Sedimentary process control on carbon isotope composition of sedimentary organic matter in an ancient shallow‐water shelf succession

Abstract

Source and delivery mechanisms of organic matter are rarely considered when interpreting changingδ13C through sedimentary successions even though isotope excursions are widely used to identify and correlate global perturbations in the carbon cycle. Combining detailed sedimentology and geochemistry we demonstrate how organic carbon abundance andδ13C values from sedimentary organic matter from Carboniferous‐aged mudstones are influenced by the proportion of terrestrial versus water column‐derived organic matter. Silt‐bearing clay‐rich shelf mudstones that were deposited by erosive density flows are characterized by 1.8–2.4% organic carbon and highδ13C values (averaging −22.9 ± 0.3‰, n = 12). Typically these mudstones contain significant volumes of terrestrial plant‐derived material. In contrast, clay‐rich lenticular mudstones, with a marine macrofauna, are the products of the transport of mud fragments, eroded from pre‐existing water‐rich shelfal muds, when shorelines were distant and biological productivity in the water column was high. Higher organic carbon (2.1–5.2%) and lowerδ13C values (averaging −24.3 ± 0.5‰, n = 11) characterize these mudstones and are interpreted to reflect a greater contribution by (isotopically more negative) amorphous organic matter derived from marine algae. Differences inδ13C between terrestrial and marine organic matter allow the changing proportions from different sources to be tracked through this succession. Combiningδ13C values with zirconium (measured from whole rock), here used as a proxy for detrital silt input, provides a novel approach to distinguishing mudstone provenance and ultimately usingδ13C to identify oil‐prone organic matter in potential source rocks. These results have important implications for using bulk organic matter to identify and characterize global C‐isotope excursions.