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 in Oceanography and Applied Ocean Science and Engineering at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 2019.

Description: The Common Era (A.D. 1– present) represents a crucial period for climatic studies, documenting the timespan over which human activities have become an increasingly domineering force in shaping Earth’s landscape, climate, and ecology. Direct, quantifiable records of climatic phenomena are severely limited over much of the Common Era, necessitating high-resolution, naturally-derived proxies to extend climatic insights beyond the satellite and instrumental era, particularly across remote high-latitude and maritime regions of the North Atlantic. Here, I use modern, data-driven and physically-based modeling approaches to gain new insights into North Atlantic climate variability from the Greenlandic ice core archive. First, I investigate the climatic fidelity of ice core glaciochemical climate proxies at the microphysical-scale. I show that several soluble chemical species – key among them methanesulfonic acid (MSA) – undergo rapid vertical migration through a super-cooled liquidadvection process along ice crystal grain-boundaries. I demonstrate that significant multi-year MSA changes occur only under low snow-accumulation and high-impurity-content conditions, thus mitigating the phenomenon over much of Greenland. Building upon these findings, I then investigate the cause of declining 19th and 20th-century MSA concentrations across the interior Greenland Ice Sheet. My results illustrate that Greenlandic MSA records provide a new proxy for North Atlantic planktonic biomass changes, illuminating a 10 ± 7% decline in marine productivity over the Industrialera. I next present a new climate record from a previously-unexplored coastal ice cap in west-central Greenland. Using a physically-constrained ice cap flowline inversion model, I identify marked centennial-scale changes in coastal precipitation during the last millennium, including a ~40% increase in coastal precipitation since the industrial-onset. These changes are drastically larger than those observed from inland Greenland records, revealing enhanced sensitivity in west Greenlandic hydroclimates to regional Atlantic and Arctic-wide temperature variability. Finally, leveraging a compilation of nearly 30 annual-resolution Greenland water-isotope records, I isolate coherent signatures of atmospheric circulation variability to reconstruct changes in the North Atlantic eddydriven jet-stream over the last millennium, exposing progressively enhanced variability during the past two-centuries consistent with amplified Arctic thermal-wind forcing. This thesis thus illuminates new Common Era climatic and ecologic changes, and expands the scope of the Greenlandic ice archive as proxies of the coupled North Atlantic climate system.

Description: To my various funders: scientific research – especially the polar variety – is wicked expensive, and this dissertation wouldn’t have been possible without the big-bucks graciously provided by the U.S. Department of Defense Office of Naval Research (National Defense Science and Engineering Graduate fellowship), the National Science Foundation’s Office of Polar Programs (1205196, 1418256), as well as generous support from the Woods Hole Oceanographic Institution. An Ocean Outlook Fellowship from the Bjerknes Centre (Bergen, Norway) bred much creativity, and is also much-appreciated.

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 marine carbon cycle plays an important role in regulating Earth’s climate. The vastness of the open ocean and the large variability in the coastal ocean provide obstacles to accurately quantify storage and transport of inorganic carbon within marine ecosystems and between marine and other earth systems. Thus far, the open ocean has been the only true net sink of anthropogenic carbon dioxide (Canthro) emissions. However, ocean storage of Canthro is not uniformly distributed. Changes in water chemistry in the Northeast Pacific were quantified to estimate the amount of Canthro stored in this region over the last decade. This additional Canthro was found to cause acidification and aragonite saturation horizon shoaling at rates towards the higher end of those found in Pacific and Atlantic Ocean basins, making the Northeast Pacific one of the most sensitive regions to the invasion of anthropogenic carbon dioxide. Due to large variability in biogeochemical signals in coastal oceans, it is challenging to accurately assess carbon fluxes across different boundaries, such as tidal exchange between coastal wetlands and coastal oceans. Coastal salt marshes have been suggested to be a large net CO2 sink, thus designated as a type of “blue carbon.” However, accurate and dynamic estimates of carbon fluxes to and from tidal marshes are still premature, particularly carbon fluxes from marshes to the coastal ocean via tidal exchange, often referred to as marsh lateral fluxes. In this thesis, lateral total alkalinity (TA) and dissolved inorganic carbon (DIC) export fluxes were realistically quantified using high frequency time-series, in situ data. High-resolution fluxes permitted a closer look at how marsh generated TA and DIC are being exported over diurnal, spring-neap, and seasonal scales. I investigated the best way to capture variability of marsh exports via traditional bottle sampling and assessed uncertainties associated with different sampling strategies. Marsh TA and DIC exports significantly modified buffering capacity of coastal waters. This work contains the first realistic estimate of TA exports from a tidal salt marsh. Accurate estimates of DIC and TA fluxes indicate the significance of salt marshes to the coastal carbon and alkalinity budgets.

