Limited Archaean continental emergence reflected in an early Archaean 18O-enriched ocean
Limited Archaean continental emergence reflected in an early Archaean 18O-enriched ocean
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DOI：
10.1038/s41561-020-0538-9
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发表时间：
2020-03-01
影响因子：
18.3
通讯作者：
Wing, Boswell A.
中科院分区：
文献类型：
--
作者：
Johnson, Benjamin W.;Wing, Boswell A.
地球生物圈的起源和演化是由海洋的物理和化学历史决定的。海洋化学沉积物和改变的洋壳保存了这些历史的地球化学记录。例如，海洋化学沉积物的 O-18/O-16 比率随时间的推移而增加。该信号的含义是不明确的，但通常根据两个端元（但不是相互排斥的）场景来表达。如果海洋的氧同位素组成与今天相似，那么在遥远的过去，海洋可能会温暖得多。或者，流体-岩石相互作用的性质（包括与大陆出现相关的风化过程）在过去可能有所不同，导致海洋氧同位素组成不断变化。在这里，我们检查了西澳大利亚皮尔巴拉克拉通全景区大约 32.4 亿年前热液蚀变的海洋地壳，作为海洋化学沉积物的替代氧同位素档案。我们发现，当时全景海水的氧同位素成分相对于现代海洋而言富含 O-18，其 Delta O-18 为 3.3 +/- 0.1 ppm VSMOW。我们认为，海水三角洲 O-18 可能随着时间的推移而减少，而海洋化学沉积物却大幅增加。为了解释这种可能性，我们构建了地质水循环的氧同位素交换模型，该模型表明，在 3 至 25 亿年前的太古代晚期大陆风化的开始，将使富含 O-18 的早期太古宙海洋下降到与现代海水相似的三角洲 O-18 值。我们得出的结论是，在大陆出现之前和之后，地球的水循环可能经历了两个不同的稳态行为阶段。根据海洋地壳记录的逆地球化学模型，自太古宙以来，海水氧同位素值不断下降，在大陆出现之前和之后，水循环处于两个不同的稳态。
The origin and evolution of Earth's biosphere were shaped by the physical and chemical histories of the oceans. Marine chemical sediments and altered oceanic crust preserve a geochemical record of these histories. Marine chemical sediments, for example, exhibit an increase in their O-18/O-16 ratio through time. The implications of this signal are ambiguous but are typically cast in terms of two endmember (but not mutually exclusive) scenarios. The oceans may have been much warmer in the deep past if they had an oxygen isotope composition similar to that of today. Alternatively, the nature of fluid-rock interactions (including the weathering processes associated with continental emergence) may have been different in the past, leading to an evolving oceanic oxygen isotope composition. Here we examine approximately 3.24-billion-year-old hydrothermally altered oceanic crust from the Panorama district in the Pilbara Craton of Western Australia as an alternative oxygen isotope archive to marine chemical sediments. We find that, at that time, seawater at Panorama had an oxygen isotope composition enriched in O-18 relative to the modern ocean with a delta O-18 of 3.3 +/- 0.1 parts per thousand VSMOW. We suggest that seawater delta O-18 may have decreased through time, in contrast to the large increases seen in marine chemical sediments. To explain this possibility, we construct an oxygen isotope exchange model of the geologic water cycle, which suggests that the initiation of continental weathering in the late Archaean, between 3 and 2.5 billion years ago, would have drawn down an O-18-enriched early Archaean ocean to delta O-18 values similar to those of modern seawater. We conclude that Earth's water cycle may have gone through two separate phases of steady-state behaviour, before and after the emergence of the continents.The water cycle was in two different steady states, before and after continental emergence, as recorded in the decreasing oxygen isotope values of seawater since the Archaean, according to an inverse geochemical model of the oceanic crustal record.