ABSTRACT Studies of Earth's surface temperature before 3.0 Ga have focused heavily on the oxygen isotopic composition of silica-rich sedimentary rocks called cherts. Interpretation of the results have suggested early surface temperatures ranging from as high as 70 ± 15 °C down to those that differ little from modern values. A major controversy centers on whether differences in the oxygen isotopic compositions of cherts over time reflect changing surface temperatures, changing ocean isotopic composition, or post-depositional diagenetic and metamorphic effects. We here present results of triple oxygen measurements of 3.472 Ga to 3.239 Ga cherts from the Barberton Greenstone Belt, South Africa. The best preserved samples based on geological evidence have Δ'17O and δ'18O values that plot generally on or near the equilibrium fractionation line for silica precipitated out of modern, ice-free sea water. Geologic considerations allow many potentially useful samples to be eliminated for paleotemperature analysis because of proximity to younger mafic intrusions or interactions with meteoric waters during deposition, both of which tend to lower preserved isotopic values. Our results of triple-O isotopic analyses of a suite of samples representing deposition under open marine, shallow shelf conditions suggest that Archean surface temperatures were well above those of the present day, perhaps as high as 66 to 76 °C. They demonstrate that geologic context, including depositional setting and post-depositional history, requires careful assessment before the significance of oxygen isotopic results can be evaluated.

中文翻译：

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基于南非巴伯顿绿岩带燧石三氧同位素对34亿年前地表温度的约束及样本选择问题

摘要 3.0 Ga 之前地球表面温度的研究主要集中在称为燧石的富含二氧化硅的沉积岩的氧同位素组成上。结果的解释表明，早期的表面温度范围从高达 70 ± 15 °C 到与现代值相差不大的温度。一个主要的争议集中在燧石氧同位素组成随时间的变化是否反映了表面温度的变化、海洋同位素组成的变化或沉积后的成岩和变质效应。我们在此展示了来自南非巴伯顿绿岩带的 3.472 Ga 至 3.239 Ga 燧石的三重氧测量结果。根据地质证据保存最好的样品有Δ'17O和δ' 18O 值通常绘制在现代无冰海水中沉淀的二氧化硅的平衡分馏线上或附近。由于靠近较年轻的基性侵入体或在沉积过程中与大气水相互作用，地质因素使得许多潜在有用的样本被排除在古温度分析中，这两者往往会降低保存的同位素值。我们对一组代表在开阔海洋、浅陆架条件下沉积的样品进行三氧同位素分析的结果表明，太古代表面温度远高于当今的温度，可能高达 66 至 76 °C。他们证明，在评估氧同位素​​结果的重要性之前，需要仔细评估地质背景，包括沉积环境和沉积后历史。