Description: The work in this thesis was supported by the National Science Foundation Graduate Research Fellowship Program, Link Foundation Ocean Engineering and Instrumentation Ph.D. Fellowship, WHOI Academic Programs Office, MIT PAOC Houghton Fund, MIT Student Assistance Fund, WHOI Innovative Technology Award (PI: Wang), National Institute of Science and Technologies (NIST no. 60NANB10D024, PIs: Camilli, Wang), USGS LandCarbon Program, USGS Coastal and Marine Geology Program, NSF (OCE-1233654, PI: Wang; OCE-1041068 PIs: Lawson, Wang, Wiebe, Lavery; OCE-1459521 PIs: Wang, Kroeger, Gonneea), and NOAA Science Collaborative (NA09NOS4190153, PIs: Leschen, Roth, Surgeon-Rogers, Tang, Kroeger, Ganju, Moseman-Valtierra, Abdul-Aziz, Emmett-Mattox, Emmer, Crooks, Megonigal, Walker, Weidman).

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: Significant changes occurred during the last deglaciation (roughly 10-20 thousand years (ka) before present) throughout the climate system. The ocean is a large reservoir of carbon and heat, however, its role during the deglaciation is still not well understood. In this thesis, I rely on radiocarbon measurements on fossil biogenic carbonates sampled from the seafloor to constrain deglacial ocean ventilation rates, using new data, an extensive data compilation, and inverse modeling. First, based on a sediment core that is absolutely dated from wooden remains, I argue that the deglacial 14C reservoir age of the upper East Equatorial Pacific was not very different from today. Combined with stable carbon isotope data, the results suggest that the deglacial atmospheric CO2 rise was probably due to CO2 released directly from the ocean (e.g., in the Southern Ocean) to the atmosphere rather than first mixed through the upper ocean. Then using a high-deposition-rate sediment core located close to deep water formation regions in the western North Atlantic, I show that compared to today, the mid-depth water production in the North Atlantic was probably stronger during the Younger Dryas cold episode, and weaker during other intervals of the late deglaciation. However, the change was not as large as suggested by previous studies. Finally, I compile published and unpublished deep ocean 14C data, and find that the 14C activity of the deep ocean mirrors that of the atmosphere during the past 25 ka. A box model of modern ocean circulation is fit to the compiled data using an inverse method. I find that the residuals of the fit can generally be explained by the data uncertainties, implying that the compiled data jointly do not provide strong evidence for basin-scale ventilation changes. Overall, this thesis suggests that, although deep ocean ventilation may have varied at some locations during the last deglaciation, the occurrence of basin-scale ventilation changes are much more difficult to be put on a firm footing. An imbalance between cosmogenic production and radioactive decay appears as the most natural explanation for the deglacial 14C activity decline observed in both the atmosphere and the deep ocean.

Description: This research was supported by NSF-OCE grants 0822854 to Lloyd Keigwin, 1031224 to Lloyd Keigwin, 1204045 to Lloyd Keigwin, 1301907 to Olivier Marchal, Geoffrey Gebbie and Lloyd Keigwin, and 1405160 to Lloyd Keigwin, WHOI Academic Programs Endowed Funds, the Ocean Ventures Fund from WHOI, and a graduate student internship from NOSAMS.