Progressive change of the water column, pore water and sediment chromium isotope composition at the Rainbow hydrothermal vent field, North Atlantic Ocean

Chromium (Cr) cycling in hydrothermal systems, particularly within sediments and pore waters, remains poorly constrained, limiting the robust application of Cr isotopes as paleo-redox proxies. Here we present the first integrated investigation of Cr concentrations and stable isotopes (δ 53 Cr) in seawater, sediments and pore waters, across a ∼60 km transect from the Rainbow hydrothermal field on the Mid-Atlantic Ridge. Our results show that the non-buoyant hydrothermal plume acts as a significant local sink for seawater Cr, driven by reductive removal of Cr(VI) by hydrothermal Fe(II) and subsequent scavenging onto Fe(oxy)hydroxide particles. This process is associated with a strong isotopic fractionation (ε = −2.90 ± 0.22 ‰). Relative to background seawater (3.01 nmol kg −1 , δ 53 Cr = +1.13 ± 0.02 ‰, n = 4), plume waters exhibit Cr removal of up to 1.27 nmol kg −1 and seawater δ 53 Cr enrichment to + 2.67 ± 0.03 ‰, with hydrothermal influence detectable up to 60 km from the vent source. The hydrothermal imprint is also preserved in sediments and pore waters. Pore water [Cr] ranges from 2.69 and 48.40 nmol kg −1 with δ 53 Cr values between −0.31 ± 0.06 ‰ to +0.88 ± 0.03 ‰, while sediment leachates display lower δ 53 Cr values (−1.19 ± 0.04 ‰ to −0.24 ± 0.02 ‰) and [Cr] between 0.36 and 3.6 µg g −1 . Bulk sediments contain substantially higher Cr concentrations (10–220 µg g −1 ) with δ 53 Cr values (−0.32 ± 0.03 ‰ to −0.01 ± 0.0 2 ‰) consistent with mixed detrital and authigenic inputs. Isotopic mass-balance calculations demonstrate that Cr release from sediments into pore waters is accompanied by significant isotope fractionation, with preferential mobilization of isotopically heavy Cr(VI). Remobilization of scavenged Cr into pore waters produces a diffusive benthic Cr flux to overlying bottom waters of 0.15 to 0.46nmol cm −2 yr −1 and is associated with additional isotopic fractionation (+0.21 ± 0.07 ‰ to + 1.01 ± 0.04 ‰; 2 SD). Together, these results reveal complex interactions between hydrothermal scavenging, sedimentary recycling and redox-driven Cr transformations. Our findings underscore the importance of sediment–pore water processes in shaping marine Cr isotope signatures and highlight the need to account for diagenetic modification when interpreting Cr isotope records as paleo-redox proxies.