Environmental redox changes of the ancient sea in the Yangtze area during the Ordo-Silurian transition

	Environmental redox changes of the ancient sea in the Yangtze area during the Ordo-Silurian transition
	Yan Detian 1,2; Chen Daizhao 1; Wang Qingchen 1; Wang Jianguo 1; Chu Yang 1
刊名	ACTA GEOLOGICA SINICA-ENGLISH EDITION
	2008-06-01
卷号	82 期号:3 页码:679-689
关键词	oceanic anoxia water column stratification Fe-S geochemistry organic carbon Late Ordovician Early Silurian Yangtze Sea South China
ISSN号	1000-9515
文献子类	Article
英文摘要	Extensive organic-matter (OM) rich facies (black shales) occur in the Ordo-Silurian boundary successions in the Yangtze area, South China. To investigate the redox changes of the Yangtze Sea during the Ordo-Silurian transition, two OM sections (Wangjiawan in Yichang, Hubei Province, and Sanjiaguan in Zhangjiajie, Hunan Province) straddling the Ordo-Silurian boundary are studied. The measurements finished in this study include contents of the total organic carbon (TOC), pyrite sulphur, and different species of Fe, including dithionite-extractable Fe (Fe-D), pyrite Fe (Fe-p), HCl-extractable Fe (Fe-H), and total Fe (Fe-T), in black shales, as well as other redox proxies, such as the S/C ratio, the ratio between highly reactive Fe (Fe-HR = Fe-D + Fe-p) and Fe-T, and the Fe-p(Fe-p + Fe-H) ratio, known as the degree of pyritization (DOP). In the Wangjiawan section, the Middle Ashgill sediments have high Fe-HR/Fe-T ratios (0.20-0.77; avg. 0.45), high DOP values (0.21-0.72; avg. 0.54), and a relatively constant sulfur content independent of the organic carbon content. By the contrast, the mid-early Hirnantian deposits generally have low Fe-HR/Fe-T ratios (0.10-0.35; avg. 0.21), low DOP values (0.11-0.40; avg. 0.28), and S/C values are clustering on the normal marine value (S/C = 0.36). The late Hirnantian and early Rhuddanian deposits, similar to those of the Middle Ashgill deposits, are characterized by high Fe-IIR/Fe-T ratios (0.32-0.49; avg. 0.41), high DOP values (0.46-0.68; avg. 0.53) and fairly constant sulfur contents. These data suggest the occurrences of marine anoxia on the Yangtze Sea shelf during intervals of the Mid Ashgill, Late Hirnantian and Early Rhuddanian, and ventilated and oxygenated marine conditions during the mid-early Hirnantian time. The mid-early Hirnantian ventilated event was concomitant with the global glacial period, likely resulted from the glacio-eustatic sea-level fall and subsequent circulation of cold, dense oxygenated waters upon the shelf seabed. Accordingly, the abrupt change from oxygenated to anoxic marine waters from the late Hirnantian and the early Rhuddanian were resulted from the post-glacial rise of eustatic sea level. Combined with the data from Sanjiaguan section near the subemergent highland, salinity stratification of water columns are strongly evidenced by very low S/C ratios (0.00-0.08, avg. 0.02), low Fe-HR/Fe-T ratios (0.10-0.37, avg. 0.27), low DOP values (0.01-0.27, avg. 0.10), and TOC contents (0.72%-4.27%, avg. 2.55%). Under this circumstance, the anoxic water columns could have formed beneath the halocline, above which desalinized waters formed. In the Wangjiawan section, TOC contents are generally high (0.949-9.32%, avg. 4.44%), but low (0.35%-2.12%, avg. 1.29%) in the mid-early Hirnantian that is coincident with the oceanic oxic stage, together with relation of the organic contents to the stratal thickness, suggesting that the accumulation of the organic matter was mainly controlled by the oxygen levels of the water columns; on the other hand, productivity and depositional rate may also have played a role in the organic accumulation and preservation.
WOS关键词	SEDIMENTARY PYRITE FORMATION ; ORGANIC-CARBON ; SOUTH CHINA ; HIRNANTIAN GLACIATION ; SULFUR RELATIONSHIPS ; IRON RELATIONSHIPS ; MARINE-SEDIMENTS ; LATE ORDOVICIAN ; FRESH-WATER ; ROCKS
WOS研究方向	Geology
语种	英语
出版者	GEOLOGICAL SOC CHINA
WOS记录号	WOS:000258544300023
资助机构	National Key Basic Research Development Program(2005CB422101) ; National Key Basic Research Development Program(2005CB422101) ; National Key Basic Research Development Program(2005CB422101) ; National Key Basic Research Development Program(2005CB422101) ; National Key Basic Research Development Program(2005CB422101) ; National Key Basic Research Development Program(2005CB422101) ; National Key Basic Research Development Program(2005CB422101) ; National Key Basic Research Development Program(2005CB422101) ; National Key Basic Research Development Program(2005CB422101) ; National Key Basic Research Development Program(2005CB422101) ; National Key Basic Research Development Program(2005CB422101) ; National Key Basic Research Development Program(2005CB422101) ; National Key Basic Research Development Program(2005CB422101) ; National Key Basic Research Development Program(2005CB422101) ; National Key Basic Research Development Program(2005CB422101) ; National Key Basic Research Development Program(2005CB422101)
内容类型	期刊论文
源URL	[http://ir.iggcas.ac.cn/handle/132A11/69564]
专题	中国科学院地质与地球物理研究所
通讯作者	Yan Detian
作者单位	1.Chinese Acad Sci, Inst Geol & Geophys, State Key Lab Lithospher Evolut, Beijing 100029, Peoples R China 2.China Univ Geosci, Key Lab Tecton & Petr Resources, Minist Educ, Wuhan 430074, Peoples R China
推荐引用方式 GB/T 7714	Yan Detian,Chen Daizhao,Wang Qingchen,et al. Environmental redox changes of the ancient sea in the Yangtze area during the Ordo-Silurian transition[J]. ACTA GEOLOGICA SINICA-ENGLISH EDITION,2008,82(3):679-689.
APA	Yan Detian,Chen Daizhao,Wang Qingchen,Wang Jianguo,&Chu Yang.(2008).Environmental redox changes of the ancient sea in the Yangtze area during the Ordo-Silurian transition.ACTA GEOLOGICA SINICA-ENGLISH EDITION,82(3),679-689.
MLA	Yan Detian,et al."Environmental redox changes of the ancient sea in the Yangtze area during the Ordo-Silurian transition".ACTA GEOLOGICA SINICA-ENGLISH EDITION 82.3(2008):679-689.