Jeremy D. Owens

Following the largest known Phanerozoic extinction, the end‐Permian mass extinction, the Early Triassic is often considered an interval of slow biotic recovery. During this time, the minor Smithian‐Spathian extinction occurred, further delaying the biotic recovery of this epoch. One mechanism linked to the slow recovery is widespread marine anoxic conditions, but the spatiotemporal changes to anoxia following the mass extinction remain poorly constrained. Here, we utilized local and global redox‐sensitive metal proxies to track variations in (de‐)oxygenation at Wallenbergfjellet, a section in Spitsbergen, Arctic Norway. Local proxies (e.g., Fe speciation and trace metal concentrations) indicate basinal anoxia and possibly euxinia prevalent in the Boreal Ocean during this interval. Importantly, thallium isotopes, a global paleoredox proxy sensitive to Mn oxide burial, have not previously been applied to the Early Triassic recovery interval that followed the end‐Permian mass extinction, though basinal restriction may limit geographic extent of interpretations. Our results show a shift in ε ²⁰⁵ Tl values from ~ −4.0 in the Smithian to ~ −2.0 at the Smithian‐Spathian boundary (SSB), consistent with a rapid expansion of global anoxia coincident with the boundary extinction. A rapid perturbation in thallium isotopes at the SSB (from −4.0 to −2.0 ε‐units) indicates this interval experienced an expansion of anoxic conditions that can be linked to the minor extinction event during the recovery interval following the end‐Permian mass extinction. This suggests that global anoxia persisted throughout the Early Triassic and contributed to delayed biotic recovery, with the SSB representing a major intensification of reducing conditions.

The long-term global oxygenation history of ancient oceans provides context for the rapidly decreasing oxygen concentrations in the present-day ocean. Here, we present thallium (Tl) isotope evidence from an epicontinental Laurasian Seaway, deposited during a greenhouse climate between 150 and 149 Ma, that suggests substantial changes in the global burial of manganese (Mn) oxide. This provides evidence for fluctuating global marine (de)oxygenation at a time when there is no evidence for major shifts in the global carbon cycle. Between periods when Tl isotope data suggest similar rates of burial of Mn oxides to the modern, reductions of up to 70% Mn oxide burial are inferred, suggesting major expansions of hypoxic bottom waters. Periods of more extensive global hypoxia are not coincident with regional enrichment of sedimentary organic matter. Fluctuations in deoxygenation occurred on geologically rapid timescales of 10,000 to 100,000 years but are still an order of magnitude lower than those occurring in the modern ocean.

Widespread marine anoxic events occurred throughout the Phanerozoic, most notably the Mesozoic oceanic anoxic events (OAEs). They were likely the result of major climatic perturbations that resulted in the burial of significant quantities of organic matter-rich sediments. During OAEs, reduced carbon and sulfur are more efficiently sequestered, which, based on stoichiometric balances, should result in a net increase of oxygen counteracting marine deoxygenation. This relationship has been the fundamental basis of models for the long-term rise in oxygen across deep-time. However, the geologic record of OAEs indicate widespread marine anoxia persisted on million-year timescales while these processes occurred. Many OAEs occur contemporaneous with the emplacement of large igneous provinces (LIPs), which released volatile compounds that likely induced climatic perturbations that could, in turn, impact marine (de)oxygenation. These volcanic systems also released reducing compounds that may provide a mechanism for the temporal exacerbation of OAEs. Additionally, the climatic effects of LIPs are associated with enhanced chemical weathering intensities, which may have promoted increased oxidative weathering that also consumed oxygen. Here, forward box models provide first-order quantifications of the excess oxygen produced via organic carbon and pyrite sulfur burial along with the effects of introducing LIP-sourced reductants and oxidative weathering to the ocean-atmosphere system during OAEs. This study focuses on Oceanic Anoxic Event 2 (~94 Ma) and the Toarcian Oceanic Anoxic Event (~184 Ma) as the most well-studied OAEs. During both events, significant increases in oxygen are produced from the burial of reduced compounds, approximately 10–100% of modern atmospheric oxygen levels. The added LIP-reductants partially or even completely buffer this excess oxygen through the oxidation of reduced volatile compounds. Short-term increases in oxidative weathering show a similar magnitude of oxygen removal from the ocean-atmosphere system. This analysis highlights the importance of quantifying the oxygen budget during OAEs and similar events.

