Persistent global marine euxinia in the early Silurian

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

No evidence for high atmospheric oxygen levels 1,400 million years ago

Zhang et al. (1) recently proposed atmospheric oxygen levels of ∼4% present atmospheric levels (PAL) based on modeling a paleoenvironment reconstructed from trace metal and biomarker data from the 1,400 Ma Xiamaling Formation in China. Intriguingly, this pO2 level is above the threshold oxygen requirements of basal animals and clashes with evidence for atmospheric oxygen levels <<1% PAL in the mid-Proterozoic (2). However, there are fundamental problems with the inorganic and organic geochemical work presented by Zhang et al. (1)

New constraints on mid-Proterozoic ocean redox from stable thallium isotope systematics of black shales

The final publication is available at Elsevier via https://doi.org/10.1016/j.gca.2021.09.006. © 2021. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/Stable thallium (Tl) isotope data from organic-rich siliciclastic sedimentary rocks have the potential to track ocean redox state on a broad scale. Here, we report new Tl isotope data from the Mesoproterozoic Velkerri Formation (Roper Group) and the Paleoproterozoic Wollogorang Formation (Tawallah Group), McArthur Basin, Northern Territory, Australia, and interpret these in the context of rhenium-osmium (Re-Os) geochronometry on the same sample suite. Previous work has shown that marine black shales from the Velkerri Formation provide evidence for closed-system Re-Os systematics, yielding a precise isochron with an age of 1361 ± 21 Ma that agrees well with independent age constraints for the unit. The isotopic composition of authigenic Tl in euxinic black shales from the upper Velkerri Formation (ε205Tl = -2.4 ± 0.8, 2SD) indicates that the Tl isotope composition of local seawater at 1.36 Ga was within a plausible range for Tl inputs to the ocean. Isotope mass balance modeling of the Tl isotope system within a Monte Carlo framework suggests that the Tl isotopic composition of seawater at 1.36 Ga was homogenous on a global scale and that the burial of Mn-oxides exerted minimal isotopic leverage on the Tl isotope composition of seawater at 1.36 Ga. Taken together with existing Mo, Cr, and U isotope data from the same samples, these observations are consistent with a low-O2 ocean-atmosphere system during this period of the Mesoproterozoic. Previous work has shown that the Re-Os systematics of black shales from the older (1.73 Ga) Wollogorang Formation are scattered and yield an erroneously young isochron age of 1359 ± 150 Ma, which has been attributed to post-depositional hydrothermal alteration at ~1640 Ma. We observe no systematic relationship between stable Tl isotope compositions and the extent of alteration as gauged by open-system Re-Os behavior (-4.7 ± 1.4 for the upper Wollogorang Formation and -4.8 ± 0.4 for the lower Wollogorang Formation), in marked contrast to previous observations for the molybdenum (Mo) and uranium (U) isotope systems. The invariant signature of the Tl isotope data suggests the Tl isotope system was largely unperturbed during hydrothermal alteration. However, it remains difficult to definitively rule out the possibility that authigenic Tl isotope signatures have been overprinted by later localized hydrothermal fluid alteration in the Wollogorang Formation shales. These observations highlight the potential insights afforded by evaluating open-system behavior via radiogenic isotope systems together with other stable isotope tracers in efforts to reconstruct the redox landscape of Earth’s oceans over time

Curation and Analysis of Global Sedimentary Geochemical Data to Inform Earth History

Large datasets increasingly provide critical insights into crustal and surface processes on Earth. These data come in the form of published and contributed observations, which often include associated metadata. Even in the best-case scenario of a carefully curated dataset, it may be non-trivial to extract meaningful analyses from such compilations, and choices made with respect to filtering, resampling, and averaging can affect the resulting trends and any interpretation(s) thereof. As a result, a thorough understanding of how to digest, process, and analyze large data compilations is required. Here, we present a generalizable workflow developed using the Sedimentary Geochemistry and Paleoenvironments Project database. We demonstrate the effects of filtering and weighted resampling on Al2O3 and U contents, two representative geochemical components of interest in sedi-mentary geochemistry (one major and one trace element, respectively). Through our analyses, we highlight several methodological challenges in a "bigger data" approach to Earth science. We suggest that, with slight modifications to our workflow, researchers can confidently use large collections of observations to gain new insights into processes that have shaped Earth's crustal and surface environments

A long-term record of early to mid-Paleozoic marine redox change

The extent to which Paleozoic oceans differed from Neoproterozoic oceans and the causal relationship between biological evolution and changing environmental conditions are heavily debated. Here, we report a nearly continuous record of seafloor redox change from the deep-water upper Cambrian to Middle Devonian Road River Group of Yukon, Canada. Bottom waters were largely anoxic in the Richardson trough during the entirety of Road River Group deposition, while independent evidence from iron speciation and Mo/U ratios show that the biogeochemical nature of anoxia changed through time. Both in Yukon and globally, Ordovician through Early Devonian anoxic waters were broadly ferruginous (nonsulfidic), with a transition toward more euxinic (sulfidic) conditions in the mid–Early Devonian (Pragian), coincident with the early diversification of vascular plants and disappearance of graptolites. This ~80-million-year interval of the Paleozoic characterized by widespread ferruginous bottom waters represents a persistence of Neoproterozoic-like marine redox conditions well into the Phanerozoic

Sustained increases in atmospheric oxygen and marine productivity in the Neoproterozoic and Palaeozoic eras

A geologically rapid Neoproterozoic oxygenation event is commonly linked to the appearance of marine animal groups in the fossil record. However, there is still debate about what evidence from the sedimentary geochemical record—if any—provides strong support for a persistent shift in surface oxygen immediately preceding the rise of animals. We present statistical learning analyses of a large dataset of geochemical data and associated geological context from the Neoproterozoic and Palaeozoic sedimentary record and then use Earth system modelling to link trends in redox-sensitive trace metal and organic carbon concentrations to the oxygenation of Earth’s oceans and atmosphere. We do not find evidence for the wholesale oxygenation of Earth’s oceans in the late Neoproterozoic era. We do, however, reconstruct a moderate long-term increase in atmospheric oxygen and marine productivity. These changes to the Earth system would have increased dissolved oxygen and food supply in shallow-water habitats during the broad interval of geologic time in which the major animal groups first radiated. This approach provides some of the most direct evidence for potential physiological drivers of the Cambrian radiation, while highlighting the importance of later Palaeozoic oxygenation in the evolution of the modern Earth system