Stable carbon isotopic composition of particulate organic matter in the Cosmonaut and Cooperation Seas in summer

This study examined particulate organic carbon (POC) and its isotopic composition (δ¹³C_POC) in the Cosmonaut and Cooperation Seas in the Antarctica during the summer of 2019. Our results show that the spatial variation of POC concentration in summer surface water generally mirrors that of δ¹³C_POC, with higher POC and δ¹³C_POC values in the Cosmonaut Sea compared to the Cooperation Sea. The δ¹³C_POC values in both seas were positively correlated with the proportion of Chl-a in smaller particles (< 20 μm). However, the relationship with the proportion of biogenic POC in smaller particles (< 20 μm) differed between the two seas. This discrepancy is attributed to differences in the dominant phytoplankton species. In the Cosmonaut Sea, smaller phytoplankton (nano- and pico-phytoplankton) were dominated by Phaeocystis antarctica, whereas in the Cooperation Sea, they were dominated by pennate diatoms. The δ¹³C_POC in deep waters of both seas increased with depth, reflecting the effects of organic remineralization. The carbon isotope fractionation factors during remineralization, estimated using Rayleigh model, were 1.5 ± 0.2‰ and 1.6 ± 0.2‰ in the Cosmonaut Sea and the Cooperation Sea, respectively. These small isotope effects indicate that the isotope signals of organic matter exported from the upper layer are well preserved in the deep ocean. Additionally, anomalously high δ¹³C_POC values were observed in the bottom water outside the Cape Darnley polynya in the Cooperation Sea, reflecting the input of ice algae-derived organic matter from the shelf during AABW formation. A simple isotopic mass balance estimate suggests that 6–19% of the POC in the AABW of the Cooperation Sea is contributed by ice algae. Our study highlights the complexity of factors affecting δ¹³C_POC in the Southern Ocean, emphasizing the importance of phytoplankton community composition.