Thesis (Ph. D.)--Joint Program in Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2004. / Includes bibliographical references (p. 188-198). / Relatively little is known about the role of eddies in controlling subduction in the eastern half of the subtropical gyre. Here, a new tool to study the eastern North Atlantic Ocean is created by combining a regional, eddy-resolving numerical model with observations to produce a state estimate of the ocean circulation. The estimate is a synthesis of a variety of in-situ observations from the Subduction Experiment, TOPEX/POSEIDON altimetry, and the MIT General Circulation Model. A novel aspect of this work is the search for an initial eddy field and eddy-scale open boundary conditions by the use of an adjoint model. The adjoint model for this region of the ocean is stable and yields useful information despite concerns about the chaotic nature of eddy-resolving models. The method is successful because the dynamics are only weakly nonlinear in the eastern region of the subtropical gyre. Therefore, no fundamental obstacle exists to constraining the model to both the large scale circulation and the eddy scale in this region of the ocean. Individual eddy trajectories can also be determined. The state estimate is consistent with observations, self-consistent with the equations of motion, and it explicitly resolves eddy-scale motions with a 1/6⁰ grid. Therefore, subduction rates, volume budgets, and buoyancy budgets are readily diagnosed in a physically interpretable context. Estimates of eddy subduction for the eastern subtropical gyre of the North Atlantic are larger than previously calculated from parameterizations in coarse-resolution models. Eddies contribute up to 40 m/yr of subduction locally. Furthermore, eddy subduction rates have typical magnitudes of 15% of the total subduction rate. To evaluate the net effect of eddies on an individual density class, / (cont.) volume budgets are diagnosed. Eddies contribute as much as 1 Sv to diapycnal flux, and hence subduction, in the density range 25.5 < [sigma] < 26.5. Eddies have a integrated impact which is sizable relative to the 2.5 Sv of diapycnal flux by the mean circulation. A combination of Eulerian and isopycnal maps suggest that the North Equatorial Current and the Azores Current are the geographical centers of eddy subduction. The findings of this thesis imply that the inability to resolve or accurately parameterize eddy subduction in climate models would lead to an accumulation of error in the structure of the main thermocline, even in the eastern subtropical gyre, which is a region of comparatively weak eddy motions. / by Geoffrey Alexander Gebbie. / 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), 2007. / Includes bibliographical references. / Basaltic ocean crust has the potential to host one of the largest endolithic communities on Earth. This portion of the biosphere, however, remains largely unexplored. In this study, we utilize molecular biological, microscopic, and geochemical tools to gain a better understanding of the geomicrobiology of the ocean crust. Specifically, we examine the phylogenetic diversity of microorganisms inhabiting basaltic lavas, the activities and abundances of these microorganisms, the spatial extent of the biosphere, and the potential effect that microbial activity has on the geochemistry of the ocean crust and overlying water column. Our study demonstrates that young, fresh volcanic lavas near mid-ocean ridges host an incredibly diverse and dense population of microorganisms dominated by Bacteria, quite distinct from the microbial communities found in surrounding deep seawater and hydrothermal vents. Furthermore, these communities may contribute to the elemental cycling of Fe, S, Mn, N, and C in this environment. The inability to definitively identify microorganisms in drill-cores of old (> 15 Ma) ocean crust, however, implies that these once prolific communities may become scarce as the crust ages and moves further away from the ridge axis. Finally, we provide evidence suggesting that these communities are fueled by oxidative alteration reactions occurring in the basaltic crust. / by Cara M. Santelli. / Ph.D.

