Environmental and biological controls on the shell geochemistry of the planktic foraminifera Orbulina Universa
Creators: name: Holland, Katherine
Publisher: name: The Australian National University
Year: 2017
Subjects: Foraminifera
Orbulina universa
LA-ICP-MS

description: Foraminifera make excellent archives of past oceans and climate because they grow calcium carbonate shells which incorporate trace element and stable isotopes that reflect the seawater conditions in which they grow. This study aims to: (i) improve understanding of the factors that influence trace element and stable isotope incorporation into foraminifera shells, and (ii) improve proxy calibrations for reconstructing past ocean carbonate system parameters and temperature. To do this, I have cultured living foraminifers in seawaters that have a wide range of cation and carbonate chemistry compositions. The shells produced by these foraminifers have been analysed by a laser ablation ICP-MS microanalysis techniques. The study has focused on three specific objectives: (i) formalising the sensitivity of shell trace element to calcium (TE/Ca) ratios to changes in seawater composition, (ii) establishing the response of bulk shell TE/Ca compositions to carbonate system chemistry and shell growth rates linked to the calcite saturation state of seawater, and (iii) characterising the responses of intra-shell TE/Ca and boron isotope composition to changes in the carbonate system chemistry of the foraminiferal microenvironment over diurnal cycles due to variation in respiration, calcification, and symbiont photosynthesis. To quantify the effect that changes in the Mg/Ca ratio of seawater has on the foraminifer shell Mg/Ca thermometer, both seawater [Mg] and [Ca] concentrations were varied independently in culture experiments. Results from these experiments has permitted the shell Mg/Ca thermometer to be recalibrated to account for the effects of past changes in seawater Mg/Ca ratio and [Ca] concentration. Culture experiments on Orbulina universa were also used to investigate the effects of different carbonate system parameters; pH, DIC, carbonate ion, seawater [Ca] and calcite saturation state on shell B/Ca, Mg/Ca, Mn/Ca, Sr/Ca and U/Ca compositions. Seawater DIC concentration was found to form significant correlations with B/Ca, Sr/Ca and Mg/Ca shell compositions, and seawater carbonate ion concentration had the dominant effect on U/Ca. Orbulina universa exhibits a narrow range in shell growth rates over a wide range in calcite saturation state of experimental seawaters, indicating shell growth is highly regulated. Intra-shell B/Ca and U/Ca ratios record significant changes in carbonate system chemistry of the foraminiferal microenvironment over diurnal time scales due to the net effects of photosynthesis-respiration and calcification of the foraminifer and its algal symbionts. Orbulina universa shell composition was also explored for internal variation in boron isotope composition. The resolution of the laser ablation MC-ICP-MS technique used is near the precision limits of modelled changes in microenvironment carbonate system chemistry. The measured bulk shell boron isotope compositions are significantly offset from the predicted boron isotope composition of the seawater borate ion. In summary, this thesis provides new insights into the factors controlling trace element and isotope incorporation into foraminiferal calcite, including the extent to which biological factor influence the composition of precipitated shell calcite relative to external seawater chemistry.