figure 4. Excess carbon (g C m 2) relative to pCO2 = 385 matm, integrated throughout the whole water column.
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ESS of the Arctic Ocean as a result of decaying organic
matter. The fate of this excess will ultimately be an outgassing to the atmosphere as it is found in the low salinity
surface water. Some of the excess will likely be trapped
under the sea ice as the water moves into the central Arctic
Ocean, but released when the water is exposed to the
atmosphere again. Furthermore we suggest that the transport
and decay of particulate eroded terrestrial organic carbon
plays a significant role in this CO2 excess. The rationale
being that the nutrient and oxygen signatures indicate a
limited marine organic matter decomposition and that the
eroded organic matter to a large degree is biodegradable
[Guo et al., 2004; van Dongen et al., 2008], whereas
riverine DOC is more stable and mainly composed of
soil-derived humic substances [Dittmar and Kattner,
2003]. However, recent findings suggest terrestrial DOC
to be variable in lability [Raymond et al., 2007].
[16] It is plausible that the previous estimates of terrestrial
particulate organic matter are on the low side in view of
the retreating summer sea ice coverage (http://nsidc.org/
arcticseaicenews/) and the increased coastal and river bank
erosion. The increased river discharge [Savelieva et al.,
2000; Peterson et al., 2002] furthermore might bring more
humic substances that will decrease transparency and primary production. Some of this effect could be compensated
for by increasing nutrient supply, mainly phosphate and
silicate, by the runoff [e.g., Frey and McClelland, 2009] that
could boost primary production. A continuing warming
adds more terrestrial organic matter to the Arctic Shelf
Seas, which increases pCO2, at the same time as decreased
transparency lowers primary production, which reduce
consumption of CO2. Both these effects result in a positive
feedback by out-gassing CO2 over the Siberian Shelf, which
comprises one half of the entire Arctic Ocean shelf area.
[17] Acknowledgments. This work was carried out by logistic support from the Knut and Alice Wallenberg Foundation and from the Swedish
Polar Research Secretariat. The science was supported by the Swedish
Research Council and the European Union projects, CarboOcean (contract
no 511176-2), DAMOCLES (contract 018509-2) and EPOCA (contract
211384). Publication 27 from Tellus, The Centre of Earth Systems Science
at University of Gothenburg.
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