Inorganic carbon removal and isotopic enrichment in Antarctic sea ice gap layers during early austral summer

Abstract

The biogeochemical composition of 2 spatially separate surface gap layers on a single Antarctic sea ice floe during early austral summer was predominantly controlled by the growth of diatoms and, especially, Phaeocystis. These algal communities in and near the gap layers imposed large geochemical changes in the chemical and isotopic composition of the gap waters, typical of intense autotrophic activity. These included a large deficit in all major dissolved inorganic nutrients (dissolved inorganic carbon [C T ], nitrate, soluble reactive phosphorus, silicic acid), O 2 accumulation above air saturation, large pH shifts into the alkaline spectrum, and a large, closely coupled 13 C enrichment of the C T pool and the accumulated particulate organic carbon, in all cases relative to the composition of surface oceanic water. The amount of inorganic carbon removed from the gap water exceeded that which can be predicted from the deficits of dissolved inorganic nitrogen or phosphorus and the elemental composition of the biogenic matter suspended in it or the mean elemental composition of oceanic phytoplankton. This stoichiometric deviation suggests either (1) the operation of the inorganic carbon overconsumption mechanism via the biological production of particular classes of intra-or extracellular carbon rich compounds, or (2) substantial utilisation of ammonium and urea as autotrophic nitrogen sources in addition to nitrate, or (3) both. KEY WORDS: Antarctica. Sea ice. Gap layers. Biogeochemistry. Nutrients. Particulate organic matter. Carbon isotopic composition Resale or republication not permitted without written consent of the publisher

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

“…Sea ice thickness in Ryder Bay rarely exceeds 0.5 m , so we assume that the relatively thin first year ice was undergoing some exchange with surrounding sea water. Although sea ice porosity data are not available for Ryder Bay specifically, we know that sea ice in Marguerite Bay is comparatively porous due to relatively warm conditions (Fritsen et al, 2008), ice formation through the pancake ice cycle (Eicken, 1992;Thomas and Dieckmann, 2002) and subsequent deformation and snow-ice formation (Perovich et al, 2004). Our observations agree with the expected occasional nutrient inputs from, and brine drainage to, surrounding sea water, and consequently less extreme environmental conditions in Ryder Bay sea ice than in more permanent sea ice environments (Gleitz et al, 1995;Kattner et al, 2004;Papadimitriou et al, 2007).…”

“…In comparison to organic carbon synthesised under closed system dynamics in multiyear ice (Papadimitriou et al, 2009), δ 13 C POC values found here in 2005/06 are somewhat lower, as would be expected for a semi-closed system setting. Conversely, sea ice δ 13 C POC in 2004/05 is more 13 C-enriched than those of Papadimitriou et al (2009) whilst [CO 2(aq) ] was lower and significantly higher POC:chl a ratios compared well with values of 1262 ± 2276 found in intact ice floes (Kennedy et al, 2002).…”

“…This would increase δ 13 C POC in the same way as discussed for surface waters and would impact on δ 13 C DIC rather than specifically δ 13 C CO 2 . CO 2 degassing and carbonate mineral precipitation due to CO 2 saturation or supersaturation in brine inclusions upon sea ice formation may further affect δ 13 C CO 2 (Romanek et al, 1992;Papadimitriou et al, 2003Papadimitriou et al, , 2007 and therefore δ 13 C POC . However, low CO 2 concentrations in Ryder Bay sea ice make this scenario unlikely and occasional flushing by seawater would overprint any small contribution of these processes to δ 13 C CO 2 .…”

“…In addition to biological production, nutrient drawdown and isotopic enrichment in the semi-closed sea ice ecosystem, relatively high sea ice δ 13 C POC may be influenced by some species utilising HCO − 3 as CO 2 becomes limiting (Papadimitriou et al, 2009). This would increase δ 13 C POC in the same way as discussed for surface waters and would impact on δ 13 C DIC rather than specifically δ 13 C CO 2 .…”

Winter-to-summer evolution of <i>p</i>CO<sub>2</sub> in surface water and air–sea CO<sub>2</sub> flux in the seasonal ice zone of the Southern Ocean

“…Samples for chlorophyll a measurement were immediately filtered through 25 mm Whatman GF/F filters, and filters were stored in a deep freezer (−80 • C) until analysis (Suzuki and Ishimaru, 1990). Chlorophyll a concentration was determined by fluorometry (Parsons et al, 1984). DIC was determined by coulometry (Johnson et al, 1999) using a coulometer (CM5012, UIC, Inc., Binghamton, NY, USA).…”