Environmental and oceanographic evolution in the Southern Qiangtang Basin (eastern Tethys) during the latest Pliensbachian to early Toarcian (Early Jurassic)

Abstract

The late Pliensbachian-early Toarcian (Early Jurassic) interval is marked by significant climatic changes that affected environmental and paleoceanographic conditions, impacting biogeochemical cycles, as well as marine and terrestrial ecosystems. The most profound environmental changes occurred across the Pliensbachian/Toarcian boundary (P/T B) and in the early Toarcian. Both events align with phases of increased greenhouse gas emissions, global warming, sea-level rises, intensified hydrological cycles, and continental weathering. The Toarcian coincides with the expansion of oxygen-deficient conditions in oceanic and at various shelf areas, marking the Toarcian Oceanic Anoxic Event (T-OAE). In this study, we present high-resolution inorganic geochemical data from the Qixiangcuo section of the Southern Qiangtang Basin, which records both the P/T B and early Toarcian events. The section studied reflects a shallow shelf environment in northeastern part of the Tethys Ocean. Element ratios, such as Ti/Al and Zr/Al, interpreted as sediment grain size proxies, reveal characteristic changes that can be linked to the interplay between sea-level changes and shifts in continental weathering and terrigenous sediment influx. Secular trends in these grain-size proxies reflect sea-level dynamics during the late Pliensbachian-early Toarcian, with notable transgressions at the P/T B and early Toarcian. The early Toarcian event further coincides with increased terrigenous material in response to an intensified hydrological cycle, as evidenced by transient increases in grain-size proxies and higher paleo-weathering indices (e.g., ln(Al₂O₃/Na₂O)). Despite the increased nutrient flux to the paleo-shelf sea, enrichment factors (EF) of productivity proxies (Zn_EF, Cu_EF, Ba_EF, Ni_EF) do not show increases in organic matter export to the sediments. Thus, organic matter-lean sediments (total organic carbon <0.5 wt%) have accumulated in the Southern Qiangtang Basin during the latest Pliensbachian-early Toarcian. The lack of enrichment in redox-sensitive elements, such as Mo, V, and U in the sedimentary rocks of the Qixiangcuo section, points to a well‑oxygenated environment. Accordingly, although the T-CIE is identified in the study section, the T-OAE did not develop in this paleogeographic area. The new data thereby highlight the role of local-regional paleoenvironmental and paleoceanographic parameters as important factors contributing to the genesis of the T-OAE and the burial of organic carbon in marine sediments.