Thesis (Ph. D.)--Joint Program in Oceanography/Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2005. / Includes bibliographical references. / Minor and trace element records of planktic and benthic foraminifera from Atlantic sediment cores, as well as output from a coupled OAGCM, were used to investigate the magnitude and distribution of the oceanic response to abrupt climate events of the past 20 kyr. The study addressed three major questions: 1) What is the magnitude of high-latitude sea surface temperature and salinity variability during abrupt climate events? 2) Does intermediate depth ventilation change in conjunction with high-latitude climate variability? 3) Are the paleoclimate data consistent with the response of a coupled OAGCM to a freshwater perturbation? To address these questions, analytical methods were implemented for the simultaneous measurement of Mg/Ca, Zn/Ca, Cd/Ca, Mn/Ca and Al/Ca in foraminiferal samples using inductively-coupled plasma mass spectrometry. Paired records of planktic foraminiferal [delta]¹⁸O and Mg/Ca from the subpolar North Atlantic reveal trends of increasing temperatures ([approx.] 3⁰C) and salinities over the course of the Holocene, which were punctuated by abrupt events. The variability does not appear to be periodic, but tends to recur within a broad millennial band. The records provide the first evidence of open-ocean cooling (nearly 2⁰C) and freshening during the 8.2 kyr event, and suggest similar conditions at 9.3 ka. / (cont.) However, the two largest temperature oscillations ([approx.] 2⁰C) occurred during the last 4,000 years, suggesting a recent increase in temperature variability relative to the mid-Holocene. Benthic foraminiferal Cd/Ca from an intermediate depth, western South Atlantic core provides insights into changes in the southward penetration of North Atlantic Intermediate Water (NAIW). Cd seawater estimates (Cdw) for the last glacial are consistent with the production of NAIW and its export into the South Atlantic. At [approx.] 14.5 ka, the NAIW contribution to the South Atlantic began to decrease, marking a transition from a glacial subsurface geometry to a Younger Dryas geometry, which occurred concurrently with the onset of the Bolling-Allerod to Younger Dryas cooling. High Cdw in both the deep North Atlantic and the intermediate South Atlantic imply reduced export of deep and intermediate water during the Younger Dryas, and a major decrease in northward heat transport. Modern subsurface geometry was established at [approx.] 9 ka, concurrently with the establishment of Holocene warmth in the North Atlantic region, further supporting a close linkage between subsurface circulation and North Atlantic climate. / (cont.) Paired benthic foraminiferal Mg/Ca and [delta]¹⁸O data from two intermediate depth low latitude western Atlantic sites - one from the Florida Current and one from the Little Bahama Bank- provide insights into the spatial distribution of intermediate depth temperature and salinity variability during the Younger Dryas. The Florida site lies within the deeper portion of the Florida Current; the Little Bahama Bank site lies within the deeper, unventilated portion of the North Atlantic subtropical gyre. During the Younger Dryas, temperatures increased at the Florida Current site and temperatures decreased at the Little Bahama Bank site. The temperature increase within the Florida Current is consistent with the reduced northward heat transport associated with a reduction in the Atlantic meridional overturning circulation (MOC); the temperature decrease at Little Bahama Bank is consistent with a cooling of high latitude surface waters. To test the possibility that a freshening of the surface North Atlantic caused the terrestrial and oceanographic changes during the Younger Dryas, the GFDL R30 coupled ocean-atmosphere general circulation model was forced using a North Atlantic freshwater perturbation of 0.1 Sv for a period of 100 years. The freshwater flux causes an overall reduction in the Atlantic overturning from 25 Sv to 13 Sv. / (cont.) However, at [approx.] 1,100 meters water depth, ventilation increases, causing decreases in both temperature and salinity throughout much of the intermediate depth North Atlantic. In the open North Atlantic, intermediate depth temperatures decrease by approximately 1⁰C; at the eastern side, intermediate depth temperatures decrease by less than 0.4⁰C. Intermediate depth temperatures at the western boundary, however, increase due to a reduction in northward heat transport, and also due to a shift in the location of the Intertropical Convergence Zone, which causes a reduction in surface salinity and a decrease in the upwelling of colder, deeper waters. Benthic foraminiferal Cd/Ca from an intermediate depth Florida Current core documents the history of the northward penetration of southern source waters within the return flow of the Atlantic meridional overturning circulation (MOC). Cd seawater estimates (Cdw) for the last glacial are consistent with the reduced influence of southern source waters at this location relative to the present. / (cont.) At [approx.] 18.5 ka, the southern source contribution to the Florida Current began to increase significantly, marking the onset of a transition from a glacial circulation pattern to a deglacial pattern, which lasted from [approx.] 17 ka to [approx.] 14 ka. At [approx.] 12.5 ka, following the onset of the Younger Dryas cooling in the North Atlantic and the reduction in North Atlantic Deep Water (NADW) production, the influence of southern source waters within the Florida Current decreased abruptly. A renewed influence of southern source waters occurred at [approx.] 9 ka, concurrent with the establishment of Holocene warmth in the North Atlantic region. / by Rosemarie Evangeline Came. / Ph.D.

