Thesis (Ph. D.)--Joint Program in Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), February 2000. / Includes bibliographical references. / Benthic foraminiferal δ13C, Cd/Ca, and Ba/Ca are important tools for reconstructing nutrient distributions, and thus ocean circulation, on glacial-interglacial timescales. However, each tracer has its own "artifacts" that can complicate paleoceanographic interpretations. It is therefore advantageous to measure multiple nutrient proxies with the aim of separating the various complicating effects. Zn/Ca is introduced as an important aid toward this goal. Benthic (Hoeglundina elegans) Cd/Ca ratios from the Bahama Banks indicate that the North Atlantic subtropical gyre was greatly depleted in nutrients during the last glacial maximum (LGM). A high-resolution Cd/Ca record from 965 m water depth suggests that Glacial North Atlantic Intermediate Water formation was strong during the LGM, weakened during the deglaciation, and strengthened again during the Younger Dryas cold period. Comparison of Cd/Ca and δ13C data reveals apparent short-term changes in carbon isotopic air-sea signatures. Benthic foraminiferal Zn/Ca could be a sensitive paleoceanographic tracer because deep water masses have characteristic Zn concentrations that increase about ten-fold from the deep North Atlantic to the deep North Pacific. A "core top calibration" shows that Zn/Ca is controlled by bottom water dissolved Zn concentration and, like Cd/Ca and BalCa, by bottom water saturation state with respect to calcite Since Zn/Ca responds to a different range of saturation states than Cd/Ca, the two may be used together to evaluate changes in deep water carbonate ion (CO32- ) concentration. Zn/Ca and Cd/Ca ratios in the benthic foraminifer Cibicidoides wuellerstorfi exhibit large fluctuations over the past 100,000 years in a deep (3851 m) eastern equatorial Pacific sediment core. The data imply that bottom water CO32- concentrations were lowest during glacial Marine Isotope Stage 4 and highest during the last deglaciation. LGM CO32- concentrations appear to have been within a few μmol kg-1 of modern values. Deep North Atlantic Cd/Ca ratios imply much higher nutrient concentrations during the LGM. Although such data have usually been explained by a northward penetration of Southern Ocean Water (SOW), it has been suggested that they could result from increased preformed nutrient levels in the high-latitude North Atlantic or by increased aging of lower North Atlantic Deep Water (NADW). Glacial Zn/Ca data, however, require a substantially increased mixing with SOW and thus a reduction in NADW formation. Large changes in carbon isotopic air-sea exchange are invoked to reconcile benthic δ13C and trace metal data. / by Thomas Mathew Marchitto, Jr. / Ph.D.

Thesis (Ph. D.)--Joint Program in Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 1988. / Includes bibliographical references (v.2, leaves 339-351). / This study focuses on the zonal weakening, eastern termination and seasonal variations of the Atlantic equatorial undercurrent (EUC). The main and most original contribution of the dissertation is a detailed analysis of the Atlantic EUC simulated by Philander and Pacanowski's (1986)general circulation model (GCM), which provides a novel description of the dynamical regimes governing various regions of a nonlinear stratified undercurrent. Only in a narrow deep western region of the simulation does one find an approximately inertial regime corresponding to zonal acceleration. Elsewhere frictional processes cannot be ignored. The bulk of the mid-basin model EUC terminates in the overlying westward surface flow while only a small fraction (the deeper more inertial layers) terminates at the eastern coast. In agreement with observations, a robust feature of the GCM not present in simpler models is the apparent migration of the EUC core from above the thermocline in the west to below it in the east. In the GCM, this happens because the eastward flow is eroded more efficiently by vertical friction above the base of the thermocline than by lateral friction at greater depths. This mechanism is a plausible one for the observed EUC. A scale analysis using a depth scale which decreases with distance eastwards predicts the model zonal transition between western inertial and eastern inertio-frictional regimes. Historical and recent observations and simple models of the equatorial and coastal eastern undercurrents are reviewed, and a new analysis of current measurements in the eastern equatorial Atlantic is presented. Although the measurements are inadequate for definitive conclusions, they suggest that Lukas' (1981) claim of a spring surge of the Pacific EUC to the eastern coast and a seasonal branching of the EUC into a coastal southeastward undercurrent may also hold for the Atlantic Ocean. To improve the agreement between observed and modelled strength of the eastern undercurrent, it is suggested that the eddy coefficient of horizontal mixing should be reduced in future GCM simulations. / by Sophie Huguette Claire Wacongne. / Ph.D.

Thesis (Ph.D.)--Joint Program in Oceanography (Massachusetts Institute of Technology, Dept. of Civil and Environmental Engineering, and the Woods Hole Oceanographic Institution), 2001. / Includes bibliographical references. / by Paulo Salles. / Ph.D.

Thesis (Ph.D.)--Joint Program in Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences and the Woods Hole Oceanographic Institution), 2000. / Vita. / Includes bibliographical references. / by Robert P. Ackert, Jr. / Ph.D.

