Coupled molybdenum and iron isotopes distinguish seawater and hydrothermal inputs to seep carbonates

Molybdenum (Mo) and its isotopic composition (δ 98 Mo) can serve as robust proxies for reconstructing paleo-oceanographic and paleoenvironmental conditions. Seep carbonates, in particular, hold potential for archiving contemporaneous seawater δ 98 Mo signatures. However, this application requires that Mo be sourced exclusively from seawater. This prerequisite is complicated by the frequently complex sources of Mo in cold seep environments, particularly within hydrothermally influenced systems. Here, we conducted elemental and Mo isotopic analyses of seep carbonates collected from the middle (MOT) and northern Okinawa Trough (NOT). Seep carbonates from NOT, in contrast to those from MOT, exhibit distinctive hydrothermal vent signatures, including generally lighter δ 56 Fe (min: −0.49‰), elevated Fe/Al ratios (max: 2.90) that show positive correlations with the enrichment factors of V, Zn, As, Sb (V EF , Zn EF , As EF , Sb EF ) and anomalous U enrichments (U EF max: 53). In addition, NOT seep carbonates show markedly lower δ 98 Mo values (−0.55‰ to +0.99‰) compared to those from MOT (+1.44‰ to +2.23‰; avg. +1.85‰), a pattern attributed to the incorporation of isotopically light, hydrothermally derived Mo into NOT carbonates. A corroborating mass balance model demonstrates that the measured δ 98 Mo values in NOT seep carbonates fall within the field defined by Mo sourced from hydrothermal plumes, whereas δ 98 Mo values of the MOT seep carbonates align with a seawater-dominated source. Our findings reveal that seep carbonates can reliably record marine chemical signals, and crucially, they underscore the critical need to account for source influences when interpreting Mo isotopes in sedimentary archives. This work thus significantly advances our understanding of Mo cycling in the ocean.