THE 13C SUESS EFFECT EXPLAINS ELEVATED CARBON ISOTOPE RATIOS OF SEDIMENT ON MODERN CARBONATE PLATFORMS

The measured carbon isotopic compositions of carbonate sediments (δ¹³C_carb) on modern platforms are commonly more ¹³C-enriched than values predicted 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. Several explanations have been proposed to explain this offset, including the hypothesis that diurnal changes in seawater carbonate chemistry and δ¹³C_DIC values result in precipitated carbonate that is substantially more ¹³C-enriched than expected for daily average δ¹³C_DIC values. However, “modern” seawater and “modern” carbonate sediments represent inherently different timescales of observation. Previous sequential radiocarbon dating has shown that ooid sediment from locations across the Lucayan Archipelago have been accumulating for >1000 years. This duration is important because the δ¹³C values of atmospheric CO₂ and shallow seawater DIC have changed substantially over the past 200 years due to anthropogenic carbon release (the ¹³C Suess effect). We hypothesize that the ¹³C Suess effect is responsible for the apparent offset between predicted and observed δ¹³C_carb values on modern carbonate platforms. To test this hypothesis, we collected a suite of seawater chemistry samples on Caicos platform to compare the magnitude of diurnal change in carbonate chemistry and δ¹³C_DIC values with previous measurements of δ¹³C_carb values in Caicos platform sediment. We observed changes in aragonite saturation state and δ¹³C_DIC consistent with the diurnal engine effect, but the δ¹³C_carb value predicted considering these diurnal changes was still ca. 1.2‰ less than the observed δ¹³C_carb value of modern platform sediment. In contrast, our measurements are consistent with previously published measurements of the δ¹³C_carb values of the young, outermost cortices of ooids from Caicos platform. These data demonstrate that δ¹³C_carb values on modern platforms are broadly representative of seawater, but only after accounting for 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.