Nitrogen, carbon, 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 nitrogen isotopes in carbonate bound organic matter (d15NCBOM) have the potential to record sea ice state over time. Recent studies illustrated that d15NCBOM values provide a similar proxy as soft tissue d15N values which are in turn are predicably related to food d15N values (Gillikin et al., 2017, GCA, 200, 55–66, doi: 10.1016/j.gca.2016.12.008). Sea-ice organic N should have higher d15N 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). 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 adults shells collected at these sites in 2008 (3 from each site) and two juveniles collected in 2016 from Explorers Cove were be serially sampled for d15NCBOM values from the growing shell margin to the umbo. d15NCBOM values from Explorers Cove with persistent sea ice cover were consistently higher (+10 ± 0.7 ‰) than those from Bay of Sails where the sea ice melts out every year (+8 ± 0.5 ‰; t-test p<0.0001). d15NCBOM data from Mid- to Late Holocene shells that grew in these locations will also be presented. We posit that nitrogen isotopes in A. colbecki shells have a high potential to record sea ice cover.