Factors influencing the stable carbon isotopic composition of suspended and sinking organic matter in the coastal Antarctic sea ice environment

Henley, S.F., Annett, A.L., Ganeshram, R.S., Carson, D.S., Weston, K., Crosta, X., Tait, A., Dougans, J., Fallick, A.E. and Clarke, A. (2012) Factors influencing the stable carbon isotopic composition of suspended and sinking organic matter in the coastal Antarctic sea ice environment. Biogeosciences, 9 (3), 1137-1157. (doi:10.5194/bg-9-1137-2012).

Abstract

A high resolution time-series analysis of stable carbon isotopic signatures in particulate organic carbon (δ ¹³CPOC) and associated biogeochemical parameters in sea ice and surface waters provides an insight into the factors affecting δ ¹³CPOC in the coastal western Antarctic Peninsula sea ice environment. The study covers two austral summer seasons in Ryder Bay, northern Marguerite Bay between 2004 and 2006. A shift in diatom species composition during the 2005/06 summer bloom to near-complete biomass dominance of Proboscia inermis is strongly correlated with a large ∼10 ‰ negative isotopic shift in δ ¹³CPOC that cannot be explained by a concurrent change in concentration or isotopic signature of CO2. We hypothesise that the δ ¹³CPOC shift may be driven by the contrasting biochemical mechanisms and utilisation of carbon-concentrating mechanisms (CCMs) in different diatom species. Specifically, very low δ ¹³CPOC in P. inermis may be caused by the lack of a CCM, whilst some diatom species abundant at times of higher δ ¹³CPOC may employ CCMs. These short-lived yet pronounced negative δ ¹³CPOC excursions drive a 4 ‰ decrease in the seasonal average δ ¹³CPOC signal, which is transferred to sediment traps and core-top sediments and consequently has the potential for preservation in the sedimentary record. This 4 ‰ difference between seasons of contrasting sea ice conditions and upper water column stratification matches the full amplitude of glacial-interglacial Southern Ocean δ ¹³CPOC variability and, as such, we invoke phytoplankton species changes as a potentially important factor influencing sedimentary δ ¹³CPOC. We also find significantly higher δ ¹³CPOC in sea ice than surface waters, consistent with autotrophic carbon fixation in a semi-closed environment and possible contributions from post-production degradation, biological utilisation of HCO3− and production of exopolymeric substances. This study demonstrates the importance of surface water diatom speciation effects and isotopically heavy sea ice-derived material for δ ¹³CPOC in Antarctic coastal environments and underlying sediments, with consequences for the utility of diatom-based δ ¹³CPOC in the sedimentary record.

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Contributors

Author: A.L. Annett

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