206 / 2022-02-28 22:13:09

龙马溪组黑色页岩沉积的全球性和区域性控制因素研究

Lungmachi black shales; large igneous provinces; global anoxia; regional paleogeography;

The Lungmachi black shales, famous for the great success of commercial shale gas exploitation, have been proposed to arise from regional paleogeography and/or volcanisms, and transgression after de-glaciation. However, recent studies indicated that their concurrent shales covered large areas and were widespread on the northern margin of Gondwana, and global anoxia was in global scale. To explore the controlling factors for their deposition, here we build the first high-resolution multi-proxy record of Hg (concentrations and isotopic compositions) combined with whole-rock geochemical data (including trace elements as oceanic paleo- productivity and redox proxies and P) from continuous cores in the Yangtze Platform, South China. Our data, combined with reported ones, confirmed late Ordovician LIP eruptions against localized volcanisms. Trace element data demonstrate high oceanic productivity and anoxic conditions, P records suggest P recycling on the vast shelves for eutrophication. We proposed that, global warmth by voluminous CO₂ input and vast pristine mafic volcanic rocks from the late Ordovician LIPs facilitated chemical weathering, increased nutrient supply to the ocean and thus enhanced oceanic productivity. Effectively enhanced productivity caused widespread anoxia and resultant deposition of black shales in epicontinental seas during the early Silurian. Intensified chemical weathering nourished the Rhuddanian epicontinental seas and sustained widespread anoxia, longer than Mesozoic OAEs. P recycling in the vast anoxic epicontinental seas were important in sustaining long-term high productivity in turn. Both chemical weathering and organic carbon burial transferred atmospheric carbon to the seafloor, leaving the atmosphere with lower pCO₂ yet heavier carbon isotopes. The paleogeography and volcanisms might also have acted as favorable factors, but second to the global ones. This study highlights the prevalent driving role of LIPs over regional factors in controlling the deposition of Lungmachi black shales and their concurrent black shales.