I. Organic Matter Diagenesis

Over the past 15 years I have studied the role of oxygen and other electron acceptors in organic matter diagenesis and developed instrumentation for measuring fine-scale profiles of porewater solutes with microelectrodes. This work is now continuing with a host of collaborators along several related avenues. Below are summaries of several projects.

Project 1.

The Biogeochemistry of Manganese and Iron Reduction in Shelf Sediments

Clare E. Reimers and George Luther, III (University of Delaware)

The goal of this project is to obtain a set of field and laboratory measurements that will give by far the most complete picture of the reductive dissolution- reprecipitation cycles of Mn and Fe in bioturbated sediments overlain by well oxygenated bottom waters. This situation is probably the most common found on continental shelves and in shallow estuaries. We are using a Remote Operated Vehicle and state-of-the-art voltammetric microsensors to obtain in situ porewater profiles, and we are also analyzing cores. The field sites chosen are within the Hudson-River Estuary and the Hudson Shelf Valley. Here concentrations of solid-Mn and -Fe are highly variable. No mechanistic explanation has been given for this variability other than it is related to grain size and source materials (including dredge spoils and sewage sludge). We are investigating the importance of rates and couplings of various chemical and biochemical reactions occurring in the sediments (including the reduction of nitrate by manganese), and the rates of (bio)physical processes (molecular diffusion, bioturbation and sediment accumulation). Modeling of the information collected as a result of field operations will be the final stage of our study. Funding has been provided by the Mid-Atlantic Bight Center of NOAA-NURP.

Project 2.

Chemical and Biological Implications of Water Flow Through Permeable Sediments

Gary L. Taghon, Clare E. Reimers, Lee J. Kerkhof, Charlotte M. Fuller (all from Rutgers)

The sediments on most continental shelves are relict sands with very low organic contents.

Paradoxically, however, even in these permeable sediments, dissolved oxygen concentrations in pore waters typically go to zero within a few mm of the sediment-water interface. These facts suggest that the low standing stock of organic matter does not accurately reflect the significance of continental shelf sediments in the oceanic carbon cycle. Rather, the data are consistent with the hypothesis that the low organic content of shelf sands is a by-product of rapid bacterial mineralization of organic matter, fueled by the high rate of supply of organic substrates and oxygen and other electron acceptors as particle-rich bottom water is advected through the upper layer of sediment. This interstitial advection results from pressure variations on the seabed resulting from the passage of surface gravity waves and bottom currents flowing along a permeable, uneven seabed.

We have proposed (to NSF) to test this hypothesis using field and laboratory studies. In order to measure the advection-driven fluxes of reactive solutes (such as oxygen) into sediments with minimal artifacts, we will employ a novel approach using voltammetric microelectrodes and molecular biology to assess microbial metabolism. An effective dispersion coefficient, incorporating the effects of pressure-driven pore water flow, will be derived by measuring the rate of penetration of a conservative dissolved tracer (I^-) from bottom water into the sediment. Depth profiles of oxygen, manganese, iron, and sulfide will be measured simultaneously by microelectrodes. Characterization of the denitrifying bacterial community (zone of activity, number of bacteria, transcript abundance) will be accomplished using molecular tools. In the laboratory, we will conduct experiments using seawater flumes to measure how variations in seabed topography, sediment permeability, suspended organic matter concentration, and flow velocity affect the penetration of bottom water, oxygen consumption rates and concentration gradients, and the responses of the bacterial community. By configuring flumes as large, sealed flux chambers, we will be able to compare directly the rate of change of oxygen in the overlying water with the oxygen flux into the sediments calculated from the microelectrode data. These laboratory studies will be used to develop and refine methods for determining organic matter mineralization rates in the field, without the errors associated with enclosing the sea bed in a chamber or assuming diffusive fluxes when using conventional microelectrodes. The field component of the project will be conducted at locations on the New Jersey continental shelf near the site of a permanent sea bed observatory (LEO-15).

Project 3.

