• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 44
  • 39
  • 3
  • Tagged with
  • 1278
  • 428
  • 117
  • 78
  • 52
  • 49
  • 44
  • 34
  • 34
  • 29
  • 24
  • 23
  • 22
  • 22
  • 21
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
181

The uptake and flux of dissolved nitrogen in marine waters

Rees, Andrew Paul January 2001 (has links)
The biological uptake and transformation of inorganic nitrogen species is described for contrasting marine environments which include a sea loch, riverine plume, polar marginal ice-zone, continental shelf break, oligotrophic ocean and a mesoscale eddy. Uptake of nitrate and ammonium by phytoplankton has been determined using the stable isotope as a tracer and continuous flow stable isotope-ratio mass spectrometry. The analysis of dissolved and atmospheric nitrous oxide was made using electron capture detector gas chromatography in a novel configuration which also allowed for the simultaneous analysis of methane from a single sample. Significant advances in the study of the marine nitrogen cycle have been made and presented to the scientific community via publication in high quality research journals and by the placement of data into the British Oceanographic Data Centre. These are important from local and global perspectives; information on the trophic status of seawaters is presented with respect to seasonal and hydrographical variability, as is the contribution made to knowledge regarding the role of nitrogen in climate change. Nitrogen availability is limiting in many oceans and attempts to constrain the global atmosphere - ocean fluxes of CO2 are based on basin scale nitrogen balances. The development of novel analytical procedures and their subsequent use during a number of field programs and modeling exercises has increased the available knowledge regarding the role of nitrogen in the removal of carbon to the deep ocean. In particular new insights into new production and ultimately carbon export have been made, these include: (i) The accepted threshold limit for phytoplankton uptake of nitrate in the oligotrophic ocean has been reduced from 0.016 to 0.005 mmol m ^ (ii) a relationship has been described relating the size structure of the phytoplankton productivity to new production which can be used as a direct estimate of carbon export, (iii) a series of algorithms have been produced to allow the basin scale estimation of new production from satellite and ship derived data, and (iv) whilst nitrous oxide plays a significant role in radiative forcing and stratospheric ozone depletion, an area in the Southern Ocean was found to provide a seasonal sink to atmospheric N2O.
Read more
182

Wind and bouyancy driven models of sub-tropical ocean gyres

Walkington, Ian A. January 2003 (has links)
No description available.
183

Millennial-scale ice-ocean-climate variability during the last glacial : high-resolution records from the NE Atlantic

Peck, Victoria Louise January 2006 (has links)
Marine sediment core MDO1-2461 recovered from the European Margin, SW of Ireland (51 45' N, 12 55' W) at a water depth of 1153m provides material for multi-decadal to centennial scale investigation into ice-ocean-climate variability during the period 60 to 8 kyrs BP. Particular focus is placed on the oceanic and climatic conditions under which periodic collapse of the North American Laurentide ice sheet (LIS) occurred, so called Heinrich (H) events, and the involvement of the NW European ice sheets (NWEIS) within episodes of abrupt climate change. Presented here are records of circum-North Atlantic ice sheet growth, dynamics and decline from lithological and geochemical analysis of ice-rafted debris. Paired Mg/Ca and 8180 data from the surface dwelling <italic> Globigerina bulloides</italic> and subsurface dwelling <italic>Neogloboquadrina pachyderma</italic> sinistral are used to determine late-glacial variability of temperature, salinity and stratification of the upper water column, and benthic 813C records from <italic>C. wuellerstorfi</italic> documents the interchange of glacial northern and southern-sourced intermediate water signatures along the European Margin when compared to similar records from the Portuguese Margin and high-latitude North Atlantic basin. A picture emerges that the BIS was in a continuing state of readjustment and never fully reached steady state. Persistent 2 kyr instability of the BIS fluxes throughout the late glacial (26.5-10 kyr BP) suggests that 'precursor events' were not directly implicated in the collapse of the LIS. However, negative salinity excursions of up to 2.6 indicate significant incursions of meltwater associated with peaks in NWEIS instability, stratifying the upper water column. Such events are associated with reduced ventilation of intermediate waters, recorded in depleted epibenthic 813C suggesting that instability and meltwater forcing of the NWEIS temporarily weakened Glacial North Atlantic Intermediate Water formation, allowing transient advance of southern-sourced waters to the site of MDO1-2461. NWEIS-instability and associated perturbation to the North Atlantic thermohaline circulation may have set the stage for H event initiation regional cooling associated with a preliminary reduction in MOC may have promoted the growth of a LIS-fringing ice-shelf and subsequent subsurface warming and sea level rise may have destabilised the ice margin. Additionally, high-resolution summer sea surface temperature records based on the Mg/Ca <italic> G. bulloides</italic> suggest that mild, even warm summer months may be a have been a feature of H events.
Read more
184

