Thesis (Ph. D.)--University of Washington, 2005. / Vita. Includes bibliographical references (leaves 100-123).

Thesis (M. S.)--Earth and Atmospheric Sciences, Georgia Institute of Technology, 2008. / Newman, Andrew, Committee Member ; Lowell, Robert, Committee Chair ; Germanovich, Leonid, Committee Member.

Hydrothermal circulation at mid-ocean ridges plays an important role in the interaction between oceanic lithosphere and the overlying ocean. Changes in fluid flux within hydrothermal systems may directly impact ocean circulation, temperature, and chemistry, and hence the lives of biological organisms in hydrothermal vent environments. The permeability structure within a hydrothermal environment is an important control on fluid flow throughout the system. Common suggestions for mechanisms that might increase permeability within the system include thermal cracking due to contraction of the hot rock from interaction with cold seawater, fluid pressure of the water moving through the rock, tectonics, or tidal forces. Additional factors such as mineral precipitation can decrease permeability in the system, further complicating the permeability structure. Though there are many factors to consider within a hydrothermal system, few quantitative studies of these cracking mechanisms exist. This study examines the role of thermal cracking near hydrothermal vents via a numerical model created in Matlab. Flow was modeled using the Laplace equation, and the heat transfer equation was used to determine temperature differences in the rock, which lead to thermal cracking. The numerical results were compared with microearthquakes observed by Tolstoy et al. (2008) near a hydrothermal vent field along the East Pacific Rise. The model suggests that thermal cracking does occur, and this cracking occurs within the area of microearthquakes observed. Though thermal cracking is important for increasing permeability within the system, there are no obvious spatial or temporal trends within the earthquake data that support a direct relation between the modeled thermal cracking and observed earthquakes. It is likely that the observed earthquakes are due to a combination of cracking mechanisms, such as cracking due to fluid pressure, tectonics, or tides, in addition to the modeled thermal stresses.

Thesis (Ph. D.)--Civil and Environmental Engineering, Georgia Institute of Technology, 2008. / Committee Chair: Germanovich, Leonid; Committee Co-Chair: Lowell, Robert; Committee Member: Di Iorio, Daniela; Committee Member: Huang, Haiying; Committee Member: Rix, Glenn; Committee Member: Xu, Wenyue.

Thesis (Ph. D.)--University of Washington, 2008. / Vita. Includes bibliographical references (leaves 131-141).

Seafloor hydrothermal systems play a key role in Earth s energy and geochemical budgets. They also support the existence and development of complex chemosynthetic biological ecosystems that use the mineral-laden fluids as a source of energy and nutrients. This dissertation focuses on two inter-related topics: (1) the heat output at mid-ocean ridge (MOR) hydrothermal sites, and (2) the hydrothermal response to earthquakes at mid-ocean ridges. The response of mid-ocean ridge hydrothermal systems to seismic and magmatic activity could provide a means of using seafloor observations to assess processes occurring at crustal depths. Three decades of study of the ocean floor have made clear that tectonic/volcanic perturbations and changes in the hydrothermal activity are correlated. Yet, this relationship is still highly misunderstood. In this dissertation, we developed a mathematical model of hydrothermal circulation affected by magmatic and/or tectonic events occurring at depth. We showed that the perturbations they generate need not propagate through the entire upflow zone, as previously suggested, but need only affect the movement of the boundary layers. In this case, the fluid residence time in the discharge zone is on the order of years. The determination of heat output at MOR hydrothermal sites provides important constraints on the physics of these processes. Yet, such measurements are still very limited, available only for ~30 sites out of the ~10^3 estimated worldwide. This dissertation reports the first measurements of hydrothermal heat output at 9°50' N on the East Pacific Rise (EPR), which has been the most intensively studied site for the past 20 years (~10^2 expeditions). The values we obtained helped constraining the afore-mentioned mathematical model, which was tested based on the March 1995 microearthquake swarm near 9°50' N, EPR. Our results showed that the current interpretation based on a thermal cracking episode at the bottom of the upflow zone, may be incorrect. We argue that a diking scenario better explains the seismic and temperature data, while being consistent with the mechanics of inflation of the axial magma chamber in the 9°50' N, EPR area between two major eruptions, in 1991 and 2006.

