NITROGEN AND OXYGEN ISOTOPES IN THE SHELL OF 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 (δ¹⁸O_s) and nitrogen isotopes in carbonate bound organic matter (δ¹⁵N_CBOM) have the potential to record sea ice state over time. To test this hypothesis we will analyze 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 adults shells collected at these sites in 2008 (3 from each site) and two juveniles collected in 2016 from Explorers Cove will be serially sampled for δ¹⁸O_s 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. Carbonate bound organic material will be sampled for δ¹⁵N_CBOM values. Recent studies illustrated that δ¹⁵N_CBOM 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 δ¹⁵N_CBOM values will be more time averaged than δ¹⁸O_s 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. We posit that oxygen and nitrogen isotopes in A. colbeckishells have a high potential to record sea ice cover.