Lacustrine organic carbon sequestration driven by volcanism: A case study of the third submember of the Chang 7 Member of the Yanchang Formation in the Ordos Basin

The Ordos Basin was recognized as the earliest terrestrial aquatic ecosystem to recover following Permian–Triassic mass extinction, significantly contributing to organic carbon sequestration during the early Mesozoic era. Volcanic activity has increased the organic carbon burial capacity of the third submember of Ch7 (Ch7 3 ) within this basin, although it has both positive and negative effects on organic carbon burial. In this study, we quantitatively characterized the organic carbon burial process by calculating the organic carbon accumulation rate (OCAR) and proposed an optimal sedimentary model influenced by volcanic activity. We conducted time series analysis on gamma ray (GR) data from Ch7 3 to determine sedimentation rates (SRs) while measuring the density of each sample via hydrostatic methods. By integrating these measurements with the total organic carbon (TOC) content, we established a dynamic OCAR for Ch7 3 ( =0.68 g/cm 2 ·kyr) and estimated that it sequestered 0.27 Tt of organic carbon. Our findings indicate that the OCAR under eunixic conditions ( =1.02 g/cm 2 ·kyr) is 2.49 times greater than that under ferruginous conditions ( =0.41 g/cm 2 ·kyr). The geochemical identification fingerprints of volcanism suggest that the top of Ch7 3 is influenced by volcanoes of appropriate intensity. In this sedimentary model, the dilution of organic matter (OM) by rapidly deposited volcanic ash is relatively low. Additionally, the cumulative effect of chemical weathering intensity due to volcanic activity leads to the input of nutrients from continental sources into the lake basin. This process promotes increased productivity, resulting in a significant increase in the OCAR ( =0.76 g/cm 2 ·kyr). This study provides new insights for dynamically assessing the impact of geological events on the OCAR.