Thesis (S.M.)--Joint Program in Marine Geology and Geophysics (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2008. / Includes bibliographical references (p. 30-37). / We evaluate the applicability of plagioclase and gabbro flow laws by comparing predicted and observed deformation mechanisms in gabbroic shear zones. Gabbros and layered gabbro mylonites were collected from the Southwest Indian Ridge (SWIR), ODP Hole 735B. Deformation temperatures are constrained by two-pyroxene thermometry, stress is estimated from grain size, and deformation mechanisms are analyzed by microstructure and the presence or absence of a lattice preferred orientation (LPO). Our analyses indicate that mylonite layers deformed at a strain rate in the range of 1012 to 101 s-1, while coarse-grained gabbro deformed at a strain rate of approximately 10-14 to 1013 s-1. Plagioclase in pure plagioclase mylonite layers exhibit strong LPOs indicating they deform by dislocation creep. Plagioclase grain size in mixed plagioclase-pyroxene mylonite layers is finer than in pure plagioclase layers, and depends on the size and proportion of pyroxenes. Progressive mixing of pyroxene and plagioclase within gabbro mylonite layers is accompanied by weakening of the LPO indicating that phase mixing promotes a transition to diffusion creep processes that involve grain boundary sliding. Our results indicate that experimental flow laws are accurate at geologic strain rates, although the strain rate for diffusion creep of fine-grained gabbro may be underestimated. At the conditions estimated for the SWIR crust, our calculations suggest that strain localization leads to a factor of two to four decrease in lower crustal viscosity. Even so, the viscosity of lower gabbroic crust is predicted to be similar to that of dry upper mantle. / by Luc Mehl. / S.M.

Thesis (S.M.)--Joint Program in Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2008. / Includes bibliographical references (p. 45-47). / Biogeochemical cycling of phosphate is a key component in the overall production rate of coastal ecosystems. Mineral phases in the near-shore sediments play a significant role in the return of phosphate remineralized in the upper sediments to the water column. Sequential Extraction (SEDEX) of the solid-phase associated P04-3 yielded reservoir profiles of phosphate at three sites off of the Massachusetts coast. These extractions found Fe-associated P04 to be the dominant phase associated with rapid porewater-solid P exchange. Additionally, a seasonal enrichment/depletion pattern of phosphate fluxes relative to total carbon was observed from the sediments. These observations established the behavior of phosphate in coastal sediments as interconnected with the ongoing Fe-cycling in the sediments as well. / by Richard Hunter Oates, Jr. / S.M.

Thesis (S.M.)--Joint Program in Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences and the Woods Hole Oceanographic Institution), 2002. / Includes bibliographical references (p. 22-28). / We have investigated the near liquidus phase relations of a primitive absarokite from the Mascota region in Western Mexico. Sample M. 102 was chosen because it has high MgO contents, a high Mg# and Fo90 olivine phenocrysts, indicating it is primitive mantle melt. Highpressure experiments on a synthetic analogue of the absarokite composition with a H20 content of either -1.7 wt% or -5.1 wt% were carried out in a piston cylinder apparatus. The composition with -1.7 wt% H20 is multiply saturated with olivine and orthopyroxene as liquidus phases at 1.6 GPa and 14000C. At the same pressure clinopyroxene appears 300C below the liquidus. With a H20 content of -5.1 wt% composition M.102 is multiply saturated with olivine and orthopyroxene on the liquidus at 1.7 GPa and 13000 C. Assuming batch melting, we suggest that absarokite M. 102 segregated from a depleted lherzolite or harzburgite residue at depth -50 km depth in the mantle wedge. Unlike most lavas in the region, the absarokite has not ponded and fractionated at the crust mantle interface (-35-40 km), and the temperatures of multiple saturation indicate that the mantle wedge beneath the Jalisco block is hotter than previously thought. The low degree batch melting of an original metasomatised harzburgite source, can produce the observed trace element abundances. The liquidus phase relations are not consistent with the presence of non-peridotitic veins at the depth of last equilibration. Therefore, we propose that the Mascota absarokites segregated at an apparent melt fraction of less than 5% from a depleted peridotitic source. They initially formed by a small degree of melting of a metasomatised original source at greater depth. / by Marc Hesse. / S.M.

