浙江长兴二叠系和三叠系界限地层的碳同位素

研究海相碳酸盐岩的碳和氧同位素已有三十多年,积累了数千个数据,其目的在于研究古海洋碳和氧同位素的演变。在此期间,一部分研究者认为,海相碳酸盐岩的δ¹³C值在0±2范围内变化,未表现出与地质时代相关的变化趋势(Clayton和Degens,1959;Degens和Epstein,1962;Keith和Weber,1964;Galimov,1965;Becker和Clayton,1972;Schidlowski等,1975)。但是,另一些学者,如Jeffery等(1955),Baertschi(1975),Compston(1960),Weber(1967),Garrels和Parry(1974)却认为,海相碳酸盐岩的δ¹³C值随地质时代而有规律地变化。

CARBON ISOTOPE OF CARBONATE STRATA AT PERMIAN-TRIASSIC BOUNDARY IN CHANGXING,ZHEJIANG

Stratigraphic section of Permian-Triassic boundary of Changxing in northern Zhejiang is well-known in the world.It is characterized by successive sequence,abundant fossils and similarity of lithofacies.Changxing section is divided into two parts.The lower part belongs to Changxing Formation (Upper Permian),which consists mainly of marine limestone,about 40 m thick.Changxing Formation is underlain conformably by Longtan Formation and overlain conformably by Qinglong Formation.Qinglong Formation,the upper part of the section,belongs to Lower Triassic.It is characteristic of marlite and limestone.This part of section only exposes a thickness no much more than 10 m.38 carbonate samples are systematically collected from Changxing section and 37 pairs of carbon and oxygen isotope data are obtained from the method described by McCrea (1950).Average precision is: ±0.2‰The maximum δ¹³C value in Changxing Formation is 5.1‰ ,see fig 1.Magaritz and Schulzc (1982) also reported a high δ¹³C (7‰) in Zechstein in Europe.Since Changxing Formation is higher than Zechstein in stratigraphic position,we can conclude that the heavier δ¹³C value (≥5‰) of marine carbonate is found not only in the lower part of Upper Permian (Zechstein),but also in the upper part of Upper Permian (Changxing Formation ).In Changxing Formation,δ¹³C value of marine limestone varies from 0.1‰ (at the ,base of the formation),through 5.1‰ (in the middle) to 0.1‰ (at the top),i.e.δ¹³C values of Changxing Formation constitute a cycle of carbon isotope variation.The variation of carbon isotopic composition of carbonate in Qinglong Formation is similar to that in Changxing Formation,however δ¹³C values are negative,i.e.δ¹³C value varies from -1.9‰ (at the base),through -5.0‰ to 0.3‰.So,it seems that the cycle of carbon isotope variation of marine carbonate is consistent with certain stratigraphic unit,see fig 2.The patterns of the curves of carbon isotopic composition of Changxing Formation and Qinglong Formation are similar,but different from each other by positive (Changxing) and negative (Qinglong) values and the abrupt shift in carbon isotopic composition precisely corresponding to Permian-Triassic boundary,see fig.2.Therefore,we suggest that the δ¹³C value curve of marine carbonates may be used as a tool for stratigraphic classification.