Sedimentological characterisation is a necessary prerequisite for interpretation of stable carbon isotope ratios of bulk sediment from reefal settings

ABSTRACT For most of Earth history, carbonate sediments deposited in shallow marine settings are the only remaining archive of changing biogeochemical cycles. All deep‐sea sediments older than >200 Ma have been subducted and are therefore lost from the record. Stable carbon isotope (δ 13 C) values of shallow marine carbonates are available for older time periods but are complicated to interpret because bulk sediments are comprised of a variety of grain types. The δ 13 C values of each grain type are impacted by the cumulative effects of local environmental conditions, mineralogy and vital effects, yet the effect of sediment allochem diversity on bulk carbonate δ 13 C values in shallow marine settings has yet to be quantified. This study aimed to determine whether allochem diversity exerts a predictable influence on the bulk carbonate δ 13 C values of modern surface sediments collected from eight atolls in the Pacific, Indian and Atlantic Ocean basins. Richness, evenness, Shannon–Wiener and Gini–Simpson Indices of allochem diversity were calculated, and correlations with bulk carbonate δ 13 C values were assessed. Bulk carbonate δ 13 C values of modern reefal sediments varied by ~5‰, on par with some of the largest perturbations to the Phanerozoic global carbon cycle. New results show that carbonate δ 13 C values were consistently influenced by changes in allochem diversity indices and the abundance of specific allochems, such as Halimeda and coral fragments. However, the effects of allochem diversity were not predictable across the studied atolls, reinforcing the need for careful sedimentological characterisation in each depositional setting prior to interpreting variations in bulk carbonate δ 13 C values as dynamic changes in global carbon cycling.