http://hdl.handle.net/1957/1313 Search the Channel search http://ir.library.oregonstate.edu/dspace/simple-search http://hdl.handle.net/1957/6715 Title: Remote measurements of waves and currents over complex bathymetry <br/> <br/>Abstract: Waves are the primary input of energy in the nearshore region, and together with the currents forced through the transfer of momentum that occurs during the wave breaking process they are the principal mechanism for sediment transport in the nearshore. The basic physics of waves and currents are thought to be well understood, and simple models for waves and current interacting with simple (alongshore-uniform bathymetry) generally agree with in situ measurements. However, alongshore variability is ubiquitous, and the predictability of waves and currents over complex bathymetry is an important research topic. Significant effort has been placed into understanding and predicting (modeling) the kinematics and dynamics of waves and currents, but the accuracy of these models are largely untested, due to the lack of appropriate wave and current measurements. This dissertation first details the development and application of an optical remote sensing technique to measure the shape of wave frequency-direction spectra over large scale complex bathymetry. The optical wave spectral estimation technique is based on the wave slope dependence of reflected skylight, and while not a metric of the absolute wave energy, the measurement technique is shown to agree with the frequency and directional distributions of in situ measurements. Simultaneously, the optical technique offers increased directional resolution. Maps of the wave direction field over a submarine canyon, using the optical technique, show dramatic changes in the wave divergence and convergence which coincides with details of the large scale bathymetry gradients. The last section of this dissertation analyzes 14 months of surf zone longshore currents measured from a two-dimensional array using a previously described optical remote sensing technique. Based on metrics developed in the study, the longshore currents are shown to be alongshore nonuniform for the majority (70%) of the observations, suggesting that a simple one-dimensional force balance circulation model is not generally applicable. Related tests of a point model only show moderate skill when predicting the peak longshore current (rms error 0.38 m/s). Though cases of two-dimensional circulation (i.e. rip currents) are shown to coincide with alongshore-variable bathymetry, a metric of the degree of alongshore bathymetry variability, γB, has poor skill of predicting the strength of spatial longshore current variability. <br/> <br/>Description: Graduation date: 2008 http://hdl.handle.net/1957/6713 Title: Remote sensing of radiation stress gradients from optical imagery <br/> <br/>Abstract: Wave-induced circulation is the defining characteristic of the nearshore. Within this region, the constant feedback cycle between incoming waves, wave-generated currents, and the mobile sediment bed is responsible for the evolution of complex patterns in nearshore and beach morphology. Central to our understanding of this system is knowledge of the forcing mechanisms. We investigate the potential of optical signals to provide an alternative remote sensing method for the direct estimation of radiation stress gradients in the nearshore. The use of an optical method will provide the ability to easily observe a large, spatially-dense array for extended periods. Additionally, the creation of a remote sensing technique will greatly expand the possibilities for data assimilation in nearshore circulation and sediment transport models. Here, we will examine the formulation of radiation stress gradients in terms of component variables with distinct optical signatures, as well as examine the relationship between optical radiation stress gradient measurements and in situ measurements of fluid response. Our results show that the spatial patterns in optical intensity due to wave breaking can be isolated from background optical intensity with a robust Gaussian breaker intensity model. However, the transformation from optical intensity due to wave breaking to a quantitative measure of local dissipation is quite complex. The use of in situ measurements of wave energy flux in calibration of this technique introduces a dependence on in situ instrumentation in an otherwise optical remote sensing technique. Normalized wave spectra are calculated based on pixel intensity time series from a 2-dimensional spatially-lagged array. These optical spectra compare well with spectra calculated from in situ puv data. The wave spectra provide a weighting term, allowing for wave frequency and directional spread. Results of each component model compare well with available field measurements, but validation of the complete optical radiation stress gradient technique is complicated by the lack of direct field measurements of radiation stress gradients. Attempts to examine the balance between wave-induced forcing and fluid response at the 1997 SandyDuck field experiment are complicated by the complex, non-linear nature of the current patterns during this deployment. <br/> <br/>Description: Graduation date: 2008 http://hdl.handle.net/1957/6141 Title: Mantle melting and heterogeneity along mid-ocean ridges : insight