[论文解读] The Case for a Hot Archean Climate and its Implications to the History of the Biosphere

David Schwartzman|arXiv (Cornell University)|Apr 1, 2015

Paleontology and Stratigraphy of Fossils参考文献 80被引用 6

一句话总结

本文主张原始地球的气候远比现今温暖，其依据包括燧石中的氧同位素数据、蛋白质稳定性重建、高pCO2水平以及海水粘度估算。尽管太阳光度较弱，这一高温气候仍能持续，表明温度约束在塑造重大演化转变（包括产氧光合作用和后生动物的出现）的时间与节奏方面发挥了关键作用。

ABSTRACT

The case for a much warmer climate on the early Earth than now is presented. The oxygen isotope record in sedimentary chert and the compelling case for a near constant isotopic oxygen composition of seawater over geologic time support thermophilic surface temperatures prevailing in the Archean, with some support for hot conditions lasting until about 1.5 billion years ago, aside from lower temperatures including glacial episodes at 2.1-2.4 Ga and possibly an earlier one at 2.9 Ga. Other evidence includes the following: 1) Melting temperatures of proteins resurrected from sequences inferred from robust molecular phylogenies give paleotemperatures at emergence consistent with a very warm early climate. 2) High atmospheric pCO2 levels in the Archean are consistent with high climatic temperatures near the triple point of primary iron minerals in banded iron formations, the formation of Mn-bicarbonate clusters leading to oxygenic photosynthesis and generally higher weathering intensities on land. These higher weathering intensities would not have occurred if seafloor weathering dominated the carbon sink, pulling down the temperature, hence this empirical evidence supports a hot climate and high carbon dioxide levels. 3) The inferred viscosity of seawater at 2.7 Ga is consistent with a hot Archean climate. 5) A cold Archean is hard to explain taking into account the higher outgassing rates of carbon dioxide, significantly smaller land areas and weaker biotic enhancement of weathering than present in the context of the long-term carbon cycle, taking into account the fainter Archean sun in climate modeling. This evidence points to an important conclusion regarding biological evolution, namely to the critical role of a temperature constraint holding back the emergence of major organismal groups, starting with phototrophs, culminating with metazoans in the latest Precambrian.

研究动机与目标

• 通过综合多种地质与生物化学证据，挑战当前关于原始地球气候寒冷或温带的主流观点。
• 通过提出持续高浓度的温室气体（尤其是CO2）来解决“古老太阳黯淡悖论”。
• 探讨热力约束如何影响关键演化创新（如产氧光合作用和后生动物演化）的时间节点。
• 证明在条带状铁建造中，强烈风化作用与矿物稳定性与一个温暖的早期地球一致。

提出的方法

• 分析原始地球沉积燧石中的氧同位素比值，以推断地表温度历史。
• 利用分子系统发育方法复活并测试祖先蛋白质的热稳定性，以估算其出现时的古温度。
• 评估矿物学约束条件，如条带状铁建造中主要铁矿物的三相点，以推断大气CO2水平。
• 将27亿年前海水粘度作为温度的代理指标进行评估。
• 整合长期碳循环模型，以评估脱气作用、风化作用与大气CO2之间的平衡。
• 利用21–24亿年前和29亿年前冰川事件的实证数据，识别在整体温暖的原始地球中出现的较冷气候区间。

实验结果

研究问题

• RQ1 尽管太阳光度较弱，有哪些证据支持原始地球气候更温暖？
• RQ2 从分子系统发育重建的蛋白质稳定性如何为早期地球地表温度提供估算依据？
• RQ3 为何需要高大气pCO2水平才能解释条带状铁建造中某些矿物组合的持久存在？
• RQ4 推断的原始海水粘度如何支持高温气候的结论？
• RQ5 温度约束在调控生物圈重大演化转变时间方面发挥了何种作用？

主要发现

• 原始燧石中的氧同位素数据表明，原始地球的海水同位素组成近乎恒定，支持地表温度长期保持嗜热状态。
• 从祖先谱系中复活的蛋白质表明，其出现时的古温度与温暖的早期地球一致，部分证据可追溯至15亿年前。
• 高大气pCO2水平是解释条带状铁建造中Mn-碳酸盐簇形成及主要铁矿物稳定性的必要条件，暗示气候温暖。
• 27亿年前海水粘度估算与高温原始地球气候一致，进一步支持地球化学数据的热力解释。
• 缺乏以洋底风化为主导的碳汇，意味着大陆风化强度更高，而这种高强度风化仅在温暖条件下可行。
• 综合证据否定了寒冷原始地球的模型，支持一个持续温暖的气候，该气候影响了生物演化的节奏。