ALBERT

Description: Submitted in partial fulfillment of the requirements for the degree of Master of Science in Aeronautics and Astronautics at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 2021.

Description: Global temperature rise and increased atmospheric carbon dioxide (CO2) levels have affected the health of the world’s ocean and water ecosystems, impacting the balances of natural carbon cycling and causing ocean acidification. Additionally, as global temperatures rise, thawing permafrost has stimulated increased release of methane (CH4), a gas with a shorter lifetime in the atmosphere but with even more heat trapping ability than CO2. In situ analysis of dissolved gas content in surface waters is currently performed with large, expensive instruments, such as spectrometers, which are coupled with gas equilibration systems, which extract dissolved gas from water and feed it to the sensor. Accurate, low cost, and portable sensors are needed to measure the dissolved CH4 and CO2 concentration in water systems to quantify their release and understand their relationship to the global carbon budget. At the same time, while greenhouse gases are well established threats to water ecosystems, the ubiquity and potential consequences of microplastics in aqueous environments are just beginning to be recognized by the environmental research community. Microplastics (MPs) are small particles of polymer debris, commonly defined as being between 1 μm and 1000 μm. Despite the pervasiveness of MPs, our ability to characterize MPs in the environment is limited by the lack of technologies for rapidly and accurately identifying and quantifying MPs. This thesis is concerned with the engineering challenges prompted by the need for high quality and quantity environmental data to better study and the impact, cycling, and prevalence of these pollutants in aqueous environments. Three distinct investigations are presented here. First, the design of the Low-Cost Gas Extraction and Measurement System (LC-GEMS) for dissolved CO2 is presented. At just under $600 dollar to build, the LC-GEMS is an ultra-portable, toolbox-sized instrument for dissolved gas sensing in near-surface waters. The LCGEMS was characterized in the lab and demonstrated linear relationships with dissolved CO2 as well as temperature. Lab calibrations and subsequent field testing in the Little Sippewissett Marsh, in Falmouth, Massachusetts showed that the LCGEMS captures both diurnal and minute-time scale trends in dissolved CO2. Second, this thesis presents the novel design of three simple and low-cost planar nanophotonic and plasmonic structures as optical transducers for measuring dissolved CH4. Through simulations, the sensitivity of the structures are evaluated and found to exhibit superior performance in the reflectance intensity readout mode to that of the standard surface-plasmon-polariton-mode Spreeta sensor. A practical, small, and low-cost implementation of this chip with a simple intensity-based measurement scheme is proposed. This design is novel in the space of dissolved gas monitoring because it shows potential to measure directly in the water phase while being robust and low-cost to implement. Finally, this thesis presents a literature review and perspective to motivate the development of field-deployable microplastic sensing techniques. A framework for field-deployable microplastic sensing is presented and seeks to inform the MP community of the potential in both traditional MP analysis techniques and unconventional methods for creating rapid and automated MP sensors. The field-deployabilty framework addresses a full scope of practical/technological trade-offs to be considered for portable MP detection.

Description: © The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Colson, B. C., & Michel, A. P. M. Flow-through quantification of microplastics using impedance spectroscopy. ACS Sensors, 6(1), (2021): 238–244, doi:10.1021/acssensors.0c02223.

Description: Understanding the sources, impacts, and fate of microplastics in the environment is critical for assessing the potential risks of these anthropogenic particles. However, our ability to quantify and identify microplastics in aquatic ecosystems is limited by the lack of rapid techniques that do not require visual sorting or preprocessing. Here, we demonstrate the use of impedance spectroscopy for high-throughput flow-through microplastic quantification, with the goal of rapid measurement of microplastic concentration and size. Impedance spectroscopy characterizes the electrical properties of individual particles directly in the flow of water, allowing for simultaneous sizing and material identification. To demonstrate the technique, spike and recovery experiments were conducted in tap water with 212–1000 μm polyethylene beads in six size ranges and a variety of similarly sized biological materials. Microplastics were reliably detected, sized, and differentiated from biological materials via their electrical properties at an average flow rate of 103 ± 8 mL/min. The recovery rate was ≥90% for microplastics in the 300–1000 μm size range, and the false positive rate for the misidentification of the biological material as plastic was 1%. Impedance spectroscopy allowed for the identification of microplastics directly in water without visual sorting or filtration, demonstrating its use for flow-through sensing.

Description: The authors thank the Richard Saltonstall Charitable Foundation and the National Academies Keck Futures Initiative (NAKFI DBS13) for their funding support.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution December 1996

Description: Two-thirds of the surface of the Earth is created at mid-ocean ridges where magmas rise from the mantle and cool to form the oceanic crust. The objective of this Thesis is to examine the influence of magma supply and eruptive processes on axial morphology, crustal construction, and the properties of crustal magma chambers at intermediate and fast spreading ridges. Variations in magma supply on time scales of ~100 Kyr generate along-axis changes in crustal thickness and temperature. Magma sill properties and hydrothermal activity are closely linked to spreading events which occur on much shorter time scales (ca. 10-100 yr) than the longer-term variations in magma supply reflected in along-axis changes in ridge morphology. The seismically constrained depths of ridge crest magma sills (〉1-2 km) are considerably deeper than the level of neutral buoyancy (100-400 m). The apparent inverse relationship between magma sill depth and spreading rate suggests that a thermally controlled permeability boundary, such as the solidus horizon, controls the depth at which magma ponds beneath mid-ocean ridges. Recent thermo-mechanical models predict that, at intermediate spreading rates, rift valley and magma sill formation are sensitive to small changes in crustal thickness and mantle temperature. Analysis of gravity at an intermediate spreading ridge shows that small differences in crustal thickness (300-700 m) and mantle temperature (10-15°C) are indeed sufficient to produce major changes in lithospheric strength and axial morphology. A stochastic model for the emplacement of dikes and lava flows with a bimodal distribution of lava flows is required to satisfy geological and geophysical constraints on the construction of the extrusive section. Most dikes are intruded within a narrow zone at the ridge axis. Short flows build up approximately half the extrusive volume. Occasional flows that pond at a considerable distance off-axis build up the remainder of the extrusive section. This Thesis underlines the importance of eruption dynamics in the emplacement of the uppermost volcanic layer of the crust and of the crustal thermal structure in controlling local variations in magma sill depth and ridge morphology.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution May 1997

