Investigating the origin of discrepancies in clumped isotope calibrations

Abstract:

The abundance of ¹³C-¹⁸O ‘clumps’ in calcite or aragonite of corals skeletons are a potentially valuable tool for reconstructing past ocean temperatures. However, corals are known to exhibit significant “vital effects” (i.e., non-equilibrium mineral compositions) in δ¹⁸O, which complicates its application in paleoclimate studies, and may also exhibit clumped isotope disequilibrium. Here we determined mass 47 anomalies (Δ₄₇) in CO₂ derived from cultured shallow water and live-collected deep-sea coral.

In a species of cultured surface water coral, we find disequilibrium Δ₄₇ and δ¹⁸O values that are consistent with a pH effect driving disequilibrium isotopic signatures. We go on to show that culturing specimens at elevated CO₂ conditions drives changes in both Δ₄₇ and δ¹⁸O that follows the same relationship defined for pH effects in inorganic carbonate precipitation experiments. This suggests that dissolved inorganic carbon speciation at the site of calcification and therefore fluid pH can effect the clumped isotope composition of biogenic minerals. In two different live-collected deep-sea coral taxa, we find distinct clumped isotope signatures and Δ₄₇-temperature calibration relationships.