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Variations in the isotopic composition of particulate organic carbon and their relation with carbon dynamics in the western Arctic Ocean
Deep-Sea Research II 81–84 (2012) 72–78
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Deep-Sea Research II journal homepage: www.elsevier.com/locate/dsr2
Variations in the isotopic composition of particulate organic carbon and their relation with carbon dynamics in the western Arctic Ocean Run Zhang a, Min Chen a,b,n, Laodong Guo b,c, Zhongyong Gao d, Qiang Ma a, Jianping Cao a, Yusheng Qiu a,b, Yanping Li a a
Department of Oceanography, Xiamen University, Xiamen 361005, China State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen 361005, China c Department of Marine Science, University of Southern Mississippi, Stennis Space Center, MS 39529, USA d The Third Institute of Oceanography, State Oceanic Administration, Xiamen 361005, China b
a r t i c l e i n f o
a b s t r a c t
Available online 19 May 2011
The relation between the dissolved carbon dioxide (CO2) and the stable isotopic composition of particulate organic matter in the water column has not been well quantified, but this information could help provide a better understanding of carbon dynamics in a warmer Arctic Ocean. The stable carbon isotopic composition of suspended particulate organic carbon (d13CPOC) in the surface waters of the western Arctic Ocean was measured during July–September 2003, to evaluate the spatial variability of d13CPOC and its key controlling factors. Values of d13CPOC fell within the range of 28.5% to 21.1%, with an average of 24.5 7 2.3%. The spatial variability of d13CPOC showed a general decreasing trend from shallow waters in the continental shelf toward the deeper, colder waters in the basin. A negative correlation between d13CPOC and the dissolved CO2 concentration in surface waters was observed, indicating that carbon isotopic fractionation during photosynthesis was largely dependent on the dissolved CO2 concentration. Compared to the solubility pump, biological processes may play a more important role in determining the distribution and variation of d13CPOC in the western Arctic Ocean during summer. The coupled relationship between CO2 concentration and stable isotopic composition of particulate organic matter has the potential to be used for reconstruction of sea-surface CO2 changes in the past, provided a quantitative relationship of d13C between POC and sediments can be established. & 2011 Elsevier Ltd. All rights reserved.
Keywords: Western Arctic Ocean Particulate organic matter Stable carbon isotopes pCO2
1. Introduction
studies, revealing that [CO2(aq)] is a major factor in controlling the
d13C composition of POC (Francois et al., 1993; Hofmann et al., 2000; The natural stable carbon isotopic composition (d13CPOC) of marine suspended particulate organic matter (POM) can potentially provide important insights into changes in the environmental conditions under which carbon fixation occurs. An understanding of the spatial variability and the key controlling factors of d13CPOC is therefore indispensable for the use of d13CPOC as a proxy in studies of the marine carbon cycle and environmental changes (Fischer, 1991; Hofmann et al., 2000; Laws et al., 1995; Popp et al., 1999; Rau et al., 1997). Planktonic d13CPOC is a function of the d13C of the inorganic carbon source for organic matter production and the isotopic fractionation factor during photosynthesis. During photosynthesis, the isotopic fractionation plays a decisive role (Keller and Morel, 1999; Laws et al., 1995). A negative correlation between d13CPOC and dissolved carbon dioxide concentration [CO2(aq)] has been observed in many previous
n Corresponding author at: Department of Oceanography, Xiamen University, Xiamen 361005, China. E-mail address: [email protected] (M. Chen).
0967-0645/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr2.2011.05.005
Kennedy and Robertson, 1995; Rau et al., 1997, 1992). The Arctic Ocean is especially sensitive to climate change and may act as an important sink or source for atmospheric CO2 (Bates et al., 2006; Cai et al., 2010; Dickson, 1999; Kaltin and Anderson, 2005; Murata and Takizawa, 2003; Walsh, 1989). Applications using stable isotopic composition should improve our understanding of carbon dynamics and its response to climate and environmental changes in the Arctic Ocean. Previous studies have shown that the biological pump in the western Arctic Ocean is actively running in summer (Chen et al., 2002, 2003; Ma et al., 2005; Moran et al., 2005; Trimble and Baskaran, 2005). The active biological uptake of CO2 in the surface ocean decreases [CO2(aq)], and at the same time contributes to the POC export flux (namely, export production). Unfortunately, measurements of the stable isotopic composition of particulate organic matter in the Arctic Ocean remain scarce. There are very few available or published data allowing examination of the relationship between d13CPOC and [CO2(aq)] in the Arctic Ocean. Thus, measurements of stable isotopic composition are sorely needed, because the dependence of d13CPOC on [CO2(aq)] observed in other marine environments
R. Zhang et al. / Deep-Sea Research II 81–84 (2012) 72–78
may not be applicable in the Arctic Ocean, and carbon isotopic fractionation may also be controlled by other factors such as growth rate, phytoplankton species composition, trophic effects, and water temperature (e.g., Cullen et al., 2001; Goericke and Fry, 1994; Guo et al., 2004; Hofmann et al., 2000; Tamelander et al., 2006). The release of anthropogenic CO2 (mainly derived from fossil fuel burning and deforestation) has been increasing, especially in recent decades, and a large fraction of these emissions has been taken up by the ocean (Gruber, 1998; Sabine et al., 2004; Siegenthaler and Sarmiento, 1993). As an isotopically ‘‘light’’ source, anthropogenic CO2 invading the upper ocean will result in an increase in surface [CO2(aq)] but a decrease in d13CO2(aq) (known as the Suess effect). Therefore, a subsequent decrease of d13CPOC in the water column can be predicted (Bauch et al., 2000; Druffel and Benavides, 1986; Quay et al., 2003, 1992; Schell, 2001). In the Arctic Ocean, a decrease of about 2% in phytoplankton d13C since the Industrial Revolution has been attributed to the input of anthropogenic CO2 (Bauch et al., 2000). In the present study, samples collected from the western Arctic Ocean were measured for stable carbon isotopic composition (d13C) and other parameters to investigate spatial variability in the d13C values of suspended POM, the role of CO2(aq) in controlling the d13CPOC, and the effects of the solubility and biological pumps on d13CPOC. The relationship between surface [CO2(aq)] and d13CPOC was further used to assess the possible impact of anthropogenic CO2 on the d13CPOC of the western Arctic Ocean. 2. Materials and methods 2.1. Study area and sampling locations Water and suspended particulate samples were collected onboard the icebreaker R/V XUELONG from July to September 2003 during the Second Chinese Arctic Research Expedition (CHINARE). A total of 31 stations were sampled for POC, PON, and d13CPOC measurements in the western Arctic Ocean, including the northern Bering Sea (3 stations near the Bering Strait), the eastern Chukchi Sea, the western Beaufort Sea, and part of the Canada Basin (Fig. 1). Seawater samples were collected using a CTD rosette system. Temperature, salinity, and
