Paper No. 3
Presentation Time: 8:30 AM

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


SONG, Haijun, State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan, Lumo Road, No 388, Wuhan, 430074, China, TONG, Jinnan, State Key Laboratory of Geological Processes, China University of Geosciences at Wuhan, Wuhan, 430074, China and ALGEO, Thomas J., Department of Geology, University of Cincinnati, Cincinnati, OH 45221-0013, haijun.song@aliyun.com

Vertical gradients in water-column δ13CDIC can be estimated for paleomarine systems based on δ13Ccarb 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 (Δδ13Ccarb) for Early Triassic seas of the South China Block. Δδ13Ccarb 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 δ13CDIC 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 Δδ13Ccarb 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 δ13Ccarb excursion during the late Dienerian (P2) due to the stripping from ocean-surface waters of 13C, followed by a negative δ13Ccarb 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.