Sean M. Newby

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.

The timing and connections between global cooling, marine redox conditions, and biotic turnover are underconstrained for the Late Ordovician. The second most severe mass extinction occurred at the end of the Ordovician period, resulting in ~85% loss of marine species between two extinction pulses. As the only “Big 5” extinction that occurred during icehouse conditions, this interval is an important modern analog to constrain environmental feedbacks. We present a previously unexplored thallium isotope records from two paleobasins that record global marine redox conditions and document two distinct and rapid excursions suggesting vacillating (de)oxygenation. The strong temporal link between these perturbations and extinctions highlights the possibility that dynamic marine oxygen fluctuations, rather than persistent, stable global anoxia, played a major role in driving the extinction. This evidence for rapid oxygen changes leading to mass extinction has important implications for modern deoxygenation and biodiversity declines.