Originally issued as the author's thesis (Ph. D.)--Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 2006. / "February 2006". "Doctoral dissertation." "Department of origin: Geology and Geophysics." "Joint Program in Oceanography/Applied Ocean Science and Engineering"--Cover. Bibliography: p. 117-126.

Thesis (Ph. D.)--Joint Program in Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2005. / Includes bibliographical references (p. 131-149).

No description available.

published_or_final_version / Physics / Doctoral / Doctor of Philosophy

published_or_final_version / Civil Engineering / Doctoral / Doctor of Philosophy

Marine sediments are a storehouse of the geochemical, biological, and physical changes in the ocean over thousands to millions of years. Intensive study of the Atlantic Ocean has well constrained the role of this basin in global climate change, but the vast Pacific Ocean, deeper and more corrosive to carbonate, has remained more elusive. This thesis leverages a new suite of sediment cores collected on the Juan de Fuca Ridge in the East Subarctic Pacific Ocean (~45˚N, 135˚W) to better understand how the paleoceanographic history of this region has evolved over the past 500kyr. In Chapter 1, I developed age models for multiple cores using benthic δ18O and physical properties of sediment as stratigraphic markers. Despite the proximity of the cores (within 50km2), the sedimentation rates varied by an order of magnitude, likely reflecting remobilization of sediment caused by the high relief of the mid-ocean ridge bathymetry. In Chapter 2, I analyzed uranium series disequilibria in the sediment in order to investigate the processes generating the highly variable sedimentation rates. This chapter presents evidence that the particle flux settling through the water column (based on excess 230Th) is relatively constant at six different sites, and the variability in sedimentation rates is largely driven by lateral sediment remobilization along the rough bathymetry of the ridge. Chapter 3, entitled “Trace element (Mn, Zn, Ni, V) and authigenic uranium (aU) geochemistry reveal sedimentary redox history on the Juan de Fuca Ridge, North Pacific Ocean”, presented high-resolution x-ray fluorescence records of metal diagenesis in response to changing oxygen conditions in the sediment. This study is the first to show strong evidence for low sedimentary oxygen conditions during interglacial periods in the North Pacific, which we suggest may be linked to hydrothermal sulfide deposition. In Chapter 4, I returned to uranium series disequilibria by utilizing 231Pa/230Th records from the Juan de Fuca Ridge to reconstruct productivity in the East Subarctic Pacific Ocean over the last 200kyr. Productivity across much of the Subarctic Pacific is low during glacial periods and high during interglacial periods, which is usually associated with changes in stratification. I investigated several different mechanisms for increasing stratification during glacial periods, and conclude that a combination of surface freshening, weak winds, and reduced subsurface nutrient concentrations likely created the stratification that led to low glacial productivity. Finally, in Chapter 5, “Dust deposition in the East Subarctic Pacific on glacial-interglacial timescales”, I reconstructed the patterns of dust fluxes in the East Subarctic Pacific Ocean over the last 500kyr to assess the climatic effects on the spatial distribution of dust in the North Pacific Ocean. I predict that migration of the westerlies would have caused a shift in dust provenance away from Asian dust and towards higher North American contributions during glacial periods. Although lithogenic endmembers are currently poorly constrained in this region, I present some evidence for variable provenance over time that may be consistent with the influence of the westerlies on dust fluxes in the East Subarctic.

No description available.

The rise in global energy demand has motivated the exploration and production of oil and gas in increasingly challenging marine environments and there is a continuous risk of accidental oil spills. One of the many fates of spilled oil is deposition on the seabed, which has been extensively studied following the Deepwater Horizon oil spill. However, post-depositional fates of oil in sediments are not well understood. Similarly, the effects of chemical dispersant on oil fate are currently under investigation as their overall contribution to mitigating oil spills environmental impacts remains debated. This project aimed to evaluate the potential for spilled oil to entrain marine sediments and the effects dispersant application had on the process under three transport regimes; (1) post-depositional transport via oil-sediment aggregate deposition in deep-sea sediments, (2) percolative transport in intertidal sands and (3) advective pore-water transport in intertidal and subtidal sands. Investigations into the sorption dynamics of two polyaromatic hydrocarbons in sediment-dispersantseawater systems were also undertaken to evaluate the influence of dispersant application on sorption of hydrocarbons to sediments. Finally, the effects of oil exposure at in situ conditions of pressure and temperature on sediment bacterial community composition were investigated. Oil transport experiments revealed that the tested regimes resulted in significant entrainment of hydrocarbons in marine sediments. Dispersant application resulted in enhanced oil entrainment into sands but not in silts and this effect depended on the water-solubility of hydrocarbons. Watersoluble components were less affected by dispersant than less water-soluble ones. Investigations into sediment bacterial responses to oil exposure at in situ conditions of pressure and temperature revealed a significant effect of both variables on diversity and community composition, highlighting the importance of conducting deep-sea microbial studies at conditions as close to in situ as possible.

