摘要
生物成因海洋生成物中的负Ｃｅ异常指示上覆水体氧化、负Ｃｅ异常消失指示上覆水体还原，这是迄今为止利用Ｃｅ异常值推测古水体氧化还原条件相对变化时所遵循的原理．根据对四川汉源晚奥陶世五峰期地层中ＲＥＥ分配模式和富Ｃｅ载体的研究，本文提出非生物成因海洋生成物中Ｃｅ富集与亏损的如下机理：当上覆水体缺氧且呈酸性时，Ｃｅ以＋３价溶解并相对邻近稀土元素富集，而海洋生成物中的Ｃｅ则相对亏损，呈现负异常；当上覆水体富氧且呈碱性时，Ｃｅ以＋４价并入铁的氢氧化物絮状物或磷酸盐絮状物中并沉淀，因此上覆水体中Ｃｅ相对亏损，而海洋生成物中Ｃｅ则相对富集，呈现正异常或负异常基本消失．该机理为运用Ｃｅ异常值对古水体氧化还原条件相对变化的推测，提供了另一种可能的解释．
Because cerium has variable valences, its relative abundance compared to the adjacent elements in seawater is mainly controlled by oxidation reduction conditions. This chemical property of cerium provides an important clue for inferring redox environments of ancient seawater masses. A principle usually followed, when the cerium anomalies in biogenically authigenic substances (such as conodonts and fish debris ) are employed as aredox indicator, is that these substances inherit the REE paterns of their overlying water masses; therefore, the negative cerium anomaly in them indicates an oxygen richened water mass, while the positive cerium anomaly records an anoxic sea environment. Some researchers have ever attempted to apply this principle to the cerium anomalies in non biogenically authigenic substances for the explanation of ancient redox conditions, but why the cerium from sediments deposited in apparently anoxic environments possesses negative anomly values is still a puzzling problem. Another possible mechanism of concentration and depletion of cerium in authigenic sea substances is suggested in this paper. The cerium anomaly values studied here are from the Ashgillian strata deposited on the west border of the Yangtze sea and characterized by vertical alternating of black and non black rock series. The black rock series, which contain plankton (graptolites. small sized inarticulate brachiopods), yield lower cerium anomaly values (0.63～0.74), and the non black rock series, including carbonate rocks and mudstones with benthos (trilobites, articulate brachiopods and algae), produce higher cerium anomaly values ( 0.90～1.30). The strong correlation of cerium with iron (r=0.86) and phosphorus (r=0.70) shows that the cerium richened carriers probably are compunds of iron and phosphates, and the correlation coefficient between cerium and organic carbon is so small (r=0.25) that the phosphates could be further thought as non biogenic. These observed results maybe let us come to the following conclusions: during times of anoxia and acidity, cerium dissolves and concentrates as the Ce 3 state in the overlying rater, but depletes and obtains a negative anomaly in the non biogenically authigenic substances settling towards seafloor; during times of oxidation and alkalinity, cerium is oxidized to Ce 4 , incorporated into the flocs of hydrous iron oxides and phosphates and precipitated together with them, depleting in the overlying water, but concentrating and getting a positive anomaly (or at least having no obvious negative anomaly ) in the non biogenically authigenic substances.
出处
《南京大学学报（自然科学版）》
CAS
CSCD
1997年第3期402-408,共7页
Journal of Nanjing University（Natural Science）
基金
国家自然科学基金
关键词
REE分配模式
铈
古海洋
富集
亏损
氧化还原条件
REE patterns, Cerium richened carriers, mechanism of cerium concentration and depletion