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Chl-a concentration data were provided by the CHINARE program (www.polar.gov.cn). 2.2. Sampling and measurements of POC, PON, and d13CPOC For POM and d13CPOC measurements, about 2 l of surface ( 1 m) seawater samples were filtered onboard ship through a precombusted (400 1C, 4 h) Whatman GF/F membrane. After filtration, the filters with suspended POM samples were rinsed with 10 ml 0.1 mol/dm3 HCl and 30 ml Milli-Q water to remove inorganic carbonate, and then stored frozen. While in the laboratory, the filter samples were dried at 60 1C and wrapped into tin capsules for measurements of POC, PON, and d13CPOC. Concentrations of POC, PON, and stable carbon isotopic composition (in term of d13C) were measured on a Finnigan MAT Deltaplus XP mass spectrometer interfaced with an elemental analyzer (Carlo Erba NC2500). The d13C value (versus PDB) of POM sample was defined as the following: Rsample d13 CPOC ¼ 1 1000 Rstandard The reproducibility of d13C measurements was within 70.2%. 2.3. Measurements of pCO2 and [CO2(aq)] The partial pressures of CO2 (pCO2, matm) in the surface waters were measured using a shipboard LI-COR 6262 CO2/H2O infrared gas analyzer. Standard carbon dioxide gas mixtures from the National Research Center of Standard Materials of China were used for calibration, with concentrations of 285, 348, and 401 ppmv [CO2/air]. The system accuracy was 1.0 matm (Chen and Gao, 2007). Concentrations of dissolved CO2 ([CO2(aq)]) were calculated from pCO2, temperature (T), and salinity (S), using the solubility constants (K0) established by Weiss (1974): ½CO2 ðaqÞ ¼ K0 pCO2 " # 100 T T T 2 þ a3 ln þ S b1 þ b2 þb3 ln K0 ¼ a1 þ a2 T 100 100 100
3. Results 3.1. Hydrographic features The spatial distributions of surface water temperature and salinity are shown in Fig. 2A and B. Surface water temperature ranged from 1.55 to 10.50 1C, with an average of 1.95 1C in the study area (Table 1). Temperature decreased northward from an average of 9 1C in the northern Bering Sea to 3 1C at stations shallower than 50 m on the Chukchi Shelf, and fell to below zero (about 1 1C) at deeper stations in the Canada Basin. Water temperature can alter the solubility of CO2, although primary production and organic matter degradation also play an important role. Surface water salinity ranged from 27.44 to 32.57, with an average of 29.76 (Table 1). Except for several extremely lowsalinity values ( 28) that were observed in the deep basin, likely caused by sea-ice melt, salinity ( 31) did not vary dramatically over the northern Bering Sea and the Chukchi Shelf. Generally, low surface-water salinity and its spatial distribution could be attributed to the significant dilution effect of sea ice melting in summer. 3.2. Distribution of POC, PON, and C/N ratios
Fig. 1. Sampling locations in the western Arctic Ocean during summer 2003.
The spatial distributions of POC and PON are depicted in Fig. 2C and D. POC concentrations in the surface water varied between 1.8
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R. Zhang et al. / Deep-Sea Research II 81–84 (2012) 72–78
Fig. 2. Distributions of (A) temperature (1C), (B) salinity, (C) POC (mM), (D) PON (mM), (E) d13CPOC (%), and (F) [CO2(aq)] (mM) in the western Arctic Ocean during Summer 2003.
and 19.1 mM, with an average value of 7.774.0 mM, while PON concentrations varied between 0.3 and 3.5 mM, with an average value of 1.270.7 mM (Table 1). The average C/N ratio (6.771.4) deviated little from the Redfield ratio of 6.6 (106:16), with a range between 5.1 and 11.9. These POC and PON concentrations and C/N ratios are similar to those reported in previous studies (Chen et al., 2003; Ma et al., 2005; Trimble and Baskaran, 2005). POC and PON concentrations generally decreased with increase in latitude, showing similar distribution patterns as surface water temperature and salinity (Fig. 2A and B). The observed POC/PON ratios in the western Arctic Ocean are considerably lower than those measured
for terrestrial POM (2778) derived from the Yukon and Mackenzie Rivers (Guo et al., 2011; Unpublished Data). The positive correlation between POC and temperature, together with the average C/N ratio of POM close to the Redfield ratio, suggested that particulate organic matter collected during the sampling period was mainly from marine-derived organic matter. 3.3. Distribution of d13CPOC The d13CPOC fell in a wide range between 28.5% and 21.1%, with an average value of 24.572.29% (Table 1).
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Table 1 Temperature, salinity, Chl-a, particulate organic carbon (POC), particulate organic nitrogen (PON), C/N molar ratio, stable carbon isotopic composition (d13C) of suspended organic matter, partial pressure of carbon dioxide (pCO2), and concentration of dissolved carbon dioxide ([CO2(aq)]) in the western Arctic Ocean during summer 2003. Station
BS01 BS07 BS10 R01 R03 R04A R05 R07 R11 R15 C12 C15 C18 C22 C24 C26 C34 M03 M07 S11 S21 S25 S32 P11 P22 B11 B13 B32 B33 B79 B80
Longitude (1W)
Latitude (1N)
Layer (m)
T (1C)
171.51 168.50 167.01 169.01 169.00 169.00 169.00 169.00 169.67 168.99 167.03 164.01 161.03 167.01 164.99 162.98 167.01 171.93 171.94 159.00 154.98 153.40 150.38 169.99 164.93 156.33 151.88 151.55 149.27 151.79 146.74
64.33 64.34 64.33 66.99 68.00 68.50 69.00 70.00 72.01 74.00 71.65 71.58 71.48 70.50 70.50 70.49 68.92 76.54 75.00 72.49 71.65 72.74 71.26 75.01 77.40 74.00 73.38 75.00 74.63 79.31 80.22
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
9.34 7.56 10.50 5.72 3.34 7.63 6.48 4.62 1.17 1.47 0.65 0.87 0.37 3.87 3.20 4.96 7.94 1.55 1.27 1.02 2.94 0.65 1.52 1.27 1.49 0.98 1.02 1.20 1.22 1.47 1.52
S
31.61 31.84 28.57 31.95 32.57 31.49 31.25 31.99 29.68 29.49 29.49 30.45 29.21 31.57 31.13 31.20 30.93 29.16 28.29 28.85 27.44 28.15 29.37 28.58 28.68 27.98 27.56 27.91 27.92 28.47 29.86
Chl-a (mg/dm3)
POC (mM)
0.8 0.9 nd nd nd 1.3 nd 0.5 nd nd nd nd nd nd nd 0.3 nd 0.2 0.1 0.6 0.5 nd 0.2 0.1 0.2 0.1 0.0 nd 0.1 0.1 0.2
10.5 10.2 8.6 17.6 11.9 13.1 7.0 6.8 7.6 8.0 1.8 11.5 19.1 4.9 5.6 5.4 8.9 5.6 7.0 9.2 9.3 2.7 5.9 5.5 4.1 4.5 4.5 9.6 3.5 2.9 4.9
PON (mM)
1.5 1.5 1.2 2.6 1.8 2.2 1.0 0.9 1.0 0.9 0.3 2.2 3.5 0.8 0.9 1.0 1.5 0.9 1.4 1.8 1.5 0.5 1.1 0.7 0.6 0.5 0.6 0.8 0.5 0.4 0.7
C/N (molar)
pCO2 (matm)
[CO2(aq)] (mM)
d13CPOC
7.2 6.7 7.2 6.7 6.7 6.0 7.0 7.2 7.4 8.7 5.2 5.3 5.5 6.2 5.9 5.4 5.8 6.0 5.1 5.2 6.2 5.9 5.2 7.4 7.4 9.0 7.3 11.9 7.8 7.1 6.9
184 238 389 198 227 195 287 212 178 253 155 181 189 209 210 244 318 275 291 244 267 309 270 274 272 308 318 496 310 370 391
8.6 11.8 17.8 10.5 13.1 9.7 14.9 11.7 12.4 17.8 10.6 12.4 12.8 11.9 12.3 13.4 15.7 19.5 20.5 17.0 16.0 21.2 16.9 19.3 19.3 21.5 22.3 34.9 21.8 26.2 27.6
22.1 23.5 24.6 22.5 22.3 24.4 24.1 22.1 24.2 21.8 22.6 21.1 21.2 24.2 22.3 23.3 22.4 26.0 25.1 27.3 22.7 28.5 24.0 26.9 27.0 27.9 25.6 27.9 27.0 28.1 27.1
(%)
Note: nd represents no data.
