ALBERT

Description: Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Physical Oceanography at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution February 2019.

Description: Each year, surface ocean ecosystems export sinking particles containing gigatons of carbon into the ocean’s interior. This particle flux connects the entire ocean microbiome and constitutes a fundamental aspect of marine microbial ecology and biogeochemical cycles. Particle flux is also variable and intricately complex, impeding its mechanistic or quantitative description. In this thesis we pair compilations of available data with novel mathematical models to explore the relationships between particle flux and other key variables – temperature, net primary production, and depth. Particular use is made of (probability) distributional descriptions of quantities that are known to vary appreciably. First, using established thermodynamic dependencies for primary production and respiration, a simple mechanistic model is developed relating export efficiency (i.e. the fraction of primary production that is exported out of the surface ocean via particle flux) to temperature. The model accounts for the observed variability in export efficiency due to temperature without idealizing out the remaining variability that evinces particle flux’s complexity. This model is then used to estimate the metabolically-driven change in average export efficiency over the era of long-term global sea surface temperature records, and it is shown that the underlying mechanism may help explain glacial-interglacial atmospheric carbon dioxide drawdown. The relationship between particle flux and net primary production is then explored. Given that these are inextricable but highly variable and measured on different effective scales, it is hypothesized that a quantitative relationship emerges between collections of the two measurements – i.e. that they can be related not measurement-by-measurement but rather via their probability distributions. It is shown that on large spatial or temporal scales both are consistent with lognormal distributions, as expected if each is considered as the collective result of many subprocesses. A relationship is then derived between the log-moments of their distributions and agreement is found between independent estimates of this relationship, suggesting that upper ocean particle flux is predictable from net primary production on large spatiotemporal scales. Finally, the attenuation of particle flux with depth is explored. It is shown that while several particle flux-versus-depth models capture observations equivalently, these carry very different implications mechanistically and for magnitudes of export out of the surface ocean. A model is then proposed for this relationship that accounts for measurements of both the flux profile and of the settling velocity distribution of particulate matter, and is thus more consistent with and constrained by empirical knowledge. Possible future applications of these models are discussed, as well as how they could be tested and/or constrained observationally.

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 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 June, 1982

Description: Oceanic fluctuations are dependent on geographical location. Near intense currents, the eddy field is highly energetic and has broad meridional extent. It is likely that the energy arises from instabilities of the intense current. However, the meridional extent of the linearly most unstable modes of such intense jets is much narrower than the observed region of energetic fluctuations. It is proposed here that weaker instabilities, in the linear sense, which are very weakly trapped to the current, may be the dominant waves in the far field. As a preliminary problem, the (barotropic) instability of parallel shear flow on the beta plane is discussed. An infinite zonal flow with a continuous cross-stream velocity gradient is approximated with segments of uniform flow, joined together by segments of uniform potential vorticity. This simplification allows an exact dispersion relation to be found. There are two classes of linearly unstable solutions. One type is trapped to the source of energy and has large growth rates. The second type are weaker instabilities of the shear flow which excite Rossby waves in the far field: the influence of these weaker instabilities extends far beyond that of the most unstable waves. The central focus of the thesis i: the linear stability of thin, twolayer, zonal jets on the beta plane, with both horizontal and vertical shear. The method used for the parallel shear flow is extended to the two-layer flow. Each layer of the jet has uniform velocity in the center, bordered by shear zones with zero potential vorticity gradient. The velocity in each layer outside the jet is constant in latitude. Separate linearly unstable modes arise from horizontal and vertical shear. The energy source for the vertical shear modes is nearly all potential while the source for the horizontal shear modes is both kinetic and potential. The most unstable waves are tightly trapped to the jet, within two or three deformation radii for small but nonzero beta. Rossby waves and baroclinically unstable waves (in the presence of vertical shear) exist outside the jet because of a nonzero potential vorticity gradient there. Weakly growing jet instabilities can force these waves when their phase speeds and wavelengths match. In particular, westward jets and any jets with vertical shear exterior to the jet can radiate in this sense. The radiating modes influence a large region, their decay scales inversely proportional to the growth rate. Two types of radiating instability are found: (1) a subset of the main unstable modes near marginal stability and (2) modes which appear to be destabilized neutral modes. Westward jets have more vigorously unstable radiating modes. Applications of the model are made to the eddy field south of the Gulf Stream, using data from the POLYMODE settings along 55°W and farther into the gyre at MODE. The energy decay scale and the variation of vertical structure with latitude in different frequency bands can be roughly explained by the model. The lower frequency disturbances decay more slowly and become more surface intensified in the far field. These disturbances are identified with the weak, radiating instabilities of the model. The higher frequency disturbances are more trapped and retain their vertical structure as they decay, and are identified with the trapped, strongly unstable modes of the jet.

