| 泥盆纪海水的碳、氧同位素变化——来自腕足化石的同位素记录 |
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| 引用本文：程红光,李心清,袁洪林,王克卓.2009.泥盆纪海水的碳、氧同位素变化——来自腕足化石的同位素记录[J].地球学报,30(1):79-87. |
| DOI：10.3975/cagsb.2009.01.11 |
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| 基金项目:国家自然科学基金项目(编号：40273011)和西北大学大陆动力学国家重点实验室开放课题基金项目(编号：07LCD13) |
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| 中文摘要:生物壳体中的碳、氧同位素一直被用来反演地质历史时期海水同位素组成。具有较强抵抗成岩后生作用(低镁方解石)的腕足化石，分布广泛，被认为是反演古生代海水同位素的理想样品之一。本文通过对四川龙门山泥盆纪保存完好的腕足化石碳、氧同位素的提取，建立该时期海水同位素变化曲线，并对其控制因素进行探讨。研究显示：碳同位素组成在~2‰~+2‰(PDB下同)之间变化。其间经历3个旋回，其相对高值分别对应龙门山地区其中3个成礁期。同时因海平面变化不同，碳同位素表现出正偏幅度的不同。表明生物生长以及海平面变化共同控制着海水碳同位素组成，且生物生长对其影响较大。氧同位素组成在4‰~ -10‰之间变化。在埃姆斯阶对应的甘溪组、谢家湾组以及二台子组时期，氧同位素从-10‰逐渐升高到-6‰。但相对于世界其他地区该时期氧同位素偏低-2‰~ -4‰。可能原因是早期处于封闭海洋环境的平驿铺群时期，淡水注入造成海水氧同位素严重偏负。埃姆斯阶全球海平面上升，广海海水同化作用，氧同位素不断上升。从养马坝至小岭坡组时段内，氧同位素在-4‰~ -6‰之间变化，同世界其他地区相当。其幅度变化表明：温度可能是其控制因素。 |
| 中文关键词:龙门山 氧同位素 碳同位素 腕足 海平面变化 |
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| Carbon and Oxygen Isotope Variations of Devonian Seawater:Isotopic Records from Brachiopod Shell |
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| Abstract:The stable isotopic information in biogenic calcite was used to calculate the isotopic composition of seawater in the geological history. Brachiopod shell, due to its resistant ability to diagenesis (low magnesium calcite) and widespread distribution, was considered to be one of the ideal samples in the study of seawater isotopic composition in Paleozoic. The authors measured carbon and oxygen isotopic compostion of well-preserved Longmenshan brachiopod shells, and established the seawater isotopic variation curve of Devonian and discussed the influencing factors. The results shows that: the δ13C value range from -2‰ to 2‰ (PDB) with three cycles, it every high value corresponding to one reef building event in Longmenshan area. The δ13C value shows different ranges in these cycles because of different variation degrees of sea level, indicating that the δ13C value of seawater was controlled jointly by biological growth and sea level variation, with the former being more important. The δ18O value ranges from -4‰ to -10‰. In Emsian stage, the oxygen isotope increased gradually from -10‰ to -6‰, being -2‰ ~ -4‰ lower than that of other areas. It may be suggested that, before Emsian stage, the Longmen sea was closed or the seawater circulation was not smooth, and its δ18O value was desalinized by the input of fresh water. With the transgression and the seawater assimilation in Emsian stage, the δ18O value unceasingly rises. From Eiferian stage to Frasian stage, the δ18O value varied from -4‰ to -6‰. Those results indicated that temperature is possibly the factor controlling the oxygen isotopic composition of sea water. |
| keywords:Longmenshan oxygen isotope carbon isotope brachiopod sea level variation |
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