Thesis: Ph. D., Joint Program in Oceanography/Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), May, 2020 / Cataloged from the official PDF of thesis. / Includes bibliographical references (pages 161-183). / Highly productive marine microbial communities in the coastal Southern Ocean sustain the broader Antarctic ecosystem and play a key role in Earth's climate via the biological pump. Regional phytoplankton growth is primarily limited by iron and co-limited by cobalamin (vitamin B₁₂), a trace cobalt-containing organometallic compound only synthesized by some bacteria and archaea. These micronutrients impact primary production and the microbial ecology of the two keystone phytoplankton types: diatoms and Phaeocystis antarctica. This thesis investigates microbe-driven cobalamin cycling in Antarctic seas across multiple spatiotemporal scales. I conducted laboratory culture experiments with complementary proteomics and transcriptomics to investigate the B₁₂-ecophysiology of P. antarctica strain CCMP 1871 morphotypes under iron-B₁₂ co-limitation. / We observed colony formation under higher iron treatments, and a facultative use of B₁₂-dependent (MetH) and B₁₂-independent (MetE) methionine synthase isoforms in response to vitamin availability, demonstrating that this strain is not B₁₂-auxotrophic. Through comparative 'omics, we identified a putative MetE protein in P. antarctica abundant under low B₁₂, which is also found in other marine microbes. Across Antarctic seas, community-scale cobalt and B₁₂ uptake rates were measured by ⁵⁷Co radiotracer incubation experiments and integrated with hydrographic and phytoplankton pigment data. I observed significant correlations between uptake fluxes and environmental variables, providing evidence for predominantly diatom-driven uptake of these micronutrients in warmer, fresher surface waters with notable regional differences. / To date, this work is the most comprehensive attempt to elucidate the processes governing the co-cycling of cobalt and B₁₂ in any marine system. At the ecosystem-scale, I developed and tested a hypothesis of micronutrient-driven community dynamics through a trait-based model with cross-feeding interactions. The model demonstrates how the observed seasonal succession of springtime P. antarctica from solitary to colonial cells, bacterioplankton, and summertime diatoms may be explained by the microbial cycling of iron, dissolved organic carbon, and B₁₂. Overall, this dissertation provides new information about the micronutrient-driven ecology of Antarctic marine microbes and adds to our understanding of the interconnections between organismal life cycle, trace metals, and trace organics in marine environments. / by Deepa Rao. / Ph. D. / Ph.D. Joint Program in Oceanography/Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution)

Thesis (Ph. D.)--Joint Program in Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2001. / "September 2001." Vita. Page 223 blank. / Includes bibliographical references. / This thesis studies interactions between mid-ocean ridges and mantle plumes using geophysics, geochemistry, and geodynamical modeling. Chapter 1 investigates the effects of the Marion and Bouvet hotspots on the ultra-slow spreading, highly-segmented Southwest Indian Ridge (SWIR). Gravity data indicate that both Marion and Bouvet impart high-amplitude mantle Bouguer anomaly lows to the ridge axis, and suggest that long-offset transforms may diminish along-axis plume flow. Building upon this observation, Chapter 2 presents a series of 3D numerical models designed to quantify the sensitivity of along-axis plume-driven mantle flow to transform offset length, spreading rate, and mantle viscosity structure. The calculations illustrate that long-offset transforms in ultra-slow spreading environments may significantly curtail plume dispersion. Chapter 3 investigates helium isotope systematics along the western SWIR as well as near a global array of hotspots. The first part of this study reports uniformly low 3He/4He ratios of 6.3-7.3 R/Ra along the SWIR from 9⁰-24⁰E, compared to values of 8 +/- 1 Ra for normal mid-ocean ridge basalt. The favored explanation for these low values is addition of (U+Th) into the mantle source by crustal and/or lithospheric recycling. Although high He/4He values have been observed along the SWIR near Bouvet Island to the west, there is no evidence for elevated 3He/4He ratios along this section of the SWIR. The second part of Chapter 3 investigates the relationship between 3He/4He ratios and geophysical indicators of plume robustness for nine hotspots. / (cont.) A close correlation between a plume's flux and maximum 3He/4He ratio suggests a link between plume upwelling strength and origination in the deep, relatively undegassed mantle. Chapter 4 studies 3D mantle flow and temperature patterns beneath oceanic ridge-ridge-ridge triple junctions (TJs). In non-hotspot-affected TJs with geometry similar to the Rodrigues TJ, temperature and upwelling velocity along the slowest-spreading of the three ridges are predicted to increase within a few hundred kilometers of the TJ, to approach those of the fastest-spreading ridge. Along the slowest-spreading branch in hotspot-affected TJs such as the Azores, a strong component of along-axis flow directed away from the TJ is predicted to advect a hotspot thermal anomaly away from its deep-seated source. / by Jennifer E. Georgen. / Ph.D.

