Systematic variations in C, O, and Sr isotopes and elemental concentrations in Neoproterozoic carbonates in Namibia: implications for a glacial to interglacial transition
Systematic variations in C, O, and Sr isotopes and elemental concentrations in Neoproterozoic carbonates in Namibia: implications for a glacial to interglacial transition
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DOI：
10.1016/s0301-9268(03)00079-2
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发表时间：
2003-06
影响因子：
3.8
通讯作者：
H. Yoshioka;Y. Asahara;B. Tojo;S. Kawakami
中科院分区：
文献类型：
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作者：
H. Yoshioka;Y. Asahara;B. Tojo;S. Kawakami
我们分析了纳米比亚冰川沉积物上新元古代盖碳酸盐的 C、O 和 Sr 同位素组成，以追踪冰川作用后的环境变化。三个同位素记录显示出与地层变化相对应的连贯和阶段性变化。较低的韵律岩单元具有三个区间，通过同位素变化和矿物类型来区分。在基础区间，δ13C 从负值（−4.3‰，Pee Dee Belemnite [PDB]）呈指数增加。 δ 13 C值在中间区间相对恒定，并且在顶部区间再次增加到正值，表明13 C贫碳从表层海洋分阶段去除。上部叠层石单元逐渐增加到较大值（>6‰）被解释为反映了冰川期环境恢复后生物活动引起的海水成分的变化。 δ18O值和Sr同位素比在基底层段下降，在中间层段接近稳定值。它们在上部区间增加并在叠层石单元中波动。根据Sr同位素成岩蚀变地球化学筛选标准，节律岩单元的中间区间87Sr/86Sr比值最小为0.70685，被解释为保留了原始值，或者说Sr同位素组成变化最小。基底层段的比值 (∼0.718) 与古老的唾液质岩石的比值一样高，被解释为 Sr 同位素比值发生了改变。该区间底部的高Sr同位素比值逐渐下降至顶部的最小值附近，其他同位素组成和微量元素浓度呈现系统变化。除了钠浓度的变化之外，这种系统变化可以通过渐进的成岩流体-岩石相互作用来解释。或者，它们可能反映了从冰川作用后局部的、运输受限的水到全球的、混合良好的海水的成分转变。
We analyzed C, O, and Sr isotopic compositions of Neoproterozoic cap carbonates overlying a glacial deposit in Namibia to trace environmental changes following the glaciation. The three isotopic records showed coherent and phased changes that corresponded to the stratigraphic variation. A lower rhythmite unit had three intervals, which were distinguished by isotopic variations and mineral type. In the basal interval, δ13C increased exponentially from a negative value (−4.3‰, Pee Dee Belemnite [PDB]). δ13C values were relatively constant in the middle interval, and increased again to positive values in the top interval, indicating a phased removal of13C -depleted carbon from the surface ocean. The gradual increase to larger values (>6‰) in the upper stromatolite unit was interpreted to reflect a change in seawater composition resulting from biological activity after environmental recovery from the glacial episode. δ18O values and Sr isotopic ratios decreased in the basal interval and approached stable values in the middle interval. They increased in the upper interval and fluctuated in the stromatolite unit. On the basis of geochemical screening criteria for detecting diagenetic alteration of Sr isotopes, the middle interval of the rhythmite unit, with a minimum87Sr /86Sr ratio of 0.70685, was interpreted to have retained the primary value, or to have the least-altered Sr isotopic composition. The basal interval, which had ratios (∼0.718) at its base as high as those of old, sialic rocks, was interpreted as having altered Sr isotopic ratios. The high Sr isotopic ratios at the base of this interval gradually decreased to near the minimum value at the top of the interval, and other isotopic compositions and trace element concentrations showed systematic variations. Such systematic variations, except for the variation in Na concentration, can be explained by a progressive diagenetic fluid–rock interaction. Alternatively, they might reflect a compositional transition from a local, transport-limited water following the glaciation to global, well-mixed seawater.