贵州乐康二叠/三叠系界线剖面的有机碳同位素异常及其意义

ANOMALIES OF ORGANIC CARBON ISOTOPES ACROSS THE P/T BOUNDARY AT THE LEKANG SECTION, GUIZHOU, SOUTH CHINA,AND THEIR SIGNIFICANCE

本文研究了贵州望谟乐康深水槽谷相二叠／三叠系界线剖面的有机碳同位素记录。研究结果表明，乐康剖面上二叠统δ13Corg值的变化幅度不大，越过第34层进入第35层以后，在WL－16号样品层位上，δ13Corg值突然从WL－15号样品的-25．419‰降低到-28．235‰，衰减幅度达到近3‰；其后δ13Corg值逐渐回升，并进入下三叠统。根据δ13Corg值的突然衰减正好发生在第34，35层的界线过渡层内这一事实，我们认为乐康剖面的P／T界线应划在第35层的WL－16号样品层位上。由于沉积有机物的碳同位素组成与海洋生物原始生产率（primaryproductivity）密切相关，因此，贵州乐康二叠／三叠系界线剖面的有机碳同位素变化曲线，不仅可以作为精确划分地层界线的一种新的有效手段，而且反映了海洋原始生产率的变化情况。

Organic carbon isotopic records in the P/T boundary sequence deposited in a deep watertrough facies at Lekang, Guizhou, South China, have been studied on the basis of the detail fieldinvestigation. The results show that the δ13C values of kerogens in the rocks at the uppermost ofthe Permian are rather close to each other except for sample WL-12 and that the δ13Corg values ex-hibit a sudden shift in the P/T boundary transitional zone from -25. 419‰(PDB) of sample WL-15 to a minimum va1ue of - 28. 235‰ of sample WL-16 and then back to normal δ13Corgvalues atthe lowermost at the Triassic. The fact that the sudden negative shift of organic δ13C values occursin the P/T boundary transitional zone of the 34th and 35th beds suggests that the P/T boundary ofthe Lekang section should be drawn at the sample WL-16 bed in the transitional zone and the re-duced surface-seawater primary productivity following the P/T mass extinction is largely responsi-ble for the observed δ13Corg shift. Therefore, the organicδ13C anomaly across the P/T boundary atthe Lekang section, Guizhou, South China, not only is an available way to accurately define thestratigraphic boundary, but also reflects the variation in primary productivity of the surface-5eawa-ter.