Thesis (Ph. D.)--Joint Program in Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 1987. / Supervised by Mike Purdy, Sean C. Solomon. / Includes bibliographical references. / Two-thirds of the Earth's surface has been formed along a global system of spreading centers that are presently manifested in several different structural forms, including the classic rift valley of the Mid-Atlantic Ridge, the more morphologically subdued East Pacific Rise, and the pronounced en echelon structure of the Reykjanes Peninsula within southwestern Iceland. In this thesis, each of these different spreading centers is investigated with microearthquake studies or tomographic inversion of travel times. Results of these studies are used to constrain the spatial variability of physical properties and processes beneath the axis of spreading and, together with other observations, the temporal characteristics of crustal accretion and rifting. In Chapter 2 the theoretical basis of seismic body-wave travel-time tomography and techniques for the simultaneous inversion for hypocentral parameters and velocity structure are reviewed. A functional analysis approach assures that the theoretical results are independent of model parameterization. An important aspect of this review is the demonstration that travel time anomalies due to path and source effects are nearly independent. The discussion of the simultaneous inverse technique examines theoretically the dependence of tomographic images on the parameterization of the velocity model. In particular, the effects of parameterization on model resolution are examined, and it is shown that an optimum set of parameters averages velocity over localized volumes. Chapter 2 ends with the presentation of the results of tomographic inversions of synthetic data generated for a model of the axial magma chamber postulated to exist beneath the East Pacific Rise. These inversions demonstrate the power of the tomographic method for imaging three-dimensional structure on a scale appropriate to heterogeneity along a spreading ridge axis. Chapter 3 is the first of two chapters that present the results of a microearthquake experiment carried out within the median valley of the Mid-Atlantic Ridge near 230 N during a three week period in early 1982. In this chapter, the experiment site, the seismic network, the relocation of instruments by acoustic ranging, the hypocenter location method, and the treatment of arrival time data are described. Moreover, hypocentral parameters of the 26 largest microearthquakes are reported; 18 of these events have epicenters and focal depths which are resolvable to within ±1 km formal error at the 95% confidence level. Microearthquakes occur beneath the inner floor of the median valley and have focal depths generally between 5 and 8 km beneath the seafloor. Composite fault plane solutions for two spatially related groups of microearthquakes beneath the inner floor indicate normal faulting along fault planes that dip at angles of 300 or more. / Douglas Ray Toomey. / Ph.D.

Thesis (Ph. D.)--Joint Program in Physical Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 1988. / Includes bibliographical references (leaves 163-172). / A freshwater plume often forms when a river or an estuary discharges water onto the continental shelf. Freshwater plumes are ubiquitous features of the coastal ocean and usually leave a striking signature in the coastal hydrography. The present study combines both hydrographic data and idealized numerical simulations to examine how ambient currents and winds influence the transport and mixing of plume waters. The first portion of the thesis considers the alongshore transport of freshwater using idealized numerical simulations. In the absence of any ambient current, the downstream coastal current only carries a fraction of the discharged fresh water; the remaining fraction recirculates in a continually growing "bulge" of fresh water in the vicinity of the river mouth. The fraction of fresh water transported in the coastal current is dependent on the source conditions at the river mouth. The presence of an ambient current augments the transport in the plume so that its freshwater transport matches the freshwater source. For any ambient current in the same direction as the geostrophic coastal current, the plume will evolve to a steady-state width. A key result is that an external forcing agent is required in order for the entire freshwater volume discharged by a river to be transported as a coastal current. The next section of the thesis addresses the wind-induced advection of a river plume, using hydrographic data collected in the western Gulf of Maine. The observations suggest that the plume's cross-shore structure varies markedly as a function of fluctuations in alongshore wind forcing. Consistent with Ekman dynamics, upwelling favorable winds spread the plume offshore, at times widening it to over 50 km in offshore extent, while downwelling favorable winds narrow the plume width to a few Rossby radii. Near-surface current meters show significant correlations between cross-shore currents and alongshore wind stress, consistent with Ekman theory. Estimates of the terms in the alongshore momentum equation calculated from moored current meter arrays also indicate an approximate Ekman balance within the plume. A significant correlation between alongshore currents and alongshore wind stress suggests that interfacial drag may be important. The final section of the thesis is an investigation of the advection and mixing of a surface-trapped river plume in the presence of an upwelling favorable wind stress, using a three-dimensional model in a simple, rectangular domain. Model simulations demonstrate that the plume thins and is advected offshore by the cross shore Ekman transport. The thinned plume is susceptible to significant mixing due to the vertically sheared horizontal currents. The first order plume response is explained by Ekman dynamics and a Richardson number mixing criterion. / by Derek Allen Fong. / Ph.D.

