by Thomas H. Orsi
Working under Dr. Aubrey Anderson on the Naval Research Laboratory-sponsored Coastal Benthic Boundary Layer Special Research Project (CBBL SRP), I examined spatial relationships between sediment macrostructure (mm-to-cm scale sedimentary features) and variations in sediment physical properties. The importance of environmental processes, such as biological and hydrodynamic mechanisms, in the development of these relationships was particularly significant. The first phase of my research addressed the lack of a suitable quantitative technique with the resolution necessary to analyze the internal structure of intact marine sediment cores. To solve this problem I adopted x-ray computed tomography (CT or CAT scanning) and modified the CAT scanner located in the Department of Petroleum Engineering for use with marine sediment cores. Then I devised a calibration scheme to quantify the scanner's submillimeter spatial resolution and strong linear response to sediment bulk density. The sensitivity of the CAT scanner to sediment bulk density is extremely valuable because this engineering parameter is often needed for numerical seafloor simulations.
[123K] In the second phase of the study I conducted a comparative CAT scan examination of sediment cores from Eckernforde Bay in the western Baltic Sea and the Louisiana continental shelf in the northern Gulf of Mexico. I found distinct environmental and geotechnical differences among regions in which the dominant sources of macrostructural variability were feeding pockets (advective benthic mixing), shells and shell debris, worm tubes, and laminae. Interestingly, sediment variability induced by burrowing (ichnogenus Planolites) was nominal overall, although it produced readily discernable structures. Sediment grain size strongly influences the character of CT variability, but the ultimate magnitude of this variability directly relates to the sediment sorting capacity (spatial segregation of particles by size) of an environmental process. Thus, the variability of physical properties does not result from a specific process per se, but rather the environmental process determines the geometry of the macrostructure. Based on these results, I developed a tiered conceptual model to relate sediment macrostructure and physical property variability for the final phase of my research. I found that mechanical compaction (consolidation) increased substantially at extremely shallow seafloor depths (10 cm or less), suggesting its importance, in addition to hydrodynamic and biological processes, in the macrostructural development of the upper decimeters of the seafloor.
Editor's Note: Thomas H. Orsi successfully defended his dissertation on June 1, 1994. He will remain in College Station with his wife, Laine, and their three children, Katie, Michael, and Rebecca, to continue his research as a research associate under Dr. Anderson. Presently, he is applying the CAT scan technique to sediment cores collected from CBBL SRP study sites off Panama City and Key West, FL, to examine similar relationships for terrigenous sands and carbonate muds.
Oceanography, Texas A&M University
Updated July 24, 1995