Visualizing tephra deposits and sedimentary processes in the marine environment: The potential of X-ray microtomography

Localized tephra deposition in marine sequences is the product of many complex primary and secondary depositional processes. These can significantl y influence the potential applicability of tephra depos- its as isochronous marker horizons and current techniques, used in isolation, may be insufficient to fully unravel these processes. Here we demonstrate the innovative application of X-ray microtomography ( m CT) to success- fully identify tephra deposits preserved within marine s ediments and use these parameters to reconstruct their internal three-dimensional structure. Three-dimensiona l visualizations and animations of tephra dispersal in the sediment permit a more thorough assessment of postdepositional processes revealing a number of complex microsedimentological features that are not revealed by conventional methods. These features include biotur- bation burrows and horizontally discontinuous tephra packages, which have important ramifications for the stratigraphic placement of the isochron in a sedimentary sequence. Our results demonstrate the potential for utilizing rigorous two and three-dimensional microsedimentological analysis of the ichnofabric to enhance and support the use of tephra deposits as isochronous marker horizons and to identify the stratigraphic position that best reflects the primary fallout of ash. The application also provides an exceptional insight into the style and rate of sedimentation processes and permits an assessment of the stratigraphic integrity of a tephra deposit. We discuss the possibility of applying these m CT methods to the identification of cryptotephras within various paleoclimatic sequences and to enhance our understanding of marine sedimentation processes