Micro X-ray CT Scanning of planktonic foraminifera tests (Globigerina bulloides) for Paleoceanographic reconstructions of ocean carbonate chemistry

The carbonate ion concentration ([CO₃²⁻]) in the deep ocean is a key parameter for reconstructing ocean carbonate chemistry and understanding its role in the global carbon cycle. The dissolution of planktonic foraminiferal tests has long been used as a proxy for past deep ocean [CO₃²⁻] variability (Lohmann, 1995; Broecker and Clark, 2001a,b, 2003). However, traditional dissolution proxies, such as size-normalized weight (SNW), have inherent limitations in quantitatively constraining past fluctuations in deep-sea carbonate chemistry. For instance, fossil tests are often filled with sediments, making it difficult to clean them without damaging the original shell. Additionally, the initial size-normalized weight (SNW) values are influenced by ambient environmental conditions (such as the surface water [CO₃²⁻]) during calcification (Barker and Elderfield, 2002; Broecker and Clark, 2004).

  To address these limitations, a new quantitative approach has been developed to reconstruct bottom water saturation with respect to calcite (Δ[CO₃²⁻]) using micro X-ray computed tomography (MXCT) to separately evaluate the density of the test surface and interior. This method has been employed to reconstruct ocean carbon storage during the Last Glacial Period (Iwasaki et al., 2022). Δ[CO₃²⁻] represents the difference between the carbonate ion concentration at saturation and the in situ carbonate ion concentration, providing an effective method for reconstructing past carbonate ion levels. This technique enables high-resolution, non-destructive three-dimensional analysis of foraminiferal test microstructures, offering more precise constraints on past ocean carbonate chemistry (Iwasaki et al., 2023; Kimoto et al., 2023).

  In this study, we applied MXCT technology at the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) to investigate foraminiferal test dissolution patterns in the North Atlantic on orbital timescales. We constructed three-dimensional models of Globigerina bulloides to examine its dissolution processes from the Pliocene to the Pleistocene. Previous dissolution experiments have shown that the inner calcite of G. bulloides dissolves selectively, and dissolution intensity can be evaluated using CT histogram patterns (Iwasaki et al., 2015). Our research results are consistent with previous studies showing that as shell dissolution progresses, the shape of the CT value histogram shifts toward a bimodal distribution. These findings contribute to improving alternative dissolution-based proxies and enhancing our understanding of the oceanic carbonate system’s response to climatic and oceanographic changes.