Trace metals such as Mo, U, and V are useful paleo-redox proxies because of their sensitivity to redox conditions and unique incorporation pathways into marine sediments. However, microbial processes can change the primary signal of specific trace metals that can record the environment at the time of deposition. To investigate the impact of microbial activity on trace metals during early diagenesis, geochemical analysis was performed via bag-incubations on samples collected from two giant box corers during RV SONNE Expedition SO260, funded by the MARUM-Center for Marine Environmental Sciences at the University of Bremen. The first core was taken at the head of the Mar del Plata Canyon, consists of mud to fine sand, and was sampled at five depths. The second core was taken on a coral mound, contains larger grains and abundant coral fragments, and was sampled at four depths. Four splits were taken at each depth; the first split was processed on-board, while splits 2-4 were stored in argon flushed aluminum bags at 4°C for processing and analysis every 4 months on-shore. Our data show strong changes in the pore-water trace metal concentrations in the first box core samples, with Mo increasing more than 5000 nM within 8 months. In samples from the second box core, pore-water Mo increases by more than 1000 nM in the first 4 months before decreasing likely due to the onset of sulfate reduction and, consequently, the formation of hydrogen sulfide leading to the (co-)precipitation of Mo. Our data indicate that the dissolution of iron and manganese oxides leads to the release of associated trace metals at different time points for each site. With comparable amounts of organic carbon at both sites, the observed changes are likely related to sediment composition and physical properties. The sediments sampled at the coral mound have a higher overall porosity and thus provide more space for fluid circulation and host microbial communities. This could explain the faster cycling of iron minerals, potential onset of sulfate reduction, and thus changes in the concentration of relevant trace metals. Therefore, our data suggest that trace metal cycling is not only related to the overall sediment composition, but also to physical properties including pore space, permeability, and grain size which affect how much area is available for microbial communities.
EPIC Alfred Wegener Institut