Improving Paleoclimate Reconstructions using Models and Observations of Foraminifera

Date of Award

Spring 2022

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Geology and Geophysics

First Advisor

Hull, Pincelli

Abstract

This study investigates the biological and morphological factors controlling carbon and oxygen stable isotope fractionation in foraminifera (δ13C and δ18O) and uses these findings to address two major questions of paleoclimate—the effect of warm climates on organic carbon cycling and the strength of polar amplification through geologic time. Using a chemical model of the foraminifera’s diffusive boundary layer, it is shown that metabolism and the physical arrangement of algal symbionts on the spines and rhizopodia of planktonic foraminifera can significantly affect the δ13C signal recorded in their shells. This finding implies that vertical gradients of δ13CDIC in the Eocene were likely less different from modern profiles than previously proposed, meaning that the temperature-dependence of organic carbon cycling may be a weaker climate feedback than previously proposed. This theoretical investigation of spines and rhizopodia is supplemented with in situ observations using the autonomous imaging platform Zooglider, which demonstrate that spines increase the effective prey encounter volume of spinose foraminifera by two to three orders of magnitude, opening new predatory ecological niches. Novel observations are presented of exceptionally large hastigerinid foraminifera with a total prey encounter volume close to 40 cm3 (about the size of a golf ball), as well as newly observed features of the hastigerinid bubble capsule. Expanding the boundary-layer model described above with a new model of carbon isotope behavior in the cell interior provides for the first time an explanation of the “carbonate ion effect” on δ13C, in which δ13C covaries strongly with seawater pH or [CO32-]. It is shown how changes in pH alter the dynamics of CO2 diffusing into or out of the calcifying fluid, fractionating 13C and creating the carbonate ion effect. This process implies a linkage between photosymbiosis and the foraminifera’s internal pH and can also explain why δ13C is abnormally low in small foraminifera, an effect previously attributed to faster metabolism. These insights are then applied to δ18O, where a suite of new techniques for estimating past seawater δ18O and converting δ18O to sea-surface temperatures (SSTs) are developed and applied to a large compilation of δ18O data from planktonic foraminifera in order to estimate the past magnitude of polar amplification. It is shown that zonal mean SSTs predictably covary with bottom-water temperatures over at least the past 95 million years, allowing the data-dense bottom-water record to be converted into a continuous reconstruction of the latitudinal SST gradient. Gradients obtained in this way yield better agreement with climate model predictions than previous proxy estimates, supporting the validity of the model physics. The consistent relationship between bottom-water temperatures and SST gradients across numerous climate states implies an important underlying predictability of the climate system despite large changes in temperature, continental configuration, and atmospheric dynamics. These findings emphasize the importance of treating the biological and environmental signals recorded in foraminiferal calcite as an integrated system. Rather than simply being an impediment to seeing the environmental signal, vital effects record valuable information about the biology and ecology of foraminifera, which in turn can be used to strengthen proxy reconstructions of ancient climate.

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