Paired X-Ray Micro CT Scanning and Individual Foraminifera Isotopic Analysis Reveal (De)coupled Changes in Carbonate Preservation and Temperature

Abstract

The composition and preservation state of biogenic carbonate archives, such as foraminiferal tests, record ocean chemistry during the lifetime of the organism and post-depositional changes in ambient conditions via carbonate compensation. Depending upon the specific paleoclimate proxy, post-depositional processes, including dissolution, may alter original paleoenvironmental signals captured by the foraminifer's test composition. Accordingly, quantifying dissolution independent of geochemical measurements can improve proxy interpretation. Developing independent tools may also be useful for investigating whether changes in paleoclimatic conditions are associated with changes in seawater carbonate chemistry. Such approaches can be improved further if they are applied to individual foraminiferal tests, as specimen-to-specimen differences can record higher-frequency environmental changes compared to conventional bulk-scale analyses. Here, we combine individual foraminiferal carbon and oxygen isotopic analyses (IFA) with X-ray MicroCT Scanning to generate paired analyses of test density (a proxy for the extent of post-depositional dissolution) and isotopic composition. As a proof-of-concept application of this approach, we analyze Globigerina bulloides tests from both coretop and latest Miocene/earliest Pliocene-aged sediment from Ocean Drilling Project (ODP) Site 1088 (Agulhas Ridge). Our measurements and mixing model calculations show that within-population differences in carbon and oxygen isotopic ratios are largely independent of dissolution extent. By comparing population averages from coretop and downcore sediments, we find that lower oxygen isotopic ratios (likely driven by higher calcification temperatures) are associated with greater extents of dissolution at ODP Site 1088. We interpret this finding to reflect coupled changes in carbonate chemistry and climatic conditions over million-year timescales.