Thesis (Ph.D.)--Joint Program in Oceanography/Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2002. / Includes bibliographical references (leaves 169-187). / The marine geochemistry of the North Atlantic Ocean varies on decadal to centennial time scales, a consequence of natural and anthropogenic forcing. Surface corals provide a useful geochemical archive to quantify past mixed layer variability, and this study presents elemental and isotopic records from the tropical and subtropical North Atlantic. A consistent method for stable lead isotope analysis via multiple collector ICP-MS is first presented. This method is then applied to western North Atlantic surface corals and seawater, constraining historical elemental and isotopic lead variability. Six stable lead isotope profiles are developed from the western and eastern North Atlantic, demonstrating consistent mixed layer, thermocline, and deep water variability. Finally, coralline trace element records, including cadmium, barium, and lead, are presented from the Cariaco Basin. First, a reliable method is developed for stable lead isotope analysis by multiple collector ICP-MS. This study presents new observations of the large (0.7% amu-l), time-dependent mass fractionation determined by thallium normalization, including preferential light ion transmission induced by the acceleration potential and nebulizer conditions. These experiments show equivalent results for three empirical correction laws, and the previously proposed [beta]Pb/[beta]TI correction does not improve isotope ratio accuracy under these conditions. External secondary normalization to SRM-981 provides one simple alternative, and a rationale is provided for this correction. With current intensities less than 1.5x10-12 A, external isotope ratio precision less than 200 ppm is observed (2[sigma]). Matrix effects are significant with concomitant calcium in SRM-981 (-280 ppm at 257 [mu]M [Ca]). / (cont.) With the appropriate corrections and minimal concomitants, MC-ICP-MS can reliably determine 206Pb/207Pb and 208Pb/207Pb ratios of marine carbonates and seawater. Anthropogenic lead represents a promising transient oceanographic tracer, and its historical isotopic and elemental North Atlantic variability have been documented by proxy reconstructions and seawater observations. Two high-resolution surface coral and seawater time series from the western North Atlantic are presented, demonstrating past variability consistent with upper ocean observations. The elemental reconstruction suggests the primary lead transient was advected to the western North Atlantic from 1955 to 1968, with an inferred maximum lead concentration of 205 pmol kg-1 in 1971. The mean 1999 North Atlantic seawater concentration (38 pmol kg-1) is equivalent to 1905, several decades prior to the initial consumption of leaded gasoline in the United States. A 206Pb/207Pb transient from 1968 to 1990 is also observed, lagging the elemental transient by ten years. The provenance of this isotopic record is distinct from Arctic and European ice core observations and supports a 40% ... / by Matthew K. Reuer. / 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), September 1998. / Includes bibliographical references (leaves 226-246). / The upper ocean crust contains a comprehensive record of the shallow geological processes active along the world's mid-ocean ridge system. This thesis examines the magnetic and seismic structure of the upper crust at two contrasting ridges-the fast spreading East Pacific Rise (EPR) and the slow spreading Mid-Atlantic Ridge (MAR)-to build a more complete understanding about the roles of volcanic emplacement, tectonic disruption and hydrothermal alteration in the near-ridge environment. A technique that inverts potential field measurements directly from an uneven observation track is developed and applied to near-bottom magnetic data from the spreading segments north of the Kane transform on the MAR. It is concluded that the central anomaly magnetization high marks the locus of focused volcanic emplacement. A cyclic faulting model is proposed to explain the oscillatory magnetization pattern associated with discrete blocks of crust being transported out of the rift valley between intensely altered fault zones. Seismic waveform and amplitude analyses of the magma sill along the EPR reveal it to be a thin (<100 m) body of partial melt. These characteristics have important implications for melt availability and transport within the cycle of eruption and replenishment. A genetic algorithm-based seismic waveform inversion technique is developed and applied to on- and near-axis multichannel data from 17'20'S on the EPR and the spreading segment south of the Oceanographer transform (MAR) to map and compare for the first time the detailed velocity structure of the upper crust at two different spreading rates. Combined with conventionally processed seismic profiles, our results show that, while final extrusive thickness is comparable at all spreading ridges (300-500 m), the style of thickening may vary. While a thin (<100 m) extrusive carapace quadruples in thickness within 1-4 km of the EPR crest, the extrusive section at the MAR achieves its final thickness within the inner valley. Both show evidence for a narrow zone of volcanic emplacement. Vigorous hydrothermalism at the EPR may produce a more rapid increase in basement velocities relative to the MAR. Rapid modification of the extrusive/dike transition at both ridges indicates that hydrothermalism is enhanced in this interval. Along-axis transport of lavas may thicken the extrusive pile at slow spreading segment ends, strengthening the magnetic highs generated by lava chemistry. / by Stefan Anthony Hussenoeder. / Ph.D.

