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

Key words: 
• brachiopod /
• oxygen isotope /
• Longmenshan of Sichuan /
• trace element

Abstract: 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.