A STRONG CARBON ISOTOPE GRADIENT IN THE DEVONIAN-CARBONIFEROUS BOUNDARY: A NEW PERSPECTIVE ON CARBONATE CARBON ISOTOPE

The canonical model of long term carbon cycle proposes that carbonate carbon (δ¹³C_carb) and organic carbon (δ¹³C_org) isotopes are coupled, and excursion of δ¹³C_carb that is believed to record the isotopic composition of dissolved inorganic carbon (DIC) in seawater (δ¹³C_sw) is driven by the change of organic carbon burial. However, decoupling of δ¹³C_carb and δ¹³C_org as well as δ¹³C_carb gradient is commonly reported from rock records, particularly in Proterozoic samples. These isotopic anomalies have been attributed to the oxidation of dissolved organic carbon (DOC) that might be enriched in anoxic deep oceans. It is proposed that the deep ocean has not been completely oxidized until mid-Paleozoic, when atmospheric O₂ content reached or exceeded the modern level. Thus, the marine carbon cycle after the early Paleozoic may resemble that of the modern ocean, in which δ¹³C_sw is more or less homogeneous. Here, we report δ¹³C_carb and δ¹³C_org data from four sections across the Devonian-Carboniferous boundary in South China. We observe a 4‰ gradient in δ¹³C_carb, while δ¹³C_org shows little variation among the four sections. Petrographic study indicates that carbonate was mainly derived from benthic carbonate-secreting organisms, such as brachiopods, corals, and echinoderms, suggesting that δ¹³C_carb recorded the isotopic composition near the water-sediment interface (WSI). In contrast, organic matter was mainly produced in the euphotic zone with homogeneous δ¹³C_sw. Therefore, the isotopic decoupling may be attributed to the different sources of organic and carbonate carbon. In addition, there is a negative correlation between δ¹³C_carb and Fe content in carbonate, and carbonate samples from the deeper environment have lower δ¹³C_carb but higher Fe content. We suggest that organic matter degradation near or at WSI provides ¹³C-depleted DIC and consumes seawater O₂, which in turn leads to a shallower depth of redox boundary in the sediment and thus higher benthic flux of Fe (II) from porewater. Therefore, δ¹³C_carb gradient may be attributed to various degree of organic matter degradation in seafloor, which is environment dependent. Finally, our study also suggests that δ¹³C_carb may not necessarily record δ¹³C_sw, and interpretation of carbon isotope data should consider the processes of carbonate formation.