Pelagic sedimentation is the primary modifier of topography created by ridge-associated volcanic and tectonic processes. The accumulation of biogenic, lithogenic, and hydrogenic sediments, at rates ranging from <1 m/Myr to >100 m/Myr, gradually reshapes the seafloor by filling in small crevices, flattening valley floors and, over millions of years, completely burying all but the highest hills. This thesis represents an effort to understand the processes of, and map the general distribution of, pelagic sedimentation on rough topography, particularly in the Atlantic basin but with applications to the world ocean as a whole.
This study of pelagic sedimentation in rough topography is facilitated by the use of a simple forward model of mass transport, implemented on a Connections Machines CM-5. By numerically applying sediment to realistic renderings of rough (Goff-Jordan) basement topography, the effects of sedimentation is quantified in terms of commonly-measured stochastic parameters. In particular, we predict the effects of sedimentation on abyssal hill spacing, RMS height, and bathymetric slope distribution. We also address the effect of sediment compaction on seafloor morphology.
The understanding gained about the effects of sedimentation on topography allows the construction of inverse problems to determine the amount and style of sedimentation on abyssal hill topography, using data from multibeam bathymetric imaging systems. By stochastically comparing data to model topographies, average sediment thicknesses are estimated to within 10-20%, and sediment mobility, as reflected by pond surface shape, is quantified in terms of apparent diffusivity.
Following an introductory chapter, Chapter 2 of my thesis introduces the forward model of pelagic sedimentation and Chapter 3 explains the inversion procedure for the estimation of average sediment thickness and sediment mobility, as applied to bathymetric (Hydrosweep) data from the Office of Naval Research's Acoustic Reverberation Corridor, located on the western flank of the Mid-Atlantic Ridge north of the Kane Fracture Zone. Results of the inversion allow the tracking of changes in the CCD during the Neogene as well as estimates of biogenic and lithogenic sediment accumulation rates. Chapter 4 will similarly use the inversion methodology to study the distribution of sediments in the South Atlantic near the Cox and Rio Grande fracture zones, with attention to regional variations in CCD, sediment mobility, and accumulation rate. Finally, in Chapter 5, topographic data of varying qualities will be used to study variations in topography in the Atlantic basin, with the hope of using inferred sediment accumulation rates to study the distribution of water masses during the Neogene.
Helen Faith Webb
Phone: (617)253-0950
Fax: (617)258-7401
email: hwebb@mit.edu