Uranium Isotope Fractionation in Non‐sulfidic Anoxic Settings and the Global Uranium Isotope Mass Balance

Abstract

Uranium isotopes (238U/235U) have been used widely over the last decade as a global proxy for marine redox conditions. The largest isotopic fractionations in the system occur during U reduction, removal, and burial. Applying this basic framework, global U isotope mass balance models have been used to predict the extent of ocean floor anoxia during key intervals throughout Earth's history. However, there are currently minimal constraints on the isotopic fractionation that occurs during reduction and burial in anoxic and iron‐rich (ferruginous) aquatic systems, despite the consensus that ferruginous conditions are thought to have been widespread through the majority of our planet's history. Here we provide the first exploration of δ238U values in natural ferruginous settings. We measured δ238U in sediments from two modern ferruginous lakes (Brownie Lake and Lake Pavin), the water column of Brownie Lake, and sedimentary rocks from the Silurian‐Devonian boundary that were deposited under ferruginous conditions. Additionally, we provide new δ238U data from core top sediments from anoxic but nonsulfidic settings in the Peru Margin oxygen minimum zone. We find that δ238U values from sediments deposited in all of these localities are highly variable but on average are indistinguishable from adjacent oxic sediments. This forces a reevaluation of the global U isotope mass balance and how U isotope values are used to reconstruct the evolution of the marine redox landscape.

The uranium isotopic record of shales and carbonates through geologic time

“…These suggestions are highly speculative and call for more field and experimental studies to better understand the controls on U removal and isotopic fractionation in shales. Interestingly, a recent study investigated the U isotopic composition of the modern and Silurian-Devonian shales, which were deposited in ferruginous settings and found that δ 238 U values were highly variable with a muted fractionation factor on average (Cole et al 2020). The authors also explored the effects on the mass balance model resulting from different reduction pathways with various apparent fractionation factors as well as different U mass accumulation rates.…”

Assessing the Effect of Large Igneous Provinces on Global Oceanic Redox Conditions Using Non‐traditional Metal Isotopes (Molybdenum, Uranium, Thallium)

“…For uranium, the "oxic and other" sink includes Mn oxides, carbonates, pelagic sediments, coastal sediments, and alteration of oceanic crust. This particular mass balance for U takes into account the possibility that U burial in anoxic/noneuxinic settings (i.e., dissolved H 2 S confined to pore-waters) is typically associated with a smaller U isotope fractionation than euxinic settings (Hood et al, 2016;Cole et al, 2020). Other mass-balance models have assumed a large U isotope fractionation in all anoxic environments (euxinic and noneuxinic) (Tissot & Dauphas, 2015;Andersen et al, 2016;Noordmann et al, 2016).…”

“…At the end of the CIE, no clear stratigraphic shift occurs (Clarkson et al, 2018), suggesting global oceanic redox conditions did not significantly change. Mass-balance modeling, which assumes the magnitude of U isotope fractionation is similar for euxinic and anoxic/noneuxinic environments (an assumption currently being scrutinized; see Hood et al, 2016;Cole et al, 2020), suggests 8%-15% anoxic seafloor during OAE2 (Clarkson et al, 2018).…”

“…As noted for OAE2, revised estimates for the extent of seafloor anoxia during the end-Permian LIP event are warranted if the U isotope fractionation in anoxic/ noneuxinic environments is smaller than euxinic settings and is closer to oxic/suboxic settings (Hood et al, 2016;Cole et al, 2020). In this scenario, quantitative estimates for the extent of overall oceanic anoxia versus oxygenated seafloor are less straightforward and even ambiguous for some global seawater δ 238 U values (notably -0.9‰ to -0.6‰), as shown by the recent U isotope mass-balance model of Gilleaudeau et al (2019).…”

“…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.…”