Chromium Cycling in Redox‐Stratified Basins Challenges δ(53)Cr Paleoredox Proxy Applications

Chromium stable isotope composition (δ(53)Cr) is a promising tracer for redox conditions throughout Earth's history; however, the geochemical controls of δ(53)Cr have not been assessed in modern redox‐stratified basins. We present new chromium (Cr) concentration and δ(53)Cr data in dissolved, sinking particulate, and sediment samples from the redox‐stratified Lake Cadagno (Switzerland), a modern Proterozoic ocean analog. These data demonstrate isotope fractionation during incomplete (non‐quantitative) reduction and removal of Cr above the chemocline, driving isotopically light Cr accumulation in euxinic deep waters. Sediment authigenic Cr is isotopically distinct from overlying waters but comparable to average continental crust. New and published data from other redox‐stratified basins show analogous patterns. This challenges assumptions from δ(53)Cr paleoredox applications that quantitative Cr reduction and removal limits isotope fractionation. Instead, fractionation from non‐quantitative Cr removal leads to sedimentary records offset from overlying waters and not reflecting high δ(53)Cr from oxidative continental weathering.