Sediment Biogeochemistry
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.
Our laboratory approaches sediment biogeochemistry primarily through:
- in situ microsensor measurements
- pore water and solid phase chemical characterizations
- diffusion-diagenesis models
We are currently focusing studies on the Oregon continental shelf where seabed respiration and nutrient regeneration are of special interest for their coupling to larger scale ocean circulation and climate phenomena, bottom boundary layer turbulence and local occurrences of water column hypoxia. In 2008 we plan to utilize a benthic tripod equipped with sensors to measure total benthic oxygen exchange by eddy-correlation, and diffusive benthic oxygen exchange and sediment oxygen penetration depth by microprofiling. A high resolution digital camera will provide time-series images of the sediment surface area that contributes to the flux.
Sandy seafloor on the Oregon shelf at 80 m water depth. The two red laser dots provide a scale reference of 10 cm.
In Situ Microsensor Measurements
We routinely 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. Thus, they can be used to derive benthic oxygen exchange after measuring oxygen concentrations in a small volume of water situated above the seabed simultaneously with vertical current velocity. This eddy-correlation method was pioneered by Peter Berg (U.Va). We also apply microsensors from benthic landers to obtain in situ pore water profiles or to follow tracer advection through permeable sediments. 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.
For a review of this area of research see:
Reimers, C.E. (2007) Applications of microelectrodes to problems in chemical oceanography. Chemical Reviews 107, 590-600.
Launching the microprofiler lander.
Three oxygen microelectrode profiles and a formation factor profile measured during a single microprofiler deployment at a muddy, highly bioturbated site on the outer Oregon shelf in October 2004.
Microprofiler instrumentation mounted on a lander. The green housing contains controller and A/D electronics, a data logger, and batteries. The black case contains a DC motor and cam system that can move the microelectrodes in 0.125 mm steps over a total vertical travel of 20 cm.
Close-up of three oxygen and one resistivity electrode mounted in oil-filled holders. Small amplifier circuit boards are contained in the holders and wired to each oxygen microelectrode.
Pore Water and Solid Phase Chemistry
Collecting surface sediments without disturbance and loss of the overlying bottom water is difficult especially when sediments are sandy. Collaborating with Rob Wheatcroft and Tony D'Andrea (OSU biological oceanographers) we recently built a hydraulically dampened slow-gravity corer capable of collecting the uppermost ~50 cm of most sediment columns with little disturbance. The corer is equipped with video and digital still cameras to monitor the coring process and to photograph bottom features. The samples obtained with the corer are used for extracting and analyzing constituents in pore waters and in solid phases (e.g., dissolved nutrients, sulfate, sulfide, total-CO2; solid-phase organic C, CaCO3, acid volatile-S, pyrite, 210-Pb).
Coring with a hydraulically-damped gravity corer from the R/V Wecoma.
Detail of the hydraulically-damped gravity corer.
Diffusion-Diagenesis Models
Interpretation of biogeochemical data can be aided greatly by conceptual and mathematical models. Diagenesis, in mathematical terms, is usually described with conservation equations that take into account transport and reaction processes. Although our laboratory does not specialize in complex model development, we frequently apply zero and one-dimensional diagenetic models to our data sets or provide biogeochemical data to colleagues for verification of new modeling approaches.
Comparison of measured and model-derived pore water profiles from Komada et al. (2004).
For examples of this research see:
Komada, T., Reimers, C.E., Luther, G.W., III., and Burdige, D.J. (2004) Factoring affecting dissolved organic matter dynamics in mixed redox to anoxic coastal sediments. Geochim. Cosmochim. Acta 68, 4099-4111.
Meysman, F.J.R et al. (2003) Reactive transport in surface sediments. II Media: an object-oriented problem-solving environment for early diagenesis. Computers & Geosciences 29, 301-318.