Waves and weathering at 3.7 Ga: Geological evidence for an equitable terrestrial climate under the faint early Sun

摘要

Chemical sedimentary rocks such as banded iron formation (BIF) and pillow basalts are persistent features of the oldest volcanic and sedimentary record by 3.8–3.7 Ga, and are direct evidence for oceans by the start of the Archean. However, their presence does not dictate an equitable 3.8–3.7 Ga terrestrial climate. This is because they could have formed in oceans below global pack ice on a frigid Earth. The oldest known depositional structures occur as locally preserved features in ca 3.7 Ga deformed, amphibolite facies rocks of the Isua supracrustal belt (Greenland). These include units up to ～1 m thick, in which there are stacked or jumbled clasts of chert. Detailed structural analysis shows that these rocks are not tectonic breccias. Also, their reported location in chemical sedimentary units capping slightly older volcanic rocks shows they are unlikely to be mass-flow deposits in a deep basin. In both composition and structure, these units resemble edgewise breccias observed on later Precambrian and Phanerozoic chemical sediment platforms, which formed when laminated sediments are disrupted by storm waves. Hence, the wave origin shows oceans were not ice covered, because in that case atmospheric storms would not generate waves. The Isua supracrustal belt also contains 3.72–3.70 Ga felsic and pelitic sedimentary rocks, derived from juvenile volcanic arc sources. These sedimentary rocks have chemical weathering indices that deviate from those of both fresh Eoarchean and modern igneous rocks. Furthermore, their weathering indices are congruent with rare examples of weathered (not hydrothermally altered) Isua volcanic rocks we have identified. Although no doubt the chemistry of Eoarchean weathering processes was different from those now, this nonetheless shows that these sedimentary rocks contain large contributions from highly weathered source materials.Rapid advanced weathering (these rocks consist of materials shed from an arc) is most feasible with an equitable to hot climate, rather than a frigid one, because higher temperatures enhance chemical reactions between rocks, atmospheric gases and precipitation. Wave-generated structures in chemical sedimentary rocks and advanced weathering at ca 3.7 Ga point to an equitable terrestrial climate, with at least partially ice-free oceans. With the faint early sun at ca 3.7 Ga, proposed mechanisms to facilitate this are either a stronger greenhouse atmosphere (e.g. more CO_2), or that early Earth had a lower albedo because it was essentially oceanic, without exposed continental crust and ice caps.