Thesis (M.S.)--Joint Program in Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 1987. / Supervised by David G. Aubrey. / Includes bibliographical references (leaves 57-61). / by Barbara Vanston Braatz. / M.S.

Thesis (Ph. D.)--Joint Program in Marine Geology and Geophysics (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 1987. / Includes bibliographical references (v.2, leaves 309-317). / Many features of the oceanic plates cannot be explained by conductive cooling with age. A number of these anomalies require additional convective thermal sources at depths below the plate: mid-plate swells, the evolution of fracture zones, the mean depth and heat flow relationships with age and the observation of small scale (150-250 km) geoid and topography anomalies in the Central Pacific and Indian oceans. Convective models are presented of the formation and evolution of these features. In particular, the effect of a shallow low viscosity layer in the uppermost mantle on mantle flow and its geoid, topography, gravity and heat flow expression is explored. A simple numerical model is employed of convection in a fluid which has a low viscosity layer lying between a rigid bed and a constant viscosity region. Finite element calculations have been used to determine the effects of (1) the viscosity contrast between the two fluid layers, (2) the thickness of the low viscosity zone, (3) the thickness of the conducting lid, and (4) the Rayleigh number of the fluid based on the viscosity of the lower layer. A model simple for mid-plate swells is that they are the surface expression of a convection cell driven by a heat flux from below. The low viscosity zone causes the top boundary layer of the convection cell to thin and, at high viscosity contrasts and Rayleigh numbers, it can cause the boundary layer to go unstable. The low viscosity zone also mitigates the transmission of normal stress to the conducting lid so that the topography and geoid anomalies decrease. The geoid anomaly decreases faster than the topography anomaly, however, so that the depth of compensation can appear to be well within the conducting lid. Because the boundary layer is thinned, the elastic plate thickness also decreases and, since the low viscosity allows the fluid to flow faster in the top layer, the uplift time decreases as well. We have compared the results of this modeling to data at the Hawaii, Bermuda, Cape Verde and Marquesas swells, and have found that it can reproduce their observed anomalies. The viscosity contrasts that are required range from 0.2-0.01, which are in agreement with other estimates of shallow viscosity variation in the upper mantle. Also, the estimated viscosity contrast decreases as the age of the swell increases. This trend is consistent with theoretical estimates of the variation of such a low viscosity zone with age. Fracture zones juxtapose segments of the oceanic plates of different ages and thermal structures. The flow induced by the horizontal temperature gradient at the fracture zone initially downwells immediately adjacent to the fracture zone on the older side, generating cells on either side of the plume. The time scale and characteristic wavelength of this flow depends initially on the viscosity near the largest temperature gradient in the fluid which, in our model, is the viscosity of the low viscosity layer. They therefore depend on both the Rayleigh number and the viscosity contrast between the layers. Eventually the flow extends throughout the box, and the time scales and the characteristic wavelengths of the flow depend on the thickness and viscosity of both layers. When the Rayleigh number based on the viscosity of the top la er, and the depth of both fluid layers, is less than 10 , the geoid anomalies of these flows are dominated by the convective signal. When this Rayleigh number exceeds 106, the geoid anomalies retain a step across the fracture zone out to large ages. We have compared our results to geoid anomalies over the Udintsev fracture zone, and have found that the predicted geoid anomalies, with high effective Rayleigh numbers, agree at longer wavelengths with the observed anomalies and can produce the observed geoid slope-age behaviour. We have also compared the calculated topographic steps to those predicted by the average depth-age relationships observed in the oceans. We have found that only with a low viscosity zone will the flow due to fracture zones not disturb the average depth versus age relationships. We have also applied the model to a numerical study of the effect of a low viscosity zone in the uppermost mantle on the onset and surface expression of convective instabilities in the cooling oceanic plates. We find that the onset and magnitude of the geoid, topography and heat flow anomalies produced by these instabilities are very sensitive to the viscosity contrast and the Rayleigh number, and that the thickness of the low viscosity zone is constrained