The north Bering Sea–Chukchi Shelf area had a higher average d13CPOC value of 22.9% (n ¼17, ranging from 24.6% to 21.1%). The average d13CPOC value decreased dramatically to about 26.5% (from 28.5% to 22.7%, n ¼14) at deep-water stations (Fig. 2E). To the best of our knowledge, such great variations in d13CPOC between the shallow shelf and the deepwater stations in the western Arctic Ocean have never been reported before. Interestingly, the distribution pattern of d13CPOC resembles those observed for temperature and POC. In addition to changes in d13CPOC along the latitudinal gradient, with higher values near the Bering Strait and Chukchi Shelf and lower values toward the deep basin, a west to east increasing trend from the Chukchi Shelf to the Beaufort Sea was also evident (Fig. 2E). The values of surface water d13CPOC measured in our study area compared well with those reported previously for other marine environments. They are close to those observed in the southeastern Bering Sea in summer (from 27.1% to 23.4% with an average of 24.5%, Guo et al., 2004), but lighter than those found in low latitude sea areas, such as in the subtropical southwestern Indian Ocean (from 28.4% to 16.5% with an average of 23.7%, Francois et al., 1993) or the equatorial Pacific Ocean (from 21.5% to 20.8% with an average of 21.1%, Laws et al., 1995). However, our d13C values are higher than those encountered in the Southern Ocean (from 29.7% to 21.9% with an average of 26.6%, Dehairs et al., 1997; Kennedy and Robertson, 1995). This is consistent with the reported latitudinal increasing trend of surface ocean d13CPOC toward the tropics in both hemispheres (Goericke and Fry, 1994; Hofmann et al., 2000). Although the Arctic Ocean receives disproportionately high continental runoff and terrestrial inputs, our comparable d13CPOC values and near-Redfield POC/PON ratios all point to
predominantly marine-derived POM during our sampling period in the western Arctic Ocean. 3.4. Distribution of [CO2(aq)] The values of the partial pressure of carbon dioxide (pCO2) varied between 155 and 496 matm, with an average of 267775 matm in the study area (Table 1). At 27 stations the pCO2 was below the equilibrium value with the atmosphere. Dissolved concentrations of carbon dioxide ([CO2(aq)]) ranged from 8.6 to 34.9 mM, with an average of 16.8 75.9 mM. [CO2(aq)] showed a mirror distribution pattern to that of d13CPOC (Fig. 2F). The northern Bering Sea–Chukchi Shelf area was characterized by very low [CO2(aq)] ( 12.8 mM). The [CO2(aq)] value then increased dramatically from the Chukchi Shelf to the deep basin and the Beaufort Sea, with a mean value of 21.7 mM, which is almost two times higher than the low value observed on the Chukchi Shelf.
4. Discussion 4.1. Variations of d13CPOC The particulate organic matter during the sampling period was derived predominantly from marine organic matter, and terrestrial organic matter did not seem to have a noticeable impact on d13CPOC variations in the study area. Three lines of evidence support this statement. First, the average C/N ratio of POM was 6.771.4 (Table 1) and similar to the Redfield ratio. Second, a significant positive correlation between POC and the Chl-a
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R. Zhang et al. / Deep-Sea Research II 81–84 (2012) 72–78
Fig. 3. Relationship between POC (mM) and Chl-a (mg/dm3) in the western Arctic Ocean during summer 2003.
concentration was observed (Fig. 3), also indicating a marine origin of the POM. A higher Chl-a concentration or higher biomass could result in a higher POC concentration. Third, d13CPOC decreased from the shelf to the deep basin (Fig. 2E), ruling out the possibility of a major contribution of terrestrial organic matter in the study area. The d13CPOC value of the terrestrial end member for the northern Bering–Chukchi–Beaufort Sea was reported as 27%, based on the data from major Arctic rivers, such as the Colville River, Mackenzie River, Lena River, and Yukon River (Guo and Macdonald, 2006; Minagawa et al., 1991; Naidu et al., 2000, 1993; Ruttenberg and Goni, 1997; Schell, 1983). Although the d13C of marine-derived POC in the study area has not yet been reported, it is speculated to be at least 2–3% heavier than that of terrestrial organic matter (Guo and Macdonald, 2006; Naidu et al., 2000) due to the difference in plant photosynthetic environments. If the influence of terrestrial organic matter is significant in the study area, the d13CPOC values would have increased from the shelf to the deep basin because the inner shelf waters should have received more freshwater and terrestrial organic matter. However, the opposite was true. Thus, POM collected during our sampling period should be predominantly derived from in situ biological production. 4.2. Dependence of d13CPOC on [CO2(aq)] The d13CPOC signatures of biogenic POM should be controlled by biological fractionation during synthesis of organic material. Our results show that the surface water d13CPOC decreased northward and had the mirror pattern with dissolved carbon dioxide (Fig. 2E and F), indicating d13CPOC was related to the abundance of CO2 and biological production in the western Arctic Ocean. Indeed, a significant negative correlation was observed between surface d13CPOC and log[CO2(aq)] (logarithmic values were adopted here after Popp et al. (1989) empirically) (Fig. 4). This indicated the dependence of the POC-d13C composition on the aqueous CO2 concentration in the western Arctic Ocean during summer, which could be described with the following relationship:
d13 CPOC ¼ 9:19212:75 log½CO2 ðaqÞ ðr 2 ¼ 0:63,p o0:0001Þ This is the first time that such a negative correlation has been revealed in the western Arctic Ocean. Similar relations have been reported previously in other areas, such as the Indian Ocean (Francois et al., 1993), the Atlantic Ocean (Rau et al., 1992), the Southern Ocean (Fischer, 1991; Kennedy and Robertson, 1995;
Fig. 4. Relationship between d13CPOC and log[CO2(aq)] in the western Arctic Ocean during summer 2003.
