Fluctuations in sea ice cover are major drivers of climate change and substantial components of the global climate cycle. Antarctica lacks notable proxy records of sea ice state (SIS); bivalves record environmental conditions and can track temporal changes in sea ice cover. Adamussium colbecki is a scallop with a wide distribution and an abundant Holocene fossil record. Our prior work showed that carbon (δ13Cs) and nitrogen (δ15NCBOM) isotopes in modern scallop shells record seasonal sea ice shifts when paired with growth markers (striae). Sea ice cover is recorded by low δ13Cs values in narrow striae, and high δ13Cs values in wide striae reflect ice-free conditions. Nitrogen isotopes also archive SIS, with ice-free conditions recorded by low values. Here we apply these paleoclimate proxies by studying A. colbecki subfossils from Explorers Cove (EC) and Bay of Sails (BOS), western McMurdo Sound, Antarctica from ~1-6,000 ybp (as per 14C ages). Today these sites have contrasting SIS: persistent (multiannual) sea ice at EC and annual sea ice (melts yearly) at BOS. 2 adult fossil shells collected at EC and 3 (+ 1 juvenile) from BOS were serially sampled for δ13C, δ18O, and δ15N from the margin to the umbo. δ13Cs and δ18Os values were paired with summer (wide striae) and winter (narrow striae) scallop growth; δ15NCBOM were not seasonally paired due to larger sample sizes. Seawater temperature proxies suggest a warmer climate 2-5,000 ybp so we expect to see variable δ13Cs values indicating annual sea ice at both sites.
Fossil BOS and EC δ13Cs means aren’t statistically different between striae groups, indicating persistent sea ice may have been present at both sites. Fossil δ18Os means are statistically higher than modern means at BOS and EC, revealing similar conditions at both sites during the scallops lives. The 0.24‰ difference (t-test p<0.0001) may mean water temperatures were 1ºC colder, meltwater input was minor, or a combination of these factors are recorded. However, colder temperatures are not consistent with estimated warmer conditions (Braddock 2014) and may imply intervals of cooling and warming. The wide range of fossil BOS δ13Cs values may be a time averaged result of multiple SIS. δ15NCBOM data in prep. Serial sampling of modern and fossil A. colbecki allow comparisons of seasonal striae with carbon and oxygen isotopes to provide high resolution proxies of SIS and temperature to understand Antarctica’s climate history.