Controls on potassium incorporation in foraminifera and other marine calcifying organisms

Seawater chemistry exerts an important control on the incorporation of trace elements into the shells of marine calcifying organisms. Variability in the major ion chemistry of seawater is also an indicator of past geological processes, and so the influence of seawater chemistry on carbonate trace element incorporation can be harnessed to determine changes in the composition of seawater through time. Here, we investigate whether key oceanographic parameters (temperature, salinity, and the carbonate system) affect the incorporation of potassium (K) into foraminiferal calcite, and explore the utility of K/Ca ratios in foraminifera as an indicator of past variability
in the seawater Ca2+ concentration. We measured both low-Mg and high-Mg modern foraminifera, including Larger Benthic Foraminifera (LBF), planktonic (Globigerinoides ruber) and shallow-dwelling larger benthic (Operculina ammonoides) species, using T
K/Ca laser-ablation sector-field inductively-coupled plasma mass spectrometry (LA-SF-ICPMS). Both species show no resolvable influence of temperature, salinity, pH, or [CO32−] on K incorporation. In order to determine the effect Seawater chemistry
of the seawater Ca concentration ([Ca2+]sw) on K incorporation, we analysed laboratory-cultured O. ammonoides, the close living relative of the abundant Eocene Nummulites, grown at four different [Ca2+]sw. We find a signifi- cant relationship between seawater and shell K/Ca, albeit with a shallower slope compared to most other trace elements which we suggest is driven by a crystal growth rate effect on K incorporation, constrained using culture experiments of O. ammonoides grown at different pH. If the K+ concentration has remained relatively constant throughout the Phanerozoic Eon, our data may pave the way forward for the use of K/Ca as a direct proxy for past [Ca2+]sw variability. Alternatively, coupling K/Ca with the Na/Ca proxy would allow more accurate reconstruction of [Ca2+]sw or verification of whether [K+]sw or [Na+]sw has indeed remained within narrow bounds.