from basalt geochemistry along axial depth and morphologic gradients for intermediate spreading rate systems <br/> <br/>Abstract: This investigation focuses on gaining a better understanding of the complex relationship between melt generation, source variability and mid-ocean ridge morphology. The approach adopted here uses a variety of geochemical techniques to evaluate the ability of 'global' models to predict regional scale geochemical variability associated with axial depth and axial morphologic changes. Three separate regional scale studies were conducted along ridges characterized by intermediate spreading rates (where the system is very sensitive to variations in magma production). The first study focuses on the development of the Australian-Antarctic Discordance (AAD), an anomalously deep portion of the modern global ridge system located in the eastern Indian Ocean, over the period from 28 to 14 Ma during which the eastern Indian Ocean basin was in a relatively young stage of formation. Major and trace element results from this study suggest a more magmatically robust ridge was present during this period. The second study investigates the link between U-series disequilibria and axial ridge depth. In this study U and Th isotopic compositions and elemental concentrations were analyzed along the Southeast Indian Ridge. The results of this study suggest that a simple relationship, such as that predicted from global MORB variations, does not exist on a regional scale in this part of the Indian Ocean. Plausible explanations of this data set require the consideration of other intrinsic variables such as residual porosity and mantle melting rates. The third investigation focuses on the western Galapagos Spreading Center (GSC), an intermediate spreading ridge whose axial morphology is affected by the addition of heat and material from the nearby Galapagos hotspot. This study investigates the origin and nature of the transfer of this material through analysis of rare earth element concentrations in melt inclusions. The results from this study support a deep (≥ 60 km), strong lateral flow of hotspot-derived mantle toward the GSC. Collectively these studies support a strong link between axial morphology and melt generation and verify that geochemical investigations along regional morphologic gradients provide a meaningful 'window' into the underlying mantle, while demonstrating that although 'global' models succeed in providing a valid platform from which to evaluate regional-scale observations, they do not accurately describe the complex process of melt generation on a regional scale. <br/> <br/>Description: Graduation date: 2008 http://hdl.handle.net/1957/6122 Title: The fluvial response to glacial-interglacial climate change in the Pacific Northwest, USA <br/> <br/>Abstract: This research focuses on the development of new techniques to explore terrestrial-ocean climate linkages along the Pacific Northwest-northeast Pacific Ocean margin. This is done by investigating river response to climate change and by unraveling this history preserved in continental margin sediments. A significant component of this work centers on developing a 40Ar-39Ar incremental heating method to fingerprint bulk fluvial sediment entering this region. Results show reproducible ages from individual rivers accounting for the majority of sediment delivered offshore. A 40Ar-39Ar detrital mixture model is developed to examine the fidelity of these results and shows that the bulk ages measured from river mouth sediments can be accurate indicators of the average age of feldspars eroded from a given catchment area. The bulk sediment ages are combined with Nd isotopic analyses into a ternary mixing model to better understand the sources of terrigenous material delivered to offshore continental margin sites. Downcore Ar-Nd isotopic compositions can be described by three general river sediment sources proximal to the core site, the Umpqua, Rogue+Klamath, and Eel Rivers, from ~14 ka to Present. Results from the ternary model also suggest that differential contributions of eroded material plays the primary role in provenance changes seen at the core site, rather than sediment transport changes due to ocean circulation. This research culminates in a modeling effort to examine downcore provenance changes. We develop a model that balances basin-averaged 40Ar-39Ar ages (detrital mixtures) of the contributing fluvial basins and predicts the bulk sediment value at the core site. We find that the Upper Klamath Basin (which contained pluvial Lake Modoc during Marine Isotope Stage 2) is the most influential source area that can contribute to younger bulk sediment 40Ar-39Ar ages at the core site, relative to present day values. The Eel River is also shown to have a considerable influence on changes in margin sedimentation. Combinations of increases in the sediment fluxes out of these two basins can describe the 40Ar-39Ar provenance evolution observed at the core site over the 22-14 ka time period. Overall, this new 40Ar-39Ar isotopic technique, together with the Nd isotopic system and the use of detrital mixture modeling show tremendous promise as a multi-faceted strategy to assess erosion and provenance change through the continuous history preserved in fine-grained marine sedimentary records. <br/> <br/>Description: Graduation date: 2008