Developing a Southern Indian Ocean Area Density Model for the Planktonic Foraminifera Globigerina bulloides

Abstract:

The increased oceanic uptake of anthropogenic carbon has resulted in a decrease of surface ocean pH levels. As a result of anthropogenic activities, the increased uptake of CO₂ decreases the carbonate ion concentrations ([CO₃^2-]) in the ocean resulting in a shift towards more acidic conditions. More corrosive conditions can impact marine organisms who secrete a calcium carbonate (CaCO₃) shell, such as corals, pterapods, and planktonic foraminifera. In particular, planktonic foraminifera shells have been shown to become thinner upon the decrease of [CO₃^2-]. Furthermore, planktonic foraminifera calcification is sensitive to nutrient concentrations and temperature at the time of the individual’s calcification. An individual’s calcification can be estimated by shell thickness – quantified by morphometric characteristics such as silhouette area, diameter, and mass. An area density (μg/μm²) calibration of shell thickness quantifies changes in foraminifera calcification to develop a proxy to expand our understanding of the marine carbonate system during periods of past CO₂ changes. Foraminifera growth increases both size and mass. However, diagenetic processes, such as dissolution and encrustation, alter the shell mass without affecting the size of the shell. Here, we present an area-density model of the asymbiotic planktic foraminifera species Globigerina bulloides for the Southern Indian Ocean. Area density allows us to estimate changes in dissolution and carbonate saturation before visible changes in the shell are apparent. This area density method is particularly useful in preserving individual variability within a population of foraminifera and is a non-destructive technique, allowing us to perform geochemical analyses on the same shells. Morphometric analyses of foraminifera from the Southern Indian Ocean remains unexplored. Our area density measurements indicate that G. bulloides in the Southern Indian Ocean are recording surface ocean [CO₃^2-]. By expanding use of this morphometric proxy, we can constrain the natural variability of the carbonate system in the Southern Indian Ocean and better understand the present and future impact of anthropogenic CO₂ on this system.