Thesis (Ph. D.)--Joint Program in Chemical Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2007. / Includes bibliographical references. / A 225-year old coral from the south shore of Bermuda (64°W, 320N) provides a record of decadal-to-centennial scale climate variability. The coral was collected live, and sub-annual density bands seen in x-radiographs delineate cold and warm seasons allowing for precise dating. Coral skeletons incorporate strontium (Sr) and calcium (Ca) in relative proportions inversely to the sea surface temperature (SST) in which the skeleton is secreted. [Delta]180 of the coral skeleton changes based on both temperature and the [delta]180 of sea water ([delta]Ow), and 6Ow is proportional to sea surface salinity (SSS). Understanding long-term climate variability requires the reconstruction of key climate parameters, such as sea surface temperature (SST) and salinity, in records extending beyond the relatively short instrumental period. The high accretion rates, longevity, and skeletal growth bands found in coral skeletons make them an ideal resource for well-dated, seasonal climate reconstructions. Growing between 2 and 6 mm/year and reaching more than im in length, slow-growing corals provide multi-century records from one colony. Additionally, unlike the fast growing (10-20 mm/year) species Porites, slow-growing species are generally found in both tropical and sub-tropical locations greatly expanding the geographical location of these records. A high resolution record (HRR, ~11 samples per year) was drilled for the entire length of the coral record (218 years). Samples were split and Sr/Ca, [delta]180, and [delta]13C were measured for each sample. Sr/Ca was used to reconstruct winter time and mean-annual SST. Oxygen isotopic measurements were used to determine directional salinity changes, in conjunction with Sr/Ca based SST reconstructions. / (cont.) Winter-time and mean annual SSTs show SSTs -1.5 'C colder during the end of the Little Ice Age (LIA) relative to today. Simultaneously, SSS is fresher during that time. Sr/Ca based climate reconstructions from coral skeletons have been met with some skepticism because some reconstructions show temperature changes back in time that are 2-4 times greater than the reconstructions of other marine proxies. In this study, we show that when using bulk-sampled, slow-growing corals, two steps are critical to producing accurate reconstructions: 1) incorporating growth rate into multi-variant regressions with SST and Sr/Ca and 2) using multiple colonies that grew at the same time with varying average growth rates and Sr/Ca. Application of these novel methods over the period of the instrumental record from Hydrostation S (monthly since 1954, 32°10'N, 64°30'W) reduces the root mean square of the residuals between the reconstructed SST and the instrumental SST by as much as 1.52'C to 0.460C for three coral colonies. Winter-time SSTs at Bermuda are correlated to phases of the North Atlantic Oscillation (NAO), a meridional oscillation in atmospheric mass. Much uncertainty remains about the relationship between the NAO and the ocean, and one critical outstanding question is whether anthropogenic changes are perturbing the system. Using winter Sr/Ca as a proxy for temperature, we show strong coherence to the NAO at multi-decadal and inter-annual frequencies. These coral records show significant changes in variance in the NAO during the late 20th century compared to the cooler LIA, but limited changes in the mean phase (positive or negative) of the NAO, implying that climate change may be pushing the NAO to extremes but not to a new mean position. / by Nathalie Fairbank Goodkin. / 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 William J. Jenkins. "September 1987." / Includes bibliographical references (p. 241-250). / The isotopic evolution of helium and lead in the Earth is coupled by virtue of their common radioactive parents uranium and thorium. The isotopic signatures in oceanic volcanic rocks provide constraints on the temporal evolution of mantle source regions and volcanic magmas. He and Pb isotopes were measured in glassy basalts from young seamounts in the East Pacific, and in phenocrysts and corresponding whole rocks, respectively, from the island of St. Helena. He isotopes were also measured in glassy mid-ocean ridge basalts from the South Atlantic, previously studied for Pb isotopes by Hanan et al. (1986). A precise reconstruction of He-Pb isotope relationships in volcanic source regions is complicated by post-eruptive radiogenic ingrowth of ⁴He in non-zero age basalts, by pre-eruptive radiogenic ingrowth of ⁴He in magmas with elevated (U+Th)/He, by multi-stage fractionation processes involving (U+Th)/He, U/Pb and Th/Pb and by convective mixing in the Earth's interior. Aspects of each of these problems are addressed. (U+Th)/He ages are estimated from the isotope disequilibrium of ³He/ ⁴He between He trapped in vesicles and that dissolved in the glass phase of young alkali basalts at seamount 6 in the East Pacific. ³He/ ⁴He in the glass phase of these alkali basalts is subatmospheric, while in the vesicles it ranges between 1.2-2.5 RA (RA = atmospheric ratio). ³He/ ⁴He in vesicles (extracted by crushing in vacuo) allows a correction to be made in the dissolved phase He (by fusion of the remaining powder) for the inherited component in order to compute the radiogenic [He]. The method