Macroalgae are abundant on shallow temperate reef environments, often forming complex communities that comprise several strata. In southern Australia, these assemblages are dominated by large canopy forming taxa from the Orders Laminariales and Fucales. The presence of subtidal fucoid macroalgae differentiates these communities from that elsewhere, and emphasises the need for local studies rather than relying on generalisations made elsewhere. Like most natural systems, temperate reefs are often threatened by human activity with degradation reported from many locations in close proximity to urban settlements. The work presented in this thesis involves an examination of the temporal and spatial variability in the structure of macroalgal communities from reefs along the Adelaide (South Australia) metropolitan coast. The work looked specifically at the effects of a dispersed sediment plume, resulting from the 1997 beach sand-replenishment dredging program, on shallow sub-tidal reef systems. An examination of the structure of canopy forming phaeophycean macroalgae in Gulf St Vincent (South Australia), noted large amounts of both spatial and temporal heterogeneity. Notwithstanding, this variation was not random, but demonstrated considerable structure that could be linked to a number of important underlying processes. In particular, macroalgal assemblages appeared as a mosaic of patches, each of which comprised a high-density state clearly dominated by a single genus (Cystophora, Sargassum, or Ecklonia), or alternatively a lower density mixed assemblage (Variable Low Abundance, VLA). Macroalgal community structure appeared to be driven by biotic interactions at small scales (metres), such that patches comprised of different species of algae in high density states rarely abutted one another. Instead, VLA assemblages frequently formed a buffer being situated between these mono generic patches. In terms of successional processes, the high-density states appeared to be relatively stable whereas the VLA state, at least in some systems, was transitory. This finding was supported by the absence of intermediary high- density states (e.g. a mix of Cystophora and Ecklonia) implying that state changes must occur via the VLA state following some form of disturbance. Larger scale patterns appeared to be driven by environmental variation, with factors such as wave exposure influencing habitat suitability for individual species and thereby affecting community composition. These phenomena were examined in terms of life history strategies that tend to promote stability, and which are common in late successional taxa. The importance of properties enhancing stability and the role of disturbance was investigated experimentally using a dispersed sediment plume, which entirely engulfed two reefs, as a pulse impact. This disturbance was of particular relevance given that degradation of macroalgal communities in close proximity to the City of Adelaide has been, at least in part, attributed to the effects of elevated levels of sediment. Follow up surveys revealed that the sedimentation from the plume had primarily affected newly recruiting individuals, with few juveniles surviving to one year of age. Over the following few years, the effect of this recruitment failure cascaded into the adult stand. In broader terms, unfavourable climatic conditions prior to the start of the study, including a particularly severe El Nino event, had a widespread effect on local assemblages, causing high levels of both adult and juvenile mortality. As such, at the commencement of the study, macroalgal communities across the study area were in the process of recovery. This was observed at control sites over the duration of the study. In contrast, recruitment failure at the sediment-affected sites retarded the recovery process, exacerbating the problems associated with prior unfavourable climatic events and leaving them in a degraded state. This study demonstrated that macroalgal assemblages are equipped (under natural conditions) to handle 'normal' environmental fluctuations (such as inter-annual variability). However, the additional stress associated with certain anthropogenic impacts has the potential to push them over the limit, causing degradation. The loss of canopy macroalgae reduces the structural complexity of the system, leading to a concomitant reduction in their ability to recover. As such, these findings are of particular relevance to those charged with the responsibility for managing near-shore marine environments. The plume was created accidentally during a dredging operation for beach sand replenishment of Adelaide's eroding shoreline. / Thesis (Ph.D.)--School of Earth and Environmental Sciences, 2004.

Thesis (M.A.)--State University of New York at Binghamton, Geological Sciences and Environmental Studies Department, 2006. / Includes bibliographical references.