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