Institut für Geologie und Mineralogie, Universität zu Köln, 50674 Köln, Germany;
Geowissenschaftliches Zentrum, Georg-August-Universität Göttingen, 37073 Göttingen, Germany.
Proc Natl Acad Sci U S A. 2021 Jun 8;118(23). doi: 10.1073/pnas.2023617118.
The low O/O stable isotope ratios (δO) of ancient chemical sediments imply ∼70 °C Archean oceans if the oxygen isotopic composition of seawater (sw) was similar to modern values. Models suggesting lower δO of Archean seawater due to intense continental weathering and/or low degrees of hydrothermal alteration are inconsistent with the triple oxygen isotope composition (Δ'O) of Precambrian cherts. We show that high CO sequestration fluxes into the oceanic crust, associated with extensive silicification, lowered the δO of seawater on the early Earth without affecting the Δ'O. Hence, the controversial long-term trend of increasing δO in chemical sediments over Earth's history partly reflects increasing δO due to decreasing atmospheric CO We suggest that δO increased from about -5‰ at 3.2 Ga to a new steady-state value close to -2‰ at 2.6 Ga, coinciding with a profound drop in CO that has been suggested for this time interval. Using the moderately low δO values, a warm but not hot climate can be inferred from the δO of the most pristine chemical sediments. Our results are most consistent with a model in which the "faint young Sun" was efficiently counterbalanced by a high-CO greenhouse atmosphere before 3 Ga.
如果古代海水的氧同位素组成与现代相似，那么古老化学沉积物中的低 O/O 稳定同位素比值（δO）表明太古海洋温度约为 70°C。由于强烈的大陆风化作用和/或低热液蚀变程度导致太古海水 δO 较低的模型与前寒武纪燧石的三重氧同位素组成（Δ'O）不一致。我们表明，与广泛的硅化作用相关的大量 CO 向海洋地壳中的封存通量降低了早期地球上海水的 δO，而不会影响 Δ'O。因此，地球历史上化学沉积物中 δO 长期增加的争议趋势部分反映了由于大气 CO 减少导致的 δO 增加。我们认为，δO 从大约 32 亿年前的-5‰增加到 26 亿年前接近-2‰的新稳态值，与这一时期 CO 大量减少相吻合。利用适度较低的 δO 值，可以从最原始的化学沉积物的 δO 推断出温暖但不炎热的气候。我们的结果与一个模型最为一致，即在 30 亿年前之前，“微弱的年轻太阳”被高 CO 温室大气有效地抵消了。