Thesis (Ph. D.)--Joint Program in Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2006. / Includes bibliographical references. / Between 90-25° E on the ultraslow-spreading Southwest Indian Ridge lie two sharply contrasting supersegments. One 630 km long supersegment erupts N-MORB that is progressively enriched in incompatible element concentrations from east to west. The second 400 km long supersegment contains three separate volcanic centers erupting E-MORB and connected by long amagmatic accretionary segments, where mantle is emplaced directly to the seafloor with only scattered N-MORB and E-MORB erupted. Rather than a major break in mantle composition at the discontinuity between the supersegments, this sharp contrast in geometry, physiography, and chemistry reflects "source" versus "process" dominated generation of basalt. Robust along-axis correlation of ridge characteristics (i.e. morphology, upwelling rate, lithospheric thickness), basalt chemistry, and crustal thickness (estimated from gravity) provides a unique opportunity to compare the influence of spreading geometry and rate on MORB generation. What had not been well established until now is the importance of melting processes rather than source at spreading rates < 20 mm/yr. / (cont.) Along the orthogonally spreading supersegment (14 mm/yr) moderate degrees of partial melting effectively sample the bulk mantle source, while on the obliquely spreading supersegment (7-14 mm/yr) suppression of mantle melting to low degrees means that the bulk source is not uniformly sampled, and thus "process" rather than "source" dominates melt chemistry. / by Jared Jeffrey Standish. / Ph.D.

Thesis (S.M.)--Joint Program in Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2008. / Includes bibliographical references (p. 45-47). / Biogeochemical cycling of phosphate is a key component in the overall production rate of coastal ecosystems. Mineral phases in the near-shore sediments play a significant role in the return of phosphate remineralized in the upper sediments to the water column. Sequential Extraction (SEDEX) of the solid-phase associated P04-3 yielded reservoir profiles of phosphate at three sites off of the Massachusetts coast. These extractions found Fe-associated P04 to be the dominant phase associated with rapid porewater-solid P exchange. Additionally, a seasonal enrichment/depletion pattern of phosphate fluxes relative to total carbon was observed from the sediments. These observations established the behavior of phosphate in coastal sediments as interconnected with the ongoing Fe-cycling in the sediments as well. / by Richard Hunter Oates, Jr. / S.M.

Thesis (S.M.)--Joint Program in Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences and the Woods Hole Oceanographic Institution), 2002. / Includes bibliographical references (p. 22-28). / We have investigated the near liquidus phase relations of a primitive absarokite from the Mascota region in Western Mexico. Sample M. 102 was chosen because it has high MgO contents, a high Mg# and Fo90 olivine phenocrysts, indicating it is primitive mantle melt. Highpressure experiments on a synthetic analogue of the absarokite composition with a H20 content of either -1.7 wt% or -5.1 wt% were carried out in a piston cylinder apparatus. The composition with -1.7 wt% H20 is multiply saturated with olivine and orthopyroxene as liquidus phases at 1.6 GPa and 14000C. At the same pressure clinopyroxene appears 300C below the liquidus. With a H20 content of -5.1 wt% composition M.102 is multiply saturated with olivine and orthopyroxene on the liquidus at 1.7 GPa and 13000 C. Assuming batch melting, we suggest that absarokite M. 102 segregated from a depleted lherzolite or harzburgite residue at depth -50 km depth in the mantle wedge. Unlike most lavas in the region, the absarokite has not ponded and fractionated at the crust mantle interface (-35-40 km), and the temperatures of multiple saturation indicate that the mantle wedge beneath the Jalisco block is hotter than previously thought. The low degree batch melting of an original metasomatised harzburgite source, can produce the observed trace element abundances. The liquidus phase relations are not consistent with the presence of non-peridotitic veins at the depth of last equilibration. Therefore, we propose that the Mascota absarokites segregated at an apparent melt fraction of less than 5% from a depleted peridotitic source. They initially formed by a small degree of melting of a metasomatised original source at greater depth. / by Marc Hesse. / S.M.

Thesis (M.S.)--Joint Program in Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 1987. / Supervised by David G. Aubrey. / Includes bibliographical references (leaves 57-61). / by Barbara Vanston Braatz. / M.S.

Thesis (Ph. D.)--Joint Program in Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2006. / This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections. / Includes bibliographical references (leaves 117-126). / In this thesis, I present high-resolution stable-isotope and planktonic-fauna records from Bering Sea sediment cores, spanning the time period from 50,000 years ago to the present. During Marine Isotope Stage 3 (MIS3) at 30-20 ky BP (kiloyears before present) in a core from 1467m water depth near Umnak Plateau, there were episodic occurrences of diagenetic carbonate minerals with very low 13C (-22.4%), high 18O (6.5%), and high [Mg]/[Ca], which seem associated with sulfate reduction of organic matter and possibly anaerobic oxidation of methane. The episodes lasted less than 1000 years and were spaced about 1000 years apart. During MIS3 at 55-20 ky BP in a core from 2209m water depth on Bowers Ridge, N. pachyderma (s.) and Uvigerina 18O and 13C show no coherent variability on millennial time scales. Bering Sea sediments are dysoxic or laminated during the deglaciation. A high sedimentationrate core (200 cm/ky) from 1132m on the Bering Slope is laminated during the Blling warm phase, Allerd warm phase, and early Holocene, where the ages of lithological transitions agree with the ages of those climate events in Greenland (GISP2) to well within the uncertainty of the age models. The subsurface distribution of radiocarbon was estimated from a compilation of published and unpublished North Pacic benthic-planktonic 14C measurements (475{2700 m water depth). There was no consistent change in 14C probles between the present and the Last Glacial Maximum, Blling-Allerd, or the Younger Dryas cold phase. N. pachyderma (s.) 18O in the Bering Slope core decreases rapidly (in less than 220 y) by 0.7-0.8h at the onset of the Blling and the end of the Younger Dryas. These isotopic shifts are accompanied by transient decreases in the relative abundance of N. pachyderma (s.), suggesting that the isotopic events are transient warnings and sustained freshenings. / by Mea S. Cook. / Ph.D.

