Experimental Geochemistry Using Planktonic Foraminifera For Palaeoceanography

The world’s oceans are facing unprecedented challenges in response to a rapidly
changing climate. Understanding Earth system response through time can enable better
predictions of the future trajectory of Earth systems. The calcium carbonate shells of
planktonic foraminifera encapsulate biogeochemical fingerprints of the oceans in which
they formed, by proxy recording ocean, atmosphere, and climate conditions of the past.
This thesis furthers our understanding and application of planktonic foraminifera in
palaeoceanography through four experimental geochemical studies.

Electron backscatter diffraction analysis (EBSD) was used to investigate foraminiferal
test microstructure and preservation. Three processes of post-depositional modification
were identified, namely: 1) dissolution; 2) interface-coupled fluid-mediated replacement
reactions of dissolution and re-precipitation; and 3) inorganic overgrowth. Paired with
electron microprobe analysis (EMPA) of Mg/Ca distribution, these processes were found
to affect foraminiferal geochemistry to differing degrees; with implications for microfossil
utility in palaeoclimate reconstructions. Single-specimen stable isotope analysis enabled
refined understanding of “Globorotalia” ecology; challenging broader assumptions of
ecological niche conservatism within foraminifera clades. Recommendations are made
on the assignment and application of foraminiferal functional groups in
palaeoceanography, and best practice using stable isotope analysis for palaeoclimate
reconstructions. S/Ca in foraminifera is hypothesised as a proxy for seawater carbonate
ion concentration (as a function of seawater [SO₄²-] / [CO3²-]), which is intrinsically linked
with atmospheric CO2. This thesis finds the relationship between seawater [SO₄²-] /
[CO3²-] and planktonic foraminiferal S/Ca more complex than first hypothesised.
Potential controls on S incorporation were investigated, with recommendations for further
research using this study as a foundation. The isotopic composition of neodymium (εNd)
in planktonic foraminifera is considered as a proxy for ocean circulation. Methodological
design and instrumental advancements are presented to improve the resolution of
potential foraminiferal εNd records. Collectively, this work improves understanding of the
nuance and application of geochemical proxies recorded in foraminiferal archives.