Environmental controls on tephra layer morphology
Thompson, Polly I. J.
The overall aim of this thesis is to assess the use of tephra layer morphology to infer surface qualities of past landscapes. By examining how environmental processes affect the development of an enduring tephra record, this thesis assesses the potential of using tephra layer morphology to infer surface environmental conditions of the past. A variety of processes operate at both the land surface and in the near sub-surface that can impact the preservation of tephra layers, and this thesis examines whether this is expressed in layer thickness, morphology and grain size distribution (GSD). A preserved tephra layer may have been fundamentally shaped by the nature of the surface environment at the time the tephra was deposited. These are assessed by examining different tephra layers (up to 20 cm thick) in a variety of different environmental settings in Iceland, collecting over 10,000 individual measurements, combined with controlled experiments in Scotland. A series of case-studied are used: 1) assessing the relationship between tephra layer thickness and GSD with the rate at which the surface tephra deposit is buried and preserved. 2) assessing the effect of small scale topographic features (cryogenic landforms found in Iceland – thúfur) on tephra layer morphology and GSD. 3) identifying the effects of slope angle on the movement of different tephra and the influence of different slope angles on layer formation. 4) evaluating morphological and GSD changes in tephra layers across cultural landscapes to infer land use and 5) testing new ways in which tephra layer morphology can measured and visualised using CT scanning. Results indicate that there are circumstances when environmental processes taking place in the days, weeks, months and years post deposition can indeed alter tephra layer morphology. Therefore, the variable preservation of tephra layers do record aspects of environmental processes and thus may enhance palaeoenvironmental reconstruction. Areas with rapid rates of burial preserve a greater proportion of the original fallout than areas with slower soil accumulation rates. The burial rate itself does not appear to correlate to the variability within the layer, indicating that processes occurring soon after the eruption are more influential. Small-scale landscape features such as thúfur formations that include locally steep slope angles (above the relevant angle of repose) can alter the thickness and grain size distribution of tephra layers across the features themselves, meaning that the variability of the layers provide evidence of micro-topographic features being present on the surface at the time of tephra deposition. Within a uniform vegetation cover that subsumes the tephra deposit, significant movement of tephra downslope only occurs when the slope angle is above the critical angle of movement. Tephra layer thickness variation is associated with changes in vegetation coverage onto which the tephra was initially deposited and thus tephra layer morphology which may be used as an indication of past land management. Due to human interventions, tephra deposits in areas actively managed (such as on occupied farms) may be actively cleared of fallout and farmed after an eruption. Combining the analysis of tephra layer morphology with historical data on social status and farming practice provides a novel insight into land use at the time of an eruption and in the years after. Whilst layer thickness (and thus mass loading) is a useful metric, grain size distribution does not appear to be an indicator of different land uses. Testing CT scanning as a method to visualise preserved tephra layers was successful and indicates that this technique can be used in future studies of tephra morphology changes to complement and extend the insights possible with studies of soil micromorphology. This work adds to the growing body of research that is using tephra layers for more than chronology in the science of environmental reconstruction. It also highlights the ways that surface environmental processes may affect the preservation of tephra deposits in subtle yet pervasive ways that may be unrecognised by those using tephra layers to reconstruct volcanic processes. Environmental controls on tephra layer morphology are therefore widespread and are likely to occur on all tephra layers as they are preserved into the stratigraphy. This has significant implications for inferences of volcanic process made using tephra, which should be taken account of in future volcanological research.
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