Understanding the relationship between foraminiferal Mg/Ca and clumped isotope thermometers

Reconstructions of past sea surface temperatures (SSTs) are essential for understanding long-term climate variability, yet different proxy methods can yield divergent results. In this study, we compare Mg/Ca-derived SSTs from Globigerinoides ruber sensu stricto and Trilobatus sacculifer with clumped isotope (Δ₄₇) SSTs measured on G. ruber s.s. from the same core, MD96-2048 (Indian Ocean), covering the last 1.25 million years (Ma). Using the same species and samples allows minimizing ecological and environmental biases. We find that Δ₄₇-derived SSTs are systematically colder than Mg/Ca-SSTs prior to 0.4 Ma, while both proxies agree well after this point. This offset is not explained by diagenetic alteration (as assessed via SEM), nor by corrections for seawater salinity, pH, or Mg/Ca composition. The Mg/Ca-SSTs from T. sacculifer are more consistent with Δ₄₇-SSTs in the older interval, but do not fully resolve the discrepancy. We found that the apparent Δ₄₇-based cooling before 0.4 Ma is not supported by seawater δ¹⁸O estimates or other climate indicators. Our results suggest that Δ₄₇-derived SSTs may be affected by an unknown bias in older intervals, although a combination of multiple factors explored in this study could also contribute to the observed offset.