published_or_final_version / Earth Sciences / Doctoral / Doctor of Philosophy

published_or_final_version / Earth Sciences / Doctoral / Doctor of Philosophy

published_or_final_version / Civil Engineering / Doctoral / Doctor of Philosophy

The production of methyl mercury, an acute neurotoxin which readily accumulates in the tissue of organisms, is a biologically mediated process facilitated by sulfate reducing bacteria in aquatic sediments. In-situ capping is a frequently considered risk management strategy for contaminated sediments. Since placement of an in-situ cap will induce anaerobic conditions that are known to be favorable for the growth of sulfate reducing bacteria, there is justifiable concern that capping could increase mercury methylation in underlying sediments. This research builds an understanding of the effects of in-situ capping on underlying biogeochemical processes and elucidates their importance in controlling methyl mercury production. Laboratory experiments and mathematical models were implemented to simulate mercury methylation in redox conditions likely to be induced by capping using sediment from different environments. Mathematical descriptions of processes known to be involved in methylation were incorporated into the model to quantify the effects of these processes. Observations in both well-mixed slurry conditions and intact sediment columns showed that methyl mercury concentrations are strongly dependent upon biogeochemical conditions. Results from experiments with sediment spanning a range of redox conditions and organic contents suggested that sulfate reduction rates, aqueous speciation, and solid phase partitioning are involved in limiting methylation depending on bulk geochemical characteristics. A model with a mechanistic basis that incorporates the effects of these processes provides a useful means of qualitatively and quantitatively considering their cumulative impact in limiting methyl mercury production. High methyl mercury concentrations observed in some lab experiments suggest that there is reason to be concerned about anoxic conditions induced by capping; however, not all anoxic conditions led to equivalent increases in methyl mercury. Experimental and modeling results suggest that in a high organic environment, in-situ capping may produce conditions which accelerate methylation in (formerly) surficial sediment while in a low organic environment, with an overall lower potential for methylation, capping can be expected to have a less dramatic effect. Over time, two processes will temper capinduced increases in methyl mercury. Increases will only last until sulfide builds up to inhibitory levels in underlying sediment or until organic carbon is depleted and overall bacterial activity slows. By providing a more fundamental understanding of the effects of capping on mercury methylation, the results of this research will aid in identifying situations and conditions in which cap-induced increases in methyl mercury have the potential to limit the effectiveness of the management strategy. / text

No description available.

The New River Estuary is a large mesotidal estuary connected to Foveaux Strait by way of an inlet, and has two principle rivers (the Oreti and the Waihopai) flowing into it. The purposes of this study have been to describe the sediment distribution within the estuary; investigate the sediment source-sink relationships.; to calculate whether on a net basis sediment is being deposited in or eroded from the New River Estuary, and to investigate some of the influences on the sediment dynamics of this estuary. Textural analysis of the sediment indicated that there are three sedimentary environments within the estuary, and each is distinguishable due to the relative mix of fine sand. It was shown that Foveaux Strait is a dominant source of medium-fine sand. The Oreti River and the channels of the main estuary body are also important sources of these sediments. Medium-fine sand is transferred into the estuary and very fine sand is transferred out of the estuary and onto the continental shelf. Aerial photographs indicated that estuarine morphology was stable over a 31 year period (from 1947 to 1978). However a network of scour rods indicated that the intertidal sediment surface was spatially and temporally dynamic. It was calculated that on a net basis sediment was deposited in the estuary over a five month period. An important influence on the sedimentation of the New River Estuary has been the reduction of the tidal com partment by 25%, due to the reclamation of 12.2 km2.

<p>This study described porosity and permeability distribution in the deep marine play of the central Bredasdorp Basin, Block 9, offshore South Africa using methods that include thin section petrography, X-ray diffraction, and scanning electron microscopy, in order to characterize their porosity and permeability distributions, cementation and clay types that affect the porosity and permeability distribution. The study included core samples from nine wells taken from selected depths within the Basin.</p>

Chemical sediments in a variety of geomorphic environments (pedogenic, fluvial, palustrine, lacustrine and pan) were investigated from seven regions (SW Kalahari, Kgalagadi, Kalkweissrand, Etosha, Linyanti, Okavango and Ngami) in the Kalahari of southern Africa. These primary and multi-phase sediments were characterised by a range of pure and intergrade silcrete, calcrete and dolocrete fabrics which contained an array of structures indicative of the crystalline and biogenic processes responsible for their precipitation, epigenesis and paragenesis. Petrography, mineralogy and isotope geochemistry provided significant insight regarding the environmental and geochemical conditions at the time of precipitation. Petrography indicated that the majority of chemical sediments were undergoing epigenetic modification as a consequence of the desiccation induced transition from phreatic to vadose diagenetic and geochemical conditions. The role of rapidly infiltrating meteoric water, associated with unstable wetting fronts, is believed to be instrumental in vadose diagenesis and the precipitation of crystalline/alpha fabric carbonate. Salinisation within the capillary fringe and deeper vadose zone is believed to be responsible for the sequence dolomitisation of crystalline calcite within mature sequences. Highly saline pan conditions instigate the precipitation of authigenic dolomite, calcite and K-feldspar within the surface sediments and authigenic silica at depth. Phreatic water beneath the islands, floodplains and fluvial systems of the Okavango, which undergoes evapotranspirational and transmission salinisation and ultimately terminal desiccation, are similarly precipitating silcrete. Pedogenic processes principally associated with C₄ vegetation are responsible for the gratification of carbonate mud within desiccating lacustrine, palustrine and pan sediments. Within the thin sandy Kalahari soils, pedogenesis is limited, but biogenic/beta fabric precipitation linked to mycorrihizae and tree/shrub savanna vegetation is instrumental in the formation of hardpans and the modification of upper calcrete horizons. The dominance of a distinct assemblage (smectite/kaolinite or sepiolite/palygorskite) of authigenic clay minerals present within all the environments provides evidence of semi-arid precipitation within Mg and Ca enriched saline/rapidly evaporating water or brackish/reduced permeability environments.

