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The isotopic signature of the global riverine molybdenum flux and anoxia in the ancient oceans

Nature Geoscience volume 1, pages 597–600 (2008)Cite this article

Abstract

Despite its important biological and biogeochemical consequences1,2,3, the identification of extensive oceanic anoxia in the geological record is controversial. In particular, global anoxia is difficult to distinguish from spatially restricted anoxia in the deep ocean, or in tectonically isolated basins such as the modern Black Sea. The marine isotope geochemistry of molybdenum (Mo) can help quantify the past oxygenation state of the ocean4,5,6,7,8,9,10,11,12,13,14, because to first approximation under oxic conditions lighter isotopes of Mo are preferentially removed to sediments, whereas in euxinic conditions quantitative removal leads to no fractionation. However, the isotopic composition of the Mo input from rivers, the main contributor of Mo to the oceans, is poorly constrained and had been assumed to be isotopically comparable to the narrow range found in a small dataset of continental rocks4,5,6,7,8,9,10,11,12,13,14. Here we present an isotopic analysis of Mo in a set of rivers that together account for 22% of the global riverine water discharge. We find a broad range of variability in the Mo isotopic composition of these rivers, with almost all samples enriched in the heavy isotopes compared with continental rocks. Our data remove key uncertainties associated with the marine Mo isotope budget14 and strongly suggest near-total anoxia in the mid-Proterozoic ocean8 as well as during Mesozoic ocean anoxic events13.

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Figure 1: Mo isotope and concentration data for major rivers.
Figure 2: Mo isotope and concentration data for riverine particulates and an estuary.
Figure 3: Quantitative model of Mo isotope balance in the present and past oceans.

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Acknowledgements

This research was supported by the NERC. We also thank J. Perkins, R. Mills, D. Teagle, A. Scrivner, A. El Sheikh, H. Kerr, J. Simpson, A. Page and N. Harris for their help with collection of the various river samples. We thank T. Elliott, C. Hawkesworth, J. McManus, A. Anbar for comments on an earlier draft. Jim’s review, particularly, fundamentally changed and improved the manuscript.

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  1. Department of Earth Sciences, Bristol Isotope Group, University of Bristol, Wills Memorial Building, Queen’s Road, Bristol BS8 1RJ, UK

    C. Archer & D. Vance

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Correspondence to C. Archer.

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Archer, C., Vance, D. The isotopic signature of the global riverine molybdenum flux and anoxia in the ancient oceans. Nature Geosci 1, 597–600 (2008). https://doi.org/10.1038/ngeo282

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