Temporal Variation in Deep Sea Benthic Boundary Layer Communities

- a collaboration with Kenneth L. Smith, Jr. (Scripps Institution of Oceanography)

A unique, autonomous, bottom-transecting vehicle (ROVER) has been developed to facilitate long time-series measurements of sediment community oxygen fluxes in the deep-sea.. I am collaborating with Ken Smith who is constructing a microprofiler to generate a time-series record of pore water O₂ at the Hawaii Ocean Time-Series Station (HOTS). Of particular interest is the coupling between the production and export of organic carbon from surface waters and the oxidation of organic carbon on the sea floor.

Project 4.

The Role of Sediment-Water Chemical Exchange in the Biogeochemistry of the Great Lakes

- a collaboration with J. Val Klump (University of Wisconsin-Milwaukee)

The central hypothesis of this research is that benthic fluxes constitute a major pathway in the biogeochemical cycles of carbon, nitrogen, phosphorus and manganese in the Great Lakes. I have measured in situ porewater profiles of O₂, pH and pCO₂ with microelectrodes operated from a ROV in order to provide estimates of oxygen and total-CO₂ fluxes. These estimates agree well with benthic chamber fluxes derived by Klump and colleagues. We hope to expand this work in the future.

Project 5

Harvesting Energy From Redox Potential Gradients In Sediments and Soils

Clare Reimers and Leonard M. Tender (Naval Research Laboratory)

This project has been funded by DARPA and represents a novel program to draw power from the natural redox potential gradients in marine sediments and saturated soils. During an initial two-year phase of research, prototype power generation devices consisting of two planar noble metal electrodes (<1 mm thick) separated by non-conducting, porous, inert and fouling resistant spacers will be designed, fabricated and tested thoroughly in sediments and soils brought to the laboratory. The electrochemical parameters responsible for producing natural Eh gradients, the range of effective power that may be generated, and the effect of duty cycle on the stability of this natural power will be studied. During the second phase of work, diagenetic models will be applied to further interpret the laboratory test results, and a device scaled to deliver continuous power on the order of a few watts will be tested in the field for three months. It is anticipated that an energy harvesting system based on ambient redox potential gradients could be used to power networked micro-unattended ground sensors and communication devices in remote environments almost indefinitely.

II. CO₂ and O₂ Variability, Trends and Fluxes in The Coastal Zone

The coastal ocean is a region with highly variable physical processes, and high and variable rates of primary production and organic matter recycling, but very little is known about the effect of these factors on the flux of CO₂ into or out of this environment. After moving to Rutgers, I became interested in addressing this question and so initiated a time-series of geochemical measurements along a 32 km transect across the inner continental shelf, off New Jersey. This time-series was maintained from April 1994-March 1997. Water column measurements of temperature, salinity, total carbon dioxide, total alkalinity, oxygen and nutrients were made approximately monthly at seven stations along the transect. An analysis of the first two years of data has been submitted to Marine Chemistry by S. Boehme, C. Sabine and C. Reimers. We report that fluxes

(-0.43 to ­0.84 mol m^-2 y^-1) calculated from local wind speed and the air-sea CO₂ difference indicate that this region acts as a small net sink for atmospheric CO₂ on a yearly averaged basis. The inner and outer stations varied on different time-scales, but in general, surface waters were a source of CO₂ to the atmosphere in the summer and fall, offset by large fluxes into the surface waters during the winter to early spring. The calculated fugacity of surface water carbon dioxide (fCO₂) between April 1994 and April 1996 ranged from 211 to 658 µatm. Superimposed on the large spatial and temporal variability typical of the coastal environment, was a clear seasonal trend in fCO₂ which was primarily responsible for the observed trend in the flux. The dominant processes responsible for the observed changes in fCO₂ are examined in detail. An important finding is that the magnitude of the effect of organic matter cycling on changes in fCO₂ generally decreased in the offshore direction.

I am now collaborating with Michael DeGrandpre (University of Montana), Christopher Sabine (Princeton University) and Mahmoud Shahrari (Rutgers, Fiber Optics Research Program) to continue these studies through a project in which pCO₂ and O₂ will both be monitored continuously (with sensors based on fiber optics) in order to improve estimates of short-term variability in air-sea fluxes, and to determine the influence of upwelling events, blooms and vertical stratification on pCO₂ and O₂ in shallow shelf environments. We will place the fiber optic oxygen sensor on a bottom mounted CTD/water column profiler at LEO-15 for field trials in the summer of 1998. Other aspects of this work depend on a proposal pending at NSF.