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The Biogeochemistry of the East Siberian Sea and Its Impact on the Upper Waters of the Deep Arctic Ocean

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The Biogeochemistry of the East Siberian Sea and Its Impact on the Upper Waters of the Deep Arctic Ocean The biogeochemistry of the East Siberian Sea and its impact on the upper waters of the deep Arctic Ocean. Leif G. Anderson, Göran Björk, Sofia Hjalmarsson, Sara Jutterström, Iréne Wåhlström. University of Gothenburg, Göteborg, Sweden, Introduction⎯ The Siberian shelf seas is a very biogeochemical dynamic region which receives a lot of river runoff (containing nutrients and dissolved and particulate organic matter), exchange waters with the deep Arctic Ocean, exchange gases with the atmosphere and has an extensive biological activity. One result of the enormous size of the Siberian shelf seas is that the conditions for primary production and microbial decay of organic matter (OM) is far from constant over the whole area. Furthermore for different reasons parts of the area, especially the East Siberian Sea, has not up to now been much studied. In the summer 2008 we investigated the waters of the eastern Laptev Sea, the East Siberian Sea and the western Chukchi Sea with respect to its hydrography (S, T, O2, nuts, DIC, TA, pH), for station positions se map down to the left. The sea ice situation was favorable with little ice over the shelf areas (see satellite image in the bottom left). River plume properties Microbial decay of organic matter at low The two sections below represent plumes from the river mouths of Lena (LS) and Salinity at surface Salinity at bottom Kolyma (KS) towards the sea (see map at bottom for locations). No south to north oxygen concentrations. increasing gradient in phosphate is seen in any section. Instead the concentration is low all throug h the LS, bu t w ith somewh at el evat ed l evel s at th e b ott om. I n th e KS th e N:P = 16:1 phosphate concentration is much higher, but with even more increasing levels at the bottom. The surface to bottom gradient is about 0.6 µmol/kg in the LS and 2.2 µmol/kg in the KS. Assuming that this signal is a result of primary production (PP) / decay of Percent of oxygen organic matter (OM), this correspond to 80 and 170 µmol/kg oxygen, respectively, saturation in the ESS assuming an RKR ratio of 138. Such a gradient is seen in AOU for the LS, but for the KS the gradient is significantly less. This implies that other electron acceptors than PO4 (µmol/kg) at surface PO4 (µmol/kg) at bottom oxygen is used during the decay of OM in the KS. Even if the phosphate and AOU sections could explain marine PP in the surface water and OM decay at depth, these are not the processes dominating in the LS. This is evident from the fugacity of CO2 that show over-saturation in the LS surface water, with a strong decreasing gradient from south to north . This can only be explained by decay of terrestrial OM low in nutrients. Such a scenario is also supported by the AOU It is well known that much of the waters in the Chukchi sea has an excess section. In the KS on the other hand the fCO2 section corresponds to the situation where of PO4 relative to NO3. This feature is also seen in the East Siberian Sea. marine PP and microbial decay dominates. NO (µmol/kg) at surface The deficit can be expressed as N** (= [NO3] – 16[·PO4] + 2.9) where the low 3 NO3 (µmol/kg) at bottom values represent the largest deficit. These are found at salinities around 33, a salinity close to that of the nutrient maximum of the Arctic Ocean. The waters with the highest PO4 are also the ones with highest fCO2. These signatures all indicate that microbial decay of organic matter is responsible, but largely in a low oxygen environment where other electron acceptors than oxygen are used. The oxygen profiles document PO4 PO4 (µmol/kg) (µmol/kg) Surface & bottom water properties decreasing values towards the bottom and there is a tendency that low oxygen percentage correspond to low N**. That other electron acceptors AOU (µmol/kg) at surface AOU (µmol/kg) at bottom than oxygen is used during microbial decay of OM is illustrated by that the relationship between AOU and PO4 has a slope of about 90 instead of the expected 135 (O2:P according to RKR). The low N** values point to dentitrification and anammox as being important processes. AOU AOU (µmol/kg) (µmol/kg) Shelf outflow to the deep basin The distribution of chemical constituents at the surface and the bottom reveal some specific features. Low salinities surface waters are found in the Laptev Sea and the (Nishino et al., 2009) southwes tern Eas t Siber ian Sea, waters tha t a lso are low i n nut ri ent s, h as AOU concentrations close to zero. The salinity is higher in the eastern East Siberian surface waters, nitrate is still low, but phosphate is high, and AOU is negative, i.e. shows a PP PO4 (µmol/kg) fCO2 fCO2 signal. In the bottom water a strong signal of microbial decay of organic matter is (µatm) (µatm) observed, with the most pronounced signal in the East Siberian Sea. This pattern support two distinct regimes, one dominated by river runoff (the LS and western ESS) and one dominated by marine conditions (eastern EAS and western CS). Conclusions ⎯ Siberian shelf seas are very active in transforming organic matter, both marine and terrestrial. The Laptev and western East Siberian Seas receive large amounts of river runoff that is rich in organic matter . Marine primary Oxygen pCO2 production is low and the summer surface water is oversaturated in CO2, adding up to (µmol/kg) (µatm) an out-gassing to the atmosphere of up to 10·1012 g C yr-1 (Anderson et al., 2009). In Lena the eastern East Siberian and Chukchi Seas marine primary production dominates the property distribution in the surface water. The draw down of DIC in the eastern East 12 Kolyma Siberian Sea adds up to about 6·10 gC. The bottom waters in all of the Siberian Water with high nutrient concentrations has for many years been observed in the Canadian shelf seas show signatures of microbial decay of organic matter, largely in low Basin of the Arctic Ocean at a depth of about 150 m and S~33. This water has been oxygen environment. This water with high fCO2 and low pH is very corrosive to suggested to be a winter water of Pacific origin but which has undergone biogeochemical calcium carbonates. The high nutrient bottom water enters the deep Arctic Ocean transformation in the Chukchi – Bering Seas area. It has been shown to enter the deep both from the East Siberian Sea and from the Chukchi Sea through the Herald Valley. central Arctic Ocean on the northern slope of the Chukchi Sea. Here we show that also the The high nutrient waters that leaves the East Siberian Sea include water of less East Siberian Sea is an area for formation of the high nutrient water and that it also leaves salinity than the one that leaves the Laptev Sea . to the deep Arctic Ocean from this area (see sections above). What we also observed was that the highest silicate concentrations where at the shallowest station. Nishino et al. (2009) recently reported a “double peak” of silicate in the area of the deep Arctic Ocean east of the Chukchi Plateau but with a minimum in N** only from the low salinity peak (see above fig). Our data support this finding as the nutrient rich water leaving the East Siberian Sea spans References a larger salinity interval, including lower salinity water. We also observed minimum N** values in this low salinity nutrient max. However, the min in N** did not coincide with the oxygen minimum as would be expected if denitrification is Anderson L.G., S. Jutterström, S. Hjalmarsson, I. Wåhlström and I.P. Semiletov, responsible for the low N**. One explanation could be that denitrification is active in the sediment, having low oxygen Geophys. Res. Lett., 36, 2009. concentration, and that the decay products leak out into a fairly well oxygenated bottom water. Nishino, S, K. Shimada, M. Itoh and S. Chiba, J. Oceanogr, Vol. 65, pp. 871 to 883, 2009. One question that needs further investigations is if the formation of this lower salinity, high nutrient rich water is a recent phenomena (when less summer sea ice cover has been in the ESS) or if this is a permanent feature?.
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