Abstract
Abstract. The Antarctic coastal zone is an area of high primary productivity, particularly within coastal polynyas, where large phytoplankton blooms and drawdown of CO2 occur. Reconstruction of historical primary productivity changes and the associated driving factors could provide baseline insights on the role of these areas as sinks for atmospheric CO2, especially in the context of projected changes in coastal Antarctic sea ice. Here we investigate the potential for using carbon isotopes (δ13C) of fatty acids in marine sediments as a proxy for primary productivity. We use a highly resolved sediment core from off the coast of Adélie Land spanning the last ∼ 400 years and monitor changes in the concentrations and δ13C of fatty acids along with other proxy data from the same core. We discuss the different possible drivers of their variability and argue that C24 fatty acid δ13C predominantly reflects phytoplankton productivity in open-water environments, while C18 fatty acid δ13C reflects productivity in the marginal ice zone. These new proxies have implications for better understanding carbon cycle dynamics in the Antarctica coastal zone in future palaeoclimate studies.
Highlights
Antarctic coastal zones are important players in the global carbon cycle
Potential sources for the C18 fatty acids (FAs) in core U1357 are discussed in Ashley et al (2021a), who suggest the prymnesiophyte Phaeocystis antarctica to be the most likely main producer. This is based on (a) previous studies of FAs produced by microalgae (Dalsgaard et al, 2003), (b) the high observed abundance of P. antarctica within modern Adélie surface waters (RiauxGobin et al, 2011; Sambrotto et al, 2003), and (c) comparison between the measured δ13C values and those reported in the literature for P. antarctica (Kopczynska et al, 1995; Wong and Sackett, 1978)
The highly branched isoprenoid alkenes (HBIs) triene is more susceptible to degradation than the diene (Cabedo Sanz et al, 2016), so while this could explain some of the differences between the diene and triene records, where the triene increases independently of the diene, this is likely to be a genuine reflection of increased production of these compounds at the surface rather than an artefact of preservation processes
Summary
Antarctic coastal zones are important players in the global carbon cycle. The deep ocean is ventilated in these regions as part of the Southern Ocean overturning circulation, allowing waters rich in nutrients and CO2 to be upwelled to the surface. Previous studies in the highly productive regions of the Southern Ocean have highlighted the potential for using compound-specific isotopes from algal biomarkers in sediments to track primary productivity changes both spatially and temporally. Villinski et al (2008) found that the spatial variation in pCO2 in the Ross Sea was associated with a variation in the δ13C of sedimentary organic carbon and sterol biomarkers, most likely due to a change in isotopic fractionation associated with the photosynthetic drawdown of CO2 Their results demonstrate that the spatial variation in surface water CO2 is recorded in sedimentary organic matter and algal biomarkers. We explore this further as well as look into other potential drivers of compound-specific carbon isotopes. Surface waters along the Adélie coast have relatively high concentrations of nitrate, silica and phosphorus, with spatially variable levels of Fe, which may be due to re-suspension of sediments and calving of ice (Vaillancourt et al, 2003; Sambrotto et al, 2003)
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