Lourey et al., 2004), the Antarctic seas (Rau et al., 1991), and model results in the world ocean (Hofmann et al., 2000). When inorganic carbon acquisition is maintained only by passive CO2 diffusion during planktonic photosynthesis, the d13C values of the POC produced can be expressed as a function of CO2(aq) (Cullen et al., 2001; Hofmann et al., 2000; Laws et al., 1995; Rau et al., 1996):
d13 CPOC ¼ d13 CCO2 ðaqÞ ef þa
m ½CO2 ðaqÞ
where d13CCO2(aq) is the 13C isotopic composition of the substrate CO2(aq), ef is the carbon isotopic fractionation associated with photosynthetic fixation, m is the specific growth rate, and a is a species-specific constant. Although direct measurements of the d13C composition of aqueous CO2 were not carried out here, short-term changes in d13CCO2(aq) should not be a major factor responsible for the large variations in d13CPOC (up to 7%) observed in the study area. Globally, the d13C value of surface ocean total dissolved CO2 (DIC) does not vary greatly (Broecker and Maier-Reimer, 1992; LynchStieglitz et al., 1995), while gradients in d13CPOC are much greater among different sea areas (Hofmann et al., 2000). Locally, the polar oceans have experienced very small changes in surface d13CDIC ( o3%) in recent decades compared with other regions (Quay et al., 2003), and the d13C value of CO2 in seawater in equilibrium with the atmosphere covaried with temperature, but at a slow rate (Mook et al., 1974). Thus, it is less likely that such a large variation in d13CPOC (up to 7%) observed in the western Arctic Ocean was mainly caused by changes in isotopic compositions of CO2(aq). Our study demonstrated that d13CPOC in the western Arctic Ocean was controlled by the aqueous CO2 concentration. However, some other studies have showed that d13CPOC could also vary independently of [CO2(aq)], such as in the southeastern Bering Sea (Guo et al., 2004) and in some parts of the Southern Ocean (Bentaleb et al., 1998; Francois et al., 1993; Popp et al., 1999), indicating the influence of other factors on d13CPOC. They may include the active uptake of bicarbonate (HCO-3, with the help of a carbon concentrating mechanism) (Cassar et al., 2004; Tortell et al., 1997), phytoplankton species composition (Cullen et al., 2001; Kopczynska et al., 1995; Rau et al., 1992), cell geometry (Burkhardt et al., 1999; Popp et al., 1998; Trull and Armand, 2001), and zooplankton-related processes (Rau et al., 1990; Tamelander et al., 2006). More studies are needed to further understand the ultimate controls on d13CPOC.
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4.3. Possible effect of anthropogenic CO2 on d13CPOC
References
The invasion of isotopically ‘‘light’’ anthropogenic CO2 into the upper ocean could result in both an increase in [CO2(aq)] and a decrease in its d13C, causing a subsequent decrease in d13CPOC in the water column (Cullen et al., 2001; Druffel and Benavides, 1986; Quay et al., 2003, 1992; Schell, 2001). The concentration of atmospheric CO2 has increased from 280 to 380 ppm since the Industrial Revolution (www.esrl.noaa.gov/gmd/ccgg/trends). Assuming the negative correlation between d13CPOC and log[CO2(aq)] observed in this study has held true over the past century or so, the following equation can be inferred:
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Uptake of atmospheric carbon dioxide in Arctic shelf seas: evaluation of the relative importance of processes that influence pCO2 in water transported over the Bering–Chukchi Sea shelf. Marine Chemistry 94 (1-4), 67–79. Keller, K., Morel, F.M.M., 1999. A model of carbon isotopic fractionation and active carbon uptake in phytoplankton. Marine Ecology Progress Series 182, 295–298. Kennedy, H., Robertson, J., 1995. Variations in the isotopic composition of particulate organic carbon in surface waters along an 881W transect from 671S to 541S. Deep-Sea Research II 42 (4–5), 1109–1122. Kopczynska, E., Goeyens, L., Semeneh, M., Dehairs, F., 1995. Phytoplankton composition and cell carbon distribution in Prydz Bay, Antarctica: relation to
Dd13 CPOC ¼ 12:75 Dlog½CO2 ðaqÞ If the surface seawater remains at a steady state where T¼0 1C and S ¼30, then an overall decrease of 1.7% in d13CPOC can be estimated since preindustrial times. In other words, an average rate of decrease of about 0.01% per year in d13CPOC can be expected owing to the addition of anthropogenic CO2. This decrease is comparable to the anthropogenic decrease rates of d13C reported for global atmospheric CO2 (Friedli et al., 1986) and surface ocean DIC (Druffel and Benavides, 1986; Quay et al., 1992; Quay et al., 2003). It is also similar to the fractionation in d13C between modern POM and surface sediments in the Arctic Ocean (Bauch et al., 2000), but it is much lower than the observed decrease in planktonic d13C in the Bering Sea in recent decades (Cullen et al., 2001; Schell, 2001). Although the overall change in d13CPOC resulting from the increase in anthropogenic CO2 on seasonal and yearly time scales could be relatively small compared to the biological fractionation in d13C and spatial changes in d13CPOC, the long-term effect on d13CPOC due to anthropogenic impacts should be detectable in the sediments, especially on decadal time scales.
5. Conclusions Our results showed that d13CPOC in the surface seawater generally decreased from the continental shelf to the deep basin in the western Arctic Ocean. d13CPOC and photosynthetic carbon isotopic fractionation were related to the dissolved CO2 concentration in surface waters. Biological processes were a major factor affecting surface water CO2 concentrations and further controlling the variation of d13CPOC. The significant correlation between d13CPOC and dissolved CO2 supports the use of d13C as a proxy for carbon dynamics in the Arctic Ocean. Thus, signatures of d13C in sediment core samples together with sediment chronologies should allow the reconstruction of changes in dissolved CO2 of the surface ocean, if a functionally coupled relationship between d13CPOC in the water column and d13CPOC in the sediment can be established.
Acknowledgments We thank the crewmembers and science party of the 2nd Chinese Arctic Research Expedition (CHINARE) for their assistance during sample collection, CHINARE for hydrographic data, and two anonymous reviewers for their constructive comments on this manuscript. This work was supported, in part, by a Chinese IPY Research Program, a Fujian Natural Science Foundation (2009J06026), a COMRA Program (DYXM115-02-4-06), a National Natural Science Foundation of China (40976116), the Scientific Research Foundation of Third Institute of Oceanography, State Oceanic Administration (2010011, 2010001), and a Cheung Kong Scholarship through Ministry of Education of China.