Thesis (Ph. D.)--Joint Program in Physical Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 1999. / Includes bibliographical references (leaves 129-139). / Observations of diapycnal mixing rates are examined and related to diapycnal advection for both double-diffusive and turbulent regimes. The role of double-diffusive mixing at the site of the North Atlantic Tracer Release Experiment is considered. The strength of salt-finger mixing is analyzed in terms of the stability parameters for shear and double-diffusive convection, and a nondimensional ratio of the thermal and energy dissipation rates. While the model for turbulence describes most dissipation occurring in high shear, dissipation in low shear is better described by the salt-finger model, and a method for estimating the salt-finger enhancement of the diapycnal haline diffusivity over the thermal diffusivity is proposed. Best agreement between tracer-inferred mixing rates and microstructure based estimates is achieved when the salt-finger enhancement of haline flux is taken into account. The role of turbulence occurring above rough bathymetry in the abyssal Brazil Basin is also considered. The mixing levels along sloping bathymetry exceed the levels observed on ridge crests and canyon floors. Additionally, mixing levels modulate in phase with the spring-neap tidal cycle. A model of the dissipation rate is derived and used to specify the turbulent mixing rate and constrain the diapycnal advection in an inverse model for the steady circulation. The inverse model solution reveals the presence of a secondary circulation with zonal character. These results suggest that mixing in abyssal canyons plays an important role in the mass budget of Antarctic Bottom Water. / by Louis Christopher St. Laurent. / Ph.D.

Thesis (Ph. D.)--Joint Program in Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), February 2000. / "September 1999." / Includes bibliographical references. / Geochemical studies are fundamental for understanding how the dynamic Earth works and evolves. These studies place constraints on the composition, formation, age, distribution, evolution and scales of geochemically distinct reservoirs such as the Earth's crust, mantle and core. In this dissertation the strategy has been to work on a broad range of topics to evaluate crustal and mantle processes. This study presents Re-Os systematics to constrain the composition, formation and age of the lower continental crust and the mantle lithosphere, examines melt inclusion from oceanic island basalts to evaluate the scale of the mantle heterogeneities, and uses U-series isotope to constrain geodynamic parameters, such as the upwelling velocities and porosities of mantle plumes. The lower continental crust plays a pivotal role in understanding the composition and evolution of the continental crust and the petrogenesis of continental basalts. / by Alberto Edgardo Saal. / Ph.D.

Thesis (Ph. D.)--Joint Program in Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), February 2000. / Includes bibliographic references. / The bottom boundary layer is an important dynamical region of shallow water flows. In this thesis, the problem of turbulent mixing in the coastal bottom boundary layer is investigated with a unique set of field measurements of velocity and sound speed that span a significant fraction of the boundary layer obtained over a six-week long period in the late summer of 1996 on the New England shelf. The energetics of the turbulent fluctuations are investigated by testing simplified budgets for turbulent kinetic energy and scalar variance. The turbulent kinetic energy budget is locally balanced while the scalar variance budget is not, probably due to turbulent diffusion. The direct effects of stratification are consistently significant only in the outer part of the boundary layer, where the flux Richardson number is approximately equal to a critical value of 0.2. Turbulence closure is investigated in terms of non-dimensional profiles of velocity and sound speed. Close to the bottom, the results are consistent with Monin-Obukhov similarity theory, while in the outer part of the boundary layer other scales including the height of the boundary layer are important for setting the turbulent length scale. / by William J. Shaw. / Ph.D.

Thesis (Ph. D.)--Joint Program in Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Dept. of Ocean Engineering; and, the Woods Hole Oceanographic Institution), 2004. / Includes bibliographical references (v. 2, leaves 211-215). / In recent years the focus of passive detection and localization of submarines has moved from the deep ocean into the littoral regions. the problem of passive detection in these regions is complicated by strong multipath propagation with high transmission loss. Large aperture planar arrays have the potential to improve detection performance due to their high resolution and high gain, but are suceptible to two main performance degradation mechanisms: limited spatial coherence of signals and nonstationarity of high bearing rate interference sources common in littoral regions of strategic importance. This thesis presents subarray processing as a method of improving passive detection performance using such large arrays. This thesis develops statistical models for the detection of performance of three adaptive, sample-covariance-based subarray processing algorithms which incorporate the effects of limited spatial coherence as well as finite snapshot support. The performance of the optimum processor conditioned on known data coveriances is derived as well for comparison. These models are then used to compare subarray algorithms and partitioning schemes in a variety of interference environments using plane wave and matched-field propagation models. / (cont.) The analysis shows a tradeoff between the required adaptive degrees of freedom, snapshot support, and adaptive resolution. This thesis shows that for both plane-wave and matched-field processing, the Conventional-Then-Adaptive (CTA) algorithm optimizes this tradeoff most efficiently. Finally, a comparison of the CTA algorithm to beam-space adaptive processing shows that for moderate beam coverage, the subarray algorithm performs as well as or superior to the adaptive beamspace algorighm. / by Jennifer Anne Watson. / Ph.D.