SHIFT IN GLOBAL OCEAN REDOX STRUCTURE CAPTURED BY URANIUM ISOTOPES FROM 1.84 GA ROVE FORMATION (ONTARIO, CANADA) BLACK SHALES

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 (δ²³⁸U_SW) 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 δ²³⁸U_SW 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 δ²³⁸U_SW at 1.84 Ga. Iron speciation constrains local bottom water redox (highly reactive vs total Fe for oxic/anoxic conditions, Fe_HR/Fe_T; pyrite Fe vs Fe_HR for ferruginous/euxinic conditions). We observe a positive δ²³⁸U–Fe_HR/Fe_T 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 (Δ_SW-sed) are used to estimate δ²³⁸U_SW. Since most ferruginous samples have δ²³⁸U of –0.2 to 0.1‰ and modern/ancient ferruginous sediments show Δ_SW-sed ≈ 0.0 ± 0.4‰, we suggest δ²³⁸U_SW was near the modern crustal/riverine flux (–0.3‰) during ferruginous interval deposition. Euxinic sediments in modern unrestricted basins incur Δ_SW-sed ≈ 0.6–0.8‰, so δ²³⁸U_SW at 1.84 Ga may range from –0.8 to –0.4‰. Our data suggest a transition from ferruginous global oceans with minimal δ²³⁸U_SW fractionation from river input to an ocean with expanded areas of euxinia, shifting δ²³⁸U_SW to lighter values.