Thesis (MTech (Oceanography))Cape Peninsula University of Technology, 2010 / The current dynamics along the Tsitsikamma coast is described from a combination of acoustic current measurements. satellite-tracked surface drifters and underwater temperature recordings made between November 2006 and March 2008. The Tsitsikamma coast is largely a Marine Protected Area (MPA) that protects a rich marine biodiversity. The nearshore currents are important in the dispersal of eggs and larvae of many marine species. including the paralarvae of the commercially caught chokka squid. LoNgo reynaudii. Changes in the environment, including the currents. can affect the successful recruitment of chokka squid, and can bring about large annual fluctuations in biomass that creates economic uncertainty in the squid fishery. Results confirm the existence of a predominantly alongshore current off the Tsitsikamma coast. At Middelbank eastward flow was slightly dominant, with a percentage occurrence of 58% vs. 41% westward flow near the surface. The percentage eastward flow decreased with depth, with 41% vs. 58% westward flow near the seabed. At Thyspunt westward and eastward flow occurred at near equal percentages. but westward flow was slightly dominant throughout the water column. The alongshore current was strongest near the surface during eastward flow (maximum = 141 crn.s1: average = 27 crn.s '). while westward surface currents were weaker (maximum velocity = 78 cm.s1: average = 19 crn.s1). Current speed generally decreased with depth and opposing surface and bottom currents, associated with a thermal stratified water column, were occasionally recorded. The nearshore flow regime was characterised by frequent barotropic alongshore reversals that occurred year round. An increase in strong eastward episodes, and opposing surface and bottom currents during spring and summer months have implications for the dispersal of squid paralarvae during the summer and winter spawning seasons. In summer, the combination of strong eastward pulses in the current and upwelling at the capes favoured dispersal onto the midshelf of the Agulhas Bank. In winter, alongshore oscillations without the offshore displacement associated with upwelling. restricted offshore dispersal which caused surface particles to be retained inshore. Drifter trajectories show that both the eastward and westward nearshore current can link the inshore spawning grounds with the nursery grounds, offshore on the central Agulhas Bank; and that passive, neutrally buoyant material in the surface layer can reach the vicinity of the cold ridge in as little as eight days. The wind-driven processes of upwelling and coastal trapped waves (CTWs). and the influence of the greater shelf circulation are discussed as possible driving forces of variability in the currents off the Tsitsikamma coast. The occurrence of coastal trapped waves during thermal stratification appears to drive the jet-like, eastward pulses in the current. and results suggest that the propagation of CTWs may regulate and even enhance upwelling and downwelling along the Tsitsikamma coast.

Thesis (Ph. D.)--Joint Program in Oceanography/Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2010. / Cataloged from PDF version of thesis. / Includes bibliographical references. / 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 (C02) 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 backare 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. / by Eoghan Reeves. / Ph.D.

