Marine Sediment Geochemistry

The increasing input of CO₂ in the atmosphere may affect the global carbon cycle, including the mineralization and preservation of organic carbon in sediments, and it is necessary to predict this effect. Unfortunately, the biogeochemical processes regulating the transformation of organic carbon in marine sediments are still poorly characterized. In particular, the kinetics of most biogeochemical reactions and the competition between oxidants of organic carbon in anoxic conditions (e.g., iron oxides and sulfate) are largely unknown. In the geochemistry group, several researchers currently study the cycling of organic carbon through a variety of approaches. Some of these research projects are provided below.

Benthic Fluxes (E. Ingall, R. A. Jahnke)

Benthic fluxes are determined to characterize the extent of natural organic matter transformation in marine sediments. Natural organic matter is respired aerobically using dissolved oxygen as electron acceptor and anaerobically using a variety of inorganic species (i.e. nitrate, nitrite, manganese and iron oxides, sulfate). In this research, fluxes of oxygen and other nutrients across the sediment-water interface are monitored over time using a benthic chamber. This chamber is placed at the sediment-water interface such that it encloses a known volume of sediment and overlying water. In the beginning of the experiment, the chamber lid is closed and oxygen removal from the overlying water is monitored with electrodes or by collecting water samples over time and analyzing them later after recovery of the lander. Benthic fluxes are calculated and used to deermine rates of natural organic matter remineralization in sediments.

Denitrification in Marine Sediments (C. Vance-Harris, E. Ingall)

Denitrification, the process of converting nitrate and nitrite to nitrogen gas, has been shown to be significant in continental shelf sediments but the cycling of nitrogen is still a mystery in these environments. The objective of this project is to quantify denitrification rates in continental shelf sediments using isotopic techniques. Inlet Mass Spectrometry is used to determine ¹⁵N : ¹⁴N ratios in N₂ generated during the incubation of continental shelf sediments.

Alternative Denitrification Pathways in SAB Sediments (A. Rao, R. A. Jahnke)

The global nitrogen and carbon cycles are linked by the dependence of living organisms on limiting nutrients such as nitrate and ammonium. Nitrogen is cycled by nitrogen fixing and denitrifying organisms between gaseous N₂ and N₂O and fixed organic and inorganic forms that are available as nutrients in solution. Denitrification in marine sediments is a key step in closing the cycle of fixed nitrogen. Originally, our understanding of denitrification included only the reduction of nitrate to N₂ with organic matter as the electron donor – a process known as heterotrophic denitrification. Recently, other pathways to N₂ have been discovered, while their significance in natural sediments is still in question.

The proposed series of experiments applies membrane inlet mass spectrometry to measure overall denitrification rates in permeable beds, which consititute 70% of global continental shelf environments. The significance of alternative pathways will then be quantified with N isotope enrichment and metabolic inhibition techniques applied to column reactors. Observations from the two sets of experiments will contribute to our understanding of both the fundamental importance and the specific pathways of denitrification.

For this project, we collected sediment cores in the South Atlantic Bight (SAB) continental shelf on the R/V Savannah. Ongoing work at the Skidaway Institute of Oceanography involves the use of continuous-flow column reactors with filtered seawater amended with NO₂^- and a NO₂^-/ NH₄⁺ mix as influent. Influent and effluent samples are presently analyzed for NO₂^- and/or NH₄⁺ in order to obtain preliminary estimates of denitrification rates on the SAB.

The Dynamics of Biogeochemical Cycles in Intertidal Saltmarsh Sediments (S. Neuhuber, M. Taillefert)

Saltmarshes are highly active and constantly evolving natural systems. Chemical (redox-changes, precipitation-dissolution), biological (microbial processes, bioturbation) and physical (erosion, sediment deposition and resuspension, tidal pumping) processes are governing the cycling of nutrients and other biogeochemically important elements in marsh sediments. In such systems, the flux of natural organic matter is extremely high, and the changes in concentrations of the main terminal electron acceptors used during the bacterial transformation of natural organic matter (i.e., oxygen, nitrate, manganese oxides, iron (hydr)oxides, and sulfate) usually occur within a few centimeters below the sediment-water interface. As a result, the sediment's biogeochemistry is easily altered by physical and biological processes from tidal to seasonal scales.

In this project, we determine the distribution of O₂ , Mn²⁺, Fe²⁺, and H₂S in porewaters with high spatial resolution using voltammetric mercury-gold microelectrodes, and we investigate the distribution of chemical species in the solid sediment. The main objective is to quantitatively describe the spatial and temporal variations in biogeochemical processes regulating the transformation of natural organic matter and inorganic chemical species between the solid sediment and the porewaters. Our ultimate goal is to determine which of physical, biological, or chemical forcing is the determining factor in the biogeochemical cycling of elements in sediments.

Formation and Reactivity of FeS and Pyrite in Saltmarsh Sediments (B. Carey, M. Taillefert)

Ferric iron plays a significant role in the mineralization of natural organic matter in marine and freshwater sediments. It can be reduced when utilized as terminal electron acceptor by bacteria, reduced by sulfides, or reduced by both.

Using incubation experiments with real sediments collected in the saltmarsh of Skidaway Island, this project investigates if soluble organic-Fe(III) complexes found in these environments may contribute to organic carbon preservation in coastal sediments by immobilizing Fe and S under the form of FeS(s) and pyrite or if these complexes rather oxidizes FeS and pyrite and recycles these elements in porewaters for further carbon oxidation.

Ongoing incubations experiments follow the temporal evolution of salinity, Br^- as tracer, pH, nutrients, Fe²⁺, soluble organic-Fe(III), and H₂S at the output of triplicate reactors placed under anaerobic conditions. The objective of this research is to quantify rates of transformation of iron and sulfur in these sediments using reactive transport modeling with an inverse approach. Preliminary results show the time evolution of the tracer and some of the nutrients at the output of the reactors.