Climate proxies in planktic foraminifera: Environmental or biomineralisation controls?

The calcite shells (or ‘tests’) of foraminifera are among the best archives of Earth’s past climate. Numerous trace elements and isotopes in foraminiferal fossils have been applied as proxies to reconstruct ocean environments over geological time. However, the proxy-environment correlations often vary among species and individuals, indicating different degrees of biological control during calcification. This introduces significant uncertainties in their interpretation. To improve the reliability of foraminiferal proxies, it is crucial to understand the biomineralisation processes that govern the incorporation of geochemical tracers into the shells. This thesis presents a series of experiments involving the planktic foraminifera Orbulina universa and Trilobatus sacculifer, cultured under laboratory conditions specifically designed to explore the mechanisms of biomineralisation. These experiments involved either growing specimens in chemically modified seawater, or feeding them with isotopically distinct dietary sources. In the seawater experiments, parameters of the carbonate system were decoupled by varying dissolved inorganic carbon (DIC), pH, and carbonate ion concentration ([CO32-]) independently. This approach makes it possible to statistically determine how foraminiferal proxies respond to different components of the seawater carbon system, which are otherwise strongly correlated in natural seawater. Geochemical measurements from these experiments reveal three critical aspects of the environmental and biological controls on proxy incorporation. First, I investigate trace element (TE) incorporation into tests under decoupled seawater carbonate chemistry conditions. A parallel analysis of multiple TE/Ca ratios across three cultured foraminiferal datasets shows that no single biomineralisation process can fully explain the observed patterns (or their absence) across all measured TE/Ca ratios. This points to a complex and dynamic interplay of biomineralisation processes, which give rise to the so-called ‘vital effects’ that cause the composition of biogenic calcite to deviate from that of inorganically precipitated calcite. Specifically, the data suggest that foraminifera regulate ion supply (e.g., calcium and carbon) and pH at the site of calcification in response to external environmental conditions. This regulation may serve to maintain a relatively stable internal calcifying fluid composition, whose chemistry is influenced by, but not identical to, that of the external seawater. Second, I examine the stable isotopes of carbon (δ13C) and oxygen (δ18O) in O. universa from a subset of cultured foraminiferal datasets. The results reveal that pH, rather than DIC or [CO32-], exerts the primary control on the isotopic composition of both C and O. This finding contrasts with the mechanism behind the well-established ‘carbonate ion effect’ on foraminiferal δ13C and δ18O. I show that the pH effect on δ18O can be attributed to kinetic precipitation processes, whereas δ13C appears to be strongly biologically regulated, likely reflecting C incorporation from an acidic microenvironment rather than direct seawater exchange. These findings advance our understanding of C and O supply during foraminiferal calcification and refine the interpretation of foraminiferal δ13C and δ18O as palaeoclimate proxies. Third, I study the incorporation and fractionation of nitrogen isotopes (δ15N) in organic material within foraminiferal biomass and shells. To trace N uptake, foraminifera were fed with two isotopically distinct dietary sources, and the resulting δ15N of biomass and shell-bound organic matter was analysed. My results reveal direct incorporation of N from the diet into shell-bound proteins, in contrast to the bulk foraminiferal biomass that reflects a mixture of original and dietary N. These findings suggest that foraminifera maintain a closed N system with minimal excretion, and that shell-bound δ15N records the isotopic composition of their diet. Together, this validates shell-bound δ15N as a reliable proxy for past nutrient conditions. These experiments provide significant new insights into the mechanisms of foraminiferal biomineralisation, the incorporation of geochemical proxy signals, and the interpretation of palaeo-environmental records. Finally, my decoupled experiments allowed me to identify a positive correlation between DIC and the strontium-to-lithium (Sr/Li) ratio in foraminiferal tests, suggesting its potential as a novel proxy for reconstructing past seawater carbon states.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2024

Author dc:creator

• Fang, Wei-Ning

Advisor dc:contributor.advisor

• Branson, oscar

• Biomineralisation
• Carbon isotopes
• Foraminifera culturing
• Nitrogen isotopes
• Oxygen isotopes
• Planktic foraminifera
• Trace elements

Licence

• https://www.repository.cam.ac.uk/bitstreams/a1d06fa6-3f0e-4b32-8a6c-74723f6a80f8/download
• https://creativecommons.org/licenses/by/4.0/

Language dc:language

eng

Author Identifier

0009-0001-3998-7207

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/392683