Environmental and biological controls on the shell geochemistry of the planktic foraminifera Orbulina Universa
Date
2017
Authors
Holland, Katherine
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Abstract
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.
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Foraminifera, Orbulina universa, LA-ICP-MS
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Thesis (PhD)
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