Global importance of oxic molybdenum sinks prior to 2.6 Ga revealed by the Mo isotope composition of Precambrian carbonates
|Author(s)||Thoby Marie1, Konhauser Kurt O.2, Fralick Philip W.3, Altermann Wladyslaw4, Visscher Pieter T.5, Lalonde Stefan1|
|Affiliation(s)||1 : Univ Bretagne Occidentale, Inst Univ Europeen Mer, Lab Geosci Ocean, CNRS UMR6538, F-29280 Plouzane, France.
2 : Univ Alberta, Dept Earth & Atmospher Sci, Edmonton, AB T6G 2E3, Canada.
3 : Lakehead Univ, Dept Geol, Thunder Bay, ON P7B 5E1, Canada.
4 : Univ Pretoria, Dept Geol, Private Bag X20, ZA-0028 Pretoria, South Africa.
5 : Univ Connecticut, Dept Marine Sci, Groton, CT 06340 USA.
|Source||Geology (0091-7613) (Geological Soc Amer, Inc), 2019-06 , Vol. 47 , N. 6 , P. 559-562|
|WOS© Times Cited||19|
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 Mo-98 values that are generally above 1 parts per thousand. 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.