Description: Many of the most important processes that create and modify continental crust occur at continental margins, but recently has the scientific community acquired the necessary intrumentation to image crustal structure across margins in detail. In this thesis we investigate the crustal structure across the U.S. East Coast rifted margin and the convergent margin of southwestern Alaska using modern, deep-penetrating marine seismic reflection/refraction data. We consider U.S. East Coast margin transects along the shelf offshore Georgia and across the mid-Atlantic margin near Chesapeak bay. Results by other workers, based on data from these transects, have shown that voluminous volcanism accompanied formation of the rifted margin during continental breakup. Results presented in this thesis constrain the landward extent of rift-related magmatic emplacement. We find that magmatic intrusion and underplating of pre-existing continental crust occurs primarily in extended crust and that crustal extension is focused in a 75-km-wide region beneath the shelf and slope. The crust thinned by 50 to 80% within this interval and then seafloor spreading began with an unusually large volume of igneous crust production. The initial volcanic extrusives were emplaced subaerially and are now present beneath the sediments in a thick seaward-dipping wedge. We use post-stack depth migration to image this wedge and use the resulting image to consider the early subsidence of the margin. The geometry of the subaerially extruded rift volcanics suggest that the margin subsided rapidly once volncanism began. We infer from the subsidence, the along-margin distribution of magmatic material, and the across-margin localization of magmatic emplacement and deformation that the U.S. East Coast rift volcanics had an anomalously-hot mantle source whose distribution beneath the lithosphere prior to rifting was long (the length of the margin) but not deep. We speculate that the distribution of this material was controlled by topography at the base of the lithosphere inherited from the Paleozoic collision of North America and Africa. Our analysis of the southwestern Alaska convergent margin is based on data from the 1994 Aleutian seismic experiment. The crust of most of Alaska has been built through terrane accretion and arc magmatism, and this experiment was conducted to study the evolution of continental crust through these processes. We consider transects across the westernmost Alaska Peninsula margin, where subduction is occurring beneath protocontinental crust composed of oceanic-arc terranes accreted in the Cretaceous, and across Bristol Bay in the back arc region where the crust has undergone a number of geologic events since accretion. Across the Peninsula, we find that the velocity structure of the accreted terranes differs little from that of the Cenozoic Aleutian oceanic-arc crust west of the Peninsula determined along another transect of this experiment. The accreted oceanicarc terranes are considerably more mafic than continental crust and the process of accretion has apparently not modified the bulk composition of these terranes toward that of average continental crust. It is possible that Cenozoic arc magmatism has been more felsic in composition than that which formed the accreted terranes and the Aleutian oceanic arc to the west, and that these magmas have been emplaced primarily within the crust inboard of the accreted terranes which lie south of the currently active arc. The geology of the Bristol Bay region suggests that the crustal components here had an origin similar to that of the Alaska Peninsula margin- that is, accreted terranes. We find, however, that the crust beneath Bristol Bay has a typically continental velocity structure. If this crust originally had a structure similar to the Alaska Peninsula margin, then at least two processes must have occured to affect the transformation to its current structure: crustal thickening and removal of the mafic lower crust. The geologic events that have affected this region since accretion are consistent with such and evolution.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1996

