Organic geochemistry of a lacustrine shale across the Toarcian Oceanic Anoxic Event (Early Jurassic) from NE China

摘要

Thirty-nine shale samples of the Lower Jurassic Beipiao Formation from drill core ZK01 in the Jinyang Basin, NE China were investigated by comprehensive organic geochemical methods in this study. A prominent negative excursion in delta C-13(org) values (similar to 2 parts per thousand) was observed in the Beipiao Formation, which is most likely associated with Toarcian Oceanic Anoxic Event (T-OAE) negative carbon isotope excursion (CIE) that is recorded globally in marine and terrestrial sediments constrained by previous studies. Elemental and Rock-Eval analyses of Beipiao shales demonstrate that the total organic carbon (TOC) ranges from 0.56 to 4.33 wt% (average of 1.40 wt%) and S-2 values range from 0.12 to 6.91 mg/g (average of 1.3 mg/g) and implicate fair to good source rock potential. A majority of the T-max values of the Beipiao shales range from 456 to 465 degrees C, indicating that the organic matter is within the mature oil window apart from few overmature samples with extreme high T-max values that might have been affected by local magmatic intrusions. Elemental analysis suggests that the kerogens are mainly Type II-III. Unlike many marine organic-rich sediments deposited during OAEs, redox proxies indicate no significant upper water column oxygen-depletion during the formation of Beipiao lacustrine shale. The decreasing upward trend of organic source indicators such as tricyclic terpanes/C-30 hopane values, relative distributions of regular C-27 steranes to C-29 steranes, and steranes/hopanes in the Beipiao Formation consistently suggest a shift to a lower abundance of eukaryotic organisms relative to bacterial community abundance during the onset of the T-OAE. Meanwhile, gradually decreasing values of gammacerane/C-30 hopane in the Beipiao Formation during the negative CIE suggest decreasing salinity and increased mixing of the Jinyang paleo-lake system, potentially corresponding with the shift toward an accelerated hydrological cycle with enhanced fresh water input to this lacustrine system. Moreover, increasing C-30 moretane/C-30 hopane values contemporaneous with the negative CIE also suggest enhanced land erosion during putative Toarcian climatic warming, which could have increased delivery of riverine nutrients and elevated regional primary productivity, eventually resulting in massive organic matter accumulation in the Beipiao shales.