is applicable to rocks containing phases with different (U+Th)/He, and the results have implications for dating lavas in the age range of 0l to 106 years, and for reconstructing the temporal evolution of young volcanic systems. / (cont.) Pb, Sr and Nd isotopic variability observed at a small seamount field between 9-14'N near the East Pacific Rise covers -80% of the variability for Pacific MORB, due to small-scale heterogeneity in the underlying mantle. Tholeiites at these seamounts have He, Pb, Sr and Nd isotope compositions which are indistinguishable from MORB. Associated alkali basalts show more radiogenic He, Pb and Sr signatures. The lower ³He/⁴Heof He trapped in vesicles of these alkali basalts (1.2-2.6 RA) iS associated with low helium concentrations (< 5x10-' ccSTP/g). Evolved alkali basalts have lower ³He/⁴He (1.2-1.8 RA) than primitive alkali basalts (2.4-2.6 RA), suggesting some degree of magmatic control on inherited ³He/ ⁴He in these alkalic lavas. Collectively, the isotopic results suggest that as the lithosphere ages, material transfer from the MORB source becomes less significant because smaller degrees of melting average the chemical characteristics of heterogeneous mantle volumes less efficiently than near the ridge. Icelandites erupted at Shimada Seamount, an isolated volcano on 20 m.y. old seafloor, have Pb, Sr and Nd isotopic compositions similar to post-erosional basalts at Samoa. ³He/⁴He at Shimada ranges between 3.9-4.8 RA and helium concentrations are too large for radiogenic contamination of magma to have lowered the ³He/⁴He appreciably. These results indicate the presence of an enriched mantle component previously unidentified beneath the East Pacific. Its low ³He/⁴He may be due to the melting of domains with high (U+Th)/He which formed during accretion of the oceanic lithosphere. Alternatively, it is an inherent characteristic of the source, which contains material recycled into the mantle at subduction zones. / (cont.) The mid-ocean ridge between 12-46°S in the South Atlantic displays ³He/⁴He lower than typical MORB values. Local anomalies occur at the latitudes of off-axis islands to the east, apparently due to contamination of depleted mantle asthenosphere by hotspot materials (as previously shown for (La/Sm)N and Pb isotopes; Schilling et al., 1985; Hanan et al., 1986). He - Pb Isotopic relationships along the 12-22°S ridge segment suggest that St. Helena has ³He/4 He less than MORB. ³He/⁴He in two St. Helena rocks (extracted by in vacuo crushing of olivine and pyroxene) is 5.8 RA when the extracteTdHe contents are greater than IxI0 - 13 ccSTP/g, consistent with the He - Pb observations along the St. Helena ridge segment. ⁴He/ ³He and radiogenic Pb isotope ratios are linearly correlated for the South Atlantic ridge segment between 2-120S. Linear correlation of ⁴He/3 He - ²⁰⁶Pb/ ²⁰⁴Pb within an oceanic rock suite reflects the temporal evolution of ²⁰⁴Pb / ³He In the source. A linear correlation between volcanic suites derived from isotopically different sources (e.g., oceanic islands) may imply a coherent fractionation of (U+Th) from He and Pb during the evolutionary history of their respective mantle source regions. / by David W. Graham. / 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), June 1990. / "April 1990." / At scale lengths less than 100 km or so, statistical descriptions of seafloor morphology can be usefully employed to characterize processes which form and reshape abyssal hills, including ridge crest volcanism, off-axis tectonics and volcanism, mass wasting, sedimentation, and post-depositional transport. The objectives of this thesis are threefold: (1) to identify stochastic parameterizations of small-scale topography that are geologically useful, (2) to implement procedures for estimating these parameters from multibeam and side-scan sonar surveys that take into account the finite precision, resolution, and sampling of real data sets, and (3) to apply these techniques to the study of marine geological problems. The seafloor is initially modeled as a stationary, zero-mean, Gaussian random field completely specified by its two-point covariance function. An anisotropic two-point covariance function is introduced that has five free parameters describing the amplitude, orientation, characteristic width and length, and Hausdorff (fractal) dimension of seafloor topography. The general forward problem is then formulated relating this model to the statistics of an ideal multibeam echo sounder, in particular the along-track auto-covariance functions of individual beams and the cross-covariance functions between beams of arbitrary separation. Using these second moments as data functionals, we then pose the inverse problem of estimating the seafloor parameters from realistic, noisy data sets with finite sampling and beamwidth, and we solve this inverse problem by an iterative, linearized, least squares method. Resolution of this algorithm is tested against ship variables such as length of data, the orientation of ship track with respect to topographic grain, and the beamwidth. This analysis is conducted by inverting sets of synthetic data with known statistics. The mean and standard deviation of the inverted parameters can be directly compared with the input parameters and the standard errors output from the inversion. The experiments conducted in this study show that the rms seafloor height can be estimated to