Sedimentary cycling and benthic fluxes of bio-essential trace metals in low-oxygen marine environments
Trace metals (TMs) are essential micronutrients required for marine life. They are indispensable components of phytoplankton enzymes which catalyse important biological functions. Due to their scarcity in the ocean, TMs can (co-)limit primary productivity and thus affect the efficiency of the biological pump. Marine sediments are an important source and sink for TMs to the ocean, especially in low-oxygen environments. However, the key processes and parameters that lead to TM release from or to fixation and burial within the sediments are not fully understood for most TMs and the corresponding fluxes are not well quantified. As the oceans are losing oxygen, oxygen minimum zones serve as a present-day example to study how benthic TM cycles will respond to future ocean conditions. In order to investigate environmental controls on benthic TM exchange and pathways from or to the sediment, this study combines sediment, pore water, bottom water and benthic flux data. The main study site is the Peruvian margin, where one of the largest and most intense oxygen minimum zones is located. Additional data stems from a seasonally anoxic fjord in the Baltic Sea. In the first scientific chapter of this thesis, Chapter II, the benthic cycling of the two TMs iron (Fe) and cadmium (Cd), which have a contrasting sulphide solubility (Fe > Cd), is compared. Hydrogen sulphide concentrations exert an important control on the benthic fluxes of both TMs at the Peruvian margin. Temporal magnitude changes of diffusive Fe effluxes into near-bottom waters are related to Fe retention via sulphide precipitation in the sediment due to high hydrogen sulphide concentrations. Further, benthic chamber incubation data indicated that Fe accumulation in anoxic near-bottom waters coincided with the depletion of nitrate and nitrite preventing Fe oxidation and subsequent (oxyhydr)oxide precipitation. Cadmium has one of the lowest sulphide solubilities among TMs. The removal of Cd from near-bottom waters during benthic chamber incubations covaried with hydrogen sulphide concentrations in the surface sediment. This suggest that Cd accumulation in the sediment is mediated by precipitation of cadmium sulphide at the sediment-water interface or within the water column. Oxygen minimum zone sediments are a source for manganese (Mn) and cobalt (Co) and a sink for nickel (Ni), cupper (Cu), zinc (Zn) and Cd. Chapter III, deals with the different mechanisms and pathways which lead to the enrichment or depletion of TMs in sediments at the Peruvian margin. Even though Mn and Co are both depleted in Peru margin sediments, the results of this thesis suggest that their cycling is partly decoupled. At least half of the Mn depletion in shelf sediments can be attributed to benthic diffusive efflux. In contrast, Co dissolution chiefly takes place in the water column as benthic diffusive Co effluxes are much lower compared to the rate of Co loss from the sediments. The majority of Ni accumulation in Peruvian shelf sediments can be explained by direct phytoplankton uptake in the photic zone and delivery with organic matter. For Cu, Zn and Cd however, this transport mechanism is rather of minor importance. Therefore, a covariation in sediments of Cu with particulate organic carbon suggests that the Cu accumulation may be primarily caused by scavenging by downward sinking organic matter. In addition, similar to Cd, the Cu delivery with sulphide minerals precipitated from the water column or near-bottom water likely contributes to the accumulation. The enrichment of Zn is driven by diffusive benthic fluxes from the near-bottom water into the sediment pore water, which matched the excess Zn accumulation. This is likely followed by sulphide precipitation, causing Zn retention in the sediment. Chapter IV presents a novel device that was developed to sample dissolved and particulate TMs in the layer of water above the seafloor, the benthic boundary layer. So far this has not been able to conduct with conventional TM sampling methods. The new device overcomes the existing limitations. Successful testing demonstrated that it enables simultaneous, uncontaminating and oxygen-free sampling of suspended particles and near-bottom water in high-resolution within the first few meters above the seafloor. The novel device will be an important tool for future studies on dissolved-particulate interactions at the ocean’s lower boundary. It will help to solve remaining questions on how benthic TM fluxes are modified in this reactive interface layer and on TM particle association. The results of this thesis demonstrate that TM behaviour in the ocean is very diverse and future ocean conditions, with declining oxygen and increasing hydrogen sulphide concentrations, may modify benthic TM fluxes individually. The differing TM fluxes at the seafloor may change TM stoichiometry in upwelling water masses and the future ocean, which can impact marine ecosystems in the surface ocean.