Revisiting Elevated δ13C Values of Sediment on Modern Carbonate Platforms

The measured carbon isotopic compositions of carbonate sediments (δ¹³C_carb) on modern platforms are commonly ¹³C-enriched compared to predicted values for minerals forming in isotopic equilibrium with the dissolved inorganic carbon (DIC) of modern seawater. This offset undermines the assumption that δ¹³C_carb values of analogous facies in the rock record are an accurate archive of information about Earth's global carbon cycle. We present a new data set of the diurnal variation in carbonate chemistry and seawater δ¹³C_DIC values on a modern carbonate platform. These data demonstrate that δ¹³C_carb values on modern platforms are broadly representative of seawater, but only after accounting for the recent decrease in the δ¹³C value of atmospheric CO₂ and shallow seawater DIC due to anthropogenic carbon release, a phenomenon commonly referred to as the ¹³C Suess effect. These findings highlight an important, yet overlooked, aspect of some modern carbonate systems, which must inform their use as ancient analogs.