within -15% and anisotropic orientation to within ~5* (for a strong lineation) using very short track lengths (down to 3 characteristic lengths, or -10 to 100 km), and characteristic lengths of seafloor topography can be estimated to within -25% using fairly short track lengths (down to 5 or 6 characteristic lengths, or 10's of km to -200 kin). The number of characteristic lengths sampled by a ship track, and hence the accuracy of the estimation, is maximized when the ship track runs perpendicular to abyssal hill lineation. Using the assumed beamwidth, the measured noise values, and the seafloor parameters recovered from the inversion, Sea Beam "synthetics" are generated whose statistical character can be directly compared with raw Sea Beam data. However, these comparisons are spatially limited in the athwart ship direction. A recent SeaMARC II survey along the flanks and crest of the East Pacific Rise between 130 and 15* N included sufficient off-axis topography to permit a comparison of a complete 2-D synthetic topographic field with a region of abyssal-hill terrain that has close to 100% data coverage. Synthetic data is compared to both Sea Beam swaths and SeaMARC II survey data. These comparisons generally indicate that we are successful in characterizing the second order properties of the seafloor. They also indicate the directions we will need to take to improve our modeling, including generalization of the second-order model and characterization of higher moments. The inversion procedure is applied to a data set of 64 near-ridge Sea Beam swaths to characterize near ridge abyssal hill morphology and its relationship to ridge properties. Much of the data (27 swaths) comes from cruises to the Pacific-Cocos spreading section of the East Pacific Rise between 9* and 15* N. These data provide very good abyssal hill coverage of this well-mapped and studied ridge section and form the basis of a regional analysis of the correlation between ridge morphology and stochastic abyssal hill parameters. This regional analysis suggests a strong relationship between magma supply and the character of abyssal hills. We also have data from near the Rivera (9) and Nazca (7) spreading sections of the East Pacific Rise, the Mid-Atlantic Ridge (18), and the Indian- African Ridge (3). Though spotty, this constitutes a good initial data set for the analysis of correlations among covariance parameters and between parameters and ridge characteristics, especially spreading rate. A working hypothesis is introduced to explain the observations within a geological framework. This hypothesis contends 1) that the maximum size of abyssal hills is related to the lithosphere's ability to elastically support the load, 2) that fissuring and horst and graben formation dominate abyssal hill formation at fast spreading ridges, and 3) that volcanic edifice formation, modified by faulting driven by lithospheric necking, dominates abyssal hill formation at slow spreading ridges. To quantify abyssal hill characteristics such as vertical and lateral asymmetry and "peakiness" we must appeal to higher statistical moments than order two. A mathematical framework is introduced for the study of higher moments of a topographic field. This framework is built upon the concept that lower-order moment provide the groundwork for studying the higher-order moments. A simple 1-D parameterized model is proposed for moments up to order 4. This model includes two parameters for the third moment, describing vertical and lateral asymmetries, and one for the fourth moment, which describes the peakiness of topography. Initial methods are developed for estimating these parameters from bathymetric profiles. Results from the near ridge data set are presented and interpreted with regard to abyssal hill forming processes. / by John Anson Goff. / 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), 2013. / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Cataloged from student-submitted PDF version of thesis. / Includes bibliographical references. / Seafloor digital imagery and bathymetric data are used to evaluate the volcanic characteristics of the 85°E segment of the ultraslow spreading Gakkel Ridge (9 mm yr-¹). Imagery reveals that ridges and volcanic cones in the axial valley are covered by numerous, small-volume lava flows, including a few flows fresh enough to have potentially erupted during the 1999 seismic swarm at the site. The morphology and distribution of volcaniclastic deposits observed on the seafloor at depths of ~3800 m, greater than the critical point for steam generation, are consistent with having formed by explosive discharge of magma and CO² from source vents. Microearthquakes recorded on a 200 m aperture seismometer network deployed on the Trans-Atlantic Geotraverse active mound, a seafloor massive sulfide on the Mid-Atlantic Ridge at 26°N, are used to image subsurface processes at the hydrothermal system. Over nine-months, 32,078 local microearthquakes (ML = -1) with single-phase arrivals cluster on the southwest flank of the deposit at depths <125 m. Microearthquakes characteristics are consistent with reaction-driven cracking driven by anhydrite deposition in the shallow secondary circulation system. Exit fluid temperatures recorded at diffuse vents on the mound during the microearthquake study are used to explore linkages between seismicity and venting. / by Claire Willis Pontbriand. / Ph.D.

