Large vertical δ~(13)C_(DIC) gradients in Early Triassic seas of the South China craton: Implications for oceanographic changes related to Siberian Traps volcanism

摘要

Vertical gradients in the δ~(13)C of seawater dissolved inorganic carbon (Δδ~(13)C_(DIC)) can be estimated for paleomarine systems based on δ~(13)C_(carb) data from sections representing a range of depositional water depths. An analysis of eight Lower Triassic sections from the northern Yangtze Platform and Nanpanjiang Basin, representing water depths of -50 to 500 m, allowed reconstruction of Δδ~(13)C_(DIC) in Early Triassic seas of the South China craton for seven time slices representing four negative (N) and three positive (P) carbon-isotope excursions: 8.5‰ (N1), 5.8‰ (P1), 3.5‰ (N2), 6.5‰ (P2), 7.8‰ (N3), -1.9‰ (P3), and 2.2‰ (N4). These values are much larger than vertical Δδ~(13)C_(DIC) gradients in the modern ocean (～1-3‰) due to intensified stratification and reduced vertical mixing in Early Triassic seas. Peaks in Δδ~(13)C_(DIC) around the PTB (N1) and in the early to mid-Smithian (P2-N3) coincided with episodes of strong climatic warming, reduced marine productivity, and expanded ocean anoxia. The Dienerian-Smithian boundary marks the onset of a major mid-Early Triassic disturbance, commencing ～1 Myr after the latest Permian mass extinction, that we link to a second eruptive stage of the Siberian Traps. Inhospitable oceanic conditions generally persisted until the early Spathian, when strong climatic cooling caused re-invigoration of global-ocean circulation, leading to an interval of negative Δδ~(13)C_(DIC) values and a sharp increase in Δδ~(13)C_(carb) driven by upwelling of nutrient-rich deepwaters. These developments marked the end of the main eruptive stage of the Siberian Traps.