Description: We analyze bathymetric and gravity anomalies at five plume-ridge systems to constrain crustal and mantle density structure at these prominent oceanic features. Numerical models are then used to explore the physical mechanisms controlling plume-ridge interaction and to place theoretical constraints on the temperature anomalies, dimensions, and fluxes of the Icelandic and Galapagos plumes. In Chapter 1 we analyze bathymetric and gravity anomalies along the hotspot-influenced Galapagos Spreading Center. We find that the Galapagos plume generates along-axis bathymetric and mantle-Bouguer gravity anomalies (MBA) that extend 〉500 km east and west of the Galapagos Islands. The along-axis MBA becomes increasingly negative towards the plume center, reaching a minimum of ~-90 mGal near 91°W, and axial topography shallows by ~1.1 km toward the plume. These variations in MBA and bathymetry are attributed to the combined effects of crustal thickening and anomalously low mantle densities, both of which are due to a mantle temperature anomaly imposed beneath the ridge by the Galapagos plume. Passive mantle flow models predict a temperature anomaly of 50±25°C is sufficient to produce the 2-4 km excess crust required to explain the along-axis anomalies. 70-75% of the along-axis bathymetric and MBA variations are estimated to arise from the crust with the remaining 25-30% generated by the anomalously hot, thus low-density mantle. Along Cocos-plate isochrons, bathymetric and MBA variations increase with increasing isochron age, suggesting the subaxial mantle temperature anomaly was greater in the past when the plume was closer, to the ridge axis. In addition to the Galapagos plume-ridge system, in Chapter 2 we examine alongisochron bathymetric and MBA variations at four other plume-ridge systems associated with the Iceland, Azores, Easter and Tristan hotspots. We show that residual bathymetry (up to 4.7 km) and mantle-Bouguer gravity anomalies (up to -340 mGal) are greatest at on-axis plumes and decreases with increasing ridge-hotspot separation distance, until becoming insignificant at a plume-ridge separation of ~500 km. Along-isochron widths of bathymetric anomalies (up to 2700 km) decrease with increasing paleo-spreading rate, reflecting the extent to which plume material flows along-axis before being swept away by the spreading lithosphere. Scaling arguments suggest an average ridgeward plume flux of -2.2x106 km/my. Assuming that the amplitudes of the MBA and bathymetric anomalies reflect crustal thickness and mantle density variations, passive mantle flow models predict maximum subaxial mantle temperature anomalies to be 150-225°C for ridge-center plumes, which decrease as the ridges migrate away from the plumes. The dynamics of mantle flow and melting at ridge-centered plumes are investigated in Chapters 3 using three-dimensional, variable-viscosity, numerical models. Three buoyancy sources are examined: temperature, melt depletion, and melt retention. The width W to which a plume spreads along a ridge axis depends on plume volume flux Q, full spreading rate U, buoyancy number B = (QΔρg)/(48η0U2), and ambient/plume viscosity contrast ϒ according to W=2.37(Q/U)l/2(Bϒ)0.04. Thermal buoyancy is first order in controlling along-axis plume spreading while latent heat loss due to melting, and depletion and retention buoyancy forces contribute second order effects. Two end-member models of the Iceland-Mid-Atlantic Ridge (MAR) system are examined. The first endmember model has a broad plume source of radius 300 km, temperature anomaly of 75°C, and volume flux of 1.2xl07 km3/my. The second model has a narrower plume source of radius 60 km, temperature anomaly of l70°C, and flux of 2.1 x106 km3/my. The first model predicts successfully the observed crustal thickness, topographic, and MBA variations along the MAR, but the second model requires substantial along-axis melt transport in order to explain the observed along-axis variations in crustal thickness, bathymetry, and gravity. We favor this second model because it predicts a mantle P-wave velocity reduction in the plume of ~2% as consistent with recent seismic observations beneath Iceland. Finally in Chapter 4 we use three-dimensional numerical models to investigate the interaction of plumes and migrating midocean ridges. Scaling laws of axial plume spreading width Ware derived first for stationary ridges and off-axis plumes, which yield results consistent with those obtained from independent studies of Ribe [1996]. Wand the maximum plume-ridge interaction distance Xmax again scale with (Q/U)l/2 as in the case of ridge-centered plumes and increase with ϒ and buoyancy number. In the case of a migrating ridge, Xmax is reduced when a ridge migrates toward the plume due to excess drag of the faster-moving leading plate, and enhanced when a ridge migrates away from the plume due to reduced drag of the slower-moving trailing plate. Thermal erosion of the lithospheric boundary layer by the previously ridge-centered plume further enhances Wand Xmax but to a degree that is secondary to the differential migration rates of the two plates. Model predictions are compared with observed along-isochron bathymetric and MBA variations at the Galapagos plume-ridge system. The anomaly amplitudes and widths, as well as the increase in anomaly amplitude with age are predicted with a plume source temperature anomaly of 80-120°C, radius of 80-100 km, and volume flux of 4.5x106 km3/m.y. Our numerical models also predict crustal production rates of the Galapagos Islands consistent with those estimated independently using the observed island topography. Predictions of the geochemical signature of the plume along the present-day ridge suggest that mixing between the plume and ambient mantle sources is unlikely to occur in the asthenosphere or shallow crust, but most likely deeper in the mantle possibly by entrainment of ambient mantle as the plume ascends through the depleted portion of the mantle from its deep source reservoir.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution March 1997

Description: The formation of new oceanic crust is the result of a complex geodynamic system in which mantle rises beneath spreading centers and undergoes decompression melting. The melt segregates from the matrix and is focused to the rise axis, where it is eventually intruded and/or erupted to form the oceanic crust. This thesis combines surface observations with laboratory studies and geodynamic modeling to study this crustal-production system. Quantitative modeling of the crustal and mantle contributions to the axial gravity and topography observed at the East Pacific Rise shows that the retained melt fraction in the mantle is small (〈3%) and is focused into a narrow column extending up to 70 km beneath the ridge axis. Consistent with geochemical constraints, the extraction of melt from the mantle therefore appears to be efficiently focus melt toward the ridge axis. A combination of laboratory and numerical studies are used to constrain the pattern of mantle flow beneath highly-segmented ridges. Even when the buoyant component of mantle flow is constrained to be two-dimensional, laboratory studies show that a segmented ridge will drive three-dimensional mantle upwelling. However, using reasonable mantle parameters in numerical models, it is difficult to induce large-amplitude three-dimensional mantle upwelling at the relatively short wavelengths of individual segments (~50 km). Instead, a simple model of three-dimensional melt migration shows that the observed segment-scale variations in crustal thickness can be explained by focusing of melt as it upwells through a more two-dimensional mantle flow field. At the Reykjanes Ridge, the melt appears to accumulate in small crustal magma chambers, before erupting in small batches to form numerous overlapping hummocky lava flows and small volcanoes. This suggests that crustal accretion, particularly at slow-spreading centers, may be a highly discontinuous process. Long-wavelength variations in crustal accretion may be dominated by variations in mantle upwelling while short-wavelength, segment-scale variations are more likely controlled by a complex three-dimensional processes of melt extraction and magma eruption.

