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Global importance of oxic molybdenum sinks prior to 2.6 Ga revealed by the Mo isotope composition of Precambrian carbonates

Marie Thoby, Kurt O. Konhauser, Philip W. Fralick, Wladyslaw Altermann, Pieter T. Visscher and Stefan V. Lalonde
Global importance of oxic molybdenum sinks prior to 2.6 Ga revealed by the Mo isotope composition of Precambrian carbonates
Geology (Boulder) (April 2019) Pre-Issue Publication

Abstract

Sedimentary molybdenum (Mo) isotope compositions are a promising paleoredox indicator because the Mo isotope composition of seawater reflects the balance between anoxic and oxic sinks. Most available data are from shales; however, the Mo isotope composition of carbonates also reflects the composition of ancient seawater. Here, we provide an expanded data set of carbonate Mo isotope compositions, including the first data for carbonates older than 2.64 Ga, which we evaluate against a compilation of published data for carbonates, shales, and iron formations spanning geological time. Archean carbonate samples reveal maximum delta (super 98) Mo values that are generally above 1 ppm. These heavy values indicate that Mn(IV)-oxide or Fe(III)-oxide sinks were sufficiently important to influence the Mo isotope composition of seawater as far back as 2.93 Ga. Comparison of Mo isotope and rare earth element data, as well as residence time considerations, indicates that this metal-oxide influence was likely global. Available Mo isotope data for shales over the same time period generally show crustal values, which we attribute to negligible authigenic enrichment of Mo from seawater due to low ambient concentrations and a paucity of euxinic conditions. Our work demonstrates that the carbonate record provides important new insights into marine paleoredox conditions, especially when shale records are absent or unsuitable, and reinforces the emerging paradigm that oxic Mo sinks were important in the marine realm prior to 2.7 Ga.


ISSN: 0091-7613
EISSN: 1943-2682
Coden: GLGYBA
Serial Title: Geology (Boulder)
Serial Volume: Pre-Issue Publication
Title: Global importance of oxic molybdenum sinks prior to 2.6 Ga revealed by the Mo isotope composition of Precambrian carbonates
Affiliation: Universite de Bretagne Occidentale, Institut Universitaire Europeen de la Mer, Plouzane, France
Published: 20190416
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 24
Accession Number: 2019-038019
Categories: Isotope geochemistryStratigraphy
Document Type: Serial
Bibliographic Level: Analytic
Annotation: GSA Data Repository item 2019205
Illustration Description: illus. incl. sketch maps
Secondary Affiliation: University of Alberta, CAN, CanadaLakehead University, CAN, CanadaUniversity of Pretoria, ZAF, South AfricaUniversity of Connecticut, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 2019
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