3D tectonostratigraphic analysis of the Messinian evaporites in the Levant Basin, Eastern Mediterranean

Bertoni, Claudia January 2006 (has links)
This thesis uses industry seismic (3D and 2D) and well data to investigate the depositional and deformational processes of the Messinian evaporites in the Levant Basin, Eastern Mediterranean. Detailed interpretation of the geometry, structural and stratigraphic context of the evaporites has been undertaken in order to improve our understanding of the events occurring during the Messinian Salinity Crisis in the region. The Messinian evaporites form a wedge up to 1.8km thick that pinches out towards the continental margin, where they pass laterally to a prominent erosional surface. The 3D seismic data have allowed the complete basinal evaporitic series to be imaged in three dimensions for the first time. Investigations showed that clastic bodies and focused incisional pattern developed at their base. The internal part of the evaporites is composed of parallel and continuous seismic reflections, alternating with transparent seismic facies. These internal reflections are truncated at the top of the Messinian evaporites, against a widespread erosional unconformity. The analysis of the 2D seismic data permitted the connection of these morphological features with a regional system of canyons, developed on the Levant continental margin since the Oligocene. The canyons acted during the Messinian Salinity Crisis as clastic fairways, erosional loci and depocentres for the evaporites. The morpho-structural observations collected have been compared to the pre- and post-evaporitic setting, in order to infer the depositional environment and post-depositional behaviour of the evaporites. Relative sea-level changes in the basin have been interpreted in relation with two major erosional events developed at the base and at the top of the evaporites. Finally, the three-dimensional analysis also allowed an early (Messinian) stage of deformation of the evaporites to be identified, and the discovery of the occurrence of evaporite dissolution during the Pliocene.
Read more
185

Investigation of high temperature enhanced hydrogen formation in mineral amended sediment slurries, as a potential novel deep biosphere energy source

Linnane, Cathal David January 2009 (has links)
The deep biosphere has been estimated to be the largest prokaryotic habitat on Earth. However, what energy sources sustain these prokaryotes at depth and over geological timescales, remote from photosynthetic energy supply, is unclear. Hydrogen generation from basalt weathering has been controversially suggested as a potential deep terrestrial energy source, although other minerals were thought to be unimportant, as they did not generate H2 under abiotic conditions. In this study these experiments were repeated but in the presence of sedimentary prokaryotes using a range of minerals, with differing iron concentrations at a range of temperatures (4-105 C) and under anoxic conditions. Results showed that at certain temperatures high levels of H2 were produced from several minerals (600-1650 umol l"1). This was associated with high levels of acetate formation (-5000-1000 umol l"1). Substantial CH4 (100-600 umol f1) was also produced. Sulphate reduction proceeded between 50-85 C, which no removal at higher temperatures, similar to hot oil reservoirs. The amounts of these compounds produced varied depending on mineral composition. The presence of iron in minerals was found not to be necessary to facilitate the production of H2 as previously thought. Under sterile conditions little H2 or other products were formed. This shows that H2 generation from minerals was a microbially mediated process and was consistent with the presence of considerable prokaryotic cell numbers (range 4.3 x 107 to 5.1 x 108 cells ml"1) in these experiments. Prokaryotic cell numbers decrease with increases in temperature, similar to the predicted decrease in numbers with depth in the sub seafloor biosphere (e.g. 99 C 6.67 x 105 cells ml"1). Sequential ramping experiments to 155 C, demonstrated that prokaryotic activity in the biotic zone <120 C, increased the formation of products such as H2 (x34) and low weight molecular acids (acetate x2.3) in the abiotic zone >120 C. This has implications for hydrous pyrolysis experiments, which are used to simulate high temperature diagenesis. This increases the sources of potential energy, which may migrate upward to feed the base of the deep biosphere. The prokaryotic community present was assessed using 16S rRNA gene analysis (PCR-DGGE). Communities varied with mineral type and included thermophilic methanogens and acetate oxidisers, iron-reducers and Deltaproteobacteria, Thermoplasmatales, as well as uncultured bacterial and archaeal sequences. Clone libraries indicate that the archaeal community is similar to proposed HyperSliMe communities, dominated by Thermococcales related species at high temperature 80- 95 C. Fresh grinding of minerals enhanced activity, which has implications for the rate of energy supply in the subsurface. A mechanochemical process based on the interaction of Si radicals, which is enhanced by the presence of prokaryotes, perhaps through microbial weathering, is hypothesised to be involved in H2 generation. But the interaction of other mineral constituents such as Fe, Al and Ti may also play a role. We found minerals stimulated prokaryotic activity up to at least 100 days. Interestingly, the carbon isotope values for the CH4 produced covered a wide range of values (-42 to -78&permil;), including depleted values that could be misinterpreted as being indicative of an abiotic origin. These results extend the range of minerals that could be potential "dark energy" sources in the subsurface and provide details of the prokaryotes and processes that such energy sources could stimulate.
Read more
186

