Anoxic depositional overprinting of 238U/235U in calcite: When do carbonates tell black shale tales?

Abstract

Abstract The fidelity of uranium isotopes (δ238U) in marine carbonates as a paleoredox proxy relies on whether carbonates can record and preserve seawater δ238U. Although modern carbonate sediments deposited under oxic conditions have been shown to track seawater δ238U, it remains unknown whether this is true for carbonates deposited under anoxic conditions. This is a crucial question because many ancient carbonates were likely deposited or reworked under anoxic bottom waters. To better understand the behavior of uranium isotopes under this scenario, we investigated U isotope geochemistry in the meromictic Fayetteville Green Lake (FGL; New York, USA), where primary calcite is precipitated from oxic surface waters, sinks past the chemocline, and is deposited under anoxic bottom waters. We observed significant depletions of dissolved U concentration (from 2.7 to 0.9 ppb) and δ238U (from –0.55‰ to –0.96‰) below the chemocline in FGL. Parallel with these depletions, δ238U of sediment traps increased progressively from –0.51‰ to –0.16‰, suggesting that U(VI) reduction was occurring in the anoxic water column. Carbonate sediments deposited under anoxic bottom waters were enriched in U by 6–18× compared to primary calcite. Our data suggest that such significant authigenic U enrichments resulted from U(VI) reduction in the anoxic water column and below the sediment-water interface. The δ238U value in the top 0.25 cm of sediments was –0.29‰ ± 0.10‰, overprinting original δ238U in primary calcite (–0.51‰ ± 0.02‰). Future applications of carbonate δ238U as a paleoredox proxy should consider depositional environments (oxic vs. anoxic) of carbonates.