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

Source and delivery mechanisms of organic matter are rarely considered when interpreting changing delta C-13 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 and delta C-13 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 high delta C-13 values (averaging -22.9 +/- 0.3 parts per thousand, 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 lower delta C-13 values (averaging -24.3 +/- 0.5 parts per thousand, 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 in delta C-13 between terrestrial and marine organic matter allow the changing proportions from different sources to be tracked through this succession. Combining delta C-13 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 using delta C-13 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.