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Rau, G.H., Riebesell, U., Wolf-Gladrow, D., 1996. A model of photosynthetic 13C fractionation by marine phytoplankton based on diffusive molecular CO2 uptake. Marine Ecology Progress Series 133 (1), 275–285. Rau, G.H., Riebesell, U., Wolf-Gladrow, D., 1997. CO2aq-dependent photosynthetic 13 C fractionation in the ocean: a model versus measurements. Global Biogeochemical Cycles 11 (2), 267–278. Rau, G.H., Sullivan, C.W., Gordon, L.I., 1991. d13C and d15N variations in Weddell Sea particulate organic matter. Marine Chemistry 35 (1–4), 355–369. Rau, G.H., Takahashi, T., Marais, D.J.D., Repeta, D.J., Martin, J.H., 1992. The relationship between d13C of organic matter and [CO2(aq)] in ocean surface water: data from a JGOFS site in the northeast Atlantic Ocean and a model. Geochimica et Cosmochimica Acta 56 (3), 1413–1419. Rau, G.H., Teyssie, J.L., Rassoulzadegan, F., Fowler, S.W., 1990. 13C/12C and 15N/14N variations among size-fractionated marine particles: implications for their origin and trophic relationships. Marine Ecology Progress Series 59, 33–38. Ruttenberg, K.C., Goni, M.A., 1997. Phosphorus distribution, C:N:P ratios, and d13Coc in arctic, temperate, and tropical coastal sediments: tools for characterizing bulk sedimentary organic matter. Marine Geology 139 (1–4), 123–145. Sabine, C.L., Feely, R.A., Gruber, N., Key, R.M., Lee, K., Bullister, J.L., Wanninkhof, R., Wong, C.S., Wallace, D.W.R., Tilbrook, B., 2004. The oceanic sink for anthropogenic CO2. Science 305 (5682), 367–371. Schell, D.M., 1983. Carbon-13 and carbon-14 abundances in Alaskan aquatic organisms: delayed production from peat in arctic food webs. Science 219 (4588), 1068–1071. Schell, D.M., 2001. Carbon isotope ratio variations in Bering Sea biota: the role of anthropogenic carbon dioxide. Limnology and Oceanography 46 (4), 999–1000. Siegenthaler, U., Sarmiento, J.L., 1993. Atmospheric carbon dioxide and the ocean. Nature 365 (6442), 119–125. Tamelander, T., Søreide, J.E., Hop, H., Carroll, M.L., 2006. Fractionation of stable isotopes in the Arctic marine copepod Calanus glacialis: effects on the isotopic composition of marine particulate organic matter. Journal of Experimental Marine Biology and Ecology 333 (2), 231–240. Tortell, P.D., Reinfelder, J.R., Morel, F.M.M., 1997. Active uptake of bicarbonate by diatoms. Nature 390 (6657), 243–244. Trimble, S.M., Baskaran, M., 2005. The role of suspended particulate matter in 234 Th scavenging and 234Th-derived export fluxes of POC in the Canada Basin of the Arctic Ocean. Marine Chemistry 96 (1–2), 1–19. Trull, T.W., Armand, L., 2001. Insights into Southern Ocean carbon export from the d13C of particles and dissolved inorganic carbon during the SOIREE iron release experiment. Deep-Sea Research II 48 (11–12), 2655–2680. Walsh, J.J., 1989. Arctic carbon sinks: present and future. Global Biogeochemical Cycles 3 (4), 393–411. Weiss, R.F., 1974. Carbon dioxide in water and seawater: the solubility of a nonideal gas. Marine Chemistry 2, 203–215.
Contents lists available at ScienceDirect
Deep-Sea Research II journal homepage: www.elsevier.com/locate/dsr2
Variations in the isotopic composition of particulate organic carbon and their relation with carbon dynamics in the western Arctic Ocean Run Zhang a, Min Chen a,b,n, Laodong Guo b,c, Zhongyong Gao d, Qiang Ma a, Jianping Cao a, Yusheng Qiu a,b, Yanping Li a a
Department of Oceanography, Xiamen University, Xiamen 361005, China State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen 361005, China c Department of Marine Science, University of Southern Mississippi, Stennis Space Center, MS 39529, USA d The Third Institute of Oceanography, State Oceanic Administration, Xiamen 361005, China b
a r t i c l e i n f o
a b s t r a c t
Available online 19 May 2011
The relation between the dissolved carbon dioxide (CO2) and the stable isotopic composition of particulate organic matter in the water column has not been well quantified, but this information could help provide a better understanding of carbon dynamics in a warmer Arctic Ocean. The stable carbon isotopic composition of suspended particulate organic carbon (d13CPOC) in the surface waters of the western Arctic Ocean was measured during July–September 2003, to evaluate the spatial variability of d13CPOC and its key controlling factors. Values of d13CPOC fell within the range of 28.5% to 21.1%, with an average of 24.5 7 2.3%. The spatial variability of d13CPOC showed a general decreasing trend from shallow waters in the continental shelf toward the deeper, colder waters in the basin. A negative correlation between d13CPOC and the dissolved CO2 concentration in surface waters was observed, indicating that carbon isotopic fractionation during photosynthesis was largely dependent on the dissolved CO2 concentration. Compared to the solubility pump, biological processes may play a more important role in determining the distribution and variation of d13CPOC in the western Arctic Ocean during summer. The coupled relationship between CO2 concentration and stable isotopic composition of particulate organic matter has the potential to be used for reconstruction of sea-surface CO2 changes in the past, provided a quantitative relationship of d13C between POC and sediments can be established. & 2011 Elsevier Ltd. All rights reserved.
Keywords: Western Arctic Ocean Particulate organic matter Stable carbon isotopes pCO2
1. Introduction
studies, revealing that [CO2(aq)] is a major factor in controlling the
d13C composition of POC (Francois et al., 1993; Hofmann et al., 2000; The natural stable carbon isotopic composition (d13CPOC) of marine suspended particulate organic matter (POM) can potentially provide important insights into changes in the environmental conditions under which carbon fixation occurs. An understanding of the spatial variability and the key controlling factors of d13CPOC is therefore indispensable for the use of d13CPOC as a proxy in studies of the marine carbon cycle and environmental changes (Fischer, 1991; Hofmann et al., 2000; Laws et al., 1995; Popp et al., 1999; Rau et al., 1997). Planktonic d13CPOC is a function of the d13C of the inorganic carbon source for organic matter production and the isotopic fractionation factor during photosynthesis. During photosynthesis, the isotopic fractionation plays a decisive role (Keller and Morel, 1999; Laws et al., 1995). A negative correlation between d13CPOC and dissolved carbon dioxide concentration [CO2(aq)] has been observed in many previous
n Corresponding author at: Department of Oceanography, Xiamen University, Xiamen 361005, China. E-mail address: [email protected] (M. Chen).
