Carbon isotope records in a Mesoproterozoic epicratonic sea: Carbon cycling in a low-oxygen world

Carbon isotope profiles of sedimentary strata are widely used as both a tool for stratigraphic correlation and as a mechanism for inferring important changes in Earth's biogeochemical cycles. Chemostratigraphic interpretations, however, often rely on the assumption that the isotopic composition of dissolved inorganic carbon (DIC) in the surface oceans is broadly homogeneous. At times of globally high sea level, when epicratonic (epeiric) environments dominate our sedimentary record, carbon isotope records reveal substantial lateral variability. Here we investigate lateral variability in the marine carbon isotope record from pericratonic to epicratonic environments of the Mesoproterozoic (∼1.1 Ga) Atar/El Mreiti Groups of the Taoudeni Basin, West Africa. Restricted-marine epicratonic environments are consistently 2–4‰ lighter than open-marine pericratonic environments, suggesting input of an isotopically light carbon source that preferentially affected epicratonic environments. In contrast to epicratonic environments in the Paleozoic, where input of isotopically light carbon is generally attributed to input of terrestrial organic matter, we suggest that in situ remineralization of organic carbon via anaerobic microbial cycling drove observed isotopic variability. Within epicratonic El Mreiti Group strata, the extent of organic matter remineralization (and thus degree of 13C depletion) is correlated with water depth, and associated with distinct differences in both total organic carbon (TOC) and pyrite concentration, suggesting a potential linkage to both the persistence of anoxia and the availability of sulfate within epicratonic environments. In such settings, the isotopic effects of organic carbon remineralization are potentially enhanced by either methane oxidation following methanogenic decomposition, or the intermittent oxidation of an enhanced dissolved organic carbon (DOC) pool.

► We examine geochemical trends across Mesoproterozoic strata of Mauritania.
► Epicratonic strata show lower δ13C values than pericratonic and open shelf strata.
► Isotopic depletion is accompanied by Fe and Mn enrichment in carbonate.
► Organic C remineralization under low O2 conditions drove isotopic depletion.
► Surface oceans were chemically heterogeneous in late Mesoproterozoic time.