Date of Award

6-2022

Document Type

Restricted (Opt-Out)

Degree Name

Bachelor of Science

Department

Geosciences

First Advisor

David Gillikin

Keywords

Paleoclimate, sea ice, Antarctica, Holocene, Adamussium colbecki, scallop

Abstract

Fluctuations in sea ice cover are major factors driving climate change and are a substantial component of the global climate feedback loop. Antarctica currently lacks notable proxy records of sea ice state; bivalves archive environmental conditions and can be studied to track changes in sea ice cover through time. Adamussium colbecki is a large sea scallop with a circum-Antarctic distribution and an abundant fossil record throughout the Holocene. Our group’s work shows that carbon (d13Cs) and nitrogen (d15NCBOM) isotopes in modern scallop shells record seasonal variation in sea ice state over time when paired with growth markers called striae. We also found that sea ice cover is recorded by low d13Cs values in narrow striae while ice-free conditions are recorded by high d13Cs values in wide striae. Nitrogen isotopes of carbonate bound organic material also recorded sea ice state, with ice-free conditions recorded by lower values. Here we apply these paleoclimate proxies by analyzing A. colbecki subfossil shells collected from terraces along Explorers Cove (EC) and Bay of Sails (BOS), western McMurdo Sound, Antarctica which grew between 1,200 and 4,000 years ago (based on 14C ages). Today, these two sites have contrasting sea ice states: persistent (multiannual) sea ice at EC and annual sea ice (that melts out every year) at BOS. Two adult fossil shells collected at EC and four fossil shells (including one juvenile) collected at BOS were serially sampled for d13C, d18O, and d15N from the growing shell margin to the umbo. Imaging of striae allowed for d13Cs and d18Os values to be paired with summer (wide striae) and winter (narrow striae) scallop growth; d15NCBOM required larger sample size, so were not seasonally paired. Seawater temperature proxy records suggest warmer conditions 2,000-5,000 yr BP, so we expect variable d13Cs values recording annual sea ice in shells from both sites.

Modern carbon and nitrogen isotopes in Adamussium colbecki shells agree with known sea ice conditions and can be considered an accurate proxy by which to extend the Antarctic paleoclimate record back through the Holocene. Overall, subfossil oxygen isotopes reveal uncertain sea ice trends, but point towards paleotemperatures that may have been analogous to current ocean temperatures (-2 to +4.3° C) or potentially slightly warmer over the past 4,000 years. Subfossil carbon isotopes display sea ice trends on a finer time scale and within individual scallops. EC1 and BOS1 are dated to 3-4 ka, which falls approximately around the second Bond Event. These valves show no significant seasonal differences in d13C values for both EC and BOS, which may indicate increased sea ice persistence during a period of theorized rapid cooling. BOS2 d13C values are seasonally different, indicating comparable annual ice persistence 1,300 yr BP. Nitrogen isotopic trends also indicate that BOS and EC sea ice was largely homologous to modern persistence characteristics: EC d15Nmean > BOS d15Nmean for subfossil scallops, indicating that BOS dominantly experienced annual ice persistence and EC experienced multiannual sea ice over the past 4,000 years. Together, the stable isotopic composition of subfossil Adamussium colbecki seem to demonstrate highly stable and constant sea ice at Explorers Cove from ~1,200 – 4,000 yr BP, while sea ice at Bay of Sails is much more responsive to climatic changes. Western Antarctica’s response to anthropogenic climate change may be observed first at Bay of Sails in McMurdo Sound, Ross Sea. Further expansion of this study could paint a more precise reconstruction of Antarctica’s response to global climate cycles and bolster predictions of future sea ice, ice sheet, and possibly glacial activity.

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