Spatial and temporal reconstructions of surface deepwater flow in the subtropical NW Atlantic at sharp climatic transitions

Evans, Helena Kay January 2007 (has links)
A suite of deep-marine sediment cores recovered from the Blake Outer Ridge (BOR) in the subtropical North West Atlantic (28-34 N, 75-71 W) provide material for centennial to millennial scale investigations of abyssal circulation and surface ocean conditions during selected intervals in the last 130 kyr. Particular focus is placed on reconstructing the position and strength of the Deep Western Boundary Current (DWBC) at a high temporal resolution. Palaeocurrent dynamics are reconstructed using the 'sortable silt" (10-63 urn terrigeneous fraction) mean grain size sedimentological proxy. Paired Mg/Ca and 8 () records from the planktonic foraminifera <italic>G. ruber</italic> (white) are used to reconstruct deglacial sea surface temperature (SSI) and salinity variations, while benthic 513C data from Cibicidoides spp. document the interchange of northern- and southern source deepwater (NSW/SSW) in the subtropical Atlantic. Comparison of Holocene sediments with modern physical hydrographic measurements reveals a DWBC high velocity core between 3,000-4,000 m water depth. A deep position for the DWBC core below 3,500 m was also observed during the peak of the last interglaciation, marine isotope stage (MIS) 5e. The benthic 5I3C during these interglacial intervals reveals little chemical stratification and a water column dominated by NSW. A shallow Labrador Sea Water (LSW)-sourced secondary fast flowing DWBC core is also apparent during the Holocene, however during MIS 5e palaeocurrent data at intermediate depths suggest a weaker and possibly shallower position for the LSW-sourced secondary DWBC limb. The last glacial maximum and Younger Dryas reconstructions show similar hydrographic regimes with nutrient-depleted, vigorously flowing NSW above 2,500 m consistent with intermediate water formation. Northern sourced intermediate water was first apparent in the records presented at 111 kyr BP and is suggested to have persisted throughout the last glacial. Benthic 5I3C data suggest the presence of a highly stratified water column with an increasing influence of SSW with depth. Coupled suborbital oscillations in DWBC flow variability and palaeo-hydrography persist throughout the records. There is evidence for a broad-scale divergence in flow speed changes in the deep subtropical North Atlantic, with the presence of a vigorous, but poorly ventilated SSW mass below 4,200 m water depth during cold episodes of the last deglaciation and LGM, when shallower palaeocurrent and geochemical data suggest that NSW was suppressed. This is consistent with the operation of a bipolar see-saw effect. This study suggests a hitherto unrecognised degree of linkage between oscillations in subtropical North Atlantic SST and DWBC flow. During the last deglaciation the SST record is dominated by the position/strength of the Gulf Stream, while the effects of tropical heat retention are restricted prior to Heinrich event 1. A combination of meridional overturning strength, meltwater inputs and hydrological changes control salinity variability.
Read more
187

Cenozoic contourite drifts and palaeoceanographic development of the Faeroe Shetland Basin

Hohbein, Michael William January 2005 (has links)
This thesis investigated the palaeoceanographic history of the Faeroe Shetland Basin, NE Atlantic, via identification and analysis of contourite drift deposits using petroleum industry seismic and well data. Integration of regional 2D seismic lines, 3D seismic volumes and high resolution seismic profiles w ith industrial well data permitted full spatial and temporal characterisation of the contourites within the basin, including identification of small scale architectural elements and subtle stratigraphic relationships. In turn, it was possible to make interpretations regarding the palaeoceanographic regime within the basin from the onset and evolution of thermohaline current flow through the basin to correlation with the present day oceanographic situation. Overall, the study serves to highlight the efficacy of industrial seismic and well data for contourite and palaeoceanographic research. A variety of contourite drift types were identified during seismic-chronostratigraphic division of the Cenozoic succession. Identification of an early middle Eocene contourite drift within the southern Faeroe Shetland Basin dates the onset of southerly flowing deep waters from the Norwegian Greenland Sea into the North Atlantic as part of a North Atlantic Conveyor Belt-style circulation system at approximately 49 million years, predating previous estimates by more than 15 million years. The presence of Oligocene, Miocene and Pliocene contourite drifts within the basin reveals that following initiation in the middle Eocene, southerly flowing deep water circulation through the basin was continuous throughout the late Palaeocene and Neogene to the present day. A gradual increase in deep water flux through the basin throughout this time is hypothesised based on contourite distribution, and is thought to relate to the global greenhouse to icehouse climatic transition that occurred during the Cenozoic. Pulses of increased deep water current velocity related to a combination of climatic and tectonic factors are thought to be responsible for the formation of major deep w ater erosional unconformities which are associated with the contourite drift successions. A link between contourite deposition and the climatic fluctuations that characterised the late Neogene northern hemisphere is also proposed based on the identification of direct indicators of significant glaciation including iceberg plough marks within the contourite drift units. Contourite drift deposition is interpreted to occur predominantly during climatically warmer episodes, while periods of glaciation are characterised by apparent disruption of ocean current circulation within the basin.
Read more
188

