The effects of geopressuring and kerogen decomposition on mineral-fluid equilibria were calculated in order to predict the diagenetic-alteration mineralogy produced in equilibrium with kerogen-rich, geopressured sediments. These calculations indicate that several processes specific to kerogen-rich geopressured sediments contribute to the development of a characteristic alteration mineralogy. These processes are: (1) the upward flow of fluids in geopressured sediments, in contrast to the generally downward flow of fluids in normally-pressured sediments; (2) the coincidence of the depths of geopressuring (2-3 km; Fertl et al., 1976), with the geothermal temperatures necessary for CO₂ release (100°-135°C; Hunt, 1979), and CH₄ release (>90°C; Hunt, 1979); and (3) the opposing rates of sediment burial and CO₂ and CH₄ transfer into the upward-flowing fluids, which result in the geopressured pore fluids becoming enriched, and in some cases saturated, with respect to CO₂ and CH₄. Three patterns of mineral deposition during diagenesis of kerogen-rich geopressured sediments are predicted. Quartz deposition should occur at the top of the geopressured zone and decrease rapidly with increased depth as a result of the decreased flux of upward fluid flow with increased depth. Carbonate deposition should occur above the zone of CO₂ release from kerogen degradation as a result of the upward flux of CO₂ saturated fluids and subsequent decreases in fluid temperature, pressure and CO₂ solubility. Kaolinite-carbonate could deposit within and above the zone of CO₂ release from kerogen as a result of silicate dissolution by CO₂-rich acid pore fluids, followed by the potential for albite-carbonate deposition upon CO₂ depletion. In contrast, laumontite and anhydrite should not deposit during diagenesis of kerogen-rich geopressured sediments, but could deposit during diagenesis of normally-pressured or kerogen-poor geopressured sediments. An additional difference between these diagenetic environments is that quartz deposition would not be expected in normally-pressured sediments in which fluids are expected to be flowing downward. These mineralogic relationships compare favorably with observed relationships in the kerogen-rich geopressured sandstones of the Frio formation from the Texas Gulf Coast.

acase@tulane.edu / Land-surface subsidence can be a major contributor to the relative sea-level rise that is threatening many coastal communities. Loosely constrained subsidence rate estimates across the Mississippi Delta make it difficult to differentiate between subsidence mechanisms and complicate modeling efforts. New data from a nearly 40 m long, 12 cm diameter core taken during the installation of a subsidence monitoring superstation near the Mississippi River, southeast of New Orleans, provides insight into the stratigraphic and geotechnical properties of the Holocene succession at that site. Stratigraphically, the core can be grouped into four units. The top 12 m is dominated by clastic overbank sediment with interspersed organic-rich layers. The middle section, 12-35 m, consists predominately of mud, and the bottom section, 35-38.7 m, is marked by a transition into a Holocene-aged basal peat (~11.3 ka) which overlies densely packed Pleistocene sediment. Radiocarbon and OSL ages are used to calculate vertical displacement and averages subsidence rates as far back as ~3.5 ka, yielding values as high as 8.0 m of vertical displacement (up to 2.34 mm/yr) as obtained from a transition from mouth bar to overbank deposits. We infer that most of this was due to compaction of the thick, underlying mud package. The top ~80 cm of the core is a peat that represents the modern marsh surface and is inducing minimal surface loading. This is consistent with the negligible shallow subsidence rate as seen at a nearby rod-surface elevation table – marker horizon station. Future compaction scenarios for the superstation can be modeled from the stratigraphic and geotechnical properties of the core, including the loading from the planned Mid-Barataria sediment diversion which is expected to dramatically change the coastal landscape in this region. / 1 / Jonathan G Bridgeman

archives@tulane.edu / Over the past two centuries, coastal wetlands have become increasingly threatened by accelerated relative sea-level rise and anthropogenic modification. Engineered structures such as sea walls, levees, and drainage systems prevent natural processes of sediment distribution, reducing the resilience of coastal ecosystems. Land subsidence and shoreline erosion combine with global sea-level rise to make low-elevation coastal zones increasingly vulnerable to submergence. This dissertation examines processes of sediment accumulation, compaction, and relative sea-level rise in coastal wetlands and assesses strategies for restoration. I find that organic content strongly controls sediment compaction in wetland sediments. At least 80% of compaction happens quickly, largely within the first 100 years after deposition and in the top 1 m of the subsurface. This rapid shallow compaction is generally not recorded by traditional methods of measuring relative sea-level rise in low-elevation coastal zones (i.e., tide gauges and global navigation satellite systems). As a result, tide gauges generally underestimate rates of relative sea-level rise in low-elevation coastal zones and these areas may be at a greater risk of flooding than previously realized. However, despite accelerated rates of relative sea-level rise and rapid sediment compaction, coastal restoration efforts such as river diversions can be successful in building new land in some areas. I find that sediment deposition responds non-linearly to water discharge, reaching a maximum at moderate discharge. Wetlands are more likely to keep up with relative sea-level rise if hydrodynamic conditions are optimized to retain mineral sediment in targeted restoration areas. / 1 / Margaret Keogh

