MLR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, CAGS, Beijing 100037, P. R. China.
Open Research Laboratory of Isotope Geology, CAGS, Beijing 100037, P. R. China.
Sci Rep. 2017 Mar 8;7:44000. doi: 10.1038/srep44000.
The silicon isotope composition of chert has recently been used to study the historic evolution of the global ocean. It has been suggested that Precambrian cherts have much higher δSi values than Phanerozoic cherts do and that the former show an increasing trend from 3.5 to 0.85 Ga, reflecting a decrease in ocean temperatures. However, cherts have various origins, and their isotopic compositions might be reset by metamorphic fluid circulation; thus, different types of cherts should be distinguished. Here, we present a new set of δSi data for cherts from early and middle Proterozoic carbonate rocks from Northern China. We found that cherts of 1.355-1.325 Ga show a peak range of 2.2-3.9‰. Based on these results, we propose that from the Archean to the middle Proterozoic, there was a drastic decrease in silicon content and an increase in the δSi value in ocean water due to a temperature decrease and biological activity increase. After that period, the silicon content of the ocean was limited to a low level by a high degree of biological absorption, and their δSi values varied in a small range around a significantly lower value.
硅同位素组成的硅质岩最近被用来研究全球海洋的历史演变。有人认为前寒武纪硅质岩的δSi 值比显生宙硅质岩高得多，而且前者从 3.5 到 0.85 Ga 呈上升趋势，反映出海洋温度的下降。然而，硅质岩有多种成因，其同位素组成可能被变质流体循环重置；因此，应该区分不同类型的硅质岩。在这里，我们提出了一组来自中国北方早古生代碳酸盐岩的中新元古代硅质岩的新的δSi 值数据。我们发现，1.355-1.325 Ga 的硅质岩的峰值范围为 2.2-3.9‰。基于这些结果，我们提出，从太古宙到中元古代，由于温度下降和生物活动增加，海洋中的硅含量急剧减少，δSi 值增加。此后，海洋中的硅含量受到高度生物吸收的限制，处于较低水平，其δSi 值在一个明显较低的值周围变化较小。
