Tuesday, September 15, 2009

High-resolution carbon isotope changes in the Permian-Triassic boundary interval, Chongqing, South China; implications for control and growth of earli


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High-resolution [delta].sup.13C.sub.CARB analysis of the Permian-Triassic boundary (PTB) interval at the Laolongdong section, Beibei, near the city of Chongqing, south China, encompasses the latest Permian and earliest Triassic major facies changes in the South China Block (SCB). Microbialites form a distinctive unit in the lowermost 190cm above the top of the Changhsing Formation (latest Permian) at Laolongdong, comparable to a range of earliest Triassic sites in low latitudes in the Tethyan area. The data show that declining values of [delta].sup.13C.sub.CARB, well-known globally, began at the base of the microbialite. High positive values (+3 to 4ppt) of [delta].sup.13C.sub.CARB in the Late Permian are interpreted to indicate storage of.sup.12C in the deep waters of a stratified ocean, that was released during ocean overturn in the earliest Triassic, contributing to the distinctive fall in isotope values; this interpretation has been stated by other authors and is followed here. The [delta].sup.13C.sub.CARB curve shows fluctuations within the microbialite unit, which are not reflected in the microbialite structure. Comparisons between microbialite branches and adjacent micritic sediment show little difference in [delta].sup.13C.sub.CARB, demonstrating that the microbialite grew in equilibrium with surrounding seawater. The Early Triassic microbialites are interpreted to be a response to upwelling of bicarbonate-rich poorly oxygenated water in low latitudes of Tethys Ocean, consistent with current ocean models for the PTB interval. However, the decline of [delta].sup.13C.sub.CARB may be due to a combination of processes, including productivity collapse resulting from mass extinction, return of deep water to ocean surface, oxidation of methane released from methane hydrate destabilisation, and atmospheric deterioration. Nevertheless, build-up of bicarbonate-rich anoxic deep waters may be expected as a result of the partial isolation of Tethys, due to continental geography; release of bicarbonate-rich deep water, by ocean upwelling, in the earliest Triassic may have been an inevitable consequence of this combination of circumstances.

Author Affiliation:

(a) State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, CAS, 39 East Beijing Road, Nanjing 210008, China

(b) Institute for the Environment, Brunel University, Uxbridge, Middlesex UB8 3PH, UK

(c) Laboratory of Micropalaeontology, Nanjing Institute of Geology and Palaeontology, CAS, 39 East Beijing Road, Nanjing 210008, China

(d) CASP, Department of Earth Sciences, University of Cambridge, Gravel Hill, 181a Huntingdon Road, Cambridge CB3 0DH, UK

(e) Experimental Techniques Centre, Brunel University, Uxbridge, Middlesex UB8 3PH, UK

Article History:

Received 26 January 2007; Revised 6 August 2007; Accepted 29 August 2007



Source Citation:Mu, Xinan, Steve Kershaw, Yue Li, Li Guo, Yuping Qi, and Alan Reynolds. "High-resolution carbon isotope changes in the Permian-Triassic boundary interval, Chongqing, South China; implications for control and growth of earliest Triassic microbialites.(Report)." Journal of Asian Earth Sciences 36.6 (Nov 2, 2009): 434(8). Academic OneFile. Gale. Alachua County Library District. 15 Sept. 2009
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