Sediment biogeochemistry is an area of chemical oceanography that encompasses the study of processes and material fluxes in recent sediments. In most marine sediments, organic matter transformations catalyzed by microorganisms drive geochemical reactions, leading to chemical gradients and diffusional transport. Other transport phenomena that can be important in surface sediments include advection due to burial, compaction and/or hydrological flow, and physical disturbance due to bioturbation, physical forcing, or anthropogenic activities (e.g., trawling). From local to global scales, sediment diagenesis and seafloor fluxes link the ocean's "biological pump" to the Earth's geological cycle.
We have recently focused studies on the Oregon continental shelf and Saanich Inlet, British Columbia, Canada where seabed respiration and nutrient regeneration are of special interest for their coupling to seasonal circulation and climate phenomena, bottom boundary layer turbulence, and local occurrences of water column hypoxia. Since 2009 we have utilized a benthic tripod we designed called BOXER that is equipped with sensors to measure total benthic oxygen exchange by eddy correlation. Oregon shelf studies include investigations of the impacts of wave motions on eddy correlation fluxes whereas in Saanich Inlet variability in bottom water oxygen concentrations is the environmental driver of greatest interest. For a full appreciation of the approaches we apply in studies of sediment biogeochemistry go to the link below.
We fabricate and use state-of-the art voltammetric, potentiometric and amperometric microsensors. Amperometric oxygen microsensors in particular have the advantages of micro-scale spatial resolution, a rapid response time and good sensitivity. One way we apply microsensors is from benthic landers to obtain in situ microprofiles of chemical properties across the sediment-water interface. These measurements can be used to quantify the role of oxygen and other electron acceptors in diagenesis. For example, based on the boundary layer gradients of oxygen across the sediment-water interface, areal diffusive fluxes of oxygen can be calculated and equated to organic matter oxidation rates.
Benthic Microbial Fuel Cells
Our laboratory is developing microbial fuel cells as power sources for ocean instrumentation. Reimers et al. (2001) and Tender et al. (2002) demonstrated that graphite electrodes placed at least a few centimeters on either side of the sediment-water interface in coastal environments can generate sustainable electrical power due to electrode reactions that are both microbial and geochemical. Early diagenetic processes separate the primary reactants (organic matter, sulfides and oxygen) and maintain the potential difference necessary to channel electrons through an external circuit. In this way the electrodes and their surroundings function as a benthic microbial fuel cell.
We are working to evaluate limiting factors for these fuel cells and testing prototypes under different geochemical conditions (e.g., in different environments, and with and without supplemental sources of organic acids). Our newest designs place the anodes above the sediment-water interface in benthic chambers. We are also exploring the generation of electrical energy from energy-rich planktonic biomass and detritus intercepted before decay in the water column (“Plankton Power”; Reimers et al., 2007), and how cathode reactions may enhance calcium carbonate precipitation. It is our expectation that new microbes important to redox reactions will be found in both plankton and sediment powered systems, as will new microbial community associations. We are in the process of identifying these microbes, studying their geochemical impacts, and using the results to document fundamental biogeochemical processes.
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Research Support: We gratefully acknowledge that our current research is supported by grants from the US National Science Foundation, Office of Naval Research and the Murdock Charitable Trust.