西藏南部晚白垩世坎潘期碳同位素偏移及其意义

Carbon isotope excursion of Late Cretaceous Campanian in southern Xizang and its implication

对西藏南部定日地区特提斯喜马拉雅晚白垩世坎潘期海相沉积进行高分辨率碳氧同位素分析,获得较为丰富的实验数据。分析表明,δ13C值在坎潘期呈现二高二低变化:坎潘早期较低,平均值约1.2‰～1.3‰;坎潘中期持续升高,平均值1.9‰～2.0‰,并形成正偏高原,到剖面196m位置达到最大值约2.3‰;坎潘晚期变低,平均值在1.2‰～1.4‰范围;坎潘末期再一次升高,均值达2.0‰左右。其中,坎潘中期的正偏高原全球对比研究暗示可能与南半球特别是南极大陆高温和高降雨量有关。根据δ13C值偏移幅度和时间跨度,在坎潘早期、中期和晚期识别出三次负偏事件Ⅰ、Ⅱ、Ⅲ,分别对应于有孔虫G.elevata带下部(剖面162～166m)、G.ventricosa带下部(188～192m)和G.ventricosa带-G.stuartiformis带界线(226～230m),时间跨度平均小于0.74Ma,它们显示了与欧洲同期δ13C值偏移事件的可比性和与Haq等全球I型层序低海平面的相关性。

Carbon isotope values are analyzed from higher resolution bulk samples of Cretaceous carbonate rock succession in Tingri, southern Xizang （Tibet）. A profile of two cycles of carbon isotope changes is recognized in the Campanian succession. A relatively lower average of δ13C value 1.2‰ - 1.3‰ displays a secular increase of average δ^13C value 1.9‰ - 2.0‰ happens in middle Campanian δ^13C value of 2. 3‰ at the horizon 196 m in the Gongzha cross section. This increase in early Campanian. Then and goes up to the highest shape is often called as a positive excursion platform highland. Upwards, it declines to an average of δ^13C value of 1.2‰ - 1.4‰ in late Campanian. And it rises again in terminal Campanian and the average δ^3C value becomes ca. 2. ‰. A global comparison of coeval carbon isotopes implies that the positive excursion platform highland of middle Campanian δ^13C value could be caused by a high atmosphere temperature and high rainfall in South Hemisphere, especially in Antarctic continent. According to amplitude and age duration of δ^13C values, three negative carbon isotope excursion events are recognized. They are the event Ⅰ, Ⅱ and Ⅲ corresponding to earliest, middle and middlelate Campanian, separately. These three isotope events are also aged by foraminiferal zones as： event I, the lower G. elevata zone （section horizon 162-166 m）; event Ⅱ, the lower G. ventricosa zone （section horizon 188-192 m）; and event Ⅲ, the interval between G. ventricosa zone-G, stuartiformis zone （section horizon 226- 230 m）. Average time duration of the events is less than 0. 74 Ma. Correlation analysis shows that the three negative carbon isotope excursion events in Xizang are comparable with those in Europe, and could be related to the type- Ⅰ sequence lowstands of sea-level cycles as shown in the Haq curve in 1987.