Description: During my first three years in the Joint Program, I was supported by an National Science Foundation Graduate Student Fellowship. Other support has been derived from National Science Foundation grants OCE-9296017, OCE-9224738, OCE-9215544, and EAR grant 93-07400.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 1993

Description: This dissertation studies several aspects of the formation of the Earth's oceanic mantle and crust, using a variety of geologic techniques, principally major elements, radiogenic isotopes and trace elements, but including petrography, mineral chemistry, x-ray diffraction, seafloor geomorphology, and analysis of the tectonics of fracture zones. The first chapter is an introduction to the problems to be addressed in this work. The second chapter examines the composition of basalts erupted near the Atlantis II Fracture Zone on the Southwest Indian Ridge. Trends in major element compositions of those basalts can be related directly to the nearby presence of the fracture zone. The effects of mantle composition and crustal level lateral transport of magma in the rift system can be ruled out by the analysis of isotopes and the geomorphology of the fracture zone floor. This is the best demonstration to date of a transform fault effect on basalt compositions. In trying to quantify putative transform fault effects documented at other fracture zones, no systematic correlation of transform offset age with mantle temperature change can be found, suggesting that mantle composition and lateral transport phenomena play a larger than expected role in the evolution of those areas. The third chapter relates to oceanic mantle rocks as they are altered at or near the Earth's surface. The major elements which make up abyssal peridotites are extensively redistributed by the alteration they have undergone. Mg is shown to be extracted from the peridotites, and a variety of trace elements added. This elemental redistribution is taken as evidence for extensive Mg transport by circulating waters. Since the solubility of Mg-bearing minerals in hydrothermal solutions is quite limited, much lower temperatures and much higher water /rock ratios are required to explain the major element compositions of the peridotites than had previously been assumed. The behavior of the Nd, Sr and Os isotopic systems during seafloor alteration was also studied. The isotope systematics of these rocks strongly support the hypothesis of high water /rock ratios in the formation of serpentinized abyssal peridotites. Nonetheless, Nd and Sr reside in a phase which is resistant to alteration (clinopyroxene) and the concentration of Os is high relative to that of seawater, so that it too appears resistant to alteration. Primary mantle isotopic signatures may be obtained from abyssal peridotites by careful analysis, even of extremely weathered rocks. Radiogenic strontium in excess of what could be introduced by seawater contamination or in situ radiogenic growth in a reasonable period of time was also found. These observations confirm earlier work which had been discredited for many years. The only plausible mechanism for the formation of this "orphan" S7Sr is that it is introduced as part of a sedimentary component which infiltrates the rock during metamorphism and/ or weathering. The 87Sr may be contained by or sorbed onto extremely fine clay particulates, or colloidal suspensions, as opposed to the dissolved ionic Sr which is normally thought of as characterizing the Sr isotopic composition of seawater. The high water/rock ratios required by the bulk isotopic analysis, as well as the pervasive elemental redistribution arguing for extensive near-surface weathering at high water /rock ra.tios strongly support this hypothesis. Given pervasive percolation of water throughout the samples, sufficient radiogenic, sediment-derived strontium may be drawn deep into the crust in the course of its weathering to cause such high B7SrfB6Sr ratios. The fourth chapter deals exclusively with primary mantle isotopic information from abyssal peridotites. This is the first study which has attempted to relate the Os isotopic system in the oceanic mantle to other isotopic systems and to trace elements. It is possible, with some extreme assumptions, to model the range of Os isotopes in the oceanic mantle alone in a standard model of formation of the depleted mantle by extraction of the crust. The additional constraints provided by the study of Nd isotopes in depleted mantle rocks from the oceans show that partial melt extraction and the formation of a depleted reservoir alone are not sufficient to account for the range of both N d and Os isotopes in the Earth's mantle. Possible mechanisms for the decoupling of the Os and Nd isotopic systems include elemental fractionation via the porous flow of basalt through the mantle, mantle metasomatism, recycling of a subducted component in the mantle and core formation. The core extraction model is pursued in some detail. Such core extraction models can account for the distributions and isotopic compositions of compatible and incompatible trace elements in the Earth's mantle, but they are highly non-unique, and thus difficult to test.

Description: This thesis was supported by an NSF Graduate Fellowship, and by a grant from the WHOI Ocean Ventures Fund.

Description: Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 1998

Description: Stable sulfur isotopes (δ34S) and trace Co are analyzed in sulfide and sulfate minerals from six sample types collected from the TAG active mound, 26°N Mid-Atlantic Ridge. δ34S values range from 2.7 to 2O.9%, with sulfate minerals isotopically indistinguishable from seawater (21%), and sulfide minerals reflecting input of 1/3 seawater and 2/3 basaltic sulfur (~0%). Co concentrations in pyrite analyzed by ion microprobe primarily reflect depositional temperatures. The δ34S and Co data are combined to provide information regarding the sources and temperatures of parent fluids, the genetic relationships among sample types, and the circulation of hydrothermal fluids and seawater in the mound. δ34S values and Co concentrations vary by sample type. Chalcopyrite from black smoker samples exhibits invariant δ34S values, indicating direct precipitation from black smoker fluids. Crust samples contain chalcopyrite with a mean δ34S indistinguishable from that of black smoker samples, and pyrite with some light δ34S and moderately high Co values, consistent with crust samples precipitating from cooled black smoker fluids. Massive anhydrite samples are a mixture of anhydrite with high δ34S, and pyrite with variable δ34S and Co values, indicative of deposition from disequilibrium mixing between black smoker fluids and seawater. White smoker samples contain chalcopyrite and sphalerite with high δ34S, and pyrite with low Co values, reflecting deposition from cooler fluids formed from mixtures of seawater and black smoker fluid, with some reduction of sulfate. Mound samples contain chalcopyrite with a mean δ34S indistinguishable from that of black smoker and crust samples, and pyrite with low Co values, suggesting deposition from a fluid isotopically similar to black smoker fluid at temperatures similar to those of white smoker fluid. Massive sulfide samples exhibit pyrite with high δ34S values and very high Co, indicating deposition from and recrystallization with very hot fluids contaminated with seawater-derived sulfate. The data demonstrate that direct precipitation from black smoker fluids, conductive cooling, disequilibrium mixing with entrained seawater, sulfate reduction, and recrystallization all contribute to the formation of the TAG mound deposit. The successful preliminary Co analyses demonstrate that ion microprobe analyses are a viable technique for measuring trace elements in sulfides.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 2010

