Proximity to Undersaturation and the Influences on G. bulloides Area‐Density in Southern Indian Ocean Marine Sediments

Abstract

AbstractThe area density proxy of foraminiferal shell thickness and calcification intensity has the potential to provide information about past ocean CO2 content and has the benefit of small sample requirements, simple analytical techniques, and the ability to re‐use the analyzed foraminifera for other paleo‐proxies. Using a series of multicore core‐tops collected from the southeastern Indian Ocean (1.8–3.8 km water depth), we evaluate the reliability of utilizing area density values of Globigerina bulloides from sediment cores to estimate surface ocean carbonate parameters. Because foraminifera in marine sediments can rarely be considered “pristine” (or “glassy”), we grouped area density measurements of shells to designate various stages of diagenesis. Visual signs of alteration were apparent at area density values as low as ∼0.122 × 104 µg/µm2, with deviations from the “pristine” endmember beginning at area density values of ∼0.087 × 104 µg/µm2. We find that increases in area density overprint the surface ocean carbonate signature in thicker (>0.122 × 104 µg/µm2 shells), but small increases associated with marine sedimentary burial and diagenesis can be accounted for, allowing this proxy to be applied back in time. Reconstructing the distribution of area density values in a given sample has the potential to provide valuable information on overall sample preservation by estimating the percent of well‐preserved shells (<0.122 × 104 µg/µm2; %wp) in a given sample. Our %wp metric has the potential for use as a proxy for lysocline variability in addition to assessing the suitability of marine sediment samples for surface ocean reconstructions.