Volcanic activity drives lacustrine carbon sequestration after Oceanic Anoxic Event 1a

The Mesozoic was punctuated by relatively brief periods of anomalous warmth associated with pulses of volcanism, intensified weathering, expansive marine anoxia, and increased organic carbon burial. These are known as oceanic anoxic events (OAEs). Several OAEs have also been linked to potentially synchronous episodes of increased carbon burial in lacustrine settings, attributed to increased weathering-derived nutrient fluxes. Here we present the Early Aptian oil shale sequences of western Liaoning Province, north China, combining sediment petrology, zircon U-Pb geochronology, carbon isotope chemostratigraphy, as well inorganic and organic geochemical palaeoenvironmental proxies to assess possible causal links to OAE1a. New zircon 206Pb/238U ages (121.06 +/- 0.63 Ma, 119.72 +/- 0.40 Ma, and 119.9 +/- 0.65 Ma) for three tuffs and delta 13Corg chemostratigraphic correlation show that the western Liaoning Province oil shale interval formed shortly after widespread marine black shale deposition characteristic of OAE1a had ceased. Elevated concentrations of alginite (dinoflagellate-derived organic matter) in oil shale samples which contain micron-thick tuff layers suggest that arc volcanism in north China was the major factor driving increased lacustrine carbon burial, via volcanic ash fertilisation. This is supported by high values of Babio, C27/C29 alpha alpha alpha-20R steranes, C28/C29 alpha alpha alpha-20R steranes, 4-methylsterane index, dinosterane index, and tetramethylsterane index in these layers, which imply that the input of volcanic ash contributed to a flourishing of brackish/saline-water adapted dinoflagellates, leading to the formation of organic-rich tuff laminae. This is a novel mechanism that is distinct to the processes known to have contributed to enhanced marine organic matter burial on a global scale during OAE1a. This research not only contributes to the study of lacustrine source rocks in north China, but also provides insights into the influence of intermediate-acidic volcanic eruptions on organic carbon burial more generally.