Terrestrial organic carbon isotopic composition (δ~(13)C_(org)) and environmental perturbations linked to Early Jurassic volcanism: Evidence from the Qinghai-Tibet Plateau of China

摘要

The Early Jurassic break-up of Pangea and the formation of the Central Atlantic Ocean are associated with changes to the atmosphere, climate, and environment, as recorded in marine carbonates from the western Tethys Ocean. However, the expression of these global changes in terrestrial successions is less well known. To better understand terrestrial-response, we investigated organic carbon isotope composition (delta C-13(org)), weathering trends and geochemistry in Lower Jurassic lacustrine strata in the Qaidam Basin, China, on the northern margin of the eastern Tethys Ocean. Four negative delta C-13(org) carbon isotope excursions are identified in lacustrine black shales during the Pliensbachian to early Toarcian stages. These negative excursions correspond to the early Pliensba-chian Warming Interval, the Subnodonus Negative Event at the beginning of the late Pliensbachian, the Pliensbachian-Toarcian Boundary Event, and the early Toarcian Oceanic Anoxia Event (T-OAE), respectively, well documented in western Tethys Ocean successions. In our terrestrial successions from the Qaidam Basin, the first three events occurred under relatively warm and humid climates associated with enhanced weathering, lake transgression, and the prevalence of swamp environments. In contrast, the T-OAE was a period of intense warming and arid climate, expressed by the deposition of red beds, a hiatus in coal deposition, a sharp reduction in plant diversity, and a floristic turnover in palynoflora. Our findings show that each negative excursion is associated with peaks in mercury and nickel concentrations, supporting the hypothesis that magma intrusion into organic-rich rocks and accompanying the volcanic eruption was responsible.