CALIBRATING FORAMINIFERAL FRAGMENTATION AS A PROXY FOR SEAWATER CARBONATE SATURATION STATE

The geologic record contains numerous examples of global carbon cycle perturbations, which are often manifested as changes in the oceanic carbonate system. For this reason, effective proxies for ocean pH and ocean carbonate saturation state are essential for understanding Earth’s carbon cycle dynamics. While current proxies for oceanic carbonate parameters are time-intensive and require technical expertise, an inexpensive means of constraining past ocean carbonate saturation may exist through quantifying foraminiferal fragmentation. Because carbonate saturation state with respect to calcite (Ω_calcite) determines the rate at which foraminifera are dissolved, measuring percent foraminiferal fragmentation in marine sediments could provide a quantitative constraint on seawater Ω_calcite through the Cenozoic and beyond. While numerous pioneering studies have demonstrated a direct correlation between ocean saturation state and different metrics of foraminiferal fragmentation, approaches have varied, and there is considerable scatter between studies. Here we present a new calibration of foraminiferal fragmentation vs. Ω_calcite based on imaged Holocene core-top samples, combining new measurements with published data. With the images used to create this novel dataset, we can directly compare different techniques used for quantifying fragmentation with the same samples, and thus normalize published datasets. Our refined compilation suggests foraminiferal fragmentation, at least in deep sea sediments, is strongly dependent on Ω_calcite. Besides its intrinsic potential as a robust, quantitative proxy for past carbon cycle changes, estimates of calcite saturation state from foraminiferal fragmentation such as those presented here may be used to aid reconstruction of ocean pH with boron isotopes. Specifically, a lack of knowledge as to the isotopic composition of boron in seawater (d¹¹B_sw) in deep time has limited the application of the proxy to the relatively recent past. However, by combining boron isotope measurements in benthic foraminifera with estimates of Ω_calcite from foraminiferal fragmentation, we may better constrain d¹¹B_sw, allowing reconstruction of surface water pH, and hence atmospheric pCO₂, further back in geologic time.