Thesis (Ph. D.)--Joint Program in Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 1986. / Microfiche copy available in Archives and Science. / Vita. Chapter 5: ²³⁹,̳²⁴⁰Pu and excess ²¹⁰Pb inventories along the shelf and slope of the northeast U.S.A. / Ken O. Buesseler, Hugh D. Livingston and Edward R. Sholkovitz, reprinted from Earth and Planetary Science Letters, 76 (1985/86) 10-22, Elsevier Science Publishers, B. V. Amsterdam. The underscored comma in the above note is superscript on the source. / Bibliography: leaves 193-207. / by Ken O. Buesseler. / Ph.D.

Thesis (Ph. D.)--Joint Program in Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 1987. / Includes bibliographical references (leaves 192-194). / The various distributions of tracer associated with the Northern Recirculation Gyre of the Gulf Stream (NRG) are studied to try to obtain information about the flow. An advective-diffusive numerical model is implemented to aid in the investigation. The model is composed of a gyre adjacent to a boundary current in which a source of tracer is specified at the upstream edge of the current. This set up attempts to simulate the lateral transfer of properties from the Deep Western Boundary Current (DWBC) to the NRG in the region where the two flows are in close contact west of the Grand Banks. The results of the model are analyzed in some detail. Tracer is entrained into the gyre as a plume which extends from the boundary current and spirals across streamlines toward the gyre center. The maintenance of the spiral during spin-up and its relationship to the occurrence of homogenization at steady state is examined. An asymmetry in the spiral exists due to the ellipticity of the gyre, which also effects homogenization. The anomalous properties that are fluxed into the NRG include salt, oxygen, and freon. These particular tracers are independent from each other, the former two because they are characterized by different vertical profiles in the deep layer. This results in a decay of oxygen but not salt, due to the presence of vertical mixing as discussed by Hogg et al. (1986, Deep-Sea Research, 33, 1139-1165). Their analysis is expanded upon here. The effect of vertical mixing on the gyre/boundary current system is examined within the context of the numerical model. Results are applied to recently collected water sample data from the region which leads to an estimate of the lateral and vertical eddy diffusion coefficients and an estimate of the amount of oxygen in the NRG that has diffused from the DWBC. The accumulation of freon within the NRG is considered in addition to salt and oxygen. Appreciable levels of freon have been present in the ocean only since 1950, and the atmospheric source functions have been increasing steadily since then. A simple overflow model is presented of the manner in which freon may be stirred in the Norwegian-Greenland basin prior to overflowing and entering the DWBC. Once in the boundary current the concentrations are diluted by way of mixing with surrounding water. Two different schemes are considered in which the immediate surrounding water accumulates a substantial amount of freon as time progresses. These models suggest that the freon-11:freon-12 ratio may not be a conserved quantity for the water in the core of the UWBC. It is found that the level of freon in the NRG is barely above the existing background level. / by Robert S. Pickart. / Ph.D.

Thesis (Ph. D.)--Joint Program in Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), June 1988. / "May 1988." / Includes bibliographical references (p. 163-169). / The deep basins, uplifted flanks, and volcanoes of the Western and Kenya rift systems have developed along the western and eastern margins of the 1300 km-wide East African plateau. Structural patterns deduced from field, Landsat, and geophysical studies in the Western rift reveal a series of asymmetric basins bounded by approximately 100 kmlong segments of the border fault system. These basins are linked by oblique-slip and strike-slip faults cross-cutting the rift valley. Faults bounding the Kenya and Western rift valleys delineate two north-south-trending, 40-75 km wide zones of crustal extension, and little or no crustal thinning has occurred beneath the uplifted flanks or the central plateau. In the Western rift, volcanism in Late Miocene time began prior to or concurrent with basinal subsidence, followed by rift flank uplift. Individual extensional basins developed diachronously, and basinal propagation may give rise to the along-axis segmentation of the rift valley. The coherence between gravity and topography data indicates that the mechanical lithosphere beneath the two rift valleys has been weakened relative to the central plateau and adjacent cratonic regions. Gravity and topography data at wavelengths corresponding to the overcompensated East African plateau can be explained by density variations within the upper mantle that are dynamically maintained. / by Cynthia J. Ebinger. / Ph.D.