Theses Doctoral

Controls on Surface and Sedimentary Processes on Continental Margins from Geophysical Data: New Insights at Cascadia, Galicia, and the Eastern North American Margin

Gibson, James Charles

Seafloor sedimentary depositional and erosional processes create a record of near and far-field climatic and tectonic signals adjacent to continental margins and within oceanic basins worldwide. In this dissertation I study both modern and paleo-seafloor surface processes at three separate and distinct study sites; Cascadia offshore Oregon, U.S.A., the Eastern North American Margin from south Georgia in the south to Massachusetts in the north, and the Deep Galicia Margin offshore Spain. I have the advantage of using modern geophysical methods and high power computing resources, however the study of seafloor processes at Columbia University's Lamont-Doherty Earth Observatory (LDEO) stretches back over ~80 yrs.

Specifically I use data collected during a variety of geophysical research cruises spanning the past ~50 yrs.-the majority of which can be directly attributed to seagoing programs managed by LDEO. The modern seafloor is the integrated result of all previous near and far field processes. As such, I look below the seafloor using multi-channel seismic reflection data, which is the result of innumerable soundings stacked together to create an image of the sub-seafloor (paleo) horizons. I map, analyze and interpret the sub-seafloor sedimentary horizons using a variety of both novel and established methods. In turn, I use multi-beam sonar data, which is also the result of innumerable soundings to map, analyze, and interpret the modern seafloor topography (bathymetry). Additionally, I look to the results from academic ocean drilling programs, which can provide information on both the composition and physical properties of sediments. The sediment composition alone can provide important information about both near and far-field processes, however when supplemented with physical properties (e.g., density/porosity) the results become invaluable.

In my second chapter, I use a compilation of multi-beam sonar bathymetry data to identify and evaluate 86 seafloor morphological features interpreted to represent large-scale erosional scours not previously recognized on the Astoria Fan offshore Oregon, U.S.A. The Astoria Fan is primarily composed of sediments transported from the margin to the deep ocean during Late Pleistocene interglacial periods. A significant portion of the sediments have been found to be associated with Late Pleistocene outburst flood events attributed to glacial lakes Bonneville and Missoula. The erosional scours provide a record of the flow path of the scouring event(s), which if well understood can provide important information for the study of past earthquakes as the sedimentary record remains intact outside of the erosional force created by the massive flood events. I design and implement a Monte Carlo inversion to calculate the event(s) flow path at each individual scour location, which results in a comprehensive map of Late Pleistocene erosion on the Astoria Fan. The results indicate that at least 4 outburst flood events are recorded by the scour marks.

In my third chapter, I build a stratigraphic framework of the Eastern North American margin using a compilation of multi-channel seismic data. Horizon Au is a primary horizon within the stratigraphic framework and is thought to represent a significant margin wide bottom-water erosional event associated with subsidence of the Greenland-Scotland Ridge and opening of Fram Strait in the late Eocene/early Oligocene. A recent study found that the bottom-water was enriched in fossil carbon, leading us to hypothesize that the bottom-water erosion recorded by horizon Au may have been facilitated by chemical weathering of the carbonate sediments. I use sediment isopach(s) to build a margin-wide model of the late Eocene/early Oligocene continental margin in order to estimate the volume of sediments eroded/dissolved during the event marked by horizon Au. The results indicate that ~170,000 km3 of sediments were removed with a carbonate fraction of 42,500 km³, resulting in 1.15e18 mol CaCO₃ going into solution. An influx of this magnitude likely played a role in significant climatic changes identified at the Eocene-Oligocene transition (EOT).

In my fourth chapter, I use a combination of 3D multi-channel seismic and multi-beam sonar bathymetry data collected during the Galicia 3D Seismic Experiment in 2013. The Galicia Bank is the largest of many crustal blocks and is located 120 km west of the coast on the Iberian Margin. The crustal blocks have been attributed to the opening of the North Atlantic Ocean in the Late Triassic/Middle Jurassic. The Galicia Bank is the source for the majority of sediments delivered to the Deep Galicia Margin, the focus of this study. I map the seafloor and 5 paleo-seafloor surfaces in order to study controls on sediment delivery provided by the crustal blocks. The results show that the crustal blocks begin as a barrier to and remain a primary control on sediment delivery pathways to the Deep Galicia basin. Additionally, the paleo-seafloor surfaces record morphological structures that can inform us on both near and far field past climatic and tectonic events e.g., the Alpine Orogeny and Pleistocene inter-glacial periods.


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More About This Work

Academic Units
Earth and Environmental Sciences
Thesis Advisors
Shillington, Donna J.
Ph.D., Columbia University
Published Here
September 28, 2022