Description: This thesis presents the results of four discrete investigations into processes governing the organic and inorganic chemical composition of seafloor hydrothermal fluids in a variety of geologic settings. Though Chapters 2 through 5 of this thesis are disparate in focus, each represents a novel investigation aimed at furthering our understanding of subsurface geochemical processes affecting hydrothermal fluid compositions. Chapters 2 and 3 concern the abiotic (nonbiological) formation of organic compounds in high temperature vent fluids, a process which has direct implications for the emergence of life in early Earth settings and sustainment of present day microbial populations in hydrothermal environments. Chapter 2 represents an experimental investigation of methane (CH4) formation under hydrothermal conditions. The overall reduction of carbon dioxide (CO2) to CH4, previously assumed to be kinetically inhibited in the absence of mineral catalysts, is shown to proceed on timescales pertinent to crustal residence times of hydrothermal fluids. In Chapter 3, the abundance of methanethiol (CH3SH), considered to be a crucial precursor for the emergence of primitive chemoautotrophic life, is characterized in vent fluids from ultramafic-, basalt- and sediment-hosted hydrothermal systems. Previous assumptions that CH3SH forms by reduction of CO2 are not supported by the observed distribution in natural systems. Chapter 4 investigates factors regulating the hydrogen isotope composition of hydrocarbons under hydrothermal conditions. Isotopic exchange between low molecular weight n-alkanes and water is shown to be facilitated by metastable equilibrium reactions between alkanes and their corresponding alkenes, which are feasible in natural systems. In Chapter 5, the controls on vent fluid composition in a backarc hydrothermal system are investigated. A comprehensive survey of the inorganic geochemistry of fluids from sites of hydrothermal activity in the eastern Manus Basin indicates that fluids there are influenced by input of acidic magmatic solutions at depth, and subsequently modified by variable extents of seawater entrainment and mixing-related secondary acidity production.

Description: The thesis research presented here was funded by the National Science Foundation through grants OCE-0327448, OCE-0136954, MCB-0702677, OCE-0549829, and by the Department of Energy grant DE-FG02-97ER14746.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 2010

Description: Volcanic accretion at the fast-spreading East Pacific Rise (EPR) occurs over a ~2-4 km wide neo-volcanic zone on either side of the axial summit trough (AST). Eruption ages are critical for understanding the distribution and timing of volcanic and magmatic activity. Uranium series nuclides are susceptible to fractionation by magmatic processes that occur beneath mid-ocean ridges, and the half-lives of 226Ra (1.6 kyrs) and 230Th (75 kyrs) make them ideally suited for determining eruption ages and placing constraints on eruption frequency and temporal changes in magma chemistry. Accordingly, major and trace element, and long-lived radiogenic and 238U-230Th-226Ra isotope compositions were measured in basalts from 9º-10ºN EPR to determine eruption ages and to place temporal constraints on volcanic and magmatic processes. At 9º30’N EPR, 238U-230Th-226Ra compositions indicate that trace elementally and isotopically enriched mid-ocean ridge basalt (MORB) collected off-axis erupted 〉8 ka and that E-MORB magmatism is interspersed with normal, depleted MORB magmatism. Lava ages are consistent with eruption from the AST and flow down the ridge flanks, which is in contrast to previous studies that suggested E-MORB erupted from off-axis vents. At 9º50’N EPR, discrete eruptive units are distinguished by high precision 238U, 232Th, and 226Ra sample concentrations, but because the resolution of the 230Th-226Ra model age dating technique is ~±1 kyrs, the surprisingly young ages of these lavas prohibit the construction of an explicit, time-constrained lava stratigraphy. Nonetheless, seven different flows identified within 0.8-2.0 km west of the AST imply greater frequency of flows to these distances than previously recognized. Model age dating of ferrobasalts, basaltic andesites, andesites, and dacites sampled from the east limb of the overlapping spreading center at 9º03’N EPR is difficult due to uncertainties in magma residence times. However, (226Ra/230Th) disequilibria indicate recent basaltic volcanism (〈〈8 ka) up to ~4 km off-axis. The axial graben at the rise crest sources the most recent volcanic activity and is the dominant location for eruption of high-silica magmas. Major element, trace element, 87Sr/86Sr, and (234U/238U) isotope compositions are consistent with the formation of dacite magmas by extensive crystallization, and 238U-230Th-226Ra systematics imply crustal residence times of ~8 kyrs.

Description: This research was made possible by funding from the Academic Programs Office, from the WHOI travel assistance funds, a Goldschmidt student travel grant, and National Science Foundation grants OCE-0623838, OCE-0527053, and OCE-0137325 to K.W.W.S.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2009.