0967-0645/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr2.2011.05.005
Kennedy and Robertson, 1995; Rau et al., 1997, 1992). The Arctic Ocean is especially sensitive to climate change and may act as an important sink or source for atmospheric CO2 (Bates et al., 2006; Cai et al., 2010; Dickson, 1999; Kaltin and Anderson, 2005; Murata and Takizawa, 2003; Walsh, 1989). Applications using stable isotopic composition should improve our understanding of carbon dynamics and its response to climate and environmental changes in the Arctic Ocean. Previous studies have shown that the biological pump in the western Arctic Ocean is actively running in summer (Chen et al., 2002, 2003; Ma et al., 2005; Moran et al., 2005; Trimble and Baskaran, 2005). The active biological uptake of CO2 in the surface ocean decreases [CO2(aq)], and at the same time contributes to the POC export flux (namely, export production). Unfortunately, measurements of the stable isotopic composition of particulate organic matter in the Arctic Ocean remain scarce. There are very few available or published data allowing examination of the relationship between d13CPOC and [CO2(aq)] in the Arctic Ocean. Thus, measurements of stable isotopic composition are sorely needed, because the dependence of d13CPOC on [CO2(aq)] observed in other marine environments
R. Zhang et al. / Deep-Sea Research II 81–84 (2012) 72–78
may not be applicable in the Arctic Ocean, and carbon isotopic fractionation may also be controlled by other factors such as growth rate, phytoplankton species composition, trophic effects, and water temperature (e.g., Cullen et al., 2001; Goericke and Fry, 1994; Guo et al., 2004; Hofmann et al., 2000; Tamelander et al., 2006). The release of anthropogenic CO2 (mainly derived from fossil fuel burning and deforestation) has been increasing, especially in recent decades, and a large fraction of these emissions has been taken up by the ocean (Gruber, 1998; Sabine et al., 2004; Siegenthaler and Sarmiento, 1993). As an isotopically ‘‘light’’ source, anthropogenic CO2 invading the upper ocean will result in an increase in surface [CO2(aq)] but a decrease in d13CO2(aq) (known as the Suess effect). Therefore, a subsequent decrease of d13CPOC in the water column can be predicted (Bauch et al., 2000; Druffel and Benavides, 1986; Quay et al., 2003, 1992; Schell, 2001). In the Arctic Ocean, a decrease of about 2% in phytoplankton d13C since the Industrial Revolution has been attributed to the input of anthropogenic CO2 (Bauch et al., 2000). In the present study, samples collected from the western Arctic Ocean were measured for stable carbon isotopic composition (d13C) and other parameters to investigate spatial variability in the d13C values of suspended POM, the role of CO2(aq) in controlling the d13CPOC, and the effects of the solubility and biological pumps on d13CPOC. The relationship between surface [CO2(aq)] and d13CPOC was further used to assess the possible impact of anthropogenic CO2 on the d13CPOC of the western Arctic Ocean. 2. Materials and methods 2.1. Study area and sampling locations Water and suspended particulate samples were collected onboard the icebreaker R/V XUELONG from July to September 2003 during the Second Chinese Arctic Research Expedition (CHINARE). A total of 31 stations were sampled for POC, PON, and d13CPOC measurements in the western Arctic Ocean, including the northern Bering Sea (3 stations near the Bering Strait), the eastern Chukchi Sea, the western Beaufort Sea, and part of the Canada Basin (Fig. 1). Seawater samples were collected using a CTD rosette system. Temperature, salinity, and
73
Chl-a concentration data were provided by the CHINARE program (www.polar.gov.cn). 2.2. Sampling and measurements of POC, PON, and d13CPOC For POM and d13CPOC measurements, about 2 l of surface ( 1 m) seawater samples were filtered onboard ship through a precombusted (400 1C, 4 h) Whatman GF/F membrane. After filtration, the filters with suspended POM samples were rinsed with 10 ml 0.1 mol/dm3 HCl and 30 ml Milli-Q water to remove inorganic carbonate, and then stored frozen. While in the laboratory, the filter samples were dried at 60 1C and wrapped into tin capsules for measurements of POC, PON, and d13CPOC. Concentrations of POC, PON, and stable carbon isotopic composition (in term of d13C) were measured on a Finnigan MAT Deltaplus XP mass spectrometer interfaced with an elemental analyzer (Carlo Erba NC2500). The d13C value (versus PDB) of POM sample was defined as the following: Rsample d13 CPOC ¼ 1 1000 Rstandard The reproducibility of d13C measurements was within 70.2%. 2.3. Measurements of pCO2 and [CO2(aq)] The partial pressures of CO2 (pCO2, matm) in the surface waters were measured using a shipboard LI-COR 6262 CO2/H2O infrared gas analyzer. Standard carbon dioxide gas mixtures from the National Research Center of Standard Materials of China were used for calibration, with concentrations of 285, 348, and 401 ppmv [CO2/air]. The system accuracy was 1.0 matm (Chen and Gao, 2007). Concentrations of dissolved CO2 ([CO2(aq)]) were calculated from pCO2, temperature (T), and salinity (S), using the solubility constants (K0) established by Weiss (1974): ½CO2 ðaqÞ ¼ K0 pCO2 " # 100 T T T 2 þ a3 ln þ S b1 þ b2 þb3 ln K0 ¼ a1 þ a2 T 100 100 100
3. Results 3.1. Hydrographic features The spatial distributions of surface water temperature and salinity are shown in Fig. 2A and B. Surface water temperature ranged from 1.55 to 10.50 1C, with an average of 1.95 1C in the study area (Table 1). Temperature decreased northward from an average of 9 1C in the northern Bering Sea to 3 1C at stations shallower than 50 m on the Chukchi Shelf, and fell to below zero (about 1 1C) at deeper stations in the Canada Basin. Water temperature can alter the solubility of CO2, although primary production and organic matter degradation also play an important role. Surface water salinity ranged from 27.44 to 32.57, with an average of 29.76 (Table 1). Except for several extremely lowsalinity values ( 28) that were observed in the deep basin, likely caused by sea-ice melt, salinity ( 31) did not vary dramatically over the northern Bering Sea and the Chukchi Shelf. Generally, low surface-water salinity and its spatial distribution could be attributed to the significant dilution effect of sea ice melting in summer. 3.2. Distribution of POC, PON, and C/N ratios
Fig. 1. Sampling locations in the western Arctic Ocean during summer 2003.
The spatial distributions of POC and PON are depicted in Fig. 2C and D. POC concentrations in the surface water varied between 1.8
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R. Zhang et al. / Deep-Sea Research II 81–84 (2012) 72–78
Fig. 2. Distributions of (A) temperature (1C), (B) salinity, (C) POC (mM), (D) PON (mM), (E) d13CPOC (%), and (F) [CO2(aq)] (mM) in the western Arctic Ocean during Summer 2003.
and 19.1 mM, with an average value of 7.774.0 mM, while PON concentrations varied between 0.3 and 3.5 mM, with an average value of 1.270.7 mM (Table 1). The average C/N ratio (6.771.4) deviated little from the Redfield ratio of 6.6 (106:16), with a range between 5.1 and 11.9. These POC and PON concentrations and C/N ratios are similar to those reported in previous studies (Chen et al., 2003; Ma et al., 2005; Trimble and Baskaran, 2005). POC and PON concentrations generally decreased with increase in latitude, showing similar distribution patterns as surface water temperature and salinity (Fig. 2A and B). The observed POC/PON ratios in the western Arctic Ocean are considerably lower than those measured
for terrestrial POM (2778) derived from the Yukon and Mackenzie Rivers (Guo et al., 2011; Unpublished Data). The positive correlation between POC and temperature, together with the average C/N ratio of POM close to the Redfield ratio, suggested that particulate organic matter collected during the sampling period was mainly from marine-derived organic matter. 3.3. Distribution of d13CPOC The d13CPOC fell in a wide range between 28.5% and 21.1%, with an average value of 24.572.29% (Table 1).
