Seismic and gravitational studies of melting in the mantle's thermal boundary layers

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 2007This thesis presents three studies which apply geophysical tools to the task of better understanding mantle melting phenomena at the upper and lower boundaries of the mantle. The first study uses seafloor bathymetry and small variations in the gravitational acceleration over the Hawaii-Emperor seamount chain to constrain the changes in the igneous production of the hot spot melting in the mantle which has created these structures over the past 80 My. The second study uses multichannel seismic reflection data to constrain the location and depth of axial magma chambers at the Endeavour Segment of the Juan de Fuca spreading ridge, and then correlates these magma chamber locations with features of the hydrothermal heat extraction system in the upper crust such as microseismicity caused by thermal cracking and high temperature hydrothermal vent systems observed on the seafloor. The third study uses two-dimensional global pseudospectral seismic wave propagation modeling to characterize the sensitivity of the SPdKS seismic phase to two-dimensional, finite-width ultra-low velocity zones (ULVZs) at the core-mantle boundary. Together these three studies highlight the dynamic complexities of melting in the mantle while offering new tools to understand that complexity.This thesis was funded by a National Science Foundation Graduate Research Fellowship, NSF grant OCE-0002551 to theWoods Hole Oceanographic Institution (WHOI), the WHOI Academic Programs Office, the Earth, Atmospheric, and Planetary Science Department at MIT, and by the WHOI Deep Ocean Exploration Institute

Time variation in igneous volume flux of the Hawaii-Emperor hot spot seamount chain

Author Posting. © American Geophysical Union, 2004. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 109 (2004): B11401, doi:10.1029/2003JB002949.Satellite gravity, ship track bathymetry, sediment thickness, and crustal magnetic age data were combined to calculate the residual bathymetry and residual mantle Bouguer gravity anomaly (RMBA) for the northwest Pacific Ocean. The Hawaii-Emperor hot spot track appears on the RMBA map as a chain of negative anomalies, implying thickened crust or less dense mantle. The hot spot swell is clearly visible in a broad band of half-width ∼500 km for about 2000 km downstream from the current hot spot location, corresponding to hot spot ages of 0–25 Ma. A much narrower expression of the hot spot is visible for the rest of the chain at hot spot ages of 25–80 Ma. Comparison of the observed RMBA with various compensation models reveals that the relatively narrow features of the Hawaii-Emperor seamounts are best explained as being supported by plate flexure, while the Shatsky Rise, Hess Rise, and Mid-Pacific Mountains oceanic plateaus are best fit by Airy isostasy with a thickened crustal root. Amplitude comparisons between the RMBA predictions of various compensation models and the observed RMBA for the Hawaiian swell are ambiguous. However, on the basis of the shape of the predicted anomalies, we favor a model of flexure in response to a buried load at 120 km depth. We further calculate igneous (i.e., crustal) volume flux along the axis of the Hawaii-Emperor hot spot by integrating cross-sectional areas of gravity-derived excess crustal thickness and seafloor elevation, respectively, with respect to the normal oceanic crust. The highest values of the calculated igneous volume flux along the Hawaiian and Emperor ridges (∼8 m3/s) occur at present and at about 20 Ma. The flux was reduced to only 50% of this maximum (∼4 m3/s) at 10 Ma. The calculated igneous volume flux is systematically smaller (maximum values of ∼4 m3/s) along the Emperor ridge. Overall, the Hawaiian and Emperor ridges appear to have experienced quasi-periodic variations in fluxes on timescales of 6–30 Ma. Furthermore, during the low-flux periods at 25–48, 57, and 75 Ma the height and size of individual hot spot seamounts appear to be noticeably less than those of the high-flux periods. We hypothesize that the variations in the fluxes of the Hawaiian ridge might be controlled by the thickness of the overlying lithosphere at the time of hot spot emplacement, while the variations along the Emperor ridge may be influenced by the dynamics of the slow absolute motion of the hot spot at the time.E. Van Ark was supported by a National Science Foundation Graduate Research Fellowship, and J. Lin was supported by NSF grant OCE-0129741 and the Andrew W. Mellon Foundation Endowed Fund for Innovative Research at WHOI

Upper crustal structure and axial topography at intermediate spreading ridges : seismic constraints from the southern Juan de Fuca Ridge

