Constraints on surface temperature 3.4 billion years ago based on triple oxygen isotopes of cherts from the Barberton Greenstone Belt, South Africa, and the problem of sample selection

IF 1.9 3区 地球科学 Q3 GEOSCIENCES, MULTIDISCIPLINARY
D. Lowe, D. Ibarra, N. Drabon, C. Chamberlain
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引用次数: 15

Abstract

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.
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限制及样品选择问题
摘要对3.0Ga以前地球表面温度的研究主要集中在富含二氧化硅的沉积岩(称为燧石)的氧同位素组成上。对结果的解释表明,早期表面温度从高达70±15°C到与现代值相差不大。一个主要的争议集中在燧石氧同位素组成随时间的差异是否反映了地表温度的变化、海洋同位素组成的变化或沉积后的成岩和变质作用。我们在这里介绍了南非Barberton Greenstone带3.472 Ga至3.239 Ga燧石的三重氧测量结果。根据地质证据,保存最好的样品具有Δ'17O和δ'18O值,这些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。由于沉积过程中靠近较年轻的镁铁质侵入体或与大气降水的相互作用,地质因素允许消除许多潜在有用的样本进行古温度分析,这两种情况都倾向于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的三-O同位素分析结果表明,太古宙表面温度远高于今天的温度,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。
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来源期刊
American Journal of Science
American Journal of Science 地学-地球科学综合
CiteScore
5.80
自引率
3.40%
发文量
17
审稿时长
>12 weeks
期刊介绍: The American Journal of Science (AJS), founded in 1818 by Benjamin Silliman, is the oldest scientific journal in the United States that has been published continuously. The Journal is devoted to geology and related sciences and publishes articles from around the world presenting results of major research from all earth sciences. Readers are primarily earth scientists in academia and government institutions.
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