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| Titel |
Revealing the three-dimensional geometry of tephra horizons found within
soft sediment deposits |
| VerfasserIn |
Elizabeth Evans, Siwan Davies, Richard Johnston, Peter Abbott, Sabine Wulf |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250150398
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| Publikation (Nr.) |
 EGU/EGU2017-14856.pdf |
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| Zusammenfassung |
| Traditionally, the stratigraphic position of tephra horizons within sedimentary deposits is
identified by visual inspection in the case of macro-deposits (e.g. by distinct changes in
colour or grain-size), or through quantified shard concentration profiles with respect to low
concentration deposits (cryptotephras). Within core samples, the one-dimensional approach
of shard concentration profiles often masks any potential displacement or post-depositional
processes that may have impacted the tephra deposit. We present a new perspective to tephra
analysis and apply X-Ray Computed MicroTomography (XRμCT) to obtain 3D
visualisations of Icelandic tephra horizons preserved within North Atlantic marine
sedimentary sequences. Geological samples present challenges for XRμCT scanning as
X-ray attenuation is controlled by density and effective atomic number. Like many geological
samples, the samples of this study are made up of complex silicates with similar atomic
numbers and so have similar attenuation properties. Therefore, contrast between
sediment and tephra can be very low. Initial experimentation on simulated tephra
deposits in the laboratory provided the basis for devising a series of parameters and
recommendations for scanning to maximise the quality of data attained from scans before
post-processing and analysis. The most successful scans were obtained when the tephra
properties (grain-size and composition) showed a marked contrast to the host material
(e.g. coarse-grained tephra >125 μm in fine mud or basaltic tephra in calcareous
sediment). These optimised scan parameters were applied to real core examples and 3D
visualisations of macro-deposits revealed tephra lined worm casts; down-going
injection-like structures; tephra sinking into the basal sediment; evidence of graded
bedding; and most significantly for those using tephra horizons as stratigraphic markers
(such as used in tephrochronology), discontinuous and fragmented deposition. Such
complex sedimentary features were not apparent in the one-dimensional visual
descriptions.
We also apply the XRμCT technique to investigate the three-dimensional structures
associated with cryptotephra deposits where tephra concentrations may be anywhere between
1 - 400,000 shards per 0.5 g dry weight (in the case of one sample studied). At this
concentration there is no visible macroscopic horizon of tephra in the sample. Our
scans, with an isotropic voxel size of ∼26 μm (giving an effective resolution of
∼50-80 μm3), detected low concentration deposits that broadly agree with the shard
concentration profile derived by conventional microscopy work. These data also
provide new insight into the sedimentary structures and indicate the discontinuous
nature of cryptotephra deposits where “cross-bedding” features between tephra-rich
and poor regions are visible. The unprecedented three-dimensional insight into the
structure of cryptotephra deposits has considerable potential as a routine first step in a
workflow for constructing a tephrostratigraphical framework in sediment cores. |
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