Cette thèse présente une étude détaillée de la composition des fluides de haute température du champ hydrothermal de Lucky Strike (37°N, dorsale médio atlantique) s'appuyant sur 3 campagnes d'échantillonnage réalisées dans le cadre de l'observatoire fond de mer EMSO-Açores. Ce champ hydrothermal s'est développé autour d'un lac de lave fossile bordé au Nord-Ouest, Nord Est et Sud Est par des cônes volcaniques plus anciens. En 2013, la découverte du site hydrothermal de Capelinhos à l'Est, et présentant des compositions de fluides inédites à Lucky Strike (concentration faible en Cl et forte en Fe et Mn), nous permet de proposer un nouveau modèle de circulation hydrothermale basé sur l'application de géothermobaromètres (Si ; Si-Cl ; Fe-Mn) chimiques sur 13 évents hydrothermaux. Nous avons défini 5 groupes de sites selon leur chlorinité et leur position par rapport au lac de lave. Les fluides de Capelinhos, dominés par la phase vapeur, sont rapidement extraits de la zone de séparation de phase (estimée à 2600 m sous le plancher océanique). Les fluides à proximité du lac de lave, avec des chlorinités variables, suggèrent un rééquilibrage à des pressions et températures plus faibles qu'à Capelinhos, cohérent avec des processus de refroidissement conductif et/ou d'entrainement de saumures prenant place lors de la remontée des fluides jusqu'à la base de la couche 2A. La fluctuation de la chlorinité témoigne de la variabilité du temps de résidence du fluide dans la zone de remontée, et ses relations avec les caractéristiques physiques de la croûte océanique. Nous avons évalué la variabilité temporelle de la composition des fluides collectés au cours des campagnes effectuées entre 2009 et 2015. Deux échelles de temps sont mises en évidence. (1) l'échelle de l'échantillonnage, i.e. de l'heure, répond à des phénomènes de subsurface, et révèle qu'un fluide hydrothermal refroidi conductivement (T<150°C) est stocké dans la roche poreuse entourant le site de décharge. (2) l'échelle de temps pluri-annuelle montre une fluctuation des conditions de P et T apparentes de la zone de séparation de phase et du degré d'altération du substratum dans la zone de réaction. Les variations intersites du rapport Ca/Na (indicateur du degré d'albitisation) sont dues à la séparation de phase, à l'exception des sites du Sud-Est du lac de lave qui indiquent un degré d'altération plus élevé. La concentration en Li et sa composition isotopique indiquent que le basalte du substratum est relativement frais avec des rapports W/R proches de 1 pour tous les groupes, avec des d7Li du fluide identiques au substratum. Pour le Sr nous calculons des rapports W/R plus élevés, autour de 7-8, qui sont dus au Sr de l'eau de mer initiale qui est partiellement stocké et à la formation de minéraux secondaires(l'albite et l'anhydrite) lors des interactions eau-roche dans le faciès schiste vert. La faible teneur en métaux des fluides situés autour du lac de lave est probablement due à un piégeage en subsurface, équivalent à ~65% du Fe mobilisé dans la zone de réaction (basé sur les teneurs des fluides de Capelinhos). La variabilité de chlorinité des fluides de Lucky Strike offre l'opportunité d'étudier le comportement des terres rares à l'échelle d'un site hydrothermal. Ainsi, nous montrons l'effet de la séparation de phase sur les terres rares légères et lourdes ainsi que le lien entre l'Eu et le cycle géochimique du Sr. Les terres rares dissoutes dans le panache hydrothermal montrent une perte au cours du mélange ainsi que des phénomènes de redissolution visible via la signature isotopique en Nd. Cette modification des compositions isotopiques en Nd de l'eau de mer profonde est similaire au " boundary exchange " observé aux interfaces océan-plateforme continentale. Au vue de la répartition des champs hydrothermaux sur les dorsales océaniques, ce phénomène pourrait avoir un impact sur le bilan océanique du Nd et donner lieu à un phénomène de " ridge exchange ". / This thesis present a detailed study of the composition of high temperature fluid from the Lucky Strike hydrothermal field (37°N, Mid Ocean Ridge) collected during three sampling campaigns within the framework of the deep sea observatory EMSO-Azores. The hydrothermal field has developped around a fossil lava lake framed by three ancient volcanic cones. In 2013, the discovery of a new active site to the East of the system, and presenting an unprecedented fluid composition at Lucky Strike (low Cl concentration and high Fe and Mn concentration), lead to a new model of hydrothermal circulation based on chemical geothermobarometer (Si; Si-Cl) and geothermometer (Fe-Mn) applied to 13 venting sites. We defined 5 groups of sites based on their chlorinity and location around the lava lake. It appears that vapor-dominated Capelinhos fluids were extracted relatively fast from the phase separation zone (estimated at ~2600mbsf). Nevertheless, fluids in the vicinity of the lava lake, both vapor and brine dominated, display P and T conditions of equilibration lower than for Capelinhos fluids. This highlights on-going equilibration process through conductive cooling and/or brine entrainment in the upflow zone up to the layer 2A of the oceanic crust. Chlorinity variations highlight the varying residence time in the upflow of the fluids between vents which depends on physical characteristics of the crust. We studied the temporal variability of fluid composition collected between 2009 and 2015. Two time scales have been evidenced. The first is the sampling scale, i.e. ~1h, and corresponds to subsurface processes indicating that a hydrothermal fluid, conductively cooled (T<150°C), was stored in the porous substratum close to the discharge. The second is at the scale of the year. It shows fluctuations of P and T conditions in the phase separation and different degree of alteration of the substratum in the reaction zone. Intersites variations of Ca/Na ratios (proxies for albitisation) are related to phase separation expected the South Eastern sites that display a more altered substratum. To avoid this issue, we use Li and Sr isotopes which are not affected phase separation. Li concentration and isotopic composition indicates that basalt substratum is relatively fresh with W/R ratio close to 1 calculated for all groups with d7Li of fluid equivalent to substratum. Sr concentration and isotopic composition suggest higher W/R ratio (~7-8) because of seawater Sr partially removed in the recharge. Moreover, other parameters are at play such as secondary mineral formation (albite, anhydrite) during water rock interaction in the greenschist facies. Because the basalt is relatively fresh, the low metal content in the fluid around the lava lake is due to storage, in the subsurface, of approximately ~60-70% of Fe that is mobilized in the reaction zone compared to Fe-Mn rich Capelinhos fluids. Furthermore, the Cl variability from the fluids at Lucky Strike brings a unique opportunity to study the REE distribution from the reaction zone to the discharge into the deep ocean. We show that the LREE are preferentially concentrated into the brine phase. Furthermore, the Eu is linked to the Sr geochemical cycle. Dissolved REE from buoyant plume fluids highlight a scavenging effect. The Nd isotopic compositions indicate redissolution process. This Nd isotopes modification of the deep seawater is similar to the process of "boundary exchange" that occurs at the ocean/continents interface. Considering the global distribution of submarine hydrothermalism, the Nd modification at the ridge could have an impact on the global Nd cycle in the oceans and act as a "ridge exchange".

