Paired X-ray Micro CT Scanning and individual foraminifera isotopic analysis reveal (de)coupled changes in carbonate preservation and temperature

To assess the coupling between drivers of isotopic variability and carbonate dissolution in foraminifera, we combine individual foraminiferal carbon and oxygen isotopic analyses (IFA) and X-ray MicroCT Scanning (XMCT)-based measurements of test densities (a proxy for the extent of post-depositional dissolution). As a proof-of-concept application of this approach, we analyze Globigerina bulloides tests from both coretop and downcore (latest Miocene/earliest Pliocene-aged) sediment from Ocean Drilling Project (ODP) Site 1088 (Agulhas Ridge). Sediments from both the core intervals were wet-sieved over a 150 μm sieve and oven-dried at 55°C overnight. Individual tests of G. bulloides were picked from the >150 μm size fraction and gently brushed with the wet tip of a paintbrush to remove any adhering clay. Broken or partially fragmented tests were avoided. 25-50 individuals were mounted on a 1.2cm x 1 cm cardboard sample holder using double-sided carbon and imaged at a time on a Bruker Skyscan 1272 Micro-CT Scanner. In total, 123 individuals from the coretop sample and 88 individuals from the downcore sample were scanned. Stable carbon (δ¹³C) and oxygen (δ¹⁸O) isotopic compositions of individual foraminifera were measured using a Kiel Carbonate Device IV coupled to a Thermo Fisher MAT 253 Plus Isotope Ratio Mass Spectrometer (IRMS). All individuals were weighed on a Sartorius ultramicrobalance before analysis, and only samples with mass >10 µg were analyzed as per the long-term precision setup of analyzing small quantities of carbonates on this machine. The XMCT-IFA dataset in this study consists of δ¹³C and δ¹⁸O values of 90 foraminifera (67 coretop and 23 downcore individuals). We compare population averages of XMCT parameters and isotopic compositions from both the coretop and downcore sediments.