Persistent global marine euxinia in the early Silurian

Richard G Stockey,Devon B Cole,Jiří Frýda,Erik A Sperling,David K Loydell,Noah J Planavsky

doi:10.1038/s41467-020-15400-y

Abstract

The second pulse of the Late Ordovician mass extinction occurred around the Hirnantian-Rhuddanian boundary (~444 Ma) and has been correlated with expanded marine anoxia lasting into the earliest Silurian. Characterization of the Hirnantian ocean anoxic event has focused on the onset of anoxia, with global reconstructions based on carbonate δ238U modeling. However, there have been limited attempts to quantify uncertainty in metal isotope mass balance approaches. Here, we probabilistically evaluate coupled metal isotopes and sedimentary archives to increase constraint. We present iron speciation, metal concentration, δ98Mo and δ238U measurements of Rhuddanian black shales from the Murzuq Basin, Libya. We evaluate these data (and published carbonate δ238U data) with a coupled stochastic mass balance model. Combined statistical analysis of metal isotopes and sedimentary sinks provides uncertainty-bounded constraints on the intensity of Hirnantian-Rhuddanian euxinia. This work extends the duration of anoxia to >3 Myrs – notably longer than well-studied Mesozoic ocean anoxic events.

Highlights

The second pulse of the Late Ordovician mass extinction occurred around the HirnantianRhuddanian boundary (~444 Ma) and has been correlated with expanded marine anoxia lasting into the earliest Silurian
We confidently show that euxinic bottomwaters were likely two orders of magnitude more widespread than today for a period of >3 Myrs following the second pulse of the Late Ordovician mass extinction, significantly longer than well-studied Mesozoic ocean anoxic events
The economic importance of the mid-Rhuddanian hot shale as a hydrocarbon source rock has led to the detailed biostratigraphic characterization of the E1-NC174 core[21,25,26], indicating that the stratigraphic record presented here spans from at least the lowermost Rhuddanian (Normalograptis tilokensis Biozone in Gondwana, Akidograptus ascensus Biozone in global biostratigraphy) to the uppermost Rhuddanian (Neodiplograptus fezzanensis Biozone in Gondwana, Coronograptus cyphus Biozone in global biostratigraphy; Fig. 1; Supplementary Note 1)

Summary

Results

The offset (~0.5‰) between δ238Ueux data presented here and published δ238Ucarb data[8] (reported mean value = −0.45‰) for the early Rhuddanian is consistent with modern observations of the fractionation between shallow-water carbonates and euxinic sedimentary sinks[44,48] (Supplementary Fig. 5), adding support that the Murzuq Basin shales and Anticosti Island carbonates both record global trace metal isotope signals. Molybdenum and uranium mass balance model results are illustrated as frequency distributions of predicted euxinic shale δ98Mo and δ238U values corresponding to a range of logarithmically scaled scenarios for global marine euxinia (Fig. 2, feux describes the fraction of global seafloor with euxinic bottomwaters). The incorporation of a second redoxsensitive δ238U archive and δ98Mo data allow us to increase quantitative constraint on the extent of Rhuddanian euxinia (relative to δ238Ucarb data alone, Fig. 2), and to extend the record of euxinia throughout the Rhuddanian Age

Discussion

Methods

Code availability