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Publication Details
AFRICAN RESEARCH NEXUS
SHINING A SPOTLIGHT ON AFRICAN RESEARCH
earth and planetary sciences
Rare Earth Element and yttrium compositions of Archean and Paleoproterozoic Fe formations revisited: New perspectives on the significance and mechanisms of deposition
Geochimica et Cosmochimica Acta, Volume 74, No. 22, Year 2010
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Description
The ocean and atmosphere were largely anoxic in the early Precambrian, resulting in an Fe cycle that was dramatically different than today's. Extremely Fe-rich sedimentary deposits-i.e., Fe formations-are the most conspicuous manifestation of this distinct Fe cycle. Rare Earth Element (REE) systematics have long been used as a tool to understand the origin of Fe formations and the corresponding chemistry of the ancient ocean. However, many earlier REE studies of Fe formations have drawn ambiguous conclusions, partially due to analytical limitations and sampling from severely altered units. Here, we present new chemical analyses of Fe formation samples from 18 units, ranging in age from ca. 3.0 to 1.8 billion years old (Ga), which allow a reevaluation of the depositional mechanisms and significance of Precambrian Fe formations. There are several temporal trends in our REE and Y dataset that reflect shifts in marine redox conditions. In general, Archean Fe formations do not display significant shale-normalized negative Ce anomalies, and only Fe formations younger than 1.9. Ga display prominent positive Ce anomalies. Low Y/Ho ratios and high shale-normalized light to heavy REE (LREE/HREE) ratios are also present in ca. 1.9. Ga and younger Fe formations but are essentially absent in their Archean counterparts. These marked differences in Paleoproterozoic versus Archean REE. +. Y patterns can be explained in terms of varying REE cycling in the water column. Similar to modern redox-stratified basins, the REE. +. Y patterns in late Paleoproterozoic Fe formations record evidence of a shuttle of metal and Ce oxides across the redoxcline from oxic shallow seawater to deeper anoxic waters. Oxide dissolution-mainly of Mn oxides-in an anoxic water column lowers the dissolved Y/Ho ratio, raises the light to heavy REE ratio, and increases the concentration of Ce relative to the neighboring REE (La and Pr). Fe oxides precipitating at or near the chemocline will capture these REE anomalies and thus evidence for this oxide shuttle. In contrast, Archean Fe formations do not display REE. +. Y patterns indicative of an oxide shuttle, which implies an absence of a distinct Mn redoxcline prior to the rise of atmospheric oxygen in the early Paleoproterozoic. As further evidence for reducing conditions in shallow-water environments of the Archean ocean, REE data for carbonates deposited on shallow-water Archean carbonate platforms that stratigraphically underlie Fe formations also lack negative Ce anomalies. These results question classical models for deposition of Archean Fe formations that invoke oxidation by free oxygen at or above a redoxcline. In contrast, we add to growing evidence that metabolic Fe oxidation is a more likely oxidative mechanism for these Fe formations, implying that the Fe distribution in Archean oceans could have been controlled by microbial Fe uptake rather than the oxidative potential of shallow-marine environments. © 2010 Elsevier Ltd.
Authors & Co-Authors
Planavsky, Noah J.
United States, Riverside
University of California, Riverside
United States, Woods Hole
Woods Hole Oceanographic Institution
Bekker, Andrey Yu
Canada, Winnipeg
University of Manitoba
Rouxel, Olivier J.
United States, Woods Hole
Woods Hole Oceanographic Institution
France, Brest
Université de Brest Ubo
Kamber, Balz Samuel
Canada, Sudbury
Université Laurentienne
Hofmann, Axel
South Africa, Durban
University of Kwazulu-natal
Knudsen, Andrew C.
United States, Appleton
Lawrence University, Appleton
Lyons, Timothy W.
United States, Riverside
University of California, Riverside
Statistics
Citations: 390
Authors: 7
Affiliations: 7
Identifiers
Doi:
10.1016/j.gca.2010.07.021
ISSN:
00167037
Research Areas
Environmental
Noncommunicable Diseases