Thesis: Ph. D., Joint Program in Chemical Oceanography (Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2014. / Cataloged from PDF version of thesis. / Includes bibliographical references. / Marine particles include all living and non-living solid components of seawater, representing an extremely dynamic and chemically diverse mixture of phases. The distributions of these phases are poorly constrained and undersampled in the oceans, despite interactions between living organisms and non-living minerals having central roles within many globally relevant biogeochemical processes. Through a combination of method development, basin-scale particulate collection and analyses, modeling, and field experiments, this thesis examines both the distributions of marine particulate trace metals and the underlying processes-inputs, scavenging, vertical and horizontal transport, and biotic uptake-in which marine particles participate. I first present the results of an intercalibration exercise among several US laboratories that analyzed filtered particles on shared polyethersulfone filters. We use inter-lab and intra-lab total elemental recoveries of these particles to determine our state of our intercalibration (</= 21% one-sigma inter-lab uncertainty for most elements; 9% intra-lab) and to identify means of future improvement. We also present a new chemical method for complete dissolution of polyethersulfone filters and compare it to other total particle digestion procedures. I then present the marine particulate distributions of the lithogenic elements Al, Fe, and Ti in the North Atlantic GEOTRACES section. Inputs of lithogenic particles from African dust sources, hydrothermal systems, benthic nepheloid layers and laterally-sourced margin influences are observed and discussed. Lithogenic particle residence times, size-fractionation patterns, Ti-mineral speciation, and relationships to biological aggregation processes are calculated and described. A one-dimensional, size-fractionated, multi-box model that describes lithogenic particle distributions is also proposed and its parameter sensitivities and potential implications are discussed. The thesis concludes with the presentation of results from a series of bottle incubations in naturally iron-limited waters using isotopically labeled Fe-minerals. We demonstrate both biotic and abiotic solubilization of the minerals ferrihydrite and fayalite via transfer of isotopic label into suspended particles. These results are the first of their kind to demonstrate that minerals can be a source of bioavailable iron to euphotic communities and that spatial and ecological variations in mineral Fe-bioavailability may exist. / by Daniel Chester Ohnemus. / Ph. D.

Thesis (Ph. D.)--Joint Program in Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Science; and the Woods Hole Oceanographic Institution), 2002. / Includes bibliographical references (p. 221-243). / This thesis investigates the evolution of lithospheric deformation and crustal structure from continental margins to mid-ocean ridges. The first part (Ch. 2) examines the style of segmentation along the U.S. East Coast Margin and investigates the relationship between incipient margin structure and segmentation at the modem Mid-Atlantic Ridge. The second part (Chs. 3-5) focuses on the mechanics of faulting in extending lithosphere. In Ch. 3, I show that the incorporation of a strain-rate softening rheology in continuum models results in localized zones of high strain rate that are not imposed a priori and develop in response to the rheology and boundary conditions. I then use this approach to quantify the effects of thermal state, crustal thickness, and crustal rheology on the predicted style of extension deformation. The mechanics of fault initiation and propagation along mid-ocean ridge segments is investigated in Ch. 4. Two modes of fault development are identified: Mode C faults that initiate near the center of a segment and Mode E faults that initiate at the segment ends. Numerical results from Ch. 5 predict that over time scales longer than a typical earthquake cycle transform faults behave as zones of significant weakness. / (cont.) Furthermore, these models indicate that Mode E faults formed at the inside-corner of a ridge-transform intersection will experience preferential growth relative to faults formed at the conjugate outside-corner due to their proximity to the weak transform zone. Finally, the last part of this thesis (Ch. 6) presents a new method to quantify the relationship between the seismic velocity and composition of igneous rocks. A direct relationship is derived to relate Vp to major element composition and typical velocity-depth profiles are used to calculate compositional bounds for the lower continental, margin, and oceanic crust. / by Mark Dietrich Behn. / Ph.D.

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, 2016. / Ph. D. Joint Program in Oceanography (Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2012 / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 179-192). / Since the Last Glacial Maximum (LGM, ~ 20,000 years ago) air temperatures warmed, sea level rose roughly 130 meters, and atmospheric concentrations of carbon dioxide increased. This thesis combines global models and paleoceanographic observations to constrain the ocean's role in storing and transporting heat, salt, and other tracers during this time, with implications for understanding how the modem ocean works and how it might change in the future. -- By combining a kinematic ocean model with "upstream" and "downstream" deglacial oxygen isotope time series from benthic and planktonic foraminifera, I show that the data are in agreement with the modem circulation, quantify their power to infer circulation changes, and propose new data locations. -- An ocean general circulation model (the MITgcm) constrained to fit LGM sea surface temperature proxy observations reveals colder ocean temperatures, greater sea ice extent, and changes in ocean mixed layer depth, and suggests that some features in the data are not robust. -- A sensitivity analysis in the MITgcm demonstrates that changes in winds or in ocean turbulent transport can explain the hypothesis that the boundary between deep Atlantic waters originating from Northern and Southern Hemispheres was shallower at the LGM than it is today. / by Daniel Edward Amrhein. / 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.