High-resolution volcanism-induced oceanic environmental change and its impact on organic matter accumulation in the Late Ordovician Upper Yangtze Sea

Volcanism is one of the most important geological processes that impact Earth's surface system. However, little is known about how a volcanic eruption in the past could cause environmental changes in the sea and its impact on organic matter (OM) accumulation due to the difficulties in gaining high-resolution sedimentary records. Here we conducted a high-resolution petrographic study and geochemical analyses including proxies reflecting the influx of volcanic and terrestrial material (Hg, Al, Fe, CIA), redox conditions (Mo, FeHR/FeT, FePy/FeHR), paleoproductivity (Baxs, Cuxs, Sixs), and biological variations (δ13Corg, δ15Norg) on a 25-cm long drilling core bracketing three cycles of volcanic activities from the Upper Ordovician Wufeng Formation in South China. This enables us to find, for the first time, the detailed process of environmental changes and different stages of OM accumulation induced by volcanism. Three nearly identical cycles have been identified, and each of them is comprised of a bentonite bed, a siliceous clay-rich mudstone interval, a radiolarian bed, and a siliceous mudstone interval with intercalations of thin pyrite. The marine redox condition shows the strongest anoxia and even euxinia on the eruption stage and began to decrease with the waning of the volcanic activities shown by Hg/TOC. The volcanic eruption will cause the microbial change that is likely reflecting the shift from nitrogen-fixation bacteria bloom to radiolarian bloom through the variation of bioavailable nitrate and some trace metals such as Cu. Of significance is that the volcanism-induced shift in microbial communities from prokaryotes to eukaryotes bloom is observed to impact the abundance of the sedimentary OM, a critical process leading to the wide occurrence of abundant oil and gas reservoirs in south China.