Bhatia, Rehemat; (2019) Geochemical signals in fossil planktonic foraminifera. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The stable isotope and trace element geochemistry of planktonic foraminifera is commonly used to reconstruct palaeoclimatic and palaeoceanographic parameters across recent and deeper timescales. However, aspects of their ecology, such as photosymbiont presence, ontogenetic growth and depth habitats, are suggested to alter these proxy signatures. Ecologies are additionally thought to change through time, creating further complications. Therefore, constraining foraminiferal palaeoecology, particularly during epochs which aid us in understanding future climate, is vital, to validate any proxy signals obtained. Two such epochs are the Eocene and Miocene, characterised by temperatures and CO2 levels comparable to a future warmer world. Many species within these periods have unresolved palaeoecologies. This thesis investigates the palaeoecology of multiple Eocene and Miocene planktonic foraminiferal species (from sites with well-preserved specimens) with unresolved palaeoecologies, using δ 13C, δ 18O, Mg/Ca, Sr/Ca and B/Ca. Whole and intra–shell geochemistry is investigated, using a combination of isotope ratio mass spectrometry and high resolution analytical techniques (laser ablation inductively coupled plasma mass spectrometry, electron microprobe analysis). It is shown for the first time that intra-chamber and size-segregated trace element/Ca data do not vary between Eocene foraminiferal species with differing ecologies (except Globigerina cf. bulloides and Pseudohastigerina wilcoxensis). Eocene and Miocene species EMPA data exhibit intra-shell Mg heterogeneity between species with varying depth habitats and photosymbiotic partnerships. Combining EMPA data with size-segregated δ 13C data is shown to be beneficial in distinguishing photosymbiotic species. Lastly, size-segregated δ 13C data from the species Globigerinatheka index indicate that it is possible for planktonic foraminifera to facultatively host photosymbionts in response to unfavourable environmental conditions. Having constrained the ecologies of various Eocene species, discrepancies between their inorganic proxy data are revealed and discussed. Overall, this thesis amplifies the importance of disentangling foraminiferal palaeoecology before constructing long-term downcore records, to mitigate the issues which vital effects can create.

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