Organic-matter source variation and the expression of a late Middle Ordovician carbon isotope excursion

The carbon isotopic composition of organic matter is a powerful tool in the study of organic carbon burial and pCO₂ through the geologic record. However, the organic-matter source can affect total organic carbon δ¹³C values and may complicate interpretation of environmental conditions. We examined the influence of organic-matter source variations on a late Middle Ordovician positive δ¹³C excursion in strata of the United States midcontinent. In a core from Iowa, the magnitude of the excursion recorded by carbonate carbon is ∼2.5‰, whereas the excursion recorded by total organic carbon is nearly 8‰. Although this difference aould arise from a profound decrease in surface-water CO₂ concentrations, association of the organic-walled micro-fossil Gloeocapsomorpha prisca with ¹³C-enriched total organic carbon (TOC) in this interval suggests that the excursion could also record a shift from normal-marine to G. prisca-dominated organic matter. Carbon isotope analyses of individual biomarkers indicate that both of these explanations are required to explain the data. Compuonds inferred to be of G. Prisca origin are enriched in ¹³C by 7‰ relative to compounds derived from other algal sources. Furthermore, compounds exclusively from either G. Prisca or other sources exhibit a positive shift of only 3.5‰. Thus, the large excursion recorded by TOC reflects, in part, a change in the proportional contribution of G. prisca to TOC. That all compounds exhibit a posotive carbon isotopic shift indicates that, independent of contributions from G. prisca, a positive organic δ¹³C excursion occurred and records a change in environmental conditions such as CO_2(aq) concentrations. The 3.5‰ shift is similar in magnitude to that observed for TOC in correlative units from Pennsylvania, suggesting that this event has broad geographic significance. This work illustrates that organic-matter source variations as well as environmental conditions govern the TOC δ¹³C values and shows how compound-specific isotope analyses can resolve these multiple controls.