Chemoautotrophy in the redox transition zone of the Cariaco Basin: A significant midwater source of organic carbon production

During the CARIACO time series program, microbial standing stocks, bacteria l production, and acetate turnover were consistently elevated in the redox transition zone (RTZ) of the Cariaco Basin, the depth interval (similar to 240-450 m) of steepest gradient in oxidation-reduction potential. Anomalous ly high fluxes of particulate carbon were captured in sediment traps below this zone (455 m) in 16 of 71 observations. Here we present new evidence th at bacterial chemoautotrophy, fueled by reduced sulfur species, supports an active secondary microbial food web in the RTZ and is potentially a large midwater source of labile, chemically unique, sedimenting biogenic debris t o the basin's interior. Dissolved inorganic carbon assimilation (27-159 mmo l C m(-2) d(-1)) in this zone was equivalent to 10%-333% of contemporaneous primary production, depending on the season. However, vertical diffusion r ates to the RTZ of electron donors and electron accepters were inadequate t o support this production. Therefore, significant lateral intrusions of oxi c waters, mixing processes, or intensive cycling of C, S, N, Mn, and Fe acr oss the RTZ are necessary to balance electron equivalents. Chemoautotrophic production appears to be decoupled temporally from short-term surface proc esses, such as seasonal upwelling and blooms, and potentially is more respo nsive to longterm changes in surface productivity and deep-water ventilatio n on interannual to decadal timescales. Findings suggest that midwater prod uction of organic carbon may contribute a unique signature to the basin's s ediment record, thereby altering its paleoclimatological interpretation.