Abstract
Abstract This review focuses on the recognition of volcanic ash occurrences in marine sediment cores and on using their appearance and properties to deduce their origin. Widespread marine tephra layers are important marker horizons for both volcanological as well as general geological investigations. We describe ash detection by visual inspection and logging of sediment cores. Ash layer structure and texture, particle morphologies and lithological compositions of primary volcanic deposits are summarized and processes modifying them are discussed, both natural processes acting on and in the seafloor, i.e. erosion and bioturbation, and anthropogenic modifications during drilling/coring and core preparation. We discuss primary emplacement processes of marine fall and flow tephra deposits derived from either subaerial or submarine sources in order to identify distinguishing properties. We also elaborate on processes generating secondary, resedimented volcaniclastic layers such as submarine landslides and shelf erosion as well as fluvial input and ice-rafting, and how they can be distinguished from primary volcaniclastic deposits, which is essential in tephrostratigraphy. Finally, methods of tephra correlation between cores and on-land deposits/volcanoes are illustrated because they allow us to extend the 1D information from single cores to 3D distribution and facies changes of tephras and to bridge the land–sea gap.
Highlights
We have reviewed the appearance of marine volcaniclastic beds, which depends on the mode of emplacement by fallout or density current, the nature of subaerial or submarine eruption or of the submarine resedimentation processes, the distance and direction from primary or secondary source, the extent of modification on the seafloor by erosion and bioturbation, and deformation during coring and core processing
The conditions of postemplacement modifications on the seafloor constrain regions where ash layers have the best chance to survive until deeper burial
While seismic pre-site surveys reveal the best locations for ocean-drilling sites, gravity-core locations are best chosen by combining high-resolution bathymetry with hydroacoustic sub-bottom profiling of the target region
Summary
The cutting effect can show pod-like 2D shapes on the drill core surface which, are part of a complex 3D structure of the (deformed, disrupted, bioturbated) ash deposit Such complex structures can nowadays be visualized by high-resolution X-ray micro-tomography (Griggs et al 2015), which can reveal the mechanism of formation more accurately. Huang et al 1975; Kennett 1981; Hess and Kuhnt 1996; Marquez 2000; Manville and Wilson 2004; Todd et al 2014; Hopkins et al 2020) This suggests that tephra horizons ,1 cm thick are insufficient to suppress benthic biota activity or post-depositional bioturbation Non-modified cryptotephra will show a sharp peak in shard concentrations and an obvious isochron (e.g. Berben et al 2020)
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