Thesis (Ph. D.)--Joint Program in Oceanography/Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Dept. of Biology; and the Woods Hole Oceanographic Institution), 2007. / Includes bibliographical references (p. 188-204). / Geochemical signatures in the otoliths of diadromous fishes may allow for retrospective analyses of natal origins. In an assessment of river-specific signatures in American shad (Alosa sapidissima), an anadromous clupeid native to the Atlantic coast of North America, stable isotope and elemental ratios in otoliths of juvenile American shad produced accurate natal tags from 12 rivers. Significant inter-annual variability in geochemical signatures from several rivers was detected, due largely to differences in 8180 values among years. The database was further expanded to include 20 rivers from Florida to Quebec, encompassing all major spawning populations. This task was accomplished by collecting juvenile otoliths along with water samples from rivers where juveniles were not sampled. Regressions between otolith and water chemistry for those rivers where both were collected showed significant relationships for Sr:Ca, Ba:Ca, 86180, and 87Sr:86Sr ratios but not for Mg:Ca or Mn:Ca. Despite reducing the combined signature to only four chemical ratios, cross-validated classification accuracies of known-origin juveniles averaged 93%. Ground-truthed signatures were used to classify migrants of unknown origins. Adults returning to spawn in the York River were classified according to their otolith composition. / (cont.) Only 6% of spawners originated from rivers other than the York, supporting the hypothesis that most American shad spawn in their natal river. Of remaining spawners, 79% originated from the Mattaponi River and 21% from the Pamunkey River. The results suggested that while most American shad home to their natal river there is less fidelity to individual tributaries, allowing subsidies to subpopulations with persistent recruitment failure. Otolith signatures were also used in mixed-stock analyses of immature migrants along the coast of Maine in the spring and Minas Basin in the summer. Mixed-stock compositions showed remarkably low diversity and were dominated by fish from the Shubenacadie and Hudson rivers, with an increasing proportion of Potomac River fish over time. In contrast to results from adult tagging studies, southern stocks were virtually absent. These data suggest ontogenetic shifts in migratory behavior. The thesis concludes with a report that water contributed 83% of Sr and 98% of Ba in the otoliths of a marine fish. / by Benjamin Walther. / Ph.D.

Includes bibliographical references (p. 211-216). / Thesis (Ph. D.)--Joint Program in Oceanography/Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Dept. of Mechanical Engineering; and the Woods Hole Oceanographic Institution), 2008. / Inspired by the swimming abilities of marine animals, this thesis presents "Finnegan the RoboTurtle", an autonomous underwater vehicle (AUV) powered entirely by four flapping foils. Biomimetic actuation is shown to produce dramatic improvements in AUV maneuvering at cruising speeds, while simultaneously allowing for agility at low speeds. Using control algorithms linear in the modified Rodrigues parameters to support large angle maneuvers, the vehicle is successfully controlled in banked and twisting turns, exceeding the best reported AUV turning performance by more than a factor of two; a minimum turning radius of 0.7BL, and the ability to avoid walls detected > 1.8BL ahead, are found for cruising speeds of 0.75BL/s, with a maximum heading rate of 400/s recorded. Observations of "Myrtle", a 250kg Green sea turtle (Chelonia mydas) at the New England Aquarium, are detailed; along with steady swimming, Myrtle is observed performing 1800 level turns and rapidly actuating pitch to control depth and speed. Limb kinematics for the level turning maneuver are replicated by Finnegan, and turning rates comparable to those of the turtle are achieved. Foil kinematics which produce approximately sinusoidal nominal angle of attack trace are shown to improve turning performance by as much as 25%; the effect is achieved despite limited knowledge of the flow field. Finally, tests with a single foil are used to demonstrate that biomimetically inspired inline motion can allow oscillating foils utilizing a power/recovery style stroke to generate as much as 90% of the thrust from a power/power stroke style motion. / by Stephen Carl Licht. / Ph.D.