The North Alpine Foreland Basin (NAFB) comprises sediments of late Eocene to middle Miocene age. The earliest deposits are the North Helvetic Flysch which are exposed in the regions of Glarus and Graubunden, eastern Switzerland. The Taveyannaz sandstones are the first thrust wedge (southerly) derived sediments of the North Helvetic Flysch. The Taveyannaz basin was divided into two sub-basins by a thrust ramp palaeohigh running ENE/WSW (parallel to the thrust front). Palaeocurrent directions were trench parallel towards the ENE. Sedimentation in the Inner basin (140m thick) is characterised by very thick bedded turbidite sands generated by thrust induced seismic events confined within the thrust-top basin. The Outer basin (240m min. thickness) comprises 10-15 sand packages (5-100m thick) formed by turbidite sands which are commonly amalgamated. Sedimentation in the Outer basin is considered to have been controlled by thrust-induced relative sea-level variations. The Inner basin underwent intense deformation at the sediment/water interface prior to the emplacement of a mud sheet over the basin whilst the sediments were partially lithified. Later tectonic deformation involved fold and thrust structures detaching in the underlying Globigerina marls. The stratigraphy of the NAFB can be considered as two shallowing upward megasequences separated by the base Burdigalian unconformity. This stratigraphy can be simulated by computer by simplifying the foreland basin/thrust wedge system into 4 parameters: 1) the effective elastic thickness of the foreland plate, 2) a transport coefficient to describe the erosion, transport and deposition of sediment, 3) the surface slope angle of the thrust wedge, 4) the thrust wedge advance rate. The Alpine thrust wedge underwent thickening during the underplating of the External Massifs at about 24-18Ma. This event is simulated numerically by slowing the thrust wedge advance rate, and increasing the slope angle and keeping all other parameters constant. This event causes rejuvenation of the forebulge, and erosion of the underlying stratigraphy, so simulating the base Burdigalian unconformity without recourse to eustasy or anelastic rheologies to the foreland plate.

Intertidal cohesive sediment systems are found throughout the world in areas of low hydrodynamic energy. These systems are ecologically and economically important but are under pressure from global warming, sea level rise and other anthropogenic influences. To protect and conserve these systems it is important to understand the sediment dynamics, especially the erosional properties of the sediment. The study of sediment erosion and transport is complex, encompassing biological, chemical and physical properties of the ecosystem. This thesis contributes towards this area of research, firstly in regard to the methods used to measure sediment erosion on exposed and submerged sediments and secondly with respect to assessing influences upon sediment stability through changes in the ecosystem, comprising of both the sediment environment and the macrofaunal community. Chapter 3: In partnership with Sediment Service a thorough re-evaluation of the Cohesive Strength Meter (CSM), a commercially available device used to measure surface sediment strength, was performed. New components, deployment method and calibration protocol were devised and tested. The new design was not effective, but the deployment and calibration have improved the ease of use and interpretation CSM data. Chapter 4: The study of intertidal sediment stability was conducted during the submerged period of the tidal cycle. Protocols and methods were devised or modified to sample submerged sediments with the aim to determine how sediment properties are affected by submersion and the resulting effect on sediment stability. Sediment stability increased with submersion. The existence of a fine layer of sediment on the surface, similar to the fluff layer found in submerged sediments, is given as a suggested explanation as it may be removed by the incoming tide. However, no other changes in sediment properties were detected. This may be due to flaws in the methods used in detecting fine scale changes in the sediment surface. In situ and laboratory experiments revealed contrasting effects of submersion on sediment stability with disturbance from the sampling and movement of sediment from the field to the laboratory given as an explanation for this. Chapter 5: The influence of the ecosystem engineering polychaete Arenicola marina on sediment properties was examined with an exclusion experiment. A. marina was excluded from five 20m² plots on an intertidal mudflat on the German island of Sylt. A holistic approach was used to measure the ecosystem, including a range of biotic and abiotic sediment properties as well as the macrofauna community. It was hypothesised that A. marina’s exclusion would alter the macrofaunal community and increase sediment stability. However, there was no consistent change in the macrofauna community or sediment environment with the exclusion of A. marina and subsequently no change in sediment stability. Chapter 6: The impact of bait digging for A. marina was examined with six 5m² plots dug up and A. marina removed, the plots were then monitored over a three month period. Bait digging disturbance was expected to have an impact upon the sediment environment and macrofauna community, resulting in a reduction in both sediment stability and microphytobenthic abundance. However, bait digging had minimal impact on the macrofauna community and caused no change in the sediment environment, despite the removal of a large proportion of the A. marina population. No change was recorded in the sediment stability or biomass of the microphytobenthos, indicating that with the exception of removing A. marina, bait digging of this nature was not detrimental to the sediment ecosystem. However, the consequences of larger, longer term digging operations can not be determined from this work and further studies are suggested. The study of intertidal sediment stability was progressed with advances made in methods and protocols. The work highlighted the importance of studying sediment stability as an ecosystem function through a holistic ecosystem approach rather than isolating individual variables.