HR: 0800h
AN: PP11A-0240    [Abstracts]
TI: Chemostratigraphies of carbon, oxygen and strontium isotopes and oxygen contents across the Precambrian-Cambrian boundary
AU: * Komiya, T
EM: tkomiya@geo.titech.ac.jp
AF: Tokyo Institute of Technology, 2-12-1 Ookayama Meguro-ku, Tokyo, 152-8551, Japan
AU: Sawaki, Y
AU: Ishikawa, T
AB: The Precambrian-Cambrian (PC-C) boundary is one of the most important intervals for evolution of life. However, the scarcity of well-preserved outcrops through the boundary makes it ambiguous to decode change of the surface environment and biological evolution. In south China, strata through the PC-C boundary are continuously exposed and contain many fossils, suitable for study of environmental and biological change. In addition, we conducted excavations at four sites of Three Gorge area to obtain continuous and fresh samples. We measured the delta¹³C, delta¹⁸O and ⁸⁷Sr/⁸⁶Sr values of the drill core samples and REE compositions of fresh carbonate rocks, respectively.
We identified two positive and two negative isotope excursions of delta¹³C_carb within this interval: a moderate increase from 0 to +2 permil and a subsequent dramatic drop to -7 permil at the PC-C boundary, and a continuous increase to +5 permil at the upper part of the Nemakit-Daldynian (ND) stage and the subsequent sharp decrease to -9 permil just below the basal Tommotian unconformity, respectively. The continuous pattern of the delta¹³C shift is irrespective of lithotype and is comparable to fragmented records of other sections within and outside of the Yangtze Platform, indicating that the profile represents global change of seawater chemistry. A chemostratigraphy of ⁸⁷Sr/⁸⁶Sr ratios of the drilled samples also displays a smooth curve and its large positive anomaly just below the PC-C boundary. The estimate of oxygen content of seawater from REE composition of carbonate minerals shows significant decreases around PC-C and ND-Tommotian boundaries, respectively. The combination of chemostratigraphies of delta¹³C, ⁸⁷Sr/⁸⁶Sr and pO₂ indicates that the ⁸⁷Sr/⁸⁶Sr excursions preceded the delta¹³C negative excursion at PC-C boundary, and suggests that global regression or formation of the Gondwana supercontinent, evident in increase of influx of continental materials, caused biological depression together with decrease of oxygen content of seawater due to dissolution of methane hydrate.
DE: 0410 Biodiversity
DE: 0428 Carbon cycling (4806)
DE: 0454 Isotopic composition and chemistry (1041, 4870)
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 4851 Oxidation/reduction reactions (0471)
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2007 Fall Meeting