A molybdenum-isotope perspective on Phanerozoic deoxygenation events

• Published in: Nature geoscience Vol. 10; no. 10; pp. 721 - 726
• Main Author: Dickson, Alexander J.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 01.10.2017; Nature Publishing Group
• Subjects: 704/106/2738; 704/106/413; 704/47/4112; Anoxia; Climate change; Contraction; Data; Deoxygenation; Earth; Earth Sciences; Earth System Sciences; Eocene; Geochemistry; Geology; Geophysics/Geodesy; Intervals; Isotope composition; Isotopes; Marine sediments; Modelling; Molybdenum; Molybdenum isotopes; Oceans; Oxidoreductions; perspective; Phanerozoic; Sediments
• ISSN: 1752-0894; 1752-0908
• DOI: 10.1038/ngeo3028

The careful compilation and interpretation of molybdenum isotopes can track the expansion of sulfidic bottom waters. A synthesis and analysis of data from two Mesozoic ocean anoxic events and the Palaeocene-Eocene thermal maximum applies these techniques to constrain past ocean deoxygenation. 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.