Accelerated hydrological cycle during the Toarcian oceanic anoxic event: Biomarker evidence in the Qiangtang Basin, eastern Tethys

The early Toarcian oceanic anoxic event (T-OAE) is characterized by major changes in the marine and terrestrial environments, which is possibly related to the accelerated hydrological cycle. However, the impact of hydro-logical cycling on continental vegetation is poorly constrained. Here, we present biomarker evidence that sug-gests an accelerated hydrological cycle during the Toarcian carbon-isotope excursion (T-CIE), leading to enhanced transport of land plant organic matter to shelf seas. The C27/C29 sterane ratios exhibit a general decreasing trend across the T-OAE, which is accompanied by increasing C30 17 alpha, 21 beta-moretane/C30 17 alpha, 21 beta-hopane (M/H) ratios and terrestrial aquatic ratios (TAR) suggesting an enhanced terrigenous input. The T-OAE interval record a decline in shelf sea oxygenation, which is indicated by Pr/Ph ratios and the enhanced pres-ervation of C35 homohopanes. Decline in shelf sea oxygenation is most likely promoted by thermal and fresh-water stratification. The higher gammacerane/C30 17 alpha, 21 beta-hopane (G/H) ratios occurring at the beginning of the T-OAE indicate an intensified water-column stratification. The sterane/ C30 17 alpha, 21 beta-hopane (ST/C30H) ratios also show a sharp decrease at the beginning of the T-OAE supporting the transformation of the biological community, and/or an increase in soil organic matter. The increased input of terrigenous materials, enhanced freshwater input, and abrupt change in the biological community across the T-OAE may be attributed to an accelerated hydrological cycle associated with global climatic and environmental changes during the T-OAE.