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East Siberian Arctic Shelf Waters Acidification by Freshwater Addition and Terrestrial Carbon

Marine ecosystems and the cycling of carbon are affected by acidification of oceans which is considered to be a direct result of the uptake of anthropogenic CO₂ from the atmosphere (ACIA, 2005; Orr et al., 2005; Hoegh-Guldberg & Bruno, 2010). The acidification of the ocean is predicted to double the acidity of the ocean by the end of the 21^st century as a result of the accumulation of CO₂ from the atmosphere (AMAP, 2013). As more CO₂ can dissolve in cold water the Arctic Ocean is particularly sensitive to acidification of ocean water (Bates, Mathis & Cooper, 2009; Guinnotte & Fabry, 2008). In this paper Semiletov et al. present observations of the chemical and physical characteristics of the waters of the East Siberian Arctic Shelf (ESAS) from 1999, 2000-2005, 2008 and 2011, and findings of extreme aragonite undersaturation which reflects levels of acidity in excess of those that have been projected in this region for 2100. Data for the dissolved inorganic carbon isotope  and Monte Carlo simulations of water sources which used salinity and δ¹⁸O data suggest the persistent acidification is driven by terrestrial organic matter degradation and discharge of Arctic river water with concentrations of CO₂ that are elevated, rather than by the uptake of atmospheric CO₂. It is suggested by Semiletov et al. that if thawing permafrost leads to enhanced inputs of terrestrial organic carbon and there is a continued increase of additions of freshwater, the East Artic Shelf waters may become more acidic, which may affect their efficiency as a CO₂ source.

It is shown clearly by the results of this study that a major ocean acidification (OA) pattern in the ESAS that resulted in a sever undersaturation of Ar in shelf waters that is caused by degradation of terr-OC exported from thawing coastal permafrost, and freshening resulting from increasing Arctic river runoff from watersheds and ice melt which underlie the extensive permafrost. In other marine ecosystems organic carbon originates with plankton and riverine sources, but in contrast to these a significant source of terrestrial organic carbon (terr-OC) to the ESAS originates in coastal erosion. It is established by the dual carbon isotope (δ¹³C and ¹⁴C) composition of ESAS OC that erosional carbon released from permafrost dominates burial of OC on the ESAS, and that 57 ± 2 % of this terr-OC is derived from permafrost-originated ice complexes from the Pleistocene (Vonk et al., 2012). A source of OA, differing from that of the uptake of atmospheric CO₂ that is generally considered, is this translocated terr-OC. The projected levels for the year 2100, which are based only on the uptake of atmospheric CO₂, are already exceeded by persistent and potentially increasing Ar undersaturation of ESAS water. It is suggested by Semiletov et al. that the suppression of the benthic calcifying community that has been observed could possibly be pervasive throughout the entire ESAS, which alone comprises more than 25 % of the open water of the Arctic, as undersaturation of Ar is characteristic of the entire bottom water of the ESAS. Arctic marine ecosystems over extensive scales could be affected as the transpolar drift exports these waters to the surface of the central Arctic Ocean, as well as into the Beaufort Gyre, the consequences of OA, which is triggered by mechanisms that are driven by climate change. The capacity of the Arctic Ocean to act as a sink for a growing amount of anthropogenic CO₂ is called into question by this study.

Sources & Further reading

1. Semiletov, I., I. Pipko, O. Gustafsson, L. G. Anderson, V. Sergienko, S. Pugach, O. Dudarev, A. Charkin, A. Gukov, L. Broder, A. Andersson, E. Spivak and N. Shakhova (2016). "Acidification of East Siberian Arctic Shelf waters through addition of freshwater and terrestrial carbon." Nature Geosci 9(5): 361-365.