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

Journal Article


Abstract


  • 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 CO2), or that

    early Earth had a lower albedo because it was essentially oceanic, without exposed continental crust

    and ice caps.

Authors


  •   Nutman, Allen Phillip.
  •   Bennett, Vickie C. (external author)
  •   Friend, Clark R. L. (external author)

Publication Date


  • 2012

Citation


  • Nutman, A. Phillip., Bennett, V. C. & Friend, C. R L. (2012). Waves and weathering at 3.7 Ga: Geological evidence for an equitable terrestrial climate under the faint early Sun. Australian Journal of Earth Sciences, 59 (2), 167-176.

Scopus Eid


  • 2-s2.0-84859321100

Ro Metadata Url


  • http://ro.uow.edu.au/scipapers/4675

Number Of Pages


  • 9

Start Page


  • 167

End Page


  • 176

Volume


  • 59

Issue


  • 2

Abstract


  • 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 CO2), or that

    early Earth had a lower albedo because it was essentially oceanic, without exposed continental crust

    and ice caps.

Authors


  •   Nutman, Allen Phillip.
  •   Bennett, Vickie C. (external author)
  •   Friend, Clark R. L. (external author)

Publication Date


  • 2012

Citation


  • Nutman, A. Phillip., Bennett, V. C. & Friend, C. R L. (2012). Waves and weathering at 3.7 Ga: Geological evidence for an equitable terrestrial climate under the faint early Sun. Australian Journal of Earth Sciences, 59 (2), 167-176.

Scopus Eid


  • 2-s2.0-84859321100

Ro Metadata Url


  • http://ro.uow.edu.au/scipapers/4675

Number Of Pages


  • 9

Start Page


  • 167

End Page


  • 176

Volume


  • 59

Issue


  • 2