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A molybdenum-isotope perspective on Phanerozoic deoxygenation events

Abstract

The expansion and contraction of sulfidic depositional conditions in the oceans can be tracked with the isotopic composition of molybdenum in marine sediments. However, molybdenum-isotope data are often subject to multiple conflicting interpretations. Here I present a compilation of molybdenum-isotope data from three time intervals: the Toarcian Oceanic Anoxic Event about 183 million years ago, Oceanic Anoxic Event 2 about 94 million years ago, and two early Eocene hyperthermal events from 56 to 54 million years ago. A comparison of data from sites located in different hydrographic settings tightly constrains the molybdenum cycle for these intervals, allowing a direct comparison of the expanse of sulfidic conditions in each interval compared to today. Nonetheless, tracing rates of redox change over such rapid climatic events using molybdenum isotopes remains challenging. Future efforts to achieve this goal might be accomplished by analysing specific mineral phases, using complementary redox-sensitive geochemical techniques and by linking isotopic observations with Earth system modelling. Such improvements will make it possible to more fully assess the links between ocean deoxygenation, climatic and oceanographic changes, and biotic turnover.

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Figure 1: Molybdenum-isotope fluxes in the modern marine environment.
Figure 2: Compilation of sediment Mo-isotope data.
Figure 3: Simplified mass-balance model of the fractional removal of molybdenum into sulfidic and oxic marine sediments as a function of the isotopic composition of seawater and riverine inputs.

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Acknowledgements

The ideas in this paper benefitted from discussions with H. Jenkyns, A.S. Cohen, M.-L. Bagard, J. Barling, D. Porcelli and E. Idiz.

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Correspondence to Alexander J. Dickson.

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Dickson, A. A molybdenum-isotope perspective on Phanerozoic deoxygenation events. Nature Geosci 10, 721–726 (2017). https://doi.org/10.1038/ngeo3028

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