Holocene variations in the North Atlantic marine radiocarbon reservoir effect

Ascough, Philippa January 2006 (has links)
The aim of this thesis is to examine the spatial and temporal characteristics of the <sup>14</sup>C marine reservoir effect (MRE) in the North Atlantic over the Holocene. The MRE is a variable offset in the <sup>14</sup>C age of carbon within the atmospheric and surface ocean reservoirs at any point in time where ocean <sup>14</sup>C is depleted relative to the coeval atmosphere. The MARINE04 calibration curve is produced by modelling the oceanic response to fluctuations in atmosphere <sup>14</sup>C and provides a time-series of MRE corrections for the global average ocean. In a specific ocean area however, MRE values may deviate from this global average as a function of specific local oceanographic and climatic variables. This deviation (known as ΔR) is determined by empirical measurement of local samples and modern (pre-bomb) ΔR values show considerable spatial variations. The overall results demonstrate observable spatial and temporal variability in ΔR within the study over the past c.8000 years that can be related to climate and oceanographic changes previously identified as potential mechanisms for producing variation in ΔR. These variations were as follows: 1. In the Early Holocene (c.6480-1940 BC) ΔR values were greater relative to the present day. 2. During the periods c.400 BC-90 AD and c.1000-1200 AD, ΔR values were reduced relative to those of the present day. 3. During the period c.1200-1400 AD, ΔR values were higher compared to the present. 4. In addition, there is evidence for spatial variation in ΔR within the study area over the Holocene. The results of this study have important implications for our understanding of the MRE and the effective application of correction values to marine samples for <sup>14</sup>C measurement within both paleoenvironmental and archaeological research.
Read more
189

Anomalous bathymetry and mass heterogeneity at the conjugate Iberia and Newfoundland rifted margins

Cooper, Christopher James January 2010 (has links)
No description available.
190

The interaction between waves and currents in the nearshore zone

Osaisai, Evans F. January 2008 (has links)
The interaction of waves and currents in the near-shore zone is an area of continuing research where interest derives in particular as a result of various environmental effects from natural phenomena such as storm surges and tsunamis. On time scales longer than that of an individual wave it is necessary to properly evaluate the interaction between waves and currents, using a consistent formulation of mass, momentum and energy within the water column. We describe the formulation of equations describing the mean current flow, driven by the radiation stress field of the waves, an equation for the mean conservation of mass, together with equations describing the conservation of wave action and the kinematics of the averaged wave field. The near-shore zone is often characterized by the presence of breaking waves, and so we develop equations to be used outside the surf zone, based on small-amplitude wave theory, and another set of equations to be used inside the surf zone, based on an empirical representation of breaking waves. Suitable matching conditions are applied at the boundary between the offshore shoaling zone and the near-shore surf zone. Both sets of equation are obtained by averaging the basic equations over the wave phase. In the shoaling zone, we supplement these equations by a simple model of sediment transport, where the bottom is allowed to move in response to the current field of the breaking waves. We use these basic equation sets to re-evaluate previous studies of wave set-up and longshore currents driven by the radiation stress field of the shoaling waves. In particular we extend previous work based on beach profiles with a linear depth dependence to more general beach profiles, including beaches with a depth dependence which varies quadratically with the onshore coordinate, and to beach profiles which approach a constant depth far offshore. We then turn to a situation where the incoming shoaling waves vary periodically in the alongshore direction, and use our basic equation sets to construct a mean current field which likewise varies periodically in the alongshore direction. The outcome, for our set of typical beach profiles, is a description of rip currents. The last part of the thesis examines .a simple model of sediment transport, induced by breaking waves in the surf zone. We show that the previous solutions for wave set-up and longshore currents now become time-dependent as the nearshore zone is eroded by the waves.
Read more

Page generated in 0.0227 seconds