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Table 1 Temperature, salinity, Chl-a, particulate organic carbon (POC), particulate organic nitrogen (PON), C/N molar ratio, stable carbon isotopic composition (d13C) of suspended organic matter, partial pressure of carbon dioxide (pCO2), and concentration of dissolved carbon dioxide ([CO2(aq)]) in the western Arctic Ocean during summer 2003. Station
BS01 BS07 BS10 R01 R03 R04A R05 R07 R11 R15 C12 C15 C18 C22 C24 C26 C34 M03 M07 S11 S21 S25 S32 P11 P22 B11 B13 B32 B33 B79 B80
Longitude (1W)
Latitude (1N)
Layer (m)
T (1C)
171.51 168.50 167.01 169.01 169.00 169.00 169.00 169.00 169.67 168.99 167.03 164.01 161.03 167.01 164.99 162.98 167.01 171.93 171.94 159.00 154.98 153.40 150.38 169.99 164.93 156.33 151.88 151.55 149.27 151.79 146.74
64.33 64.34 64.33 66.99 68.00 68.50 69.00 70.00 72.01 74.00 71.65 71.58 71.48 70.50 70.50 70.49 68.92 76.54 75.00 72.49 71.65 72.74 71.26 75.01 77.40 74.00 73.38 75.00 74.63 79.31 80.22
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
9.34 7.56 10.50 5.72 3.34 7.63 6.48 4.62 1.17 1.47 0.65 0.87 0.37 3.87 3.20 4.96 7.94 1.55 1.27 1.02 2.94 0.65 1.52 1.27 1.49 0.98 1.02 1.20 1.22 1.47 1.52
S
31.61 31.84 28.57 31.95 32.57 31.49 31.25 31.99 29.68 29.49 29.49 30.45 29.21 31.57 31.13 31.20 30.93 29.16 28.29 28.85 27.44 28.15 29.37 28.58 28.68 27.98 27.56 27.91 27.92 28.47 29.86
Chl-a (mg/dm3)
POC (mM)
0.8 0.9 nd nd nd 1.3 nd 0.5 nd nd nd nd nd nd nd 0.3 nd 0.2 0.1 0.6 0.5 nd 0.2 0.1 0.2 0.1 0.0 nd 0.1 0.1 0.2
10.5 10.2 8.6 17.6 11.9 13.1 7.0 6.8 7.6 8.0 1.8 11.5 19.1 4.9 5.6 5.4 8.9 5.6 7.0 9.2 9.3 2.7 5.9 5.5 4.1 4.5 4.5 9.6 3.5 2.9 4.9
PON (mM)
1.5 1.5 1.2 2.6 1.8 2.2 1.0 0.9 1.0 0.9 0.3 2.2 3.5 0.8 0.9 1.0 1.5 0.9 1.4 1.8 1.5 0.5 1.1 0.7 0.6 0.5 0.6 0.8 0.5 0.4 0.7
C/N (molar)
pCO2 (matm)
[CO2(aq)] (mM)
d13CPOC
7.2 6.7 7.2 6.7 6.7 6.0 7.0 7.2 7.4 8.7 5.2 5.3 5.5 6.2 5.9 5.4 5.8 6.0 5.1 5.2 6.2 5.9 5.2 7.4 7.4 9.0 7.3 11.9 7.8 7.1 6.9
184 238 389 198 227 195 287 212 178 253 155 181 189 209 210 244 318 275 291 244 267 309 270 274 272 308 318 496 310 370 391
8.6 11.8 17.8 10.5 13.1 9.7 14.9 11.7 12.4 17.8 10.6 12.4 12.8 11.9 12.3 13.4 15.7 19.5 20.5 17.0 16.0 21.2 16.9 19.3 19.3 21.5 22.3 34.9 21.8 26.2 27.6
22.1 23.5 24.6 22.5 22.3 24.4 24.1 22.1 24.2 21.8 22.6 21.1 21.2 24.2 22.3 23.3 22.4 26.0 25.1 27.3 22.7 28.5 24.0 26.9 27.0 27.9 25.6 27.9 27.0 28.1 27.1
(%)
Note: nd represents no data.
The north Bering Sea–Chukchi Shelf area had a higher average d13CPOC value of 22.9% (n ¼17, ranging from 24.6% to 21.1%). The average d13CPOC value decreased dramatically to about 26.5% (from 28.5% to 22.7%, n ¼14) at deep-water stations (Fig. 2E). To the best of our knowledge, such great variations in d13CPOC between the shallow shelf and the deepwater stations in the western Arctic Ocean have never been reported before. Interestingly, the distribution pattern of d13CPOC resembles those observed for temperature and POC. In addition to changes in d13CPOC along the latitudinal gradient, with higher values near the Bering Strait and Chukchi Shelf and lower values toward the deep basin, a west to east increasing trend from the Chukchi Shelf to the Beaufort Sea was also evident (Fig. 2E). The values of surface water d13CPOC measured in our study area compared well with those reported previously for other marine environments. They are close to those observed in the southeastern Bering Sea in summer (from 27.1% to 23.4% with an average of 24.5%, Guo et al., 2004), but lighter than those found in low latitude sea areas, such as in the subtropical southwestern Indian Ocean (from 28.4% to 16.5% with an average of 23.7%, Francois et al., 1993) or the equatorial Pacific Ocean (from 21.5% to 20.8% with an average of 21.1%, Laws et al., 1995). However, our d13C values are higher than those encountered in the Southern Ocean (from 29.7% to 21.9% with an average of 26.6%, Dehairs et al., 1997; Kennedy and Robertson, 1995). This is consistent with the reported latitudinal increasing trend of surface ocean d13CPOC toward the tropics in both hemispheres (Goericke and Fry, 1994; Hofmann et al., 2000). Although the Arctic Ocean receives disproportionately high continental runoff and terrestrial inputs, our comparable d13CPOC values and near-Redfield POC/PON ratios all point to
predominantly marine-derived POM during our sampling period in the western Arctic Ocean. 3.4. Distribution of [CO2(aq)] The values of the partial pressure of carbon dioxide (pCO2) varied between 155 and 496 matm, with an average of 267775 matm in the study area (Table 1). At 27 stations the pCO2 was below the equilibrium value with the atmosphere. Dissolved concentrations of carbon dioxide ([CO2(aq)]) ranged from 8.6 to 34.9 mM, with an average of 16.8 75.9 mM. [CO2(aq)] showed a mirror distribution pattern to that of d13CPOC (Fig. 2F). The northern Bering Sea–Chukchi Shelf area was characterized by very low [CO2(aq)] ( 12.8 mM). The [CO2(aq)] value then increased dramatically from the Chukchi Shelf to the deep basin and the Beaufort Sea, with a mean value of 21.7 mM, which is almost two times higher than the low value observed on the Chukchi Shelf.
4. Discussion 4.1. Variations of d13CPOC The particulate organic matter during the sampling period was derived predominantly from marine organic matter, and terrestrial organic matter did not seem to have a noticeable impact on d13CPOC variations in the study area. Three lines of evidence support this statement. First, the average C/N ratio of POM was 6.771.4 (Table 1) and similar to the Redfield ratio. Second, a significant positive correlation between POC and the Chl-a
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Fig. 3. Relationship between POC (mM) and Chl-a (mg/dm3) in the western Arctic Ocean during summer 2003.
