Composition of organic materials and the control factors of suspended particulates in the surface water of the Ross Sea-Amundsen Sea in marginal sea of the southwestern Antarctic in austral summer 2019-2020

The study on the composition and source of suspended particulate organic matter in the marginal sea helps understand marine material biogeochemical cycle. The geographical location and climate environment of Antarctica are special. Antarctic marginal seas are affected by the interaction among marine, atmospheric, and glacial systems. The composition and source of particulate organic carbon (POC) has unique regional characteristics and global significance. Based on the dataset of organic carbon, nitrogen, and their isotopes and source-specific biomarkers extracted from 59 samples of suspended particulate matter collected from surface seawater in the western Antarctic marginal seas during the 36th Antarctic scientific expedition of China, the distribution and composition of organic matter in the suspended particulate matter were studied. The factors controlling the spatial distribution of particulate organic matter were examined, and the application potential of different organic geochemical proxies as the composition and source indicators to particulate organic matter in the Ross Sea-Amundsen Sea was evaluated. Results show that POC concentration in the surface water of the Ross Sea-Amundsen Sea is consistent with the spatial distribution of surface water fluorescence, seawater pCO₂, and marine phytoplankton or animal derived biomarkers, which implies that POC in the surface water of Ross Sea-Amundsen Sea in summer is mainly produced by marine phytoplanktons and animals. The C/N ratio is generally below 4, indicating that POC in these regions is obviously degraded by microorganisms. The bulk δ¹³C value is generally lower than −25.2‰, and the spatial distribution is complex, which reflects the unique mixed POC signals of ¹³C-depleted phytoplankton source, ¹³C-enriched animal source , and ¹³C-depleted terrestrial origins from the Antarctic landscape. Lipid biomarker proxies archived from surface suspended particulate matter is an effective tool to distinguish sources of POC. The concentrations of phytoplankton-derived biomarkers such as brassicasterol and dinosterol reflect the contribution of phytoplankton-derived POC. The nearshore polynya shows a high value, and the spatial variability of its concentration is controlled by the phytoplankton activity in water column. Cholesterol reflects the contribution of animal derived POC. The nearshore at edge of ice shelf shows a high value, and the spatial variability of its concentration is controlled by the secondary productivity in water column and the biomass of penguins and seals. Long chain alkyl lipid biomarkers represent the contribution of lithologic POC from the Antarctic continental region. The spatial variation of their concentration is controlled by glacial activities, and often shows high values in the nearshore area at the edge of ice shelf where obvious ice melting is taken place. This research shows that the comprehensive evaluation of biomarker molecular indicators combined with bulk organic matter indicators provides an effective method for accurately resolving the complex POC matrix in the Antarctic marginal sea.