Lipid biomarkers in modern marine sediments and surface snow: Towards a multi-archive reconstruction of phytoplankton conditions in the southwestern Ross Sea region, Antarctica

Antarctic phytoplankton (microscopic marine plants) form the base of the marine food web and influence the rate of oceanic carbon dioxide drawdown. The Ross Sea, Antarctica is one of the most seasonally productive areas globally, where different types of phytoplankton such as diatoms and Phaeocystis antarctica, have different functions in the ecosystem and impacts on the carbon cycle. Seasonally resolved instrumental records of past Ross Sea phytoplankton changes cover less than three decades, too short to understand potential future changes in phytoplankton productivity, composition, and the resulting impacts on climate. There is a pressing need to reconstruct high-resolution (seasonal to annual) records of phytoplankton productivity and composition in the Southwestern Ross Sea over the last 2000 years, using a spatial array of ice and sediment records to contextualise recent climate change. This study investigates the modern-day spatial distribution of lipid biomarkers (fatty acids and highly branched isoprenoids; HBIs) sourced from phytoplankton in the Southwestern Ross Sea. Lipid biomarkers (organic chemical tracers of past life) show promise as a proxy to reconstruct past phytoplankton productivity and composition in ice and sediment cores. Five sediment cores, thirteen snow samples, and two seawater samples representing the last century were investigated due to the proximal location to two seasonally re-occurring polynya in McMurdo Sound and Terra Nova Bay (TNB). Biomarker analysis reveals fatty acids as a unifying proxy of phytoplankton community composition including diatoms (C14:0, C16:0, C16:1ω7, C24:1ω9), Phaeocystis antarctica (C14:0, low C16:1ω7, C18:0, C18:1ω9), and bacteria (C18:1ω7, iC14:0, iC15:0, aC15:0, iC17:0, aC17:0), between snow and sediment. Annually resolved snow samples reflect the diatom dominance in TNB and the diverse community of diatoms and Phaeocystis antarctica in the McMurdo Sound. The spatial trends in fatty acid concentration and composition are influenced by locally derived dust, the proximity of the site to nearby polynyas, phytoplankton productivity, the composition of the seasonal phytoplankton bloom, the atmospheric transport fetch region, as well as biomarker degradation. Sediment cores spanning the last century contain higher concentrations of diatom, Phaeocystis antarctica, and bacterial-sourced fatty acids, as well as an HBI biomarker of sea-ice diatoms. Sediment profiles reflect biomarker degradation and indicate biomarkers are archived in the sediment at >30-40 cm depth or after ~100 years. Other sources of lipids were identified, such as copepods (C22:1ω9) in snow and sediment. Overall, the Southwestern Ross Sea is a promising site to reconstruct a multi-archive 2000-year record of past phytoplankton productivity and composition.