published_or_final_version / Biological Sciences / Master / Master of Philosophy

Thesis: Ph. D., Joint Program in Oceanography (Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2014. / 122 / Cataloged from PDF version of thesis. / Includes bibliographical references (pages 149-158). / Understanding the fate of primary production in the ocean is a challenging task because once produced, organic material is oxidized over timescales which range from minutes, to millions of years. This timescale diversity is matched by an equal heterogeneity in both the local physical and chemical environment. In this thesis we explore the relationship between the distinct reservoirs of organic carbon in the ocean and their underlying complexity. First, we show how the heterogeneity of portions of the carbon cycle can be packaged in terms of age structured models and their accompanying age and rate distributions. We further relate the moments of the rate distributions to bulk reservoir properties like average age and flux. Explicit relationships are then derived for the specific case of a single turnover time and a lognormal distribution. We apply these ideas to the problem of dissolved organic carbon (DOC) cycling in the ocean. Current models of bulk concentration and isotope data suggest a microbially sourced DOC reservoir consisting of two components. A nearly homogeneous background component with a long turnover time (> 6000 years) is joined by a component of fast turnover time (~ 1 year) and equal concentration in the surface ocean. We confirm the presence of isotopically enriched, modern DOC co-cycling with an isotopically depleted older fraction in the upper ocean. However, our results show that up to 30% of the deep DOC reservoir is modern and supported by a 1 Pg per year carbon flux, ten times higher than inferred from bulk isotope measurements. Isotopically depleted material turns over at an apparent time scale of 30, 000 years, far slower than indicated by bulk isotope measurements. These results are consistent with global DOC measurements and explain both the fluctuations in deep DOC concentration and the anomalous radiocarbon values of DOC in the Southern Ocean. Finally, the thesis explores methods for determining the validity of diffusion limitation as the mechanism behind the power-law slowdown in organic remineralization in sediment. We find that diffusion limitation connects the decay behavior of organic material to the correlations found between mineral surface area and organic matter content in sediments. / by Christopher L. Follett. / Ph. D.

Thesis (S.M.)--Joint Program in Marine Geology and Geophysics (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2010. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 33-37). / Mid-ocean ridge basalts (MORBs) exhibit a wide range of CO2 concentrations, reflecting saturation to supersaturation (and rarely undersaturation) relative to their emplacement depths. In this study, we explore the mechanisms of CO2 degassing and the implications this has for estimating the advance rates and durations of seafloor eruptions. We present dissolved volatile concentrations (mainly of CO 2 and H20) and vesicle size distributions (VSDs) for a unique suite of MORB glasses collected at the East Pacific Rise, ~9° 50' N. These MORB glasses were collected at -200 m intervals along an across-axis track over a single flow pathway within the recently emplaced 2005-06 eruption boundaries; systematic sample collection provides one of the first opportunities to characterize intra-flow geochemical and physical evolution during a single eruption at a fast-spreading ridge. Compared to measurements of MORB volatiles globally, dissolved H20 concentrations are relatively uniform (0.10 - 0.16 weight percent), whereas dissolved CO2 contents exhibit a range of concentrations (154 - 278 ppm) and decrease with distance from the EPR axis (i.e., eruptive vent). Ion microprobe analysis of dissolved volatiles within the MORB glasses suggest that the magma erupted supersaturated (pressure equilibrium with 920 - 1224 mbsf) and in near-equilibrium with the melt lens of the axial magma chamber (~1250 - 1500 mbsf), and degassed to near equilibrium (299 - 447 mbsf) with seafloor depths over the length of the flow. The decrease in CO 2 concentrations spans nearly the full range of dissolved CO2 contents observed at the EPR and shows that the varying degrees of volatile saturation that have been observed in other MORB sample suites may be explained by degassing during emplacement. Vesicularity (0.1 - 1.2%) increases with decreasing dissolved CO2 concentrations. We use vesicle size distributions (VSDs)-vesicle sizes and number densities-to quantify the physical evolution of the CO2 degassing process. VSDs suggest that diffusion of CO2 into preexisting vesicles, and not nucleation of new vesicles, is the dominant mechanism of increasing CO2 in the vapor phase. We also use VSDs, along with estimates of vesicle growth rates, to constrain emplacement time of the 2005-06 eruption to <~24 hours and to resolve variations in advance rate with down flow distance. / by Dorene Samantha Nakata. / S.M.

Thesis (Ph. D.)--Joint Program in Oceanography/Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Dept. of Biology; and the Woods Hole Oceanographic Institution), 2013. / Cataloged from PDF version of thesis. / Includes bibliographical references. / The sinking flux of particles is an important removal mechanism of carbon from the surface ocean as part of the biological pump and can play a role in cycling of other chemical species. This work dealt with improving methods of measuring particle export and measuring export on different scales to assess its spatial variability. First, the assumption of ²³⁸U linearity with salinity, used in the ²³⁸U-²³⁴Th method, was reevaluated using a large sample set over a wide salinity range. Next, neutrally buoyant and surface-tethered sediment traps were compared during a three-year time series in the subtropical Atlantic. This study suggested that previously observed imbalances between carbon stocks and fluxes in this region are not due to undersampling by traps. To assess regional variability of particle export, surface and water-column measurements of ²³⁴Th were combined for the first time to measure fluxes on ~20 km scales. Attempts to relate surface properties to particle export were complicated by the temporal decoupling of production and export. Finally, particle export from ²³⁴Th was measured on transects of the Atlantic Ocean to evaluate basin-scale export variability. High-resolution sampling through the water-column allowed for the identification of unique ²³⁴Th features in the intermediate water column. / by Stephanie Anne Owens. / Ph.D.