Large Igneous Provinces

Abstract

Large igneous provinces (LIPs) have occurred episodically throughout Earth’s history, with the most severe events causing profound disturbances to Earth’s climate and biosphere that likely influenced the course of metazoan evolution. One environmental perturbation caused by LIP emplacement is a change in global oceanic redox conditions. The uranium (U) and molybdenum (Mo) isotope systems are relatively established tracers of global oceanic redox conditions, particularly for the extent of anoxic and euxinic seafloor, whereas the thallium (Tl) isotope system is emerging as a tracer for the extent of well‐oxygenated seafloor characterized by manganese (Mn) oxide burial. In this review, we discuss how these metal isotope systems can be used to infer changes to global oceanic redox conditions through the cascade of environmental perturbations caused by LIP emplacement, focusing on the three events (Cenomanian‐Turonian, Toarcian, and Permian‐Triassic) that have received the most attention. Existing isotope mass‐balance models for these metals indicate an expansion of oceanic anoxia and euxinia (by ~1 to 2 orders of magnitude greater than the modern ocean) accompanied LIP emplacement during these events. Future studies, ideally utilizing a multi‐isotope approach on the same samples and coupled with improvements in oceanic metal isotope mass balances and modeling, are expected to provide more precise and accurate estimates of the spatiotemporal extent of oceanic anoxia/euxinia expansion and how this relates to the magnitude, location, and style of LIP events. 13 1 Department of Earth and Environmental Sciences, University of Waterloo, Waterloo, Ontario, Canada 2 School of Earth and Ocean Sciences, Cardiff University, Cardiff, UK 3 Department of Earth, Ocean and Atmospheric Science and National High Magnet Field Laboratory, Florida State University, Tallahassee, Florida, USA 306 LARGE IGNEOUS PROVINCES balance models to link and infer changes in the global extent of seafloor covered by oxic versus anoxic (euxinic and noneuxinic) waters. For ancient LIP events with negligible open‐ocean seafloor records, changes in global oceanic redox conditions must be inferred from the metal isotope ratios of continental margin sedimentary rocks. Differences in marine input/output fluxes, ocean residence times, and isotope fractionation mechanisms for Mo, U, and Tl mean that each metal provides its own perspective on changes in oceanic redox conditions during LIP emplacement. Because Mo is highly insoluble in sulfidic environments and is thus enriched in euxinic sediments (Helz et al., 1996; Erickson & Helz, 2000; Scott & Lyons, 2012), Mo isotope data from euxinic organic‐rich mudrocks are typically used to infer the global extent of seafloor where dissolved sulfide occurs in the water column and sediments (e.g., Arnold et al., 2004; Dahl et al., 2011; Goldberg et al., 2016). Uranium is insoluble in its reduced form and, in contrast to Mo, does not require dissolved sulfide for its incorporation and burial into sediments (Morford & Emerson, 1999; Algeo & Tribovillard, 2009). Despite this geochemical difference, it is not fully clear if U isotopes are best used as a tracer for the global extent of general oceanic anoxia (euxinic and noneuxinic) or more specifically the global extent of oceanic euxinia because of knowledge gaps for isotope fractionation factors and U removal mechanisms in anoxic marine environments (Stylo et al., 2015; Hood et al., 2016; Andersen et al., 2017; Brown et al., 2018; Cole et al., 2020). Nevertheless, U isotope data from organic‐rich mudrocks and carbonates have gained prominence as a tool for tracing global oceanic redox changes during Phanerozoic anoxic events. Thallium isotope compositions from euxinic organic‐rich mudrocks have been used to infer the extent of well‐oxygenated seafloor because Tl adsorption to Mn oxides in oxygenated settings is associated with the largest known marine Tl isotope fractionation (Nielsen et al., 2011; Peacock & Moon, 2012; Owens et al., 2017a). The lower ocean residence time of Tl and the sensitivity of this metal’s oceanic mass balance to Mn oxide burial fluxes means that the seawater isotopic composition of Tl will respond faster than Mo or U to regional/global ocean deoxygenation at the onset of Phanerozoic anoxic events (Ostrander et al., 2017; Them et al., 2018). This chapter first reviews the modern oceanic mass balance for these three isotope systems (Fig. 13.1; see Table 13.1 for data reporting conventions). Application of these isotope systems as tracers for global oceanic redox changes during major Phanerozoic LIP events is then discussed. Future utilization will ultimately include multiple isotope systems on time equivalent rocks to decipher a holistic global redox structure as related to LIP events and their climatic consequences. 13.2. REDOX‐SENSITIVE METAL ISOTOPE SYSTEMS AS GLOBAL OCEANIC REDOX PROXIES 13.2.1. Molybdenum Isotopes The use of Mo isotopes as an oceanic redox proxy exploits its bimodal redox behavior (+6 and +4 valences; Barling et al., 2001). Molybdate (MoO4 2–) is soluble in oxygenated river water and seawater but is converted to particle‐reactive thiomolybdates (MoO4−xSx 2−) and polysulfides and removed to sediments in modern sulfidic settings (Collier, 1985; Erickson & Helz, 2000; Scott & Lyons, 2012; Dahl et al., 2013, 2017; Helz & Vorlicek, 2019). Oceanic Mo inputs are dominated by dissolved riverine and groundwater Mo (>90%), which has an average δ98Mo similar to the upper crust average of ~0.3–0.6‰ (Table 13.1 defines δ‐notation; Archer & Vance, 2008; Willbold & Elliott, 2017; King & Pett‐Ridge, 2018; Neely et al., 2018). Seafloor hydrothermal systems may constitute a minor dissolved Mo source but its δ98Mo is poorly constrained (Wheat et al., 2002; Reinhard et al., 2013; Neely et al., 2018). Oceanic Mo outputs are divided into oxic, sulfidic‐at‐ depth (weakly oxic or anoxic bottom waters; dissolved sulfide confined to sediment pore‐waters), and euxinic settings that make up ~30% to 50%, ~50% to 65%, and ~5% to 15% of the modern dissolved oceanic Mo input flux, respectively (Fig. 13.1a; Scott et al., 2008; Reinhard et al., 2013). On average, the Mo burial flux in modern euxinic environments not severely restricted from the open ocean (i.e., excluding the Black Sea) is ~1 and ~3 orders of magnitude higher than sulfidic‐at‐depth and oxic settings, respectively (Scott et al., 2008; Reinhard et al., 2013). Comparing the magnitude of these sinks with their areal seafloor extent (~2% for sulfidic‐at‐depth and ~0.1% for euxinic; Reinhard et al., 2013) reveals that the oceanic Mo mass balance is highly sensitive to the extent of sulfidic (especially euxinic) environments. Hence, seawater Mo concentrations and residence times will be lower in ancient oceans with a greater extent of euxinic seafloor than today (Reinhard et al., 2013). In all marine settings, Mo isotope fractionation results in preferential removal of light Mo isotopes to sediments. The largest Mo isotope fractionation (ΔMoseawater‐sediment ~3‰) occurs in well‐oxygenated settings where Mo adsorbs to Mn oxides (Siebert et al., 2003; Barling & Anbar, 2004; Wasylenki et al., 2008). In sulfidic‐at‐depth environments, Mo isotope fractionation is variable and depends on the type of (oxyhydr)oxides delivering Mo to sediments and pore‐water H2S content, with Mo burial most efficient when pore‐waters have high dissolved sulfide (average fractionation of ~0.7–0.9‰; Poulson et al., 2006; Siebert et al., 2006; Poulson Brucker Rivers & groundwaters f = 95% δ = 0.6% Hydrothermal fluids