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Spherule Beds 3.47-3.24 Billion Years Old in the Barberton Greenstone Belt, South Africa: A Record of Large Meteorite Impacts and Their Influence on Early Crustal and Biological EvolutionFour layers, S1-S4, containing sand-sized spherical particles formed as a result of large meteorite impacts, occur in 3.47-3.24 Ga rocks of the Barberton Greenstone Belt, South Africa. Ir levels in S3 and S4 locally equal or exceed chondritic values but in other sections are at or only slightly above background. Most spherules are inferred to have formed by condensation of impact-produced rock vapor clouds, although some may represent ballistically ejected liquid droplets. Extreme Ir abundances and heterogeneity may reflect element fractionation during spherule formation, hydraulic fractionation during deposition, and/or diagenetic and metasomatic processes. Deposition of S1, S2, and S3 was widely influenced by waves and/or currents interpreted to represent impact-generated tsunamis, and S1 and S2 show multiple graded layers indicating the passage of two or more wave trains. These tsunamis may have promoted mixing within a globally stratified ocean, enriching surface waters in nutrients for biological communities. S2 and S3 mark the transition from the 300-million-year-long Onverwacht stage of predominantly basaltic and komatiitic volcanism to the late orogenic stage of greenstone belt evolution, suggesting that regional and possibly global tectonic reorganization resulted from these large impacts. These beds provide the oldest known direct record of terrestrial impacts and an opportunity to explore their influence on early life, crust, ocean, and atmosphere. The apparent presence of impact clusters at 3.26-3.24 Ga and approx. 2.65-2.5 Ga suggests either spikes in impact rates during the Archean or that the entire Archean was characterized by terrestrial impact rates above those currently estimated from the lunar cratering record.
Document ID
20030063016
Acquisition Source
Headquarters
Document Type
Reprint (Version printed in journal)
Authors
Lowe, Donald R.
(Stanford Univ. Stanford, CA, United States)
Byerly, Gary R.
(Louisiana State Univ. Baton Rouge, LA, United States)
Kyte, Frank T.
(California Univ. San Diego, CA, United States)
Shukolyukov, Alexander
(Scripps Institution of Oceanography San Diego, CA, United States)
Asaro, Frank
(California Univ., Lawrence Berkeley National Lab. Berkeley, CA, United States)
Krull, Alexander
(Stanford Univ. Stanford, CA, United States)
Date Acquired
August 21, 2013
Publication Date
January 1, 2003
Publication Information
Publication: Astrobiology
Publisher: Mary Ann Lieber, Inc.
Volume: 3
Issue: 1
Subject Category
Geosciences (General)
Funding Number(s)
CONTRACT_GRANT: NCC2-721
CONTRACT_GRANT: NSF EAR-9909684
CONTRACT_GRANT: NAG5-9842
CONTRACT_GRANT: NAG5-9411
Distribution Limits
Public
Copyright
Other

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