The geochemistry of late Archaean microbial carbonate: Implications for ocean chemistry and continental erosion history

Kamber, B. S. and Webb, G. E. (2001) The geochemistry of late Archaean microbial carbonate: Implications for ocean chemistry and continental erosion history. Geochimica Et Cosmochimica Acta, 65 15: 2509-2525. doi:10.1016/S0016-7037(01)00613-5


Author Kamber, B. S.
Webb, G. E.
Title The geochemistry of late Archaean microbial carbonate: Implications for ocean chemistry and continental erosion history
Journal name Geochimica Et Cosmochimica Acta   Check publisher's open access policy
ISSN 0016-7037
Publication date 2001
Sub-type Article (original research)
DOI 10.1016/S0016-7037(01)00613-5
Volume 65
Issue 15
Start page 2509
End page 2525
Total pages 17
Place of publication Oxford
Publisher Elsevier Science
Collection year 2001
Language eng
Subject C1
260301 Geochronology and Isotope Geochemistry
780104 Earth sciences
0499 Other Earth Sciences
Abstract Trace element concentrations and combined Sr- and Nd-isotope compositions were determined on stromatolitic carbonates (microbialites) from the 2.52 Ga Campbellrand carbonate platform (South Africa). Shale-normalised rare earth element and yttrium patterns of the ancient samples are similar to those of modern seawater in having positive La and Y anomalies and in being depleted in light rare earth elements. In contrast to modem seawater (and microbialite proxies), the 2.52 Ga samples lack a negative Ce anomaly but possess a positive Eu anomaly. These latter trace element characteristics are interpreted to reflect anoxic deep ocean waters where, unlike today, hydrothermal Fe input was not oxidised, and scavenged and rare earth elements were not coprecipitated with Fe-oxyhydroxides. The persistence of a positive Eu anomaly in relatively shallow Campbellrand platform waters indicates a dramatic reversal from hydrothermally dominated (Archaean) to continental erosion-dominated (Phanerozoic) rare earth element flux ratio. The dominant hydrothermal input is also expressed in the initial Sr- and Nd-isotope ratios. There is collinear variation in Sr-Nd systematics, which range from primitive values (Sr-87/Sr-86 of 0.702386 and epsilon (Nd) of +2.1) to more evolved crustal ratios. Mixing calculations show that the range in trace element ratios (e.g., Y/Ho) and initial isotope ratios is not a result of contamination by trapped sediment, but that the chemical band isotopic variation reflects carbonate deposition in an environment where different water masses mixed. Calculated Nd flux ratios yield a hydrothermal input into the 2.52 Ga oceans one order of magnitude larger than continental input. Such a change in flux ratio most likely required substantially reduced continental inputs, which could, in turn, reflect a plate tectonic causation (e.g., reduced topography or expansion of epicontinental seas). Copyright (C) 2001 Elsevier Science Ltd.
Keyword Geochemistry & Geophysics
Rare-earth Elements
Banded Iron-formations
Nd Isotopic Variations
Late-archean Seawater
Mid-atlantic Ridge
Transvaal Supergroup
South-africa
Hydrothermal Fluids
Sm-nd
Precambrian Carbonates
Q-Index Code C1

Document type: Journal Article
Sub-type: Article (original research)
Collection: School of Earth Sciences Publications
 
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Created: Tue, 14 Aug 2007, 15:45:31 EST