MINERALOGICAL CONTROLS OF CLUMPED ISOTOPE COMPOSITIONS OF BIMINERALIC PEARL OYSTER SHELLS

Carbonate clumped isotope analysis is a powerful tool for reconstructing past ocean temperatures. This technique examines the paired carbon and oxygen isotope substitution of carbonate ion groups and can estimate mineral growth temperature without prior knowledge of the C or O isotope composition of ambient water. This study investigated the applicability of this technique to modern bimineralic (i.e., containing calcite and aragonite) pearl oysters (Pinctada radiata) collected from the Arabian Gulf. Specifically, focused on comparing clumped isotope apparent temperatures of aragonite and calcite layers of P. radiata shells, to each other and to records of environmental temperature obtained from nearby moored buoys. To identify the aragonite and calcite layers and accurately show seasonal change, samples were microsampled prior to analysis by acid digestion and IRMS of evolved CO₂. Clumped isotope analysis of aragonite and calcite samples yields a range of ∆₄₇ values from 0.52‰ to 0.61‰. TΔ₄₇ values derived from the calcite layer closely match the actual seasonal temperature variations, with only a ±1.5 - 4 °C relative difference, providing robust evidence that this layer faithfully and precisely records environmental temperature. Conversely, the clumped isotope composition of the aragonite layer does not accurately reflect the ambient temperatures and tends to overestimate them by approximately +12 °C. These findings highlight the complexity of biogenic carbonate mineral precipitation and emphasize the importance of considering the specific mineralogy when using biogenic carbonate clumped isotope analysis for temperature reconstructions. The differential temperature sensitivity between the aragonite and calcite layers suggests distinct biomineralization mechanisms and physiological processes. Further research is needed to explore the underlying factors responsible for this disparity and to elucidate the potential implications for paleoclimate reconstructions using clumped isotopes.