Wind-borne dust supply of iron (Fe) to the oceans plays a crucial role in Earth’s biogeochemical cycles. Iron, a limiting micronutrient for phytoplankton growth, is fundamental in regulating ocean primary productivity and in turn the global carbon cycle. The flux of bioavailable Fe to the open ocean affects oscillations in atmospheric CO₂ due to its control on inorganic carbon fixation into organic matter that is eventually exported to the sediments. However, the nature of dust-delivered Fe to the ocean and controls on its bioavailability remain poorly constrained. To evaluate the supply of wind-borne bioavailable Fe and its potential impact on Fe-based climate feedbacks over the last 120,000 years, we examine sediment profiles from four localities that define a proximal to distal transect relative to Saharan dust inputs. Bulk δ⁵⁶Fe isotope compositions (average = -0.05‰) and Fe_T/Al ratios suggest crustal values, thus pointing to a dominant dust origin for the sediments at all four sites. We observed no variability in grain size distribution or in bioavailable Fe supply at individual sites as a function of glacial-versus-interglacial deposition. Importantly, there is no correlation between sediment grain size and Fe bioavailability. Spatial trends do, however, suggest increasing Fe bioavailability with increasing distance of atmospheric transport, and our sediments also indicate the loss of this Fe and thus potential bioavailability utilization once deposited in the ocean. Our study underscores the significance of Fe dynamics in oceanic environments using refined speciation techniques to elucidate patterns in Fe reactivity. Such insights are crucial for understanding nutrient availability and productivity in various ocean regions, including the Southern Ocean, where wind-delivered Fe may play a pivotal role. It is expected that dust delivery on glacial-interglacial timescales would be more pronounced in these high-latitude regions. Our findings suggest that studies linking Fe availability to marine productivity should benefit significantly from refined Fe speciation approaches, which provide insights into the patterns and controls on Fe reactivity, including atmospheric processing. These insights are essential for understanding the impacts on primary production and thus carbon cycling in the oceans and consequences for the atmosphere.

The Silurian was marked by repeated extinctions, carbon cycle volatility and significant intervals of climatic change. The most notable of these events were the Ludfordian Lau/Kozlowskii extinction and the associated Mid-Ludfordian Lau C isotope excursion, both of which have been linked to a period of global cooling and expanded reducing conditions in the global ocean. We present new data that characterize the marine palaeoredox conditions of the Prague Basin, a peri-Gondwanan terrane. We use I/Ca ratios to assess the local redox conditions in a shallow water carbonate succession and Fe speciation and redox-sensitive trace element concentrations to assess the local redox conditions of a deeper water sequence. Consistently low I/Ca values in the shallow water section suggest either persistent local low-oxygen conditions or possibly diagenetic overprinting. Fe speciation data suggest that the bottom water redox conditions in the deeper shelf setting were consistently anoxic with possible intermittent euxinia. Concentrations of redox-sensitive trace elements consistently higher than upper continental crust values also indicate persistent reducing conditions in the deeper part of the basin. These local redox proxy data from the Prague Basin, including trends in new pyrite S isotope ( δ ³⁴ S _pyr ) data, are consistent with previous findings of the expansion of anoxic and/or euxinic oceanic conditions. These data, derived from a mid-palaeolatitude marine setting, fill an important gap in our current global dataset for this interval of the late Silurian. Supplementary material: All the geochemical data reported and discussed herein are available in the Supplemental Data Tables at https://doi.org/10.6084/m9.figshare.c.7008107 Thematic collection: This article is part of the Chemical Evolution of the Mid-Paleozoic Earth System and Biotic Response collection available at: https://www.lyellcollection.org/topic/collections/chemical-evolution-of-the-mid-paleozoic-earth-system

Marine biodiversity increased markedly during the Ordovician Period (~487–443 million years ago). Some intervals within the Ordovician were associated with unusually rapid and prominent rises in taxonomic richness, the reasons for which remain debated. Links between increased oxygenation and biodiversification have been proposed, although supporting marine oxygen proxy data are limited. Here we present an expansive multi-site iodine-to-calcium (I/Ca) record from Lower–Middle Ordovician marine carbonates in Baltoscandia that provides a detailed account of the spatio-temporal development of oxygen conditions across this palaeoshelf. The data document progressive oxygenation of regional seafloor environments, with well-oxygenated waters sourced from the palaeoequatorward Iapetus Ocean and peak I/Ca values (that is, dissolved oxygen concentrations) coinciding with the most pronounced biodiversity increases and ecosystem reorganizations during this time interval. This occurred while the climate cooled, global sea level dropped and carbonate deposits became regionally dominant. The results suggest that ventilation of shelves played a critical role in regulating early Palaeozoic marine biodiversity via development of ecospace and aerobic–metabolic conditions.