Statistical analysis of iron geochemical data suggests limited late Proterozoic oxygenation


Sedimentary rocks deposited across the Proterozoic–Phanerozoic transition record extreme climate fluctuations, a potential rise in atmospheric oxygen or re-organization of the seafloor redox landscape, and the initial diversification of animals1,2. It is widely assumed that the inferred redox change facilitated the observed trends in biodiversity. Establishing this palaeoenvironmental context, however, requires that changes in marine redox structure be tracked by means of geochemical proxies and translated into estimates of atmospheric oxygen. Iron-based proxies are among the most effective tools for tracking the redox chemistry of ancient oceans3,4. These proxies are inherently local, but have global implications when analysed collectively and statistically. Here we analyse about 4,700 iron-speciation measurements from shales 2,300 to 360 million years old. Our statistical analyses suggest that subsurface water masses in mid-Proterozoic oceans were predominantly anoxic and ferruginous (depleted in dissolved oxygen and iron-bearing), but with a tendency towards euxinia (sulfide-bearing) that is not observed in the Neoproterozoic era. Analyses further indicate that early animals did not experience appreciable benthic sulfide stress. Finally, unlike proxies based on redox-sensitive trace-metal abundances1,5,6, iron geochemical data do not show a statistically significant change in oxygen content through the Ediacaran and Cambrian periods, sharply constraining the magnitude of the end-Proterozoic oxygen increase. Indeed, this re-analysis of trace-metal data is consistent with oxygenation continuing well into the Palaeozoic era. Therefore, if changing redox conditions facilitated animal diversification, it did so through a limited rise in oxygen past critical functional and ecological thresholds, as is seen in modern oxygen minimum zone benthic animal communities7,8,9.

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Figure 1: Iron geochemical data compared using five time bins.
Figure 2: Unbinned analysis of the proportion of anoxic samples from each region for the time period 800–360 Myr ago.
Figure 3: Ocean–atmosphere oxygenation through the Proterozoic–Phanerozoic transition.


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We thank D. Cole, A. Masterson, E. Beirne, G. Resendiz, A. Miller and W. Mai for laboratory assistance, E. Smith, T. Petach, M. Laflamme, S. Darroch, P. Myrow, J. Strauss, C. Carbone and G. Narbonne for field assistance, L. Levin, P. Girguis, C. Frieder, L. Duncan, C. Partin, S. Darroch and U. Farrell for discussions, and E. Hammarlund, C. Scott, R. Raiswell, G. Gilleaudeau, S. Sahoo, Y. Shen, J. Creveling, T. Dahl, C. Partin and D. Stolper for providing raw data from published studies. E.A.S. is funded by Agouron Geobiology and NAI Postdoctoral Fellowships and by the NSF-EAR 1324095 grant to L. Levin. This work is supported by the NSF-EAR 1324095 grant to D.T.J. and by the NASA Astrobiology Institute (A.H.K. and F.A.M.).

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E.A.S. and D.T.J. conceived the project, E.A.S., A.S.M., B.C.G., G.P.H., F.A.M. and D.T.J. collected samples, E.A.S., A.S.M., B.C.G. and M.K. completed geochemical measurements, E.A.S. and C.J.W. compiled data and analysed the global data set, and E.A.S. wrote the paper with input from all co-authors.

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Correspondence to Erik A. Sperling or David T. Johnston.

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Sperling, E., Wolock, C., Morgan, A. et al. Statistical analysis of iron geochemical data suggests limited late Proterozoic oxygenation. Nature 523, 451–454 (2015).

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