全球寒武系第三统第七阶GSSP候选剖面的碳同位素地层

湘西罗依溪剖面包含有被国际地层委员会寒武系分会表决通过的、确定全球寒武系第三统第七阶底界的球接子三叶虫Lejopygelaevigata的首现点位,是全球寒武系第三统第七阶GSSP的候选剖面。野外按0.25—0.5m的样品间距,对该点位附近的碳酸盐岩采取了264件碳、氧同位素样品,分析结果表明全球寒武系第三统Drumian阶上部δ13C值呈恒幅波状演化、波动范围较小,δ18O波动强烈、幅度较大;而第七阶下部δ13C值跳跃强烈、幅度较大,总体表现为高值,δ18O值波动减弱、趋向低值;全球寒武系第三统第七阶底部的候选GSSP正好处于δ13C值由恒幅波动向剧烈跳跃的转折处。第七阶早期δ13C值、δ18O值及海平面三者之间存在较好的耦合关系,表现为海平面上升到最高位置、δ13C值较高、δ18O值较低,造成耦合的原因可能为全球气候变暖,大量淡水流向海洋、海域扩大,初始生产率提高,较多12C为生物所摄取,导致海水中13C的浓度相对升高,沉积的碳酸盐岩具有较高的δ13C值。同时,由于最大海泛期洋流活动频繁,对江南斜坡带沉积作用有一定影响,致使碳酸盐岩沉积物的碳、氧同位素组成变化较大。

CARBON ISOTOPIC STRATIGRAPHY IN THE LUOYIXI SECTION,A POTENTIAL BOUNDARY STRATOTYPE SECTION IN SOUTH CHINA: IMPLICATIONS FOR ERECTION OF THE GLOBAL SEVENTH STAGE IN THE CAMBRIAN SYSTEM

This paper presents carbon and oxygen isotopic data from the Luoyixi section,a candidate of the Global Standard Stratotype-section and Point(GSSP) defining the base of the as-yet-undefined seventh stage of the Cambrian System at the first appearance of the cosmopolitan agnostid Lejopyge laevigata.This level is favored in a vote of International Subcommission on Cambrian Stratigraphy(ISCS) as the key biohorizon for defining the base of the stage.Two hundred and sixty-four samples for carbon and oxygen isotope analysis were collected from the carbonate succession at an interval of 0.25 to 0.5 meter in the Luoyixi section.During the late Drumian Stage,carbon isotope values fluctuate within a stable narrow range,but the δ 18 O values oscillate more strongly.However,the δ 13 C shows greater variations in the early part of the Global Seventh Stage,and is generally characterized by greater values,whereas δ 18 O decreases to lower values during the same time.The proposed base of the Global Seventh Stage seems to coincide with the transition from more stable to more variable δ 13 C values.Distinct covariant-relationship among δ 13 C,δ 18 O and sea level fluctuations suggests that a warming event took place during the early Global Seventh Stage which led to greater influx of fresh water flew into the oceans,enlarged areas of ancient sea,and enhanced the primary productivity which selectively consumed 12 C from the sea water,resulting in an increase in 13 C values of dissolved inorganic carbon and carbonates.During the highest sea level stage,ocean currents which actively affected carbonates deposited on the Jiangnan Slop Belt,South China,led to strong variation in δ 13 C and δ 18 O values.