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Elements in foraminiferal shells as recorders of past climates
Geerken, E. (2019). Elements in foraminiferal shells as recorders of past climates. Utrecht Studies in Earth Sciences, 203. Postgraduate Thesis. Utrecht University: Utrecht. 198 pp. https://hdl.handle.net/1874/387734
Deel van: Utrecht Studies in Earth Sciences. Instituut voor Aardwetenschappen Utrecht: Utrecht. ISSN 2211-4335, meer
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| Author keywords |
Foraminifera, shells, salinity, climate, paleoceanography, biomineralization, proxy's, calcite, biogeochemistry |
| Abstract |
Foraminifera are unicellular marine organisms that are globally ubiquitous and abundant throughout the geological record. The calcite chemistry of fossil foraminiferal shells reflect seawater conditions during the time of formation and is therefore an excellent tool (i.e. a proxy) to reconstruct ocean variables. However, it is evident that the organism itself exerts a strong additional control on element incorporation. The overarching aim of this thesis is to better understand how foraminiferal element ratio's (El/Ca) are controlled by the organism and by the environment, at different scales: within the shell wall, between specimens and between species. Culture- and labeling studies with living foraminifera are central in this thesis. To test a recently proposed proxy for salinity, Na/Ca, individuals of Ammonia tepida and Amphistegina lessonii were grown at salinities between 20 and 40. Na/Ca shows a positive correlation to salinity, however inter-specimen variability in Na/Ca is larger than the salinity effect. Electron microprobe maps show that Na and Mg occur in bands of elevated concentrations within the shell wall. We suggest that inter-species, inter-specimen and intra-shell variability in El/Ca, are all caused by (variability in) the organismal controls during biomineralization. Element banding within chamber walls is studied in more detail using Nanoscale-Secondary Ion Mass Spectrometry (NanoSIMS). Specimens of Ammonia tepida and Amphistegina lessonii were grown at different temperatures and salinities, to assess how these parameters affect the element distribution within the shell wall. We show that both the high and low element bands are elevated in specimens grown at a higher temperature or salinity, implying that these environmental parameters have a consistent effect on the whole-shell El/Ca values. A poorly quantified, but potentially key factor impacting element incorporation in foraminiferal biomineralization, is the rate of calcite precipitation. To obtain this rate we use Sr to label the shell wall and visualise the label using Nano-SIMS. Results show that precipitation rates are surprisingly uniform between specimens at ~22±2 nmol/cm2/min, comparable to the fastest rates obtained by inorganic calcite precipitation experiments. This may explain foraminiferal Sr incorporation as precipitation from a Mg-free seawater like solution. To test various novel El/Ca-based proxies down-core, a piston-core was retrieved from the Eastern Mediterranean Sea, covering the last 350,000 years. We show that increased foraminiferal Ba/Ca and Mn/Ca reflect seawater freshening in conjunction with sapropels: organic-rich sediments formed due to bottom water deoxygenation, likely related to increased Nile-river runoff. We conclude that Na/Ca is not a suitable recorder of salinity in the setting studied here, yet ΔMg/Ca appears a promising candidate as a salinity proxy when large changes in salinity are involved. Finally, we synthesize observations of El/Ca variability at different scales and hypotheses on the biomineralization mechanisms involved, presented throughout this thesis, by combining and extending existing biomineralization models. A numerical model for element incorporation as a function of shell wall thickness, based on precipitation of calcite from a confined space of a specified volume and involving inward Ca2+-transport and inorganic partitioning coefficients, is presented and compared to observations. |
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