Continental weathering and palaeoclimatic changes through the onset of the Early Toarcian oceanic anoxic event in the Qiangtang Basin, eastern Tethys

Abstract

The Bilong Co North section, located in the South Qiangtang Depression, northern Tibetan Plateau, provides key information on continental weathering and palaeoclimatic changes through the onset of the Early Toarcian oceanic anoxic event (T-OAE) in the eastern Tethys. New mineralogical and geochemical data from the Bilong Co North section suggest that the Lower Unit of the section corresponds to low chemical index of alteration (CIA, 50–57) and ln(Al2O3/Na2O) (0.88–1.16) and high index of compositional variability (ICV, 19–27), indicating a weak weathering and arid climate preceding the onset of the T-OAE. A prominent increase in CIA and ln(Al2O3/Na2O) values is clearly recorded through the onset of the T-OAE, coinciding with a prominent change in Srxs/Ba and clay fractions, suggesting a sharp increase in humidity at the onset of the T-OAE. The palaeoclimatic index of low Srxs/Ba (0.48–4.98) ratios indicates that the climate during the T-OAE interval was most likely semiarid to semimoist. Some redox proxies, including Mo and TOC relationships, Mo-U enrichment factors, and Corg:Ptotal ratios suggest euxinic conditions during the T-OAE interval. The primary productivity parameters show two main fluctuations within the Bilong Co North section, and their patterns are in good agreement with the observation of palaeoclimatic variation patterns, suggesting an influence of palaeoclimate on primary productivity. However, the palaeoproductivity increase during the T-OAE in the Bilong Co North section should mainly be attributed to freshwater inputs rather than solely the result of paleoclimatic change.