concentration was observed (Fig. 3), also indicating a marine origin of the POM. A higher Chl-a concentration or higher biomass could result in a higher POC concentration. Third, d13CPOC decreased from the shelf to the deep basin (Fig. 2E), ruling out the possibility of a major contribution of terrestrial organic matter in the study area. The d13CPOC value of the terrestrial end member for the northern Bering–Chukchi–Beaufort Sea was reported as 27%, based on the data from major Arctic rivers, such as the Colville River, Mackenzie River, Lena River, and Yukon River (Guo and Macdonald, 2006; Minagawa et al., 1991; Naidu et al., 2000, 1993; Ruttenberg and Goni, 1997; Schell, 1983). Although the d13C of marine-derived POC in the study area has not yet been reported, it is speculated to be at least 2–3% heavier than that of terrestrial organic matter (Guo and Macdonald, 2006; Naidu et al., 2000) due to the difference in plant photosynthetic environments. If the influence of terrestrial organic matter is significant in the study area, the d13CPOC values would have increased from the shelf to the deep basin because the inner shelf waters should have received more freshwater and terrestrial organic matter. However, the opposite was true. Thus, POM collected during our sampling period should be predominantly derived from in situ biological production. 4.2. Dependence of d13CPOC on [CO2(aq)] The d13CPOC signatures of biogenic POM should be controlled by biological fractionation during synthesis of organic material. Our results show that the surface water d13CPOC decreased northward and had the mirror pattern with dissolved carbon dioxide (Fig. 2E and F), indicating d13CPOC was related to the abundance of CO2 and biological production in the western Arctic Ocean. Indeed, a significant negative correlation was observed between surface d13CPOC and log[CO2(aq)] (logarithmic values were adopted here after Popp et al. (1989) empirically) (Fig. 4). This indicated the dependence of the POC-d13C composition on the aqueous CO2 concentration in the western Arctic Ocean during summer, which could be described with the following relationship:
d13 CPOC ¼ 9:19212:75 log½CO2 ðaqÞ ðr 2 ¼ 0:63,p o0:0001Þ This is the first time that such a negative correlation has been revealed in the western Arctic Ocean. Similar relations have been reported previously in other areas, such as the Indian Ocean (Francois et al., 1993), the Atlantic Ocean (Rau et al., 1992), the Southern Ocean (Fischer, 1991; Kennedy and Robertson, 1995;
Fig. 4. Relationship between d13CPOC and log[CO2(aq)] in the western Arctic Ocean during summer 2003.
Lourey et al., 2004), the Antarctic seas (Rau et al., 1991), and model results in the world ocean (Hofmann et al., 2000). When inorganic carbon acquisition is maintained only by passive CO2 diffusion during planktonic photosynthesis, the d13C values of the POC produced can be expressed as a function of CO2(aq) (Cullen et al., 2001; Hofmann et al., 2000; Laws et al., 1995; Rau et al., 1996):
d13 CPOC ¼ d13 CCO2 ðaqÞ ef þa
m ½CO2 ðaqÞ
where d13CCO2(aq) is the 13C isotopic composition of the substrate CO2(aq), ef is the carbon isotopic fractionation associated with photosynthetic fixation, m is the specific growth rate, and a is a species-specific constant. Although direct measurements of the d13C composition of aqueous CO2 were not carried out here, short-term changes in d13CCO2(aq) should not be a major factor responsible for the large variations in d13CPOC (up to 7%) observed in the study area. Globally, the d13C value of surface ocean total dissolved CO2 (DIC) does not vary greatly (Broecker and Maier-Reimer, 1992; LynchStieglitz et al., 1995), while gradients in d13CPOC are much greater among different sea areas (Hofmann et al., 2000). Locally, the polar oceans have experienced very small changes in surface d13CDIC ( o3%) in recent decades compared with other regions (Quay et al., 2003), and the d13C value of CO2 in seawater in equilibrium with the atmosphere covaried with temperature, but at a slow rate (Mook et al., 1974). Thus, it is less likely that such a large variation in d13CPOC (up to 7%) observed in the western Arctic Ocean was mainly caused by changes in isotopic compositions of CO2(aq). Our study demonstrated that d13CPOC in the western Arctic Ocean was controlled by the aqueous CO2 concentration. However, some other studies have showed that d13CPOC could also vary independently of [CO2(aq)], such as in the southeastern Bering Sea (Guo et al., 2004) and in some parts of the Southern Ocean (Bentaleb et al., 1998; Francois et al., 1993; Popp et al., 1999), indicating the influence of other factors on d13CPOC. They may include the active uptake of bicarbonate (HCO-3, with the help of a carbon concentrating mechanism) (Cassar et al., 2004; Tortell et al., 1997), phytoplankton species composition (Cullen et al., 2001; Kopczynska et al., 1995; Rau et al., 1992), cell geometry (Burkhardt et al., 1999; Popp et al., 1998; Trull and Armand, 2001), and zooplankton-related processes (Rau et al., 1990; Tamelander et al., 2006). More studies are needed to further understand the ultimate controls on d13CPOC.
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4.3. Possible effect of anthropogenic CO2 on d13CPOC
References
The invasion of isotopically ‘‘light’’ anthropogenic CO2 into the upper ocean could result in both an increase in [CO2(aq)] and a decrease in its d13C, causing a subsequent decrease in d13CPOC in the water column (Cullen et al., 2001; Druffel and Benavides, 1986; Quay et al., 2003, 1992; Schell, 2001). The concentration of atmospheric CO2 has increased from 280 to 380 ppm since the Industrial Revolution (www.esrl.noaa.gov/gmd/ccgg/trends). Assuming the negative correlation between d13CPOC and log[CO2(aq)] observed in this study has held true over the past century or so, the following equation can be inferred:
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Dd13 CPOC ¼ 12:75 Dlog½CO2 ðaqÞ If the surface seawater remains at a steady state where T¼0 1C and S ¼30, then an overall decrease of 1.7% in d13CPOC can be estimated since preindustrial times. In other words, an average rate of decrease of about 0.01% per year in d13CPOC can be expected owing to the addition of anthropogenic CO2. This decrease is comparable to the anthropogenic decrease rates of d13C reported for global atmospheric CO2 (Friedli et al., 1986) and surface ocean DIC (Druffel and Benavides, 1986; Quay et al., 1992; Quay et al., 2003). It is also similar to the fractionation in d13C between modern POM and surface sediments in the Arctic Ocean (Bauch et al., 2000), but it is much lower than the observed decrease in planktonic d13C in the Bering Sea in recent decades (Cullen et al., 2001; Schell, 2001). Although the overall change in d13CPOC resulting from the increase in anthropogenic CO2 on seasonal and yearly time scales could be relatively small compared to the biological fractionation in d13C and spatial changes in d13CPOC, the long-term effect on d13CPOC due to anthropogenic impacts should be detectable in the sediments, especially on decadal time scales.
5. Conclusions Our results showed that d13CPOC in the surface seawater generally decreased from the continental shelf to the deep basin in the western Arctic Ocean. d13CPOC and photosynthetic carbon isotopic fractionation were related to the dissolved CO2 concentration in surface waters. Biological processes were a major factor affecting surface water CO2 concentrations and further controlling the variation of d13CPOC. The significant correlation between d13CPOC and dissolved CO2 supports the use of d13C as a proxy for carbon dynamics in the Arctic Ocean. Thus, signatures of d13C in sediment core samples together with sediment chronologies should allow the reconstruction of changes in dissolved CO2 of the surface ocean, if a functionally coupled relationship between d13CPOC in the water column and d13CPOC in the sediment can be established.
Acknowledgments We thank the crewmembers and science party of the 2nd Chinese Arctic Research Expedition (CHINARE) for their assistance during sample collection, CHINARE for hydrographic data, and two anonymous reviewers for their constructive comments on this manuscript. This work was supported, in part, by a Chinese IPY Research Program, a Fujian Natural Science Foundation (2009J06026), a COMRA Program (DYXM115-02-4-06), a National Natural Science Foundation of China (40976116), the Scientific Research Foundation of Third Institute of Oceanography, State Oceanic Administration (2010011, 2010001), and a Cheung Kong Scholarship through Ministry of Education of China.
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