Little is known about the role of microbial biofilms in nutrient cycling and ecosystem processes within desert springs. Furthermore, the difference between nutrient limitation of biofilms in desert springs and other ecosystems is unknown. Biofilms produce micro-scale physicochemical variation important to ecosystem function. We measured the variation in microscale physicochemical heterogeneity in biofilms of Devils Hole, Nevada. Microelectrodes were used to measure micro-scale chemical gradients of temperature, pH, O2, and H2S in addition to water column and pore water nutrient measurements in Spirogyra, cyanobacteria, and Beggiatoa biofilms over one year. Biofilm physicochemical gradients were used to calculate diffusion and metabolic rates. The rate of O2 and H2S diffusion ranged over two orders of magnitude. Biofilm production and respiration were influenced by biofilm type, light exposure, and sample month. Maximum O2 production occurred in spring and summer during direct light exposure. Oxygen production and consumption varied with light exposure and season. The H2S production and consumption varied with biofilm type. Higher concentrations of SO4 in Beggiatoa suggested that H2S production in Beggiatoa was quickly oxidized in the ecosystem. Spirogyra and cyanobacteria followed similar physicochemical trends; however, Spirogyra had more pronounced diurnal and seasonal variation. The differences between cyanobacteria and Spirogyra have implications on the ecosystem function of Devils Hole as well as other ecosystems with diverse biofilm communities. The heterogeneous physicochemistry of microbial biofilms and the differences in biofilm nutrient limitation suggests that a change in microbial biofilms or nutrient concentrations could alter ecosystem biogeochemical dynamics. Additionally, we assessed the nutrient limitation of two desert springs in comparison with a temperate stream. A nutrient diffusing substrata experiment was used to measure chlorophyll a, respiration, and biomass with phosphorus, nitrogen, and sulfide treatments. Autotrophic and heterotrophic biofilms responded differently to treatments and the temperate stream had higher chlorophyll a biofilm accrual but lower respiration relative to the desert springs. / Spatial and temporal variation of microbial biofilm biogeochemistry in a desert spring, Devils Hole, Nevada -- Autotrophic and heterotrophic nutrient limitation across biomes. / Department of Biology

Thesis (Ph. D.)--Joint Program in Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Dept. of Mechanical Engineering; and the Woods Hole Oceanographic Institution), 2009. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 169-174). / Internal tides are internal waves of tidal period generated by tidal currents flowing over submarine topography. Tall ridges that are nominally two-dimensional (2-D) are sites of particularly strong generation. The subsequent dissipation of internal tides contributes to ocean mixing, thereby playing an important role in the circulation of the ocean. Strong internal tides can also evolve into internal wave solitons, which affect acoustic communication, offshore structures and submarine navigation. This thesis addresses the generation of internal tides by tall submarine ridges using a combined analytical and experimental approach. The first part of the thesis is an experimental investigation of a pre-existing Green function formulation for internal tide generation by a tall symmetric ridge in a uniform density stratification. A modal decomposition technique was developed to characterize the structure of the experimental wave fields generated by 2D model topographies in a specially configured wave tank. The theory accurately predicts the low mode structure of internal tides, and reasonably predicts the conversion rate of internal tides in finite tidal excursion regimes, for which the emergence of non-linearities was notable in the laboratory. In the second part of the thesis, the Green function method is advanced for asymmetric and multiple ridges in weakly non-uniform stratifications akin to realistic ocean situations. / (cont.) A preliminary investigation in uniform stratification with canonical asymmetric and double ridges reveals asymmetry in the internal tide that can be very sensitive to the geometric configuration. This approach is then used with realistic topography and stratification data to predict the internal tide generated by the ridges at Hawaii and at the Luzon Strait. Despite the assumption of two-dimensionality, there is remarkably good agreement between field data, simulations and the new theory for the magnitude, asymmetry and modal content of the internal tide at these sites. The final part of the thesis investigates the possibility of internal wave attractors in the valley of double-ridge configurations. A one-dimensional map is developed to identify the existence and stability of attractors as a function of the ridge geometry. The Green function method is further advanced to include a viscous correction to balance energy focusing and dissipation along an attracting orbit of internal wave rays, and very good agreement is obtained between experiment and theory, even in the presence of an attractor. / by Paula Echeverri Mondragón. / Ph.D.