This thesis work was directed at aspects of two related problems: (1) sediment compaction and fluid expulsion processes in a subduction margin accretionary prism, and (2) the nature and concentration of gas hydrates that form bottom-simulating reflectors (BSRs) observed in the accretionary prism sediments of the northern Cascadia margin. The formation of the gas hydrate and the occurrence of BSRs in the study area are believed to be mainly a consequence of upward fluid expulsion in the accretionary prism. Therefore, the two study objectives are closely correlated. Most of this thesis work was carried out analyzing multichannel seismic data and incorporating available information including downhole and other geophysical measurements. Seismic techniques, such as velocity analysis, forward modelling, and waveform velocity inversion, were used in analyzing the data to advance our understanding of the tectonic and geophysical processes in a dynamic accretionary prism environment. The velocity structure and the inferred porosity variations across the frontal region of the accretionary prism have been quantitatively assessed by a detailed seismic velocity analysis. Within the Cascadia basin sediments approaching the deformation front, and within the frontal thrust zone of the accretionary prism, seismic velocities increase landward as a result of sediment consolidation. An important conclusion is that more than one third of the pore fluid content of the incoming sediment is lost by the time they are incorporated into the accretionary prism. In the lower slope region of the deformation front, a pronounced velocity decrease is evident. This low-velocity zone is explained by underconsolidation resulting from rapid horizontal shortening and vertical thickening of the sediment column, accommodated by displacements along thrust faults or by distributed deformation. A prominent BSR becomes visible immediately landward of the deformation front in the accreted sediment, and is developed over much of the low-to-mid continental slope. The upward pore-fluid migration is believed to play an important role in the formation of a gas hydrate BSR. From the estimated fluid loss of 35% over the 3-km-thick Cascadia Basin sediments with an average sediment porosity of 30%, the quantity of the expelled fluid reaches 315 m3/m2 over a distance of 12 km before the basin sediments are incorporated into the accretionary prism. Assuming that 100 mmol/L of methane is removed from the expelled fluid as it moves into the hydrate stability field, a 90-m-thick layer with an average hydrate saturation of 10% of the pore space can be formed by the rising fluids. A velocity-depth function in the lower slope region, representing a no-hydrate/no-gas reference profile, has been established from the detailed semblance velocity analyses and the ODP log data. The observed and measured sediment velocities near the ODP drill sites increase downward more rapidly than the reference profile above the BSR. Based on the reference profile, the velocity inversion results imply that the velocity increase due to hydrate above the BSR accounts for ~2/3 of the impedance contrast required to produce the BSR reflection amplitudes. The remainder of the impedance contrast appears to come from the velocity decrease associated with small concentrations of free gas below the BSR. The integrated analysis of the multichannel seismic and ODP downhole velocity data has allowed the velocity enhancement associated with the formation and concentration of gas hydrate to be estimated. If the BSR is overlain by a 100 m zone of sediment with a mean porosity of 50% in which the hydrate saturation increases linearly from zero at the top of the zone to 20% at the BSR, the estimated hydrate concentration-depth profiles indicate a total hydrate amount of about 5 m3/m2 of ocean floor or methane amount of 820 m3/m2 at STP. Throughout the Vancouver Island continental margin, where the clear BSR have been observed in an area of 30x200 km, the total methane gas estimated can amount to about 175 Tcf (trillion cubic feet) or 2.6 Gt of carbon. / Graduate

A one-dimensional, numerical model of sediment compaction has been developed using porosity, velocity of sediment particles, and depth of the evolving basin as master variables. The governing set of nonlinear, partial differential equations are solved by a finite difference scheme devised to be stable for calculations involving tens of millions years and depths up to 4 km. Input parameters include a sedimentation function and a permeability-porosity function representative of the modeled sediment. Additional terms can be incorporated to mimic the effect of fluid volume generated by dehydration from clay mineral transformations and by temperature and pressure variations. Evolution of pressure, porosity, permeability, and fluid and sediment particle velocities are documented in a vertical sediment column as well as properties of a sedimentary package being successively buried. Although this model has many potential applications, it is used here to demonstrate that the major cause of overpressuring in sediments accumulating along passive margins is nonequilibrium compaction. In general, smectite dehydration and aquathermal pressuring play minor roles in the development and sustenance of overpressuring. Comparison of model cases and Gulf Coast overpressured cases shows that sedimentation rates and strata permeability are the most important geologic factors in the formation of overpressured zones. / Master of Science

Thesis (Ph. D.)--University of Texas at Austin, 2004. / Supervisors: Earle F. McBride and Kitty L. Milliken. Vita. Includes bibliographical references. Also available from UMI.

