Reconstructing Late Miocene and Early Pliocene Sea Surface Temperatures: A Comparison of △47 and Mg/Ca Thermometry

Carbonate clumped isotope thermometry is a powerful tool increasingly utilized across earth science disciplines. This proxy measures the thermodynamically controlled bonding of heavy rare isotopes ¹³C and ¹⁸O within the same CO₂ molecule (mass 47) derived from carbonate acid digestion. Unlike the widely used Mg/Ca thermometer, △47 thermometry is independent of seawater chemistry changes, including past pH, dissolved inorganic carbon, and carbonate saturation states, although it typically requires larger sample sizes and yields lower precision.

Here, we report on the establishment of a small-sample carbonate △47 measurement method using a Thermo Fisher Kiel IV coupled to a MAT253Plus mass spectrometer at the State Key Laboratory of Marine Geology, Tongji University. Using an aliquot mass of ~120 μg, we achieved a reproducibility for the IAEA-603 standard (n = 35) of 0.04‰ in △47, 0.02‰ in ¹³C, and 0.05‰ in ¹⁸O (1 SD). We applied this method to planktonic foraminifera G. sacculifer from ODP Site 762 in the eastern Indian Ocean. The measured △47 values were 0.666‰ (n = 17, 1 SE = 0.009‰) for the 5.95 Ma interval and 0.673‰ (n = 35, 1 SE = 0.005‰) for the 3.95 Ma interval. Using the recalculated Kele et al. (2015) calibration (Bernasconi et al., 2018), we reconstructed SSTs of 26.8±6.0°C at 5.95 Ma and 24.7±3.4°C at 3.95 Ma. For comparison, temperatures reconstructed using Mg/Ca analysis were 25.9°C at 5.95 Ma and 23.5°C at 3.95 Ma, based on the calibration by Anand et al. (2003). The results demonstrate strong consistency between the two proxies, validating the utility of △47 for paleotemperature reconstruction even when temperature variations are subtle.

Keywords: Clumped isotope (△47), Mg/Ca thermometry, Temperature reconstruction, Foraminifera