Carbon isotopes, sulfur isotopes, and trace elements of the dolomites from the Dengying Formation in Zhenba area, southern Shaanxi: Implications for shallow water redox conditions during the terminal Ediacaran  

作　　者：CHEN YaLi CHU XueLei ZHANG XingLiang ZHAI MingGuo 

机构地区：[1]State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences [2]College of Earth Science, University of Chinese Academy of Sciences [3]State Key Laboratory for the Continental Dynamics, Northwest University

出　　处：《Science China Earth Sciences》2015年第7期1107-1122,共16页中国科学（地球科学英文版）

基　　金：supported by the National Basic Research Program of China(Grant No.2011CB808805);MOST Special Fund from the State Key Laboratory of Continental Dynamics,Northwest University,and National Natural Science Foundation of China(Grant No.41172029)

摘　　要：Carbon isotope, sulfur isotope, and trace element(including Rare Earth Elements, REE) analyses were conducted on the carbonates of the Dengying Formation at Lianghekou section in southern Shaanxi to reconstruct the terminal Ediacaran shallow-water environment on the northwestern margin of the Yangtze Platform. At Lianghekou section, samples in the middle 50-m of the Beiwan Member show characteristics of low ΣREE concentrations, no MREE-enriched REE distribution patterns, high Ce/Ce* values close to 1, and enriched redox-sensitive elements, whereas samples in the lower 30-m and upper 10-m show opposite characteristics of high ∑REE concentrations, MREE-enriched REE distribution patterns, low Ce/Ce* values around 0.6, and no redox-sensitive elements enriched, indicating that oxygenation did occur in the shallow water on the northwestern margin of the Yangtze Platform and redox conditions of the shallow water fluctuated from relatively oxygenated to anoxic and then back to oxygenated again. We propose that the anoxia appeared in middle of the Beiwan time may associate with the anoxic upwelled water. On one hand, abundant nutrients were brought in by this upwelling event, which stimulated the photosynthetic carbon fixation and increased the organic carbon burial under this anoxic condition, causing a peak of 3.6‰ in δ 13 C. On the other hand, because the anoxic upwelled water replaced the oxic shallow water, together with the increasing organic matter in the water column, bacterial sulfate reduction was enhanced and therefore quickly reduced the sulfate concentration, which eventually caused δ 34 S increasing to 50‰. However, as the upwelling gradually disappeared, δ 13 C and δ 34 SCAS values decreased as well in the late Beiwan time, indicating the shallow water went back to suboxic or oxic again.Carbon isotope, sulfur isotope, and trace element（including Rare Earth Elements, REE） analyses were conducted on the carbonates of the Dengying Formation at Lianghekou section in southern Shaanxi to reconstruct the terminal Ediacaran shallow-water environment on the northwestern margin of the Yangtze Platform. At Lianghekou section, samples in the middle 50-m of the Beiwan Member show characteristics of low ΣREE concentrations, no MREE-enriched REE distribution patterns, high Ce/Ce＊ values close to 1, and enriched redox-sensitive elements, whereas samples in the lower 30-m and upper 10-m show opposite characteristics of high ∑REE concentrations, MREE-enriched REE distribution patterns, low Ce/Ce＊ values around 0.6, and no redox-sensitive elements enriched, indicating that oxygenation did occur in the shallow water on the northwestern margin of the Yangtze Platform and redox conditions of the shallow water fluctuated from relatively oxygenated to anoxic and then back to oxygenated again. We propose that the anoxia appeared in middle of the Beiwan time may associate with the anoxic upwelled water. On one hand, abundant nutrients were brought in by this upwelling event, which stimulated the photosynthetic carbon fixation and increased the organic carbon burial under this anoxic condition, causing a peak of 3.6‰ in δ 13 C. On the other hand, because the anoxic upwelled water replaced the oxic shallow water, together with the increasing organic matter in the water column, bacterial sulfate reduction was enhanced and therefore quickly reduced the sulfate concentration, which eventually caused δ 34 S increasing to 50‰. However, as the upwelling gradually disappeared, δ 13 C and δ 34 SCAS values decreased as well in the late Beiwan time, indicating the shallow water went back to suboxic or oxic again.

关 键 词：Yangtze Platform southern Shaanxi EDIACARAN trace elements carbon isotopes sulfur isotopes 

分 类 号：P588.245[天文地球—岩石学] P597[天文地球—地质学]