Revisiting Elevated δ 13 C Values of Sediment on Modern Carbonate Platforms

Abstract The measured carbon isotopic compositions of carbonate sediments (δ 13 C carb ) on modern platforms are commonly 13 C‐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 δ 13 C 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 δ 13 C DIC values on a modern carbonate platform. These data demonstrate that δ 13 C carb values on modern platforms are broadly representative of seawater, but only after accounting for the recent decrease in the δ 13 C value of atmospheric CO 2 and shallow seawater DIC due to anthropogenic carbon release, a phenomenon commonly referred to as the 13 C Suess effect. These findings highlight an important, yet overlooked, aspect of some modern carbonate systems, which must inform their use as ancient analogs.