Description: The radionuclides 231Pa and 230Th, produced in the water column and removed from the ocean by particle scavenging and burial in sediments, offer a means for paleoceanographers to examine past dynamics of both water column and sedimentary processes. I show for the first time that a state of balance exists between 230Th production and burial in the Central Arctic basins, based on measured sedimentary 230Thxs inventories in box cores, establishing this nuclide’s utility as a paleoceanographic indicator of sedimentary processes and as a normalization tool. I present the first 230Th-normalized particle fluxes calculated for the central Arctic: vertical particle fluxes were extremely low during the late glacial, rose during the deglaciation due to particle inputs from shelf inundation, increased productivity and ice-rafted debris, and fell again following the establishment of interglacial conditions. A major event of lateral sediment redistribution, inferred from surplus 230Thxs inventories, occurred in the Makarov Basin during the deglaciation and may have been due to destabilization of slope and shelf sediments as sea level rose. I present the first high-resolution, radiocarbon-dated downcore records of sedimentary 231Pa/230Th from the Arctic Ocean. Low ratios indicate that 231Pa was exported from all sites during the late glacial period, with export decreasing during the deglaciation and Holocene. 231Pa/230Th measurements in cores from three continental slope sites show no evidence for a 231Pa sink related to boundary scavenging on the continental slopes. Holocene 231Pa/230Th ratios show a very significant variation by depth, with strong export of 231Pa at deep sites but little or no export at shallow sites, a result which echoes findings for the South Atlantic and the Pacific. The Arctic thus appears fundamentally similar to other ocean basins in its 231Pa and 230Th dynamics, despite its peculiar qualities of sea ice cover, low particle flux, and relatively isolated deep waters.

Description: My graduate work has been funded by NSF grants OCE-0402565 and OCE- 0550637 to Jerry McManus, ARC-0520073 to Bill Curry, and OCE-0118126 to Daniel McCorkle. My graduate education was also supported by an IODP Schlanger Ocean Drilling Fellowship, WHOI Fellowships from the WHOI Academic Programs Office, and an MIT Presidential Fellowship.

Description: Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution August 2002

Description: 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 F090 olivine phenocrysts, indicating it is primitive mantle melt. High-pressure experiments on a synthetic analogue of the absarokite composition with a H2O content of either -1 7 wt% or -5.1 wt% were cared out in a piston cylinder apparatus. The composition with -1.7 wt% H2O is multiply saturated with olivine and orthopyroxene as liquidus phases at 1.6 GPa and 1400°C. At the same pressure clinopyroxene appears 30°C below the liquidus. With a H2O content of -5.1 wt% composition M.102 is multiply saturated with olivine and orthopyroxene on the liquidus at 1.7 GPa and 1300°C. Assuming batch melting, we suggest that absarokite M.102 segregated from a depleted lherzolite or harburgite 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.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2008

Description: Chapter 1 presents the first published measurements of Sr-isotope variability in olivine-hosted melt inclusions. Melt inclusions in just two Samoan basalt hand samples exhibit most of the total Sr-isotope variability observed in Samoan lavas. Chapter 3 deals with the largest possible scales of mantle heterogeneity, and presents the highest magmatic 3He/4He (33.8 times atmospheric) discovered in Samoa and the southern hemisphere. Along with Samoa, the highest 3He/4He sample from each southern hemisphere high 3He/4He hotspot exhibits lower 143Nd/144Nd ratios than their counterparts in the northern hemisphere. Chapter 2 presents geochemical data for a suite of unusually enriched Samoan lavas. These highly enriched Samoan lavas have the highest 87Sr/86Sr values (0.72163) measured in oceanic hotspot lavas to date, and along with trace element ratios (low Ce/Pb and Nb/U ratios), provide a strong case for ancient recycled sediment in the Samoan mantle. Chapter 4 explores whether the eclogitic and peridotitic portions of ancient subducted oceanic plates can explain the anomalous titanium, tantalum and niobium (TITAN) enrichment in high 3He/4He ocean island basalts (OIBs). The peridotitic portion of ancient subducted plates can contribute high 3He/4He and, after processing in subduction zones, a refractory, rutile-bearing eclogite may contribute the positive TITAN anomalies.

Description: Funding was provided by a National Science Foundation Graduate Research Fellowship, the National Science Foundation grants EAR- 0509891 and EAR-0652707 to Stanley R. Hart, the Woods Hole Oceanographic Institution Academic Programs Office, the Woods Hole Oceanographic Institution Deep Ocean Exploration Institute, the Woods Hole Oceanographic Institution Coastal Ocean Institute, and the Ocean Ventures Fund.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1978

Description: This thesis consists of three papers examining problems related to the crustal structure, isostasy and subsidence history of aseismic ridges and mid-plate island chains. Analysis of gravity and bathymetry data across the Ninetyeast and eastern Walvis Ridges indicates these features are locally compensated by an over thickening of the oceanic crust. Maximum crustal thicknesses are 15-30 km. The western Walvis Ridge is also compensated by crustal thickening; however, the isostasy of this part of the ridge is best explained by a plate model of compensation with elastic plate thicknesses of 5-8 km. These results are consistent with the formation of the Ninetyeast and Walvis Ridges near spreading centers on young lithosphere with flexural rigidities at least an order of magnitude less than those typically determined from flexural studies in older parts of the ocean basins. As the lithosphere cools and thickens, its rigidity increases, explaining the differences in isostasy between aseismic ridges and mid-plate island chains. The long-term subsidence of aseismic ridges and island/ seamount chains can also be explained entirely by lithospheric cooling. Aseismic ridges form near ridge crests and subside at nearly the same rate as normal oceanic crust Mid-plate island chains subside at slower rates because they are built on older crust. However, some island chains have subsided faster than expected based on the age of the surrounding sea floor, probably because of lithospheric thinning over midplate hot spots, like Hawaii. This lithospheric thinning model has major implications both for lithospheric and mantle convection studies as well as the origin of continental rift systems.

