Individual Microfossil δ13C Shows That δ13Corg Excursions in the Neoproterozoic Chuar Group Do Not Reflect the Exogenic Carbon Cycle

Abstract

Changes in δ¹³C value of bulk sedimentary organic matter (OM) throughout Earth's history are thought to reflect carbon cycle perturbations, but as sedimentary OM may derive from multiple sources, it could also record other processes. We measured δ¹³C of microscale components of shale OM using nano-EA-IRMS to investigate drivers of large-magnitude carbon isotope excursions (CIE) in the late Tonian Chuar Group, USA. Components included organic-walled microfossils, kerogen, graphite, and macerate size-fractions. Microfossils δ¹³C has a broad range within samples, but average values vary little throughout stratigraphy and are decoupled from bulk δ¹³C_org, showing that these positive CIEs are not driven by secular changes in the carbon cycle. Instead, our fine-scale approach identified enriched components that can account for the CIE: exogenous clasts of kerogen and graphite, a finer macerate fraction, and abundant Eosynechococcus—a bloom-forming phytoplankter. The presence of these ¹³C-enriched particles indicates that the positive CIE signals were driven by a combination of allochthonous input/enhanced productivity, as well as thermal alteration. Fine-scale measurements can tease apart contributors to bulk δ¹³C_org records and offer insights into the Proterozoic carbon cycle.