Graduation Year

2026

Document Type

Campus Only Senior Thesis

Degree Name

Bachelor of Arts

Department

Biology

Reader 1

Branwen Williams

Reader 2

Sarah Gilman

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Terms of Use for work posted in Scholarship@Claremont.

Rights Information

@2025 Geeta K Karlcut

Abstract

Marine calcifiers—organisms that create calcium carbonate shells or skeletons—are proxy archives that provide historical reconstructions of environmental variability. Since marine calcifiers use dissolved inorganic carbon (DIC) from the surrounding ocean to create their shells and skeletons, measured calcifier δ13C values reflect ambient seawater 13C-DIC. Data from these organisms supplement instrumental data and offer insight into the ocean’s carbon chemistry, such as the uptake of anthropogenic carbon and biological carbon processing. However, these data are limited to regions that support marine calcifiers. Climate model outputs can also provide information about past carbon cycling. The Ocean Circulation Inverse Model (OCIM2) with a Biogeochemical Component (BGC) is a computational framework that simulates the transient carbon cycle from 1750 to 2022 under the assumption of a climatological mean ocean circulation and biological carbon pump. Here, we explore processes to explain offsets of marine calcifier δ13C values from the OCIM2-BGC output and differences in δ13C values among four marine calcifier taxa: coral, coralline algae, bivalves, and sclerosponges. The main driver of temporal variability for δ13C in both the model output and the marine calcifier shells and skeletons is the flux of fossil-fuel-derived carbon into the oceans (“δ13C Suess effect”). While the ambient δ13C-DIC pools drive calcifier δ13C values, calcifier δ13C values are lower than seawater δ13C data due to carbon isotopic fractionation during calcification. Shell and skeleton composition as well as growth rate both influence fractionation and resulting calcifier δ13C values. Furthermore, biological processes can alter the local carbon pool, affecting marine calcifier δ13C values. Photosynthesis—whether by the calcifier or its symbionts—increases δ13C values, while respiration decreases it. Thus, factors that affect the balance of these processes, including depth, light availability, and temperature also impact marine calcifier δ13C values. High growth rate increases metabolic carbon use, which is closer to ambient seawater δ13C values than respired carbon, thus increasing marine calcifier δ13C values. As sclerosponges lack photosynthetic symbionts and have low growth rates, their δ13C values are closest to ambient seawater 13C-DIC. This work highlights the role of calcifier biological carbon processing in shell and skeleton geochemistry; understanding these processes allows clearer historical reconstructions of environmental variability.

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