龙门山地区早泥盆世古海洋环境——来自腕足类壳体的地球化学证据

四川龙门山地区泥盆系是解决我国华南区与西北区及国外泥盆系对比的纽带，也是全球古生代气候变化对比研究的一个重要地区。但该地区早泥盆世古海洋环境研究一直缺乏同位素地球化学方面的证据，影响到该地区泥盆纪地球化学记录与世界其他地区的对比，进而影响全球范围内的古生代气候变化对比研究。针对这一问题，我们通过龙门山地区早泥盆世保存完好的腕足化石稳定同位素地球化学研究，对该地区早泥盆世海洋的古地理环境特征进行了探讨。保存鉴定结果显示:腕足化石壳体结构保存完整；阴极射线照射下壳体不发光或部分壳体呈微弱橙黄色；大多数壳体微量元素Mn < 250 μg/g、Sr > 400 μg/g。表明腕足化石保存完好，后期成岩作用影响微弱。同位素结果揭示龙门山地区δ18O值在-4.5‰ ~ -9.9‰(PDB下同)之间，明显低于该时期世界其他相同纬度地区的δ18O值 (-1.7‰ ~ -6.9‰)；微量元素Fe、Mn含量变化与氧同位素组成变化呈反相关系。这说明龙门山海域当时处于海水同开阔海水交流不畅，海水化学成分受陆源淡水注入控制。这一发现从同位素地球化学角度对龙门山古海洋环境做出了判别，为龙门山地区同位素及微量元素研究在世界范围内进行短尺度对比提供依据。

The Oceanic Environment During the Early Devonian in Longmen Shan Area: geochemical evidence from brachiopod shells

The Longmenshan section is a classical Devonian section in the world, and hence becomes the focus of Paleozoic paleocliamte studies. But in the early Devonian, it is short of geochemical isotopic attestation for the paleoenvironment. It will affect the comparison of geochemical signals between the Longmenshan and the other areas in the world, and also hinder the studying of the paleoclimate in Paleozoic. In this study, geochemical signals that came from well preserved brachiopod shells were used to discuss the paleoenvironment. The preservation show: brachiopod shells are well; noncathodoluminscence and Mn<250 μg/g, Sr>400 μg/g. It is witness that diagenetic alteration was small and even not. The δ18O values -4.5‰ ~-9.9‰ was evidently lower than that of other region’s δ18O values (-1.7‰ ~-6.9‰) in the world. And the trace elements Fe and Mn have inverse isochronous trend to the δ18O values. It is possible that the seawater circulation in the Longmenshan was unsmooth, and its δ18O value was desalinized by the input of continental fresh water. Current study will provide a convincing theoretical basis for contrast of isotope and trace elements on the worldwide scale.