by the wavelength of the observables. If the Rayleigh number of the low viscosity zone exceeds a critical value then the convection will be confined to the low viscosity zone for a period which depends on the viscosity contrast and the Rayleigh number. The small scale convection will eventually decay into longer wavelength convection which extends throughout the upper mantle, so that the small scale convective signal will eventually be succeeded by a longer wavelength signal. We compare our model to the small scale geoid and topography anomalies observed in the Southeast Pacific. The magnitude (0.50-0.80 m in geoid and 250 m in topography), early onset time (5-10 m.y.) and lifetime (over 40 m.y.) of these anomalies suggest a large viscosity contrast of greater than two orders of magnitude. The trend to longer wavelengths also suggests a high Rayleigh number of near or over 10 and their original 150-250 km wavelength indicates a low viscosity zone of 75- 125 km thickness. We have found that the presence of such small scale convection does not disturb the slope of the depth-age curve but elevates it by up to 250 m, and it is not until the onset of long wavelength convection that the depth-age curves radically depart from a cooling halfspace model. In the Pacific, the depth-age curve is slightly elevated in the region where small scale convection is observed and it does not depart from a halfspace cooling model until an age of 70 m.y.. Models that produce the small scale anomalies predict a departure time between 55 and 65 m.y. / by Elizabeth M. Robinson. / 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), 2013. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 30-38). / Thiol substances can form stable complexes with metals (especially copper and mercury) in the surface ocean that can impact cycling and bioavailability of those elements. In this study, I present seven concentration profiles of cysteine and glutathione, two low-molecular weight thiols, from the coastal northwest Atlantic Ocean and the Bermuda Atlantic Time Series (BATS) sampling site in the Sargasso Sea, a first for these regions. These two thiols were found in the upper 200 meters of the ocean at all sites, and the total thiol concentration varied from 0.2 to 3.2 nM. The highest concentration of both thiols was found at the deep chlorophyll maximum in most samples. Thiol concentrations were higher on the continental shelf than in the open ocean. The observed distribution of cysteine and glutathione and thermodynamic stability of copper complexes suggests that Cu(I)-dithiol complexes may be the dominant surface ocean copper and thiol species. Mercury-thiol complexes were also present in thermodynamically modeled seawater, which may provide a vector for mercury uptake in the surface ocean. / by Tristan Kading. / Ph.D.

Thesis: Ph. D. in Marine Geology, Joint Program in Marine Geology and Geophysics (Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2017. / Cataloged from PDF version of thesis. / Includes bibliographical references. / This thesis analyzes compositions of seafloor massive sulfide (SMS) deposits and related hydrothermal vent fluids to identify proxies of reaction zone conditions (host-rock lithology, hydrothermal fluid temperature and chemistry). Chapter 2 investigates the morphology, mineralogy, and geochemistry of SMS deposits from six vent fields along the Eastern Lau Spreading Center (ELSC), demonstrating that ELSC SMS deposits record differences in hydrothermal fluid temperature, pH, sulfur fugacity and host-rock lithology related to proximity to the nearby Tonga Subduction Zone. Chapters 3 and 4 focus on partitioning of Co, Ni, Ga, Ag, and In between hydrothermal vent fluids and chalcopyrite lining fluid conduits in black smoker chimneys. Chapter 3 develops secondary ion mass spectrometry (SIMS) as a technique to measure Co, Ni, Ga, Ag, and In in chalcopyrite and identifies a correlation between Ga and In in chalcopyrite and hydrothermal fluid pH. Chapter 4 presents new data on these elements in ELSC hydrothermal fluids that, combined with SIMS analyses of chalcopyrite chimney linings and previously published data on vent fluids from the Manus Basin, provide evidence that supports partitioning of Ag a lattice substitution for Cu. Together, concentrations of Ga, In, and Ag in chalcopyrite provide proxies of hydrothermal fluid pH and metal (i.e., Ag and Cu) contents. / by Guy Nathaniel Evans. / Ph. D. in Marine Geology

