Strontium isotope evidence for regional enhanced continental weathering during the early Toarcian in the Tethys Himalaya

The early Toarcian of the Early Jurassic was characterized by an interval of global warming, enhanced continental weathering, and an oceanic anoxic event (T-OAE, ∼183 Ma). However, the changes in continental weathering intensity in low latitudes still remain poorly constrained. Here, we present strontium isotope (87Sr/86Sr) data of the Pliensbachian–Toarcian (Pl–To) interval from the Tibetan Himalaya, which was formerly located in the tropical/subtropical zone of the southeastern Tethys. An abrupt positive shift in 87Sr/86Sr starting around the Pl–To boundary and continuing during the interval of the negative carbon-isotope excursion (CIE) suggests enhanced continental weathering and increased terrigenous influx. Stratigraphically higher, the 87Sr/86Sr ratios gradually decrease during the recovery phase of the T-OAE CIE, suggesting a decline in continental weathering. Notably, the absolute values and positive-recovery pattern of 87Sr/86Sr data from bulk carbonate (pure micrite) of this study are not consistent with that from belemnites and brachiopods of Europe. Based on strong relationship between 87Sr/86Sr and diagenetic and silicate-derived Sr proxies, the absolute 87Sr/86Sr values here could have been affected to some extent by diagenesis and regional terrestrial input, and thus greatly amplified relative to those of Europe. However, the strontium-isotope positive-recovery pattern broadly corresponds with the pattern and interpretation of other weathering proxies (δ44/40Ca and 187Os/188Os) during the T-OAE. These observations suggest that the 87Sr/86Sr trend still likely reflects the real regional signal but was superimposed by global influence, although the data were biased by diagenesis. Taken together, the 87Sr/86Sr records from Tibetan Himalaya probably provide a useful insight into the regional weathering response, superimposed by the global weathering signal, to the events of the early Toarcian.