Uranium isotopes in marine carbonates as a global ocean paleoredox proxy: A critical review

Abstract

The protracted oxygenation of the ocean-atmosphere system is one of the most fundamental changes to the Earth system through its history. The uranium isotopic composition (238U/235U, denoted as δ238U) of marine carbonates has been developed as a proxy to quantitatively track the timing, duration, and extent of global marine redox chemistry changes. This proxy has been applied to many critical evolutionary intervals in the last decade, significantly advancing our understanding of how life on Earth and its environment have co-evolved through geological history. Successful application of the uranium isotope paleoredox proxy requires a thorough understanding of the marine uranium budget, the processes by which seawater U-isotope signatures are recorded in marine carbonates, and the potential for alteration of these primary signatures by syn-and post-depositional diagenetic processes. Here, we provide a critical review of the U isotope proxy in marine carbonates with a focus on the current problems and areas where future work is needed to further develop this proxy. We also use a recently developed global C-P-U cycle model to illustrate that when the carbon cycle is perturbed by volcanic carbon injections, the ensuing transient relationship between seafloor anoxic area and δ238U can be complex and sometimes counter-intuitive.

Upper limits on the extent of seafloor anoxia during the PETM from uranium isotopes

“…The deep-sea core samples used for this study have experienced relatively simple diagenetic histories compared to carbonates used in the majority of paleo-studies using U isotopes, which to date have focused mainly on platform carbonates and/or outcrop material 32,33,[45][46][47] . In the case of platform carbonates, that originally consisted of metastable aragonite or high-Mg calcite, diagenetic corrections are often applied to δ 238 U CAU values 33,[45][46][47] to account for authigenic U(IV) uptake under reducing porewater conditions during early diagenesis (including recrystallization and cementation) that results in a high degree of scatter with a generally positively offset compared to seawater (~+0.27 ± 0.14‰; 1 SD) 48,49 . Previous work has suggested that some pelagic carbonates record lower δ 238 U CAU signatures, closer to seawater values compared to platform counterparts, and thus do not require correction 32,41 .…”

“…The invariance of the Site 865 record is taken to indicate the lack of a resolvable perturbation to the global U isotope mass balance during the PETM. Here we use an established biogeochemical box model 32,47 to assess the quantitative implications of this result. We use the Site 865 δ 238 U CAU dataset and higher resolution δ 13 C data from Site 401 (ref.…”

The uranium isotopic record of shales and carbonates through geologic time

“…The carbonates samples were then crushed into fine powder using an agate mortar and pestle, and the carbonate content was estimated from major element abundances determined on compacted powder pellets using a JEOL JSM-5800LV scanning electron microscope (SEM). The carbonate content was used to calculate the amount of acid to add to each sample to fully digest the carbonate fraction without leaching detrital phases (Tissot et al 2018;Clarkson et al 2020;Zhang et al 2020b).…”

“…Excluding dolomitic units would also leave large gaps in the carbonate archive. Instead, for some intervals, we have analyzed both limestones and dolostones to assess the effect of dolomitization on uranium isotopic ratios (see also the discussions in Herrmann et al 2018;Zhang et al 2020aZhang et al , 2020b.…”

“…Most of these studies focused on variations in the U isotopic composition of carbonates during short time intervals to trace the expansion or contraction of anoxia in the oceans. A complication to the carbonate δ 238 U paleoredox is that it is affected by diagenesis (see review in Zhang et al 2020b), which can shift the δ 238 U values of carbonates by ~+0.2 ‰ to +0.4 ‰ relative to seawater (Romaniello et al 2013;Chen et al 2018a;Tissot et al 2018). Precipitation of abiotic (Chen et al 2016(Chen et al , 2017 and biogenic (Chen et al 2018b) carbonates can also induce a small shift in the U isotopic composition of carbonates relative to seawater (<+0.1 ‰).…”