Double Trouble: Foraminiferal calcification in a changing ocean

Abstract

Within the project ‘Double Trouble: Foraminiferal Calcification in a Changing Ocean’, I tried to illuminate mechanisms determining element incorporation in foraminifera with different calcification strategies. In particular, I aimed to assess the interplay between ocean acidification and biomineralization processes. Insight in processes in calcification likely improves foraminifera as reconstruction tools for past environmental changes, as well as it increases the accuracy of predictions regarding future carbonate production by these organisms. Constraining the mechanisms involved in foraminiferal (trace) elements uptake is pivotal for improving existing proxies and potentially discovering new proxies to reconstruct past ocean chemistry. The incorporation of certain elements in foraminiferal calcite is correlated to carbonate ion concentration ([CO32-]) of the surrounding seawater and these elements can therefore be used as proxies to reconstruct past changes in marine carbonate chemistry. We studied incorporation of Na, Mg, S, Sr, Zn and Ba as a function of changes in seawater carbonate chemistry. Modelling the chemical speciation of Ba2+ and Zn2+ shows that the amount of free ions in seawater strongly depends on changes in seawater [CO32-]. The amount of incorporated Zn and Ba (in contrast to Na, Mg and Sr) in foraminiferal carbonate changes as a function of [CO32-], suggesting only free ions are taken up and incorporated during calcification. Zn/CaCALCITE might be used as a pH-independent proxy to reconstruct inter-glacial to glacial changes in seawater [CO32-], whereas Ba/Ca shows potential as a proxy for [CO32-] over a relatively limited range. Furthermore, we observed foraminiferal S/Ca to change with seawater SO42-/CO32-, which is probably due to be substitution of sulphate for carbonate in the crystal lattice. Both hyaline and miliolid foraminiferal shells show the same sensitivity to changes in [CO32-] (resp. 19% and 21% per 100 µmol/kg [CO32-]), but the absolute values differ, indicating a superimposed bio-calcification effect. We investigated differences in trends in element incorporation between hyaline (perforate) and porcelaneous (imperforate) foraminifera in order to unravel processes involved in element uptake and subsequent foraminiferal calcification. In hyaline foraminifera we observed a correlation of element incorporation of different elements between species, reflected by a general higher built-in of elements in species with higher Mg content. Between porcelaneous species inter-element differences are much smaller. Besides these contrasting trends in element incorporation, however, similar trends are observed in element incorporation (e.g. Ba/Ca and Zn/Ca) as a function of seawater carbonate chemistry in both hyaline and porcelaneous species. This hints at similar mechanisms responsible for the transportation of ions to the site of calcification for these groups of foraminifera, although the contribution of these processes might differ across species.