Paper No. 217-4
Presentation Time: 9:00 AM
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 (δ13Ccarb) and organic carbon (δ13Corg) isotopes are coupled, and excursion of δ13Ccarb that is believed to record the isotopic composition of dissolved inorganic carbon (DIC) in seawater (δ13Csw) is driven by the change of organic carbon burial. However, decoupling of δ13Ccarb and δ13Corg as well as δ13Ccarb 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 δ13Csw is more or less homogeneous. Here, we report δ13Ccarb and δ13Corg data from four sections across the Devonian-Carboniferous boundary in South China. We observe a 4‰ gradient in δ13Ccarb, while δ13Corg 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 δ13Ccarb recorded the isotopic composition near the water-sediment interface (WSI). In contrast, organic matter was mainly produced in the euphotic zone with homogeneous δ13Csw. Therefore, the isotopic decoupling may be attributed to the different sources of organic and carbonate carbon. In addition, there is a negative correlation between δ13Ccarb and Fe content in carbonate, and carbonate samples from the deeper environment have lower δ13Ccarb 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, δ13Ccarb gradient may be attributed to various degree of organic matter degradation in seafloor, which is environment dependent. Finally, our study also suggests that δ13Ccarb may not necessarily record δ13Csw, and interpretation of carbon isotope data should consider the processes of carbonate formation.