LARGE SHALLOW-TO-DEEP D13C GRADIENTS IN EARLY TRIASSIC SEAS OF THE SOUTH CHINA CRATON: EVIDENCE FOR EXTREME WATER-COLUMN STRATIFICATION

Vertical gradients in water-column δ¹³C_DIC can be estimated for paleomarine systems based on δ¹³C_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 shallow-to-deep gradients (Δδ¹³C_carb) for Early Triassic seas of the South China Block. Δδ¹³C_carb was calculated 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 δ¹³C_DIC gradients in the modern ocean (~1-3‰) because of Early Triassic warming, strengthening water-column stratification and reducing vertical mixing, and enhanced marine productivity, intensifying the biological pump. Peaks in Δδ¹³C_carb during the earliest Griesbachian (N1) and late Dienerian to mid-Smithian (P2-N3) coincided with episodes of expanded ocean anoxia. The second episode was marked by an initial positive δ¹³C_carb excursion during the late Dienerian (P2) due to the stripping from ocean-surface waters of ¹³C, followed by a negative δ¹³C_carb excursion during the early to mid-Smithian (N3) recording a shallowing of the chemocline. The Dienerian-Smithian transition thus marks a second major interval of climatic warming (after that at the Permian-Triassic boundary) commencing ~1 Myr after the latest Permian mass extinction, suggesting that the delayed recovery of marine ecosystems may have been due largely to a renewal of strong environmental stresses during the mid-Early Triassic.