ALBERT

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 2018

Description: Many chemical constituents are removed from the ocean by attachment to settling particles, a process referred to as “scavenging.” Radioisotopes of thorium, a highly particle-reactive element, have been used extensively to study scavenging in the ocean. However, this process is complicated by the highly variable chemical composition and concentration of particles in oceanic waters. This thesis focuses on understanding the cycling of thorium as affected by particle concentration and particle composition in the North Atlantic. This objective is addressed using (i) the distributions 228,230,234Th, their radioactive parents, particle composition, and bulk particle concentration, as measured or estimated along the GEOTRACES North Atlantic Transect (GA03) and (ii) a model for the reversible exchange of thorium with particles. Model parameters are either estimated by inversion (chapter 2-4), or prescribed in order to simulate 230Th in a circulation model (chapter 5). The major findings of this thesis follow. In chapters 2 and 3, I find that the rate parameters of the reversible exchange model show systematic variations along GA03. In particular, 𝑘1, the apparent first-order rate "constant" of Th adsorption onto particles, generally presents maxima in the mesopelagic zone and minima below. A positive correlation between 𝑘1 and bulk particle concentration is found, consistent with the notion that the specific rate at which a metal in solution attaches to particles increases with the number of surface sites available for adsorption. In chapter 4, I show that Mn (oxyhydr)oxides and biogenic particles most strongly influence 𝑘1 west of the Mauritanian upwelling, but that biogenic particles dominate 𝑘1 in this region. In chapter 5, I find that dissolved 230Th data are best represented by a model that assumes enhanced values of 𝑘1 near the seafloor. Collectively, my findings suggest that spatial variations in Th radioisotope activities observed in the North Atlantic reflect at least partly variations in the rate at which Th is removed from the water column.

Description: This work was supported by the US National Science Foundation. Two US NSF grants have supported the research in this thesis (OCE-1232578 and OCE-155644).

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 2017

Description: This thesis documents the origin, distribution, and fate of methane and several of its isotopic forms on Earth. Using observational, experimental, and theoretical approaches, I illustrate how the relative abundances of 12CH4, 13CH4, 12CH3D, and 13CH3D record the formation, transport, and breakdown of methane in selected settings. Chapter 2 reports precise determinations of 13CH3D, a “clumped” isotopologue of methane, in samples collected from various settings representing many of the major sources and reservoirs of methane on Earth. The results show that the information encoded by the abundance of 13CH3D enables differentiation of methane generated by microbial, thermogenic, and abiogenic processes. A strong correlation between clumped- and hydrogen-isotope signatures in microbial methane is identified and quantitatively linked to the availability of H2 and the reversibility of microbially-mediated methanogenesis in the environment. Determination of 13CH3D in combination with hydrogen-isotope ratios of methane and water provides a sensitive indicator of the extent of C–H bond equilibration, enables fingerprinting of methane-generating mechanisms, and in some cases, supplies direct constraints for locating the waters from which migrated gases were sourced. Chapter 3 applies this concept to constrain the origin of methane in hydrothermal fluids from sediment-poor vent fields hosted in mafic and ultramafic rocks on slow- and ultraslow-spreading mid-ocean ridges. The data support a hypogene model whereby methane forms abiotically within plutonic rocks of the oceanic crust at temperatures above ca. 300 C during respeciation of magmatic volatiles, and is subsequently extracted during active, convective hydrothermal circulation. Chapter 4 presents the results of culture experiments in which methane is oxidized in the presence of O2 by the bacterium Methylococcus capsulatus strain Bath. The results show that the clumped isotopologue abundances of partially-oxidized methane can be predicted from knowledge of 13C/12C and D/H isotope fractionation factors alone.

Description: The research activities documented in this thesis were made possible by grants to my advisor from the U.S. National Science Foundation (NSF award EAR-1250394), the National Aeronautics and Space Administration (NASA) Astrobiology Institute (NAI, University of Colorado, Boulder, CAN 7 under Cooperative Agreement NNA15BB02A), the Department of Energy (DOE, Small Business Innovation Research program, contract DE-SC0004575), the Alfred P. Sloan Foundation via the Deep Carbon Observatory, and a Shell Graduate Fellowship through the MIT Energy Initiative. I completed the bulk of the work in this thesis while being supported by a National Defense Science and Engineering Graduate (NDSEG) Fellowship awarded through the Office of Naval Research of the U.S. Department of Defense. The StanleyW.Watson Fellowship Fund provided support during my first summer term at WHOI.The Charles M. Vest Presidential Fellowship at MIT supported me in the first year of my Ph.D. studies. I received additional support that year through NSF award EAR-1159318 (to S. Ono and T. Bosak) and theWalter & Adel Hohenstein Graduate Fellowship of Phi Kappa Phi. The MIT Earth Resources Laboratory and PAOC Houghton Fund funded my attendance at several conferences.

