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

Other literature type, Unknown, Article English OPEN
Henley, S. F. ; Annett, A. L. ; Ganeshram, R. S. ; Carson, D. S. ; Weston, K. ; Crosta, X. ; Tait, A. ; Dougans, J. ; Fallick, A. E. ; Clarke, A. (2012)
  • Publisher: Copernicus Publications
  • Journal: (issn: 1726-4189, eissn: 1726-4189)
  • Related identifiers: doi: 10.5194/bg-9-1137-2012
  • Subject: Carbon dioxide | Sediment trap | Southern Ocean | /dk/atira/pure/subjectarea/asjc/1100/1105 | Ecology, Evolution, Behavior and Systematics | Antarctica | /dk/atira/pure/subjectarea/asjc/1900/1904 | Global and Planetary Change | Diatoms | Organic Chemistry | Sea ice | Carbon concentrating mechanisms | Oceanography | Biochemistry | Phytoplankton | Organic matter | /dk/atira/pure/subjectarea/asjc/1300/1303 | /dk/atira/pure/subjectarea/asjc/1900/1910 | Carbon isotopes | /dk/atira/pure/subjectarea/asjc/2300/2306 | Earth-Surface Processes | /dk/atira/pure/subjectarea/asjc/1600/1605

A high resolution time-series analysis of stable carbon isotopic signatures in particulate organic carbon (&delta;<sup>13</sup>C<sub>POC</sub>) and associated biogeochemical parameters in sea ice and surface waters provides an insight into the factors affecting &delta;<sup>13</sup>C<sub>POC</sub> 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 <i>Proboscia inermis</i> is strongly correlated with a large ~10 &permil; negative isotopic shift in &delta;<sup>13</sup>C<sub>POC</sub> that cannot be explained by a concurrent change in concentration or isotopic signature of CO<sub>2</sub>. We hypothesise that the &delta;<sup>13</sup>C<sub>POC</sub> shift may be driven by the contrasting biochemical mechanisms and utilisation of carbon-concentrating mechanisms (CCMs) in different diatom species. Specifically, very low &delta;<sup>13</sup>C<sub>POC</sub> in <i>P. inermis</i> may be caused by the lack of a CCM, whilst some diatom species abundant at times of higher &delta;<sup>13</sup>C<sub>POC</sub> may employ CCMs. These short-lived yet pronounced negative &delta;<sup>13</sup>C<sub>POC</sub> excursions drive a 4 &permil; decrease in the seasonal average &delta;<sup>13</sup>C<sub>POC</sub> signal, which is transferred to sediment traps and core-top sediments and consequently has the potential for preservation in the sedimentary record. This 4 &permil; difference between seasons of contrasting sea ice conditions and upper water column stratification matches the full amplitude of glacial-interglacial Southern Ocean &delta;<sup>13</sup>C<sub>POC</sub> variability and, as such, we invoke phytoplankton species changes as a potentially important factor influencing sedimentary &delta;<sup>13</sup>C<sub>POC</sub>. We also find significantly higher &delta;<sup>13</sup>C<sub>POC</sub> 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 HCO<sub>3</sub><sup>&minus;</sup> and production of exopolymeric substances. This study demonstrates the importance of surface water diatom speciation effects and isotopically heavy sea ice-derived material for &delta;<sup>13</sup>C<sub>POC</sub> in Antarctic coastal environments and underlying sediments, with consequences for the utility of diatom-based &delta;<sup>13</sup>C<sub>POC</sub> in the sedimentary record.
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