Author Posting. © American Geophysical Union, 2005. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 110 (2005): B12104, doi:10.1029/2005JB003630.We use multichannel seismic reflection data to image the upper crustal structure of 0-620 ka crust along the southern Juan de Fuca Ridge (JdFR). The study area comprises two segments spreading at intermediate rate with an axial high morphology with narrow (Cleft) and wide (Vance) axial summit grabens (ASG). Along most of the axis of both segments we image the top of an axial magma chamber (AMC). The AMC along Cleft deepens from south to north, from 2.0 km beneath the RIDGE Cleft Observatory and hydrothermal vents near the southern end of the segment, to 2.3 km at the northern end near the site of the 1980’s eruptive event. Along the Vance segment, the AMC also deepens from south to north, from 2.4 km to 2.7 km. Seismic layer 2A, interpreted as the basaltic extrusive layer, is 250-300 m thick at the ridge axis along the Cleft segment, and 300-350 m thick along the axis of the Vance segment. However off-axis layer 2A is similar in both segments (500-600 m), indicating ~90% and ~60% off-axis thickening at the Cleft and Vance segments, respectively. Half of the thickening occurs sharply at the walls of the ASG, with the remaining thickening occurring within 3-4 km of the ASG. Along the full length of both segments, layer 2A is thinner within the ASG, compared to the ridge flanks. Previous studies argued that the ASG is a cyclic feature formed by alternating periods of magmatism and tectonic extension. Our observations agree with the evolving nature of the ASG. However, we suggest that its evolution is related to large changes in axial morphology produced by small fluctuations in magma supply. Thus the ASG, rather than being formed by excess volcanism, is a rifted flexural axial high. The changes in axial morphology affect the distribution of lava flows along the ridge flanks, as indicated by the pattern of layer 2A thickness. The fluctuations in magma supply may occur at all spreading rates, but its effects on crustal structure and axial morphology are most pronounced along intermediate spreading rate ridges.This study was supported by the National Science Foundation grants OCE-0002551 to Woods Hole Oceanographic Institution, OCE-0002488 to Lamont-Doherty Earth Observatory, and OCE-0002600 to Scripps Institution of Oceanography

SKS and SPdKS sensitivity to two-dimensional ultralow-velocity zones

Seismic wave propagation through two-dimensional core-mantle boundary (CMB) ultralow-velocity zones (ULVZs) is modeled using a global pseudospectral algorithm. Seismograms are synthesized for several types of ULVZ models to investigate the effect of these structures on SKS and SPdKS phases. One-dimensional models and two-dimensional models with different, quasi-1-D velocity structures on the source and receiver sides of the CMB provide a baseline for comparison with other 2-D models. Models with finite length ULVZs are used to test the sensitivity of the SPdKS travel time and waveform to different portions of the P diffracted wave path. This test shows that SPdKS waves are only sensitive to ULVZs with lengths >100 km. Our results give three tools for identifying and characterizing 2-D ULVZ structures. First, dual SPdKS pulses indicate exposure to at least two separate CMB velocity structures, either different source and receiver-side CMB velocities or different adjacent velocity regions for which Pdiff inception occurs outside of and propagates into a ULVZ. Second, high-amplitude SKS precursors indicate a very “strong” (i.e., thick and/or large velocity perturbations) ULVZ. Hence, the absence of SKS precursors in most previous ULVZ studies indicates that very strong, sharp ULVZs are not common. Third, mean SPdKS delays relative to PREM constrain the minimum ULVZ strength and length combinations required to produce a given travel time delay.National Science Foundation (U.S.) (EAR 02-29586)National Science Foundation (U.S.) (EAR 07-38492)Massachusetts Institute of Technology. Kerr-McGee Development ChairWoods Hole Oceanographic Institution. Deep Ocean Exploration Institut

102 - Emily Kay Fischer

Includes bibliographical references.Atherosclerotic cardiovascular disease (CVD) is the leading cause of death worldwide. Endothelial dysfunction is a major initiating step in the pathophysiology of CVD. Red beetroot juice (RBJ) contains bioactive compounds including phenolic acids, flavonoids, betalains, ascorbic acid, and nitrate. Previous research suggests RBJ it can improve endothelial function. In a randomized, double-blind, placebo-controlled crossover clinical trial, 16 healthy, overweight/obese men and postmenopausal women aged 40-65 years are consuming RBJ, nitrate-free RBJ, placebo + nitrate, or placebo for 4 weeks. Endothelial function is assessed at baseline and 4 weeks. RBJ is anticipated to exert the greatest effects on endothelial function

Identification of acute respiratory distress syndrome subphenotypes de novo using routine clinical data: A retrospective analysis of ARDS clinical trials

Objectives The acute respiratory distress syndrome (ARDS) is a heterogeneous condition, and identification of subphenotypes may help in better risk stratification. Our study objective is to identify ARDS subphenotypes using new simpler methodology and readily available clinical variables. Setting This is a retrospective Cohort Study of ARDS trials. Data from the US ARDSNet trials and from the international ART trial. Participants 3763 patients from ARDSNet data sets and 1010 patients from the ART data set. Primary and secondary outcome measures The primary outcome was 60-day or 28-day mortality, depending on what was reported in the original trial. K-means cluster analysis was performed to identify subgroups. Sets of candidate variables were tested to assess their ability to produce different probabilities for mortality in each cluster. Clusters were compared with biomarker data, allowing identification of subphenotypes. Results Data from 4773 patients were analysed. Two subphenotypes (A and B) resulted in optimal separation in the final model, which included nine routinely collected clinical variables, namely heart rate, mean arterial pressure, respiratory rate, bilirubin, bicarbonate, creatinine, PaO 2, arterial pH and FiO 2. Participants in subphenotype B showed increased levels of proinflammatory markers, had consistently higher mortality, lower number of ventilator-free days at day 28 and longer duration of ventilation compared with patients in the subphenotype A. Conclusions Routinely available clinical data can successfully identify two distinct subphenotypes in adult ARDS patients. This work may facilitate implementation of precision therapy in ARDS clinical trials