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 2016

Description: Eddies in the ocean move westwards. Those shed by western boundary currents must then interact with continental shelf-slope topography at the western boundary. The presence of other eddies and mean lows complicates this simple picture, yet satellite images show that mesoscale eddies translating near the shelfbreak routinely affect the continental shelves of the Mid-Atlantic Bight, the Gulf of Mexico etc. The consequent cross-shelfbreak transports are currently of unknown importance to shelf budgets of heat, salt and volume. Thus motivated, this thesis uses idealized continuously stratified numerical experiments to explore eddy-slope interactions under four questions: 1. Can the continental slope prevent an eddy from reaching the shelfbreak? 2. What is the structure of the eddy-driven offshore low? 3. How is the continental shelf affected by an eddy at the shelfbreak? 4. Given surface observations, can one estimate the volume of water transported across the shelfbreak? The experiments show that the efficiency of Rossby wave radiation from the eddy controls whether it can cross isobaths: by radiating energy the eddy becomes shallow enough to move into shallower water. For wide continental slopes, relative to an eddy diameter, a slope can prevent an anticyclone from reaching the shelfbreak by shutting down such radiation. For narrow continental slopes, the interaction repeatedly produces dipoles, whose cyclonic halves contain shelf-slope water stacked over eddy water. The formation of such cyclones is explained. Then, the structure of shelf lows forced by the eddy are studied: their vertical structures are rationalized and scalings derived for their cross-isobath scales; for example, the extent to which the eddy influences the shelf. A recipe for estimating cross-isobath transports based on eddy surface properties is put forward. Finally, the findings are tested against observations in the Middle Atlantic Bight of the northeastern United States.

Description: I acknowledge high-performance computing support from Yellowstone (ark:/85065/d7wd3xhc) provided by NCAR’s Computational and Information Systems Laboratory, sponsored by the National Science Foundation. The research presented here was funded by National Science Foundation grants OCE-1059632 and OCE-1433953. Funding support from the Academic Programs Oice, WHOI is also gratefully acknowledged.

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 2018

Description: Highly migratory marine fishes support valuable commercial fisheries worldwide. Yet, many target species have proven difficult to study due to long-distance migrations and regular deep diving. Despite the dominance of oceanographic features, such as fronts and eddies, in the open ocean, the biophysical interactions occurring at the oceanic (sub)mesoscale (〈 100 km) remain poorly understood. This leads to a paucity of knowledge on oceanographic associations of pelagic fishes and hinders management efforts. With ever-improving oceanographic datasets and modeling outputs, we can leverage these tools both to derive better estimates of animal movements and to quantify fish-environment interactions. In this thesis, I developed analytical tools to characterize the biophysical interactions influencing animal behavior and species’ ecology in the open ocean. A novel, observation-based likelihood framework was combined with a Bayesian state-space model to improve geolocation estimates for archival-tagged fishes using oceanographic profile data. Using this approach, I constructed track estimates for a large basking shark tag dataset using a high-resolution oceanographic model and discovered a wide range of movement strategies. I also applied this modeling approach to track archival-tagged swordfish, which revealed affinity for thermal front and eddy habitats throughout the North Atlantic that was further corroborated by synthesizing these results with a fisheries-dependent conventional tag dataset. An additive modeling approach applied to longline catch-per-unit effort data further highlighted the biophysical interactions that characterize variability in swordfish catch. In the final chapter, I designed a synergistic analysis of high-resolution, 3D shark movements and satellite observations to quantify the influence of mesoscale oceanography on blue shark movements and behavior. This work demonstrated the importance of eddies in structuring the pelagic ocean by influencing the movements of an apex predator and governing the connectivity between deep scattering layer communities and deep-diving, epipelagic predators. Together, these studies demonstrate the breadth and depth of information that can be garnered through the integration of traditional animal tagging and oceanographic research with cutting-edge analytical approaches and high-resolution oceanographic model and remote sensing datasets, the product of which provides a transformative view of the biophysical interactions occurring in and governing the structure of the pelagic ocean.