Thesis (S.M.)--Joint Program in Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Dept. of Mechanical Engineering; and the Woods Hole Oceanographic Institution), 2005. / Includes bibliographical references (leaves 96-98). / The changing state of warfare has driven the US Navy's area of operations closer to shore into littoral coastal waters. Mine Warfare has been proven as an extremely effective means of battlespace control in these waters. Mines can be inexpensively mass produced and rapidly deployed over large areas. The most common type of mine in use is the bottom placed mine, an object with simple geometry that sits on the seafloor. These mines often exhibit scour induced burial below the seafloor, making detection through traditional mine hunting methods difficult or impossible, while the mines themselves remain lethal. The Office of Naval Research (ONR) has developed a computer model that predicts the extent of mine burial to aid mine hunting and mine clearing operations. Investigations under ONR's Mine Burial Program are presently being conducted to calibrate and validate this model. This thesis uses data from the deployment of an acoustically instrumented model mine near the Martha's Vineyard Coastal Observatory in part of a larger, 16 total object investigation. A 2-axis pencil beam sonar was deployed concurrently with the mine to obtain microbathymetric measurements of the scour pit development and the progression of mine burial. Data correction techniques to correct for beam pattern induced bathymetry errors and a transformed coordinate system are detailed within. / (cont.) An analysis of scour pit dimensions includes scour depth, area, and volume as well as a look into percent burial by depth as a characteristic measurement important for operational mine hunting. The progression of mine burial is related to the wave climate, unsteady flow hydrodynamic forcing, and bed-load transport. The analysis examines the relative roles of these mechanisms in the scour-infill-bury process. / by Brendan Reed Gotowka. / S.M.

Thesis (S.M.)--Joint Program in Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Dept. of Mechanical Engineering; and the Woods Hole Oceanographic Institution), 2005. / Includes bibliographical references (leaves 124-125). / A detailed study was conducted to prove the concept of an iterative approach to single transponder navigation for REMUS Autonomous Underwater Vehicles (AUVs). Although the concept of navigation with one acoustic beacon is not new, the objective was to develop a computer algorithm that could eventually be integrated into the REMUS architecture. This approach uses a least squares fit routine coupled with restrictive geometry and simulated annealing vice Kalman filtering and state vectors. In addition, to provide maximum flexibility, the single transponder was located on a GPS equipped surface ship that was free to move instead of the more common single bottom mounted beacon. Using only a series of spread spectrum ranges logged with time stamp, REMUS standard vehicle data, and reasonable initial conditions, the position at a later time was derived with a figure of merit fit score. Initial investigation was conducted using a noise model developed to simulate the errors suspected with the REMUS sensor suite. Results of this effort were applied to a small at sea test in 3,300 meters with the REMUS 6000 deep water AUV. A more detailed test was executed in Buzzard's Bay, Massachusetts, in 20 meters of water with a REMUS 100 AUV focusing on navigation in a typical search box. / (cont.) While deep water data was too sparse to reveal conclusive results, the Buzzard's Bay work strongly supports the premise that an iterative algorithm can reliably integrate REMUS logged data and an accurate time sequence of ranges to provide position fixes through simple least squares fitting. Ten navigational legs up to 1500 meters in length showed that over 90% of the radial position error can be removed from an AUV's position estimate using the STRONG algorithm vice dead reckon navigation with a magnetic compass and Doppler Velocity Log alone (DVL). / by J. Carl Hartsfield, Jr. / S.M.

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), 2005. / Bibliography: p. 119-129. / Obtaining accurate and repeatable navigation for robotic vehicles in the deep ocean is difficult and consequently a limiting factor when constructing vehicle-based bathymetric maps. This thesis presents a methodology to produce self-consistent maps and simultaneously improve vehicle position estimation by exploiting accurate local navigation and utilizing terrain relative Measurements. It is common for errors in the vehicle position estimate to far exceed the errors associated with the acoustic range sensor. This disparity creates inconsistency when an area is imaged multiple times and causes artifacts that distort map integrity. Our technique utilizes small terrain "sub-maps" that can be pairwise registered and used to additionally constrain the vehicle position estimates in accordance with actual bottom topography. A delayed state Kalman filter is used to incorporate these sub-map registrations as relative position measurements between previously visited vehicle locations. The archiving of previous positions in a filter state vector allows for continual adjustment of the sub-map locations. The terrain registration is accomplished using a two dimensional correlation and a six degree of freedom point cloud alignment method tailored for bathymetric data. / (cont.) The complete bathymetric map is then created from the union of all sub-maps that have been aligned in a consistent manner. Experimental results from the fully automated processing of a multibeamn survey over the TAG hydrothermal structure at the Mid-Atlantic ridge are presented to validate the proposed method. / by Christopher N. Roman. / Ph.D.