The impact of volcanic activity on the deposition of organic-rich shales: Evidence from carbon isotope and geochemical compositions

The organic-rich black shales from the Late Ordovician and early Silurian were identified as essential source rocks for shale gas exploration and development in South China. To investigate the influence of volcanic activity on the deposition of organic-rich shales in the Wufeng and Longmaxi formations, this study presents the carbon isotope and geochemical compositions of the Upper Ordovician-lower Silurian succession of the Yangtze Platform. The organic-rich Wufeng and lower Longmaxi shales are characterized by isotopically negative carbon shifts; however, obviously high δ13Corg values are shown in the organic-poor shales of the upper Longmaxi Formation. The negative δ13Corg excursion is attributed to 12C-depleted CO2 sourced from volcanic activity. Furthermore, the positive δ13Corg excursion in the upper Longmaxi Formation occurs because a large amount of 12C was removed from the ocean-atmosphere system through continuous burial of organic carbon. The biogenic silica (Sibio) contents and (Cu + Ni)/Al ratios in the lower Longmaxi Formation display higher values than those in the upper Longmaxi Formation, suggesting that the primary productivity gradually decreased from the Wufeng Formation to the Longmaxi Formation. This study proposes that high primary productivity was related to accelerated chemical weathering and release of micro-nutrients. The former emphasizes that high pCO2 and globally high temperatures can enhance chemical weathering, so that more nutrients are transported from continents to oceans (Yan et al., 2010, 2019). The latter highlights that volcanic material input can act as fertilizer by releasing many nutrients. Moreover, the values of UEF and MoEF are higher in the Wufeng and lower Longmaxi formations than in the upper Longmaxi Formation, suggesting that the oxygen concentration increased upward through the section. Given the positive correlations between indicators of productivity and redox conditions, the constantly high primary productivity would consume much O2, resulting in anoxic conditions in the Wufeng and lower Longmaxi formations. Overall, this study considers that volcanic activity-driven primary productivity is the first-order control for organic matter (OM) enrichment.