Computed tomography of macrostructure and physical property variability of seafloor sediments

Vita.X-ray computed tomography (CT), with concurrent conventional x-radiographic and high-resolution physical property analyses, revealed distinct environmental differences in sediment macrostructure (millimeter-to-centimeter scale) and physical property variability for sediment cores from Eckernforde Bay (western Baltic Sea) and the northern Gulf of Mexico continental shelf, shelf-break, and upper slope. Despite the popular traditional notion--that hydrodynamic activities create and benthos destroy sediment macrostructures--both processes were observed to do both in this study. Traditional interrelationships exist for Eckernforde Bay where the development of lamination and the type and intensity of bioturbation are inversely related. By contrast, hydrodynamic processes in the form of hurricanes homogenize the upper decimeters of the Louisiana continental shelf, but in doing so, generate considerable heterogeneity due to the creation of random shell orientations and localized macrostructures. Macrostructural variability is substantially reduced on the northern Gulf of Mexico shelf-break and upper slope. Although characterized by both laminae and biological reworking, the density contrasts of the structures are insufficient to result in substantial property variability. As demonstrated by CT, the dominant sources of macrostructural variability in the studied sediments are feeding voids, shells and shell debris, worm tubes, and laminae; in contrast, sediment variability induced by burrowing (ichnogenus Planolites) is nominal. Sediment type (grain size) governs the magnitude of CT variability, as the highest variability is associated with the silty sands, and the lowest, with the clays. Hence, the magnitude of macrostructural variability is directly related to the ability of an environmental process to spatially segregate particles by size (sort) or in some way induce a density contrast, and is not a result of the particular process per se. Instead, the type of process determines the geometry of the macrostructure. A tiered conceptual model is presented, based on the vertical distributions of selected statistical CT parameters, to relate sediment macrostructure and physical property variability. In addition to hydrodynamic and biological processes, mechanical compaction (consolidation) is suggested as a third process critical in the development and/or modification of sediment macrostructure and property variability within the upper decimeters of the seafloor