Department of Geology and Geophysics, Yale University, 210 Whitney Avenue, New Haven, Connecticut 06511, USA.
Nature. 2009 Nov 12;462(7270):205-8. doi: 10.1038/nature08518.
Stable oxygen isotope ratios (delta(18)O) of Precambrian cherts have been used to establish much of our understanding of the early climate history of Earth and suggest that ocean temperatures during the Archaean era ( approximately 3.5 billion years ago) were between 55 degrees C and 85 degrees C (ref. 2). But, because of uncertainty in the delta(18)O of the primitive ocean, there is considerable debate regarding this conclusion. Examination of modern and ancient cherts indicates that another approach, using a combined analysis of delta(18)O and hydrogen isotopes (deltaD) rather than delta(18)O alone, can provide a firmer constraint on formational temperatures without independent knowledge of the isotopic composition of ambient waters. Here we show that delta(18)O and deltaD sampled from 3.42-billion-year-old Buck Reef Chert rocks in South Africa are consistent with formation from waters at varied low temperatures. The most (18)O-enriched Buck Reef Chert rocks record the lowest diagenetic temperatures and were formed in equilibrium with waters below approximately 40 degrees C. Geochemical and sedimentary evidence suggests that the Buck Reef Chert was formed in shallow to deep marine conditions, so our results indicate that the Palaeoarchaean ocean was isotopically depleted relative to the modern ocean and far cooler (<or=40 degrees C) than previously thought.
稳定氧同位素比值（δ¹⁸O）的前寒武纪硅质岩已被用于建立我们对地球早期气候历史的理解，并表明太古宙（约 35 亿年前）期间海洋温度在 55 摄氏度和 85 摄氏度之间（参考文献 2）。但是，由于原始海洋的 δ¹⁸O 存在不确定性，对于这一结论存在很大争议。对现代和古代硅质岩的研究表明，另一种方法，即同时分析 δ¹⁸O 和氢同位素（δD），而不是仅分析 δ¹⁸O，可以在不依赖于环境水同位素组成的情况下，对形成温度提供更可靠的限制。在这里，我们表明，来自南非 34.2 亿年前的 Buck Reef 硅质岩的 δ¹⁸O 和 δD 样本与低温下形成的水一致。最富 δ¹⁸O 的 Buck Reef 硅质岩记录了最低的成岩温度，并且与低于约 40 摄氏度的水处于平衡状态。地球化学和沉积证据表明，Buck Reef 硅质岩形成于浅海到深海环境，因此我们的结果表明，古太古宙海洋的同位素相对于现代海洋贫化，而且比以前认为的要冷得多（<或=40 摄氏度）。