Description: Supported by the NASA Earth and Space Science Fellowship, the MIT John S. Hennessy Fellowship, the MIT Martin Family Society of Fellows for Sustainability Fellowship, the WHOI Ocean Venture, Grassle, and James Stratton Fellowships and the WHOI Academic Programs Office. This research and its dissemination was supported by funds from National Geographic, Amazon Web Services, the Explorers Club, Rolex, Sigma Xi, the MIT Center for International Studies, WHOI Access to the Sea and Coastal Ocean Institute Funds, MIT Graduate Student Council, MIT Student Assistance Fund, WHOI Biology Department, American Fisheries Society, WHOI Academic Programs Office

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Oceanography and Applied Ocean Science and Engineering at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 2019.

Description: Submesoscale currents, with horizontal length scales of 1-20 km, are an important element of upper ocean dynamics. These currents play a crucial role in the horizontal and vertical redistribution of tracers, the cascade of tracer variance to smaller scales, and in linking the mesoscale circulation with the dissipative scales. This thesis investigates submesoscale flows and their properties using Lagrangian trajectories of observed and modeled drifters. We analyze statistics of observed drifter pairs to characterize turbulent dispersion at submeso-scales. Contrary to theoretical expectations, we find that nonlocal velocity gradients associated with mesoscale eddies dominate the separation of drifters even at the kilometer scale. At submeso-scales, we observe energetic motions, such as near-inertial oscillations, that contribute to the energy spectrum but are inefficient at dispersion. Using trajectories in a model of submesoscale turbulence, we find that, if drifters have a vertical separation, vertical shear dominates the dispersion and conceals horizontal dispersion regimes from drifter observations. Particularly in submesoscale flows, vertical shear is orders of magnitude larger than horizontal gradients in velocity. Since conventional drifters in the ocean are not affected by vertical shear, it is likely that drifter-derived diffusivity underestimates the diffusivity that a tracer would experience. Lastly, we test and apply cluster-based methods, using three or more drifters, to estimate the velocity gradient tensor. Since velocity gradients become large at submesoscales, the divergence, strain, and vorticity control the evolution and deformation of clusters of drifters. Observing the velocity gradients using drifters, enables us to further constrain the governing dynamics and decipher submesoscale motions from inertia-gravity waves. These insights provide a Lagrangian perspective on submesoscale flows that illuminates scales that are challenging to observe from other platforms. We reveal observational and theoretical challenges that need to be overcome in future investigations.

Description: My doctoral studies in the WHOI/MIT Joint Program were funded by the National Science Foundation (OCE-I434788) and the Office of Naval Research (N00014-13-1-0451, Grant N00014-16-1-2470).

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 2017

Description: Salt marshes are physically, chemically, and biologically dynamic environments found globally at temperate latitudes. Tidal creeks and marshtop ponds may expand at the expense of productive grass-covered marsh platform. It is therefore important to understand the present magnitude and drivers of production and respiration in these submerged environments in order to evaluate the future role of salt marshes as a carbon sink. This thesis describes new methods to apply the triple oxygen isotope tracer of photosynthetic production in a salt marsh. Additionally, noble gases are applied to constrain air-water exchange processes which affect metabolism tracers. These stable, natural abundance tracers complement traditional techniques for measuring metabolism. In particular, they highlight the potential importance of daytime oxygen sinks besides aerobic respiration, such as rising bubbles. In tidal creeks, increasing nutrients may increase both production and respiration, without any apparent change in the net metabolism. In ponds, daytime production and respiration are also tightly coupled, but there is high background respiration regardless of changes in daytime production. Both tidal creeks and ponds have higher respiration rates and lower production rates than the marsh platform, suggesting that expansion of these submerged environments could limit the ability of salt marshes to sequester carbon.

Description: Financial support for my doctoral research was provided by the United States Department of Defense through the National Defense Science and Engineering Graduate Fellowship Program, the National Science Foundation under grant OCE-1233678, and the Woods Hole Oceanographic Institution (WHOI) under grants from the WHOI Coastal Ocean Institute, Ocean and Climate Change Institute, and Ocean Life Institute. WHOI Academic Programs Office also provided funding support for research, through the Ocean Ventures Fund, and for my stipend, as graduate research assistantships including an assistantship from the United States Geological Survey administered by WHOI.