In order to explain the observed time-dependent salinity variations in seafloor hydrothermal vent fluids, quasi-numerical and fully numerical fluid flow models of the NaCl-H2O system are constructed. For the quasi-numerical model, a simplified treatment of phase separation of seawater near an igneous dike is employed to obtain rough estimates of the thickness and duration of the two-phase zone, the amount of brine formed, and its distribution in the subsurface. For the fully numerical model, the equations governing fluid flow, the thermodynamic relations between various quantities employed, and the coupling of these elements together in a time marching scheme is discussed. The fully numerical model is benchmarked against previously published heat pipe and Elder problem simulation results, and is shown to be largely in agreement with those results. A number of simulation results are presented in the context of two-phase flow and phase separation within the framework of the single pass model. It is found that a quasi-stable two-phase (liquid + vapor) zone at depth below the hydrothermal discharge outlet gives rise to vent fluid with lower than normal seawater salinity. Additionally, it is shown that increasing the spatial extent of the two-phase zone can lower vent fluid salinity. The numerical approach used in this thesis is able to generate salinity patterns predicted by a widely held conceptual model of vent fluid salinity variation, and may be able to explain the vent fluid salinities and temperatures found at the Main Endeavour Vent Field on the Juan de Fuca Ridge, as this approach is able to produce simulated vent fluid salinities that match observed values from the Endeavour Field vents Dante and Hulk.