Thesis (Ph. D.)--Joint Program in Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2006. / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Includes bibliographical references (leaves 117-126). / In this thesis, I present high-resolution stable-isotope and planktonic-fauna records from Bering Sea sediment cores, spanning the time period from 50,000 years ago to the present. During Marine Isotope Stage 3 (MIS3) at 30-20 ky BP (kiloyears before present) in a core from 1467m water depth near Umnak Plateau, there were episodic occurrences of diagenetic carbonate minerals with very low 13C (-22.4%), high 18O (6.5%), and high [Mg]/[Ca], which seem associated with sulfate reduction of organic matter and possibly anaerobic oxidation of methane. The episodes lasted less than 1000 years and were spaced about 1000 years apart. During MIS3 at 55-20 ky BP in a core from 2209m water depth on Bowers Ridge, N. pachyderma (s.) and Uvigerina 18O and 13C show no coherent variability on millennial time scales. Bering Sea sediments are dysoxic or laminated during the deglaciation. A high sedimentationrate core (200 cm/ky) from 1132m on the Bering Slope is laminated during the Blling warm phase, Allerd warm phase, and early Holocene, where the ages of lithological transitions agree with the ages of those climate events in Greenland (GISP2) to well within the uncertainty of the age models. The subsurface distribution of radiocarbon was estimated from a compilation of published and unpublished North Pacic benthic-planktonic 14C measurements (475{2700 m water depth). There was no consistent change in 14C probles between the present and the Last Glacial Maximum, Blling-Allerd, or the Younger Dryas cold phase. N. pachyderma (s.) 18O in the Bering Slope core decreases rapidly (in less than 220 y) by 0.7-0.8h at the onset of the Blling and the end of the Younger Dryas. These isotopic shifts are accompanied by transient decreases in the relative abundance of N. pachyderma (s.), suggesting that the isotopic events are transient warnings and sustained freshenings. / by Mea S. Cook. / 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), 1986. / Microfiche copy available in Archives and Science. / Vita. Chapter 5: ²³⁹,̳²⁴⁰Pu and excess ²¹⁰Pb inventories along the shelf and slope of the northeast U.S.A. / Ken O. Buesseler, Hugh D. Livingston and Edward R. Sholkovitz, reprinted from Earth and Planetary Science Letters, 76 (1985/86) 10-22, Elsevier Science Publishers, B. V. Amsterdam. The underscored comma in the above note is superscript on the source. / Bibliography: leaves 193-207. / by Ken O. Buesseler. / 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. / Includes bibliographical references (leaves 192-194). / The various distributions of tracer associated with the Northern Recirculation Gyre of the Gulf Stream (NRG) are studied to try to obtain information about the flow. An advective-diffusive numerical model is implemented to aid in the investigation. The model is composed of a gyre adjacent to a boundary current in which a source of tracer is specified at the upstream edge of the current. This set up attempts to simulate the lateral transfer of properties from the Deep Western Boundary Current (DWBC) to the NRG in the region where the two flows are in close contact west of the Grand Banks. The results of the model are analyzed in some detail. Tracer is entrained into the gyre as a plume which extends from the boundary current and spirals across streamlines toward the gyre center. The maintenance of the spiral during spin-up and its relationship to the occurrence of homogenization at steady state is examined. An asymmetry in the spiral exists due to the ellipticity of the gyre, which also effects homogenization. The anomalous properties that are fluxed into the NRG include salt, oxygen, and freon. These particular tracers are independent from each other, the former two because they are characterized by different vertical profiles in the deep layer. This results in a decay of oxygen but not salt, due to the presence of vertical mixing as discussed by Hogg et al. (1986, Deep-Sea Research, 33, 1139-1165). Their analysis is expanded upon here. The effect of vertical mixing on the gyre/boundary current system is examined within the context of the numerical model. Results are applied to recently collected water sample data from the region which leads to an estimate of the lateral and vertical eddy diffusion coefficients and an estimate of the amount of oxygen in the NRG that has diffused from the DWBC. The accumulation of freon within the NRG is considered in addition to salt and oxygen. Appreciable levels of freon have been present in the ocean only since 1950, and the atmospheric source functions have been increasing steadily since then. A simple overflow model is presented of the manner in which freon may be stirred in the Norwegian-Greenland basin prior to overflowing and entering the DWBC. Once in the boundary current the concentrations are diluted by way of mixing with surrounding water. Two different schemes are considered in which the immediate surrounding water accumulates a substantial amount of freon as time progresses. These models suggest that the freon-11:freon-12 ratio may not be a conserved quantity for the water in the core of the UWBC. It is found that the level of freon in the NRG is barely above the existing background level. / by Robert S. Pickart. / Ph.D.