Genesis of Siderites in the Xiamaling Formation of Jixian Section and Its Paleoceanic Significance
详细信息
- 责任者：Zhang Kan (Key Laboratory of Isotope Geology ; MLR ; Institute of Geology ; Chinese Academy of Geological Sciences ; Beijing 100037 ; China) ; Zhu Xiangkun
- 刊名：Acta Geologica Sinica
- 出版年：2013
- 卷期：87(9)
- 关键词：菱铁矿 ; siderite ; 古海洋学 ; paleooceanography ; 天津 ; Tianjin
- 页码：p.1430-1438
- 图表：3 illus., 1 table, 64 refs.
- 分类号：1001
- issnNo：ISSN0001-5717
- isbnNo：CN11-1951/P
摘要
The new-exposed Xiamaling Formation near Tielingzi village of Jixian are rich of siderites.Recognition of the siderite formation mechanism is crucially important for judging the redox state and geochemical nature of the ancient ocean during Xiamaling Period,even the whole Mesoproterozoic.In this paper,we used carbon isotope to constrain the genesis of these siderites,and then supported analysis of the paleoceanic environment during this period.The δ13C values of these siderites range from-19.2‰to-7.8‰,with an average of-15.0‰,showing significantly enrichment of the lighter carbon isotope.Based on the carbon isotope compositions of different carbon sinks in nature,we can identify that the carbon in these siderites was mainly derived from organic carbon.Although the oxidation of organic matter in the ocean can provide much more 12C into the seawater,and then precipitate carbonates rich in the lighter carbon isotope,that always needs a quit oxidizing water environment,and there can be no siderites precipitated in such a condition.So we can draw a conclusion that the siderites in our study area were products of combination of CO2 derived from organic matter and Fe2+ in pore water during the diagenetic process.Firstly,the iron precipitated from the ocean as iron oxides or hydroxides,and buried with organic matter in the seafloor hypoxia zone.Then during the early stage of diagenesis,organic matter was oxidized to CO2 by Fe3+,while Fe3+ was reduced to Fe2+ by organic matter,and siderites were formed by the combination of these resultants.Accordingly,thermally-induced decarboxylation of organic matter offered aportion of CO2 to make more Fe2+ exist in strata as siderites.The precipitation form of iron in the ocean indicates that in Yanliao basin,at least,the ancient ocean during Xiamaling Period had been wildly oxidic,and the degree of oxidation can make iron oxidized to ferric iron.Also the strata are low in sulfur,indicating that the ancient ocean was sulfur-poor.However,this wildly oxidic and sulfur-poor paleoceanic state in Yanliao basin is different from the traditional stratified paleoceanic model,so more in-depth and systematic research is needed to address this issue.