Description: Submitted in the partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution August, 1996

Description: Two-thirds of the Earth's surface is oceanic crust formed by magmatic and tectonic processes along mid-ocean ridges. Slow-spreading ridges, such as the Mid-Atlantic Ridge, are discontinuous and composed of ridge segments. Segments are thus fundamental units of magmatic accretion and tectonic deformation that control the evolution of the crust. The objective of this Thesis is to constrain the tectonic processes that occur at the scale of slowspreading segments, to identify the factors controlling segment propagation, and to provide constraints on lithospheric strength with laboratory deformation experiments. In chapter 2, bathymetry and gravity from various areas along the global mid-ocean ridge system are analyzed to quantify systematic variations at the scale of individual segments. There is a marked asymmetry in bathymetry and gravity in the vicinity of segment offsets. We develop a model of faulting to explain these observations. Low-angle faults appear to accommodate tectonic extension at the inside corners of ridge-offset intersections, and result in substantially uplifted terrain with thin crust with respect to that at the outside corners or centers of segments. Results from Chapter 3 indicate that the crust magmatically emplaced on axis is not maintained off-axis. This transition is revealed by both statistical and spectral analyses of bathymetry and gravity. Tectonic extension varies along the length of a segment, resulting in thinning and uplift of the crust at ridge-offset inside corners, and a decorrelation between bathymetry and gravity patterns. Tectonic deformation substantially reshapes the oceanic crust that is magmatically emplaced on-axis, and strongly controls the crustal structure and seafloor morphology off-axis. Satellite gravity data over the Atlantic shown in Chapter 4 reveal a complex history of ridge segmentation, and provides constraints on the processes driving the propagation of segments. The pattern of segmentation is controlled mainly by the geometry of the ridge axis, and secondarily by hot spots. Segments migrate primarily down regional gradients associated with hot spot swells. However, the lack of correlation between gradients and propagation rate, and the propagation up gradient of some offsets, suggest that additional factors control propagation (e.g., variations in lithospheric strength). Most non-transform offsets are short-lived and migrating, while transform offsets are long-lived and stable. Both the propagation of segments (Chapter 4) tectonism along a segment (Chapters 2 and 3) are controlled by the lithospheric rheology. In Chapter 5 I present results from laboratory deformation experiments on serpentinite. These experiments demonstrate that serpentinites are considerably weaker than peridotites or gabbros, display a non-dilatant style of brittle deformation, and strain is accommodated by shear cracking. Serpentinites may weaken the lithosphere, enhance strain localization along faults, and control the style of faulting.

Description: A fellowship from Caixa de Pensions "La Caixa" in Barcelona provided me with all the required financial support to come to WHOI. The work presented in this thesis was also supported by the National Science Foundation grants OCE-90l2576, OCE-930078, OCE-9313812, and Office of Naval Research grant N00014-9l-J-1433.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and Woods Hole Oceanographic Institution June 2004

Description: This study is a geochemical investigation into the accretion of lower oceanic crust and processes of shallow melt-rock reaction at mid-ocean ridges. Major-, trace-elements, and isotopes from whole-rocks and minerals from the Lyngen Gabbro, a 480-My old dismembered ophiolite from the Scandinavian Caledonides, indicate that this igneous complex was produced from hydrous supra-subduction zone magmas, a remnant of an incipient ocean-arc. Such ophiolites are better models for the structural evolution than the geochemical evolution of the lower oceanic crust at mid-ocean ridges. Minerals in gabbros from Atlantis Bank, Southwest Indian Ridge, a modern, insitu example of lower ocean-crust, were analyzed for major and trace-elements. The MELTS algorithm indicates that these gabbros formed by near-fractional crystallzation at mid-crustal pressures. The gabbroic crust is more evolved than the lavas and represents melts fractionated 50-95% relative to a mantle-derived melt-composition, supported by trace-element models. This argues against the often-cited gabbro-glacier accretion model, where mantle-derived melts are transported to a shallow melt-lens and fractionates there before eruption. There remain 〉 770-m of additional primitive cumulates below 1500-m deep Hole 735B or within the underlying mantle. Thus, the seismic Moho, beneath Hole 735B, could be the crust-mantle boundary, rather than an alteration front as suggested elsewhere. The Atlantis Bank gabbros have augites that are more primitive than plagioclases and olivines with which they coexist. Melt-rock interaction, where ascending melts dissolve the pre-existing gabbroic rocks and create hybrid magma may have caused this. Dissolution-experiments for plagioclase-olivine and plagioclase-augite mineral pairs were performed at 1180°-1330°C and 20-min - 24hrs. Dissolution occurs rapidly and out of equilibrium, with the dissolution rates dependent on the ΔT above the solidus. Rocks with small grain-boundary areas (coarse grained or nearly mono-mineralic) heat internally when enclosed in hot magma, causing xenoliths or wall-rock to melt and disaggregate. The dissolution-derived magma crystallizes minerals more refractory-looking than the melts that precipitated the original gabbroic rocks. Assimilation of gabbroic rocks increases the Na content and decreases the Fe content of the melt that digests it, thus basaltic glasses formed after this hybridization will falsely reflect a lower degree and pressure of mantle melting.

