Nitrogen and oxygen isotopes in the Antarctic scallop Adamussium colbecki as a proxy for sea ice cover in Antarctica

Adamussium colbecki is a large thin-shelled scallop common in Antarctic waters and well represented in the fossil record. Shell oxygen (δ18Os) and nitrogen isotopes in carbonate bound organic matter (δ15NCBOM) have the potential to record sea ice state over time. To test this hypothesis, we analyzed A. colbecki shells from Explorers Cove and Bay of Sails, western McMurdo Sound, Antarctica. These sites have different sea ice states: persistent (multiannual) sea ice at Explorers Cove and annual sea ice (that melts out every year) at Bay of Sails. Six adult shells collected at these sites in 2008 (3 from each site) and two juveniles collected in 2016 from Explorers Cove were serially sampled for δ18Os values from the growing shell margin to the umbo. We hypothesize that melting glacial ice will pulse freshwater with low δ18O values into the system, which will be recorded in as larger amplitude pulses in shells from the Bay of Sails, but as dampened pulses in Explorers Cove. However, the measured δ18O did not give us any conclusive evidence for our hypothesis. Carbonate bound organic material was sampled for δ15NCBOM values. Recent studies illustrated that δ15NCBOM values provide a similar proxy as soft tissue δ15N values (Gillikin et al., 2017, GCA, 200, 55-66, doi: 10.1016/j.gca.2016.12.008). The organic content of the shell is low, and the shells are thin, so δ15NCBOM values will be more time averaged than δ18Os values. Nevertheless, sea-ice organic N should have higher δ15N values compared to open water organics due to nitrate draw down in the ice (Fripiat et al., 2014, Global Biogeochem. Cycles, 28, 115-130, doi:10.1002/2013GB004729). Thus, we expect large differences between Explorers Cove with persistent sea ice cover and Bay of Sails where the sea ice melts out every year. The average measured δ15N with each shell at Explores Cove is higher than at Bay of Sails (10.03±0.03"° vs 7.92±0.05"°), which agrees with our hypothesis. We posit that nitrogen isotopes in A. colbecki shells have a high potential to record sea ice cover. This work is co-authored with Sally Walker and Kelly Cronin at the University of Georgia.