Iron isotope biogeochemistry of Neoproterozoic marine shales

Iron isotopes have been widely applied to investigate the redox evolution of Earth’s surface environments. However, it is still unclear whether iron cycling in the water column or during diagenesis represents the major control on the iron isotope composition of sediments and sedimentary rocks. Inter...

Full description

Saved in:
Bibliographic Details
Published in:Geochimica et cosmochimica acta 2017-07, Vol.209, p.85-105
Main Authors: Kunzmann, Marcus, Gibson, Timothy M., Halverson, Galen P., Hodgskiss, Malcolm S.W., Bui, Thi Hao, Carozza, David A., Sperling, Erik A., Poirier, André, Cox, Grant M., Wing, Boswell A.
Format: Article
Language:English
Subjects:
Diagenesis
Iron isotopes
Neoproterozoic
Oxygenation
Shale
Citations: Items that this one cites
Items that cite this one
Online Access:Get full text
Tags: Add Tag
No Tags, Be the first to tag this record!
Description
Summary:Iron isotopes have been widely applied to investigate the redox evolution of Earth’s surface environments. However, it is still unclear whether iron cycling in the water column or during diagenesis represents the major control on the iron isotope composition of sediments and sedimentary rocks. Interpretation of isotopic data in terms of oceanic redox conditions is only possible if water column processes dominate the isotopic composition, whereas redox interpretations are less straightforward if diagenetic iron cycling controls the isotopic composition. In the latter scenario, iron isotope data is more directly related to microbial processes such as dissimilatory iron reduction. Here we present bulk rock iron isotope data from late Proterozoic marine shales from Svalbard, northwestern Canada, and Siberia, to better understand the controls on iron isotope fractionation in late Proterozoic marine environments. Bulk shales span a δ56Fe range from −0.45‰ to +1.04‰. Although δ56Fe values show significant variation within individual stratigraphic units, their mean value is closer to that of bulk crust and hydrothermal iron in samples post-dating the ca. 717–660Ma Sturtian glaciation compared to older samples. After correcting for the highly reactive iron content in our samples based on iron speciation data, more than 90% of the calculated δ56Fe compositions of highly reactive iron falls in the range from ca. −0.8‰ to +3‰. An isotope mass-balance model indicates that diagenetic iron cycling can only change the isotopic composition of highly reactive iron by
ISSN:0016-7037
1872-9533
DOI:10.1016/j.gca.2017.04.003