Description: This research was supported by the Charles D. Hollister Endowed Fund, the NSF Plutonic Foundation Grant #OCE-9618442 to Henry Dick, and NSF grant #OCE-9907630, SW Indian Ridge to Henry Dick, and by the Academic Programs Office General Fellowship Funds.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Woods Hole Oceanographic Institution and the Massachusetts Institute of Technology October 1979

Description: An investigation was carried out to observe the geologic effects of steady bottom currents on sediments of East Katla Ridge on the southern insular rise of Iceland. Near-bottom southwest to west-flowing currents exceeded 20 cm sec-1 for two weeks over a 25-kilometer wide section of the ridge flank between approximately 1400 and 1800 meters water depth; maximum density and minimum temperature were observed at 1800 meters. Total transport of Iceland-Scotland Overflow Water was calculated to be 5.0 x 106 m3 sec-1; suspended sediment transport is approximately 0.4 x 106 grams sec-1, with a net deposition of 10 to 15 cm/1000 years estimated from the flux difference in and out of the station array. Sediment distribution patterns indicate that the current axis, where flow exceeds approximately 15 cm sec-1, is a site of erosion and winnowing (sand layer formation) while the current margin is a site of rapid accumulation (from observed Holocene rates of 25 to 35 cm per 1000 yr to estimated rates of greater than 100 cm/1000 yr based on 3.5 kHz echo-sounder records). Holocene silty turbidites are locally thick in a sub-marine channel; sandy turbidites and current-winnowed 'sandy contourites' are present in the axis of the major submarine canyon. ‘ Sandy contourite' deposits beneath the axis of the Iceland-Scotland Overflow Current are very poorly sorted muddy sands lacking primary sedimentary structures. Bioturbation is inferred to cause the unique characteristics of these deposits, as well as the absence of fine silt laminae in 'muddy contourites' at the current margin.

Description: Financial support for shipboard operations and most of the post-cruise data analysis was provided by NSF Grant OCE76-Sl49l to Dr. Charles Hollister. Sediment trap and hydrocast operations received partial support under ONR Contract N00014-74-C-0262.

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September, 1975

Description: The Mid-Atlantic Ridge is one of the most well known and yet poorly understood spreading centers in the world. A detailed investigation of the Mid-Atlantic Ridge crest near 37°N (FAMOUS) was conducted using a deeply towed instrument package. The objective was to study the detailed structure and spreading history of the Mid-Atlantic Ridge median valley, to explore the roles of volcanism and faulting in the evolution of oceanic crust, and to study the morphologic expression and structural history of the zone of crustal accretion. In addition, microearthquake surveys were conducted using arrays of free-floating hydrophones. The most recent expression of the accreting plate boundary in the Famous Rift is an alternating series of linear central volcanoes and depressions 1.5 km wide which lie within the inner floor. This lineament is marked by a sharp maximum in crustal magnetization only 2-3 km wide. Magnetic studies indicate that over 90% of the extrusive volcanism occurs within the rift inner floor, a zone 1 to 12 km wide, while volcanism is extremely rare in the rift mountains. Volcanoes created in the inner floor are transported out on, block faults, becoming a lasting part of the topography. Magnetic anomaly transition widths vary from 1 km to 8 km with time and appear to reflect a bi-stable median valley structure. The valley has either a wide inner floor and narrow terraces, in which case the volcanic zone is wide and magnetic anomalies are poorly recorded (wide transition widths); or it has a narrow inner floor and wide terraces, the volcanic zone is then narrow and anomalies are clearly recorded (narrow transition widths). The median valley of any ridge segment varies between these two structures with time. At present the. Famous Rift has a narrow inner floor and volcanic zone (1-3 km) while the south Famous Rift is at the opposite end of the cycle with a wide inner floor and volcanic zone (10-12 km). Over 95% of the large scale (〉2 km) relief of the median valley is accounted for by normal faults dipping toward the valley axis. Normal faulting along fault planes dipping away from the valley begins just past the outer walls of the valley. Outward facing normal faulting accounts for most of the decay of median valley relief in the rift mountains while crustal tilting accounts for less than 20%. The pattern of normal faulting creates a broad, undulating horst and graben relief. Volcanic features contribute little to the large scale relief, but contribute to the short wavelength (〈2km) roughness of the topography. Spreading in the Famous area is highly asymmetric with rates twice as high to the east as to the west. At 1.7 m.y.b.p. the sense of asymmetry reverses in direction with spreading faster to the west, resulting in a gross symmetry when averaged through time. The change in spreading asymmetry occurred in less than 0.15 m.y. Structural studies indicate that the asymmetric spreading is accomplished through asymmetric crustal extension as well as asymmetric crustal accretion. Spreading in the Famous area is 17° oblique. Even on a fine scale there is no indication of readjustment to an orthogonal plate boundary system. Spreading has been stably oblique for at least 6 m.y., even through a change in spreading direction. Magnetic studies reveal that the deep DSDP hole at site 332 was drilled into a magnetic polarity transition, and may have sampled rocks which recorded the earth i s field behavior during a reversal. The presence of negative polarity crust within the Brunhes normal epoch in the inner floor has been determined, and may be due to old crust left behind or recording of a geomagnetic field event. Crustal magnetization decays to lie of its initial value in less than 0.6 m.y. The rapid decay may be facillitated by very intense crustal fracturing observed in the inner floor. Microearthquake, magnetic and structural studies indicate that both the spreading and transform plate boundaries are very narrow (1-2 km) and well-defined for short periods, but migrate over zones 10-20 km wide through time.