Marine sediments

Iron and manganese redox chemistry are important drivers of sulfur cycling in marine sediments. Florida Bay sediments are extremely sulfidic, having been attributed to mass mortality of seagrass and oxygen depletion in the water column. This research used conventional sediment analyses and a diagenetic model to infer the overall capacity for Florida Bay sediments to eliminate hydrogen sulfide and prevent high rates of sediment dissolved oxygen consumption via hydrogen sulfide reoxidation. Previous studies have suggested that iron is important for buffering hydrogen sulfide in Florida Bay sediments, while the results of this project show for the first time that this phenomenon is relevant only in specific locations and times of the year. However, my research indicates that Fe has the potential to sequester sulfides and minimize hypoxia in the Everglades system. Thus, under a scenario that greater amounts of Fe are delivered to Florida Bay sediments from freshwater flows under Everglades restoration, Fe could be a component of ecosystem management.

Harmful organic contaminants, such as petroleum hydrocarbons, are ubiquitous in coastal marine ecosystems around the world, a problem that will only be exacerbated with rising sea level and increased inundation of coastal urban areas. Therefore, it is necessary to understand the fate of these contaminants following their deposition on marine sediment, where they can potentially persist for long periods of time. As organic carbon remineralization rates depend on the respiration process employed by the bacteria in the sediment, it was the goal of this study to determine how the sediment redox environment, with an emphasis on Fe redox chemistry, affects the biodegradation of recalcitrant petroleum hydrocarbon compounds. While amendment of natural sediment with Fe minerals that are commonly transported to coastal areas following erosion from continental crust did successfully catalyze Fe reduction and inhibit sulfate reduction, the effect on the hydrocarbon biodegradation rate was negligible. However, inoculation of the sediment with Shewanella oneidensis, an exoelectrogenic, Fe reducing bacteria known to catalyze the degradation of hydrocarbon compounds found in crude oil, did
significantly affect the redox environment and sediment microbial communities and alter the pattern of hydrocarbon loss in the sediment over time.

In this dissertation, a fuzzy logic impedance inversion model is developed to classify marine sediments. Expert knowledge and fuzzy decision making constrain the inversion procedures to the resolving ability of the transmitted. The model is validated by comparing the estimated impedance profile with the measured impedance profile. A coherent surface scattering and incoherent volume scattering model are incorporated into a single geoacoustic scattering model that is applied to acoustic subbottom measurements. The reflected signal is modeled as the convolution of the transmitted processed wavelet and the impulse response of the sea bottom. The impedance of the acoustic return is inverted at the layer interfaces and the volume scattering strength is measured between layer interfaces. The model is applied to acoustic subbottom measurements obtained by an X-STAR subbottom profiler sonar system. The inversion techniques are developed for a 2-10 kHz 20 msec swept FM pulse. A fuzzy logic layer tracking procedure identifies the coherent surface scattering layer interfaces in a subbottom profile image. The peak amplitudes and locations are used as fuzzy inputs in the layer tracking rule base. The rule base determines which peak is assigned to the layer when two peaks compete for assignment or which layer is assigned to the peak when two layers compete for assignment. The fuzzy event detection algorithm estimates the impulse response of the acoustic return by complex least squares fitting parts of the transmitted wavelet with sections of the acoustic return. Reflectors are iteratively identified and removed from the return and the residual return is reprocessed. The detection procedure is constrained by the resolving ability of the matching signals and the peak envelope shape of the acoustic return. A genetic algorithm allows up to five low error reflector estimates to be processed until converging on the correct estimated impulse response (the tree branch whose summed error is minimized). The impedance is correlated with sediment bulk density by empirical relation. Experimental results validate that the fuzzy logic impedance inversion model reliably estimates the impedance of the sea bottom. The estimated impedance profiles of fifty acoustic returns are averaged and compared with measured impedance values.

Sediment exchange characteristics of tidal inlets subjected to tidal excitations are investigated and the results compared to field data measured at Jupiter Inlet, Florida. A specially written computer program combines inlet/nearshore hydrodynamic expressions with bed-load and suspended load sediment transport relationships to examine the building mechanisms of the near-shore tidal ebb shoal. The ebb tidal flow is modeled as a turbulent, plane jet which includes lateral mixing and entrainment, bottom friction, and offshore bathymetric changes. Flood tidal flow is modeled as a potential flow sink with the water being drawn into the inlet from one or more dominant offshore areas depending on the offshore bottom slope. Sediment transport expressions are evaluated at various locations within the offshore flow field and the sediment deposition depth is calculated at that location over one tidal cycle. Model results are plotted and compared to field data for analysis.

Laboratory scale measurements were conducted to validate numerical prediction models used to predict the acoustic field in a shallow water ocean environment. Experimental measurements were conducted in a range independent environment which included the effects of shear in marine sediments and in a depth varying range dependent environment. Good agreement between the experimental measurements and the numerical prediction codes were obtained using optimized values for the input parameters of the environmental model. In comparing experimental measurements to the numerical prediction codes it became apparent that the codes were very sensitive to the input parameters describing the bottom boundary of the ocean waveguide.

The benthic polychaete community and its associated sedimentary
environment were examined in February and August, 1978, at five
stations ranging in depth from 2.3 to 19.8 m off Highland Beach,
Florida. A total of 176 species were collected; the numbers of
species and individuals increased from the nearshore station to the
station farthest from shore. These increases are thought to be a
result of decreased wave-induced turbulence and increased food supply
in deeper waters. Motile infaunal feeders comprised the majority of
the community at the shallower stations (especially in winter) while
discretely motile and sessile surface feeders dominated in deeper
water. Dominant species at each station showed little functional
overlap, suggesting that resource partitioning was occurring.
Sediment parameters, although not as important as depth and
turbulence in regulating community structure, showed significant
correlations with biological parameters.

Currently, there is a great deal of interest in the role of sulfur in the seagrass ecosystems and for sulfide, a known phytotoxin, in particular. This research used a 35S tracer technique to examine sulfur metabolism in the seagrass Thalassia testudinum. The uptake of the 35S radiotracer was documented at similar rates under both oxic and anoxic treatments. The highest total radioactivity was in root and rhizome tissue, as compared to the leaves. 35S translocation from roots to leaves was found to be more efficient in young versus mature leaves. Total sulfur uptake was estimated and found to be significantly different between root and rhizome tissue under oxic conditions. In the anoxic treatment, 1 mM sulfide may have been a threshold, at which the seagrass showed reduced uptake of 35S into the below-ground tissue. While the plants assisted in the production of sulfide in this experiment, sulfide accumulation may inhibit 35S uptake. This is counter to the idea of increased sulfide intrusion under sediment hypoxia. This study represents the first attempt to use 35S to trace sulfur incorporation into seagrass; further research will be required to understand the complex sulfur biochemistry of these important marine plants using this method.