Description: This work was supported by a grant from the National Science Foundation, Office of Atmospheric Science.

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: The Gulf Stream and Deep Western Boundary Current (DWBC) shape the distribution of heat and carbon in the North Atlantic, with consequences for global climate. This thesis employs a combination of theory, observations and models to probe the dynamics of these two western boundary currents. First, to diagnose the dynamical balance of the Gulf Stream, a depth-averaged vorticity budget framework is developed. This framework is applied to observations and a state estimate in the subtropical North Atlantic. Budget terms indicate a primary balance of vorticity between wind stress forcing and dissipation, and that the Gulf Stream has a significant inertial component. The next chapter weighs in on an ongoing debate over how the deep ocean is filled with water from high latitude sources. Measurements of the DWBC at Line W, on the continental slope southeast of New England, reveal water mass changes that are consistent with changes in the Labrador Sea, one of the sources of deep water thousands of kilometers upstream. Coherent patterns of change are also found along the path of the DWBC. These changes are consistent with an advective-diffusive model, which is used to quantify transit time distributions between the Labrador Sea and Line W. Advection and stirring are both found to play leading order roles in the propagation of water mass anomalies in the DWBC. The final study brings the two currents together in a quasi-geostrophic process model, focusing on the interaction between the Gulf Stream’s northern recirculation gyre and the continental slope along which the DWBC travels. We demonstrate that the continental slope restricts the extent of the recirculation gyre and alters its forcing mechanisms. The recirculation gyre can also merge with the DWBC at depth, and its adjustment is associated with eddy fluxes that stir the DWBC with the interior. This thesis provides a quantitative description of the structure of the overturning circulation in the western North Atlantic, which is an important step towards understanding its role in the climate system.

Description: My research was funded by National Science Foundation grants OCE-0241354, OCE- 0726720 and OCE-1332667 as well as a graduate fellowship from the American Meteorological Society. Support for travel and educational supplies was also provided by the MIT Houghton Fund and the WHOI Academic Programs Office.

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.

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: The daily heating of the ocean by the sun can create a stably stratified near-surface layer when the winds are slight and solar insolation is strong. This type of shallow stable layer is called a Diurnal Warm Layer (DWL). This thesis examines the physics and dynamics of DWLs from observations of the subtropical North Atlantic Ocean associated with the Salinity Processes in the Upper ocean Regional Study (SPURS-I). Momentum transferred from the atmosphere to the ocean through wind stress becomes trapped within the DWL, generating shear across the layer. During SPURS-I, strong diurnal shear across the DWL was coincident with enhanced turbulent kinetic energy (TKE) dissipation (𝜖, 𝜖 〉 10−5 W/kg) observed from glider microstructure profiles of the near-surface. However, a scale analysis demonstrated that surface forcing, including diurnal shear, could not be the sole mechanism for the enhanced TKE dissipation. High-frequency internal waves (𝜔 ≫ 𝑓) were observed in the upper ocean during the daytime within the DWL. Internal waves are able to transfer energy from the deep ocean into the DWL through the unstratified remnant mixed layer, which is the intervening layer between the DWL and seasonal thermocline. As the strength of the stratification of the DWL increases, so does the shear caused by the tunneling internal waves. The analysis demonstrates that internal waves can generate strong enough shear to cause a shear-induced instability, and are a plausible source of the observed enhanced TKE dissipation. Vertically-varying horizontal transport across the upper ocean occurs because a diurnal current exists within the DWL, but not in the unstratified remnant mixed layer below. Therefore, when a DWL is present, the water within DWL is horizontally transported a different distance than the water below. Coupled with nocturnal convection that mixes the DWL with the unstratified layer at night, this cycle is a mechanism for submesoscale (1-10 km) lateral diffusion across the upper ocean. Estimates of a horizontal diffusion coefficient are similar in magnitude to current estimates of submesoscale diffusion based on observations, and are likely an important source of horizontal diffusion in the upper ocean.

Description: Supported by the Department of Defense (DoD) through the National Defense Science & Engineering Graduate Fellowship (NDSEG) Program and the National Science Foundation under Grant No. OCE-1129646. The collection and analysis of data from the SPURS-I central mooring were supported under National Aeronautics and Space Administration (NASA) Grant No. NNX11AE84G and NNX14AH38G.