Thesis (Ph. D.)--University of Texas at Austin, 2004. / Includes bibliographical references (p. 278-297). Available also in an electronic version.

Ever-growing discharges of various emerging chemical contaminants are imposing a great threat of pollution to the coastal environment. Adsorption by sediment plays an essential role in the transport and fate of pollutants in the aquatic system. The sorption of emerging contaminants onto sediment is believed to be largely dependent on the sediment organic matter (SOM). In the present study, laboratory experiments were carried out on the changes of the adsorption behavior of sediment during the sediment aging and diagenesis process. A few EDCs and antibiotics were selected as the model emerging compounds for the adsorption tests. The results demonstrated that both the quantity and the quality of the SOM affected the adsorption of the model pollutants, such as 17α-ethinyl estradiol (EE2) and bisphenol A (BPA), onto the marine sediment collected from Victoria Harbour, Hong Kong. The adsorption isotherms can be well described by the linear partition model. Natural and artificial sediment with a high SOM content was incubated for 4-6 months to simulate the natural diagenesis process. The most rapid degradation of labile SOM occurred in the first 1 month or so and afterward, SOM reduction became slower. Microbial activity played an important role in SOM degradation and transformation. A rapid initial bacterial growth was observed in the sediment, followed by a slow endogenous decay. The dynamics of biomass growth and decay first transformed the labile SOM into biomass and microbial byproducts. After the exhaust of readily biodegradable SOM, the biomass decay produced humic-like substances, resulting in more refractory and condensed SOM residues in the sediment. More importantly, the degradation and transformation of SOM displayed a profound impact on the adsorption behavior of the sediment. For the selected EDCs and antibiotics, including BPA, EE2, nonylphenol (NP), phenanthrene (PHE) and tetracyclines (TCs), the adsorption capacity indicated by the partition coefficient, Kd, decreased at the beginning of SOM diagenesis. The Kd values for different chemicals recovered lately to different extents as the result of the SOM condensation and humification. All of the organic matter normalized partition coefficients, KOM, of the concerned pollutants increased considerably in the late phase of SOM diagenesis. Based on the experimental results, a general conceptual model was established to describe SOM diagenesis and its impact on chemical adsorption by the sediment. According to the model prediction, the SOM profile would become more dominated by the condensed and refractory fractions during sediment diagenesis with an increasing affinity and partition capacity for organic contaminants. Moreover, the release of adsorbed contaminants from marine sediment in the simulated digestive fluids was investigated. In general, the presence of gastric pepsin and bile salts helped the desorption of hydrophobic pollutants from the sediment into the digestive solutions. The influence of the SOM diagenetic status on chemical desorption from the sediment varied between EDCs of different chemical properties. It is apparent that aged sediment could bring more emerging pollutants into the digestive system of receiving organisms, imposing a potential risk to human health through the food chain. / published_or_final_version / Civil Engineering / Doctoral / Doctor of Philosophy

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The health of Florida's beaches are vital to the survival of loggerhead sea turtles (Caretta caretta), as nearly half of the world's loggerheads nest on the states beaches. Many of the beaches utilized by the turtles have undergone nourishment projects in hopes of combating erosion of the shoreline, protecting beachfront property, and creating more suitable beaches for tourism. Although it is argued that beach nourishment benefits sea turtles by providing more nesting habitat, the effects of the Pinellas County nourishment projects on loggerhead nesting are unknown. Beach nourishment can alter the compaction, moisture content, and temperature of the sand, all of which are variables that can affect nest site selection and the proper development of eggs. This research has four objectives: (1) to create a GIS dataset using historic loggerhead sea turtle data collected at the individual nest level along the West coast of Florida, (2) to examine the densities of loggerhead nests, the densities of false crawls (i.e. unsuccessful nesting attempts), and the nest-to-false crawl ratio on natural and nourished beaches for the 2006-2010 nesting seasons; (3) to determine the effects of beach nourishment projects on the hatchling success rates and emergence success rates; and (4) to determine areas preferred or avoided by turtles for nesting. The study found that nesting and false crawl densities significantly differed between natural and nourished beaches during three of the five nesting seasons. Nesting densities increased directly following nourishment and false crawl densities were higher in nourishment areas during every nesting season. False crawl densities were higher than statistically expected on nourished beaches and lower than expected on natural beaches. No significant differences were found between hatchling and emergence success rates between natural and nourished beaches. However, when the rates were analyzed by nesting season, the average hatching and emergence success rates were always lower on nourished beaches than on natural beaches. A hotspot analysis on nests and false crawls revealed that turtles preferred natural beaches that border nourished areas for nesting while false crawls were more evenly distributed through the study area. Although this study documents the negative effects of beach nourishment on loggerhead sea turtle nesting, nourishment projects are likely to continue because of their benefits to human populations. Further examining of the impacts that humans have on nesting and developing loggerheads will ultimately aid policy formation as we continue to manage and protect the future of the species.