Biogeochemistry of Aquatic Systems:
1. Spatial and temporal variation of porewater profiles
and microbial respiratory pathways in redox-stratified sediments
Summary: Porewater diffusion equilibrators ("peepers") have been deployed in six distinctive
sites in saltmarshes at Sapelo Island, GA,
a barrier island located off the coast of Georgia between Savannah, GA and
Jacksonville, FL. These sites include transects through two saltmarshes located
on the southern tip of Sapelo Island, near the UGA Marine Institute and near
the historic Sapelo Island Lighthouse. Porewaters to depths of approximately
45 cm have been collected seasonally (May & August 1997, January, June
&November 1998, January & April 1999, May 2000) and have been analyzed
at 1-2 cm intervals for pH, salinity, alkalinity, dissolved sulfate, sulfide,
manganese, ammonium, iron and phosphate. Pore water profiles exhibit strong
redox stratification, yet microbial community structure inferred from culture
enumerations and from 16S rRNA studies do not show the expected succession
of respiratory pathways. Seasonal oscillations in geochemical conditions
and in microbial community structure suggest that microbial structure is
dominated by competition among different microbial populations for sources
of labile organic carbon and by strong competition for terminal electron
acceptors with abiotic reduction pathways. Variations in pore water geochemistry
observed along transects through these saltmarsh sediments suggest that bioirrigation,
the enhanced solute transport caused by macrofaunal activity, as well as
labile organic carbon availability are the dominant factors affecting pore
water geochemistry at these sites.
2. Inverse models
of pore water irrigation
Summary: Transport of solutes through sediments at Sapelo
Island, GA appears to be strongly influenced by the seasonal influence of
burrowing infauna, particularly fiddler crabs. In collaboration with Christof
Meile and Philippe
Van Cappellen, the affects of bioirrigation at three of the Sapelo Island
saltmarsh sites have been investigated using a numerical reaction-transport
model that accounts for diffusion of solutes through the sediments, as well
as for non-local transport of solutes from the sediment-water interface (due
to the burrowing activities of infauna) using a bioirrigation coefficient.
This is an inverse model, meaning that measured data is used to iteratively
solve for the statistically-optimal profile of bioirrigation coefficients.
Sulfate concentration and reaction rate profiles from Sapelo Island saltmarsh
sites were used with the inverse model to calculate bioirrigation intensity
as a function of depth in the sediment. Results of the modeling suggest that
burrowing infauna such as fiddler crabs exert a very strong influence
on sediment biogeochemistry in this saltmarsh.
3. Stochastic models
of infaunal burrow shapes and distributions
Summary: The shapes, sizes and distribution of infaunal
burrows in sedimentary environments may exert a strong control on solute
transport. This is because the burrow "flushing" activity of infauna leads
to non-local transport of solutes from the sediment-water interface to depth
within the sediment. This transport process is spatially and probably temporally
heterogeneous; therefore, it may be difficult to represent using a strictly
deterministic model. To better understand the influence of the spatial and
temporal heterogeneity of bioirrigation, a stochastic model has been developed
to relate changes in burrow distributions to changes in non-local solute
transport in sediments. Preliminary results of this model are in qualitative
agreement with results of numerical reaction-tranport models of irrigation
in a saltmarsh environment. In the future, this stochastic model may be
used to predict variations in irrigation intensity in other sedimentary
environments.
4. Incubation experiments
to elucidate microbial and abiotic reaction pathways in marine sediments
Summary: Seasonal oscillations in marine sediment
pore waters and microbial respiratory pathways are likely due to changes
in a variety of factors, including (1) temperature, (2) availability of
labile organic carbon, (3) bioirrigation intensity, (4) competition between
microbial populations for available substrates and (5) competition between
abiotic and biotic reaction pathways. Sediment incubation experiments allow
many of these factors to be addressed. Ongoing experiments, in collaboration
with Tom DiChristina and Charlie Moore, using sediments collected at a Sapelo
Island, GA saltmarsh suggest that sulfate-reducing bacteria may outcompete
iron reducing bacteria through production of sulfide. This sulfide abiotically
reduces Fe(III), which supresses microbial iron reducing populations at conditions
which favor sulfate reducing bacteria (e.g., high temperature, high availability
of lactate or acetate).
Mineral-Water Interface Geochemistry:
1. Characterization of surface hydroxyl groups using diffuse
reflectance FTIR
Summary: Oxide and silicate mineral surfaces exposed to
air or aqueous solution are thought to be covered by hydroxyl groups that
may function as active sites for adsorption of solutes, as sites for solute
incorporation in mineral growth and as sites of mineral dissolution. Surface
hydroxyl groups on a suite of oxide and silicate minerals were directly
detected using diffuse reflectance Fourier-transform infrared spectroscopy.
The results of this work indicate that the types of surface hydroxyl groups
that occur on silicate minerals are not simply combinations of those groups
that are observed on "simple" oxide solids such as silica and gamma-alumina.
The results of this work have important implications for surface complexation
modeling of adsorption on and dissolution of silicate mineral surfaces.
2. Characterization
of mineral surfaces using crystal chemical considerations
Summary: Surface complexation models, which are commonly
used to quantify solute adsorption, require estimates of both the density
of reactive surface sites as well as, in the case of multi-site models,
information concerning the number of different site types and their configurations
on different minerals. Measured site density data and spectroscopic characterization
of site types are not currently available for most rock-forming minerals.
Therefore, a theoretical approach was used to characterize the types and
densities of sites on cleavage and growth surfaces of twelve oxide and silicate
minerals. Bond strengths and the charges of coordinatively unsaturated surface
atoms were used to predict the most stable configurations of the mineral
surface. Predictions were found to be in reasonable agreement with measured
site densities using tritium exchange methods and characterizations of site
types using FTIR spectrscopy. Site densities and types estimated in this
way may be used in the future to form the basis of a multi-site surface complexation
model.
3. Silicate dissolution
rate models
Summary: Far-from-equilibrium silicate dissolution rates
are an important factor controlling nutrient availability in soils, buffering
of soils in acidified watersheds and in the global carbon cycle. It has
been shown that far-from-equilibrium dissolution rates are determined by
the concentration of activated complexes, and further, that the concentration
of such complexes may be related to the concentrations of various surface
complexes (e.g., Wieland and Stumm et al. 1988, GCA 52, 1969). Using an extended
triple-layer model with predicted surface densities and surface stability
constants for sorption at silicate minerals (Sahai and Sverjensky, 199?,
GCA 61, 2827), a new silicate dissolution rate law was used to obtain apparent
rate coefficients for a variety of minerals dissolving in various electrolytes.
This type of approach allows measured silicate dissolution rates to be extrapolated
over ranges of ionic strength and pH. In addition, this approach holds great
promise for allowing completely predicted silicate dissolution rates as a
function of electrolyte type, ionic strength and pH to be derived in
the future..
