GSA Connects 2021 in Portland, Oregon

Paper No. 130-1
Presentation Time: 8:05 AM


KUNERT, Alex1, POULTON, Simon W.2 and KENDALL, Brian1, (1)Department of Earth and Environmental Sciences, University of Waterloo, 200 University Ave W, Waterloo, ON N2L 3G1, CANADA, (2)School of Earth and Environment, Univ. of Leeds, Leeds, LS2 9JT, United Kingdom

The Paleoproterozoic Rove Fm captures a shift from mainly ferruginous to stratified oceans with euxinic mid-depths and ferruginous deep waters. Marine euxinia may have stunted eukaryotic evolution during the mid Proterozoic, so quantifying its extent is essential. Molybdenum concentration and isotope data previously obtained for Rove Fm euxinic black shales suggest <10% global seafloor euxinia. Some marine carbonates provide estimates for mid Proterozoic seawater uranium isotope values (δ238USW) of –0.43 to –0.73‰, suggesting <7% global mid Proterozoic seafloor euxinia. Others propose that U reduction into anoxic Precambrian sediments did not impart isotopic fractionations since many carbonates have δ238U near average upper crust. This raises concern on using δ238U to estimate areas of seafloor anoxia as redox systematics form the basis of the U mass balance. Constraints on δ238USW for specific temporal intervals are needed to resolve redox variations and the utility of the δ238U proxy through these “boring billion”.

We present new δ238U data for Rove Fm black shales in core 89-MC-1 to pinpoint δ238USW at 1.84 Ga. Iron speciation constrains local bottom water redox (highly reactive vs total Fe for oxic/anoxic conditions, FeHR/FeT; pyrite Fe vs FeHR for ferruginous/euxinic conditions). We observe a positive δ238U–FeHR/FeT trend: older ferruginous shales have lighter δ238U (–0.48 to 0.11‰) and younger euxinic shales have heavier δ238U (–0.04 to 0.27‰). This is expected from larger fractionations between SW and modern euxinic sediments compared to ferruginous sediments. An intervening layer with coarse pyrite—indicating a ferruginous–euxinic transition or fluctuating chemocline—has δ238U from –0.09 to 0.38‰. Heavy δ238U in euxinic/pyrite-bearing intervals are 0.4–0.6‰ above average upper crust and modern rivers, contrary to the hypothesis of limited fractionation in Precambrian oceans. Known isotopic offsets between SW and each setting (ΔSW-sed) are used to estimate δ238USW. Since most ferruginous samples have δ238U of –0.2 to 0.1‰ and modern/ancient ferruginous sediments show ΔSW-sed ≈ 0.0 ± 0.4‰, we suggest δ238USW was near the modern crustal/riverine flux (–0.3‰) during ferruginous interval deposition. Euxinic sediments in modern unrestricted basins incur ΔSW-sed ≈ 0.6–0.8‰, so δ238USW at 1.84 Ga may range from –0.8 to –0.4‰. Our data suggest a transition from ferruginous global oceans with minimal δ238USW fractionation from river input to an ocean with expanded areas of euxinia, shifting δ238USW to lighter values.