Foraminiferal diversity and novel trace elements for carbonate system proxies

Abstract

This thesis is part of ongoing efforts to improve reconstructions of past environments and climates. These reconstructions are highly urgent because of the rate and magnitude of current climate change, our planet rapidly changes. Finding past analogues for future conditions can provide insight into ongoing climate change. Fossils of foraminifera (single-celled marine organisms that build calcium carbonate shells) provide information on past seawater temperature, salinity, pH, etc. due to differences in the incorporation of elements and fractionation of isotopes during the formation of their tiny shells. This thesis investigates the process of biomineralization, both to understand the foraminiferal shell composition and to seek novel ways to use the elemental composition of the carbonate to reconstruct past conditions. One of the main outcomes of this work is that calcification pathways among foraminifera are highly diverse. Variability in biological mechanisms by which foraminifera produce their shells needs to be considered when evaluating geochemical signatures. Another result highlights the potential of a new climate reconstruction tool in the form of incorporated potassium (K), commonly expressed as its ratio to calcium, or K/Ca. Contrary to incorporation of some other elements (for example the popular reconstruction tools Mg and Na which are used to reconstruct past temperature and salinity), incorporation of potassium does not depend on seawater calcium concentrations. This means that K/Ca in fossil foraminifera reflect primarily seawater potassium concentrations, allowing for reconstruction of the oceanic potassium inventory, which in turn reflects processes like continental weathering , in which rocks are slowly dissolving onto the oceans. In my thesis, I conclude that potassium may be particularly suitable for such reconstructions as incorporation of other elements often rely on calcium concentrations in seawater, which may or may not vary in concert with the other major seawater elements.