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Title: Numerical, geomorphic and sedimentological approaches to characterise catchment-fan systems as archives of past environmental change
Author: Brooke, Sam
ISNI:       0000 0004 7658 9114
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2019
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The Earth's surface is complex and constantly in flux, its dynamism driven by the competing influence of climatic and tectonic processes. Unravelling these processes to better understand landscape sensitivity is a key challenge in the Earth sciences, and we are tasked with finding environments that may contain valuable clues to the past. The movement of sediment across the Earth's surface can be considered in both time and space, with sedimentologists focussing on the preserved record of earth past, or stratigraphy, and geomorphologists studying the landforms and residual patterns of water, wind and ice driven sediment transport processes. If we are to improve our understanding of the stratigraphic record as an archive of past environmental change we must combine both perspectives. River systems, or fluvial systems, offer a unique opportunity to study the metaphorical conveyor of sediment as it is eroded, transferred and deposited downstream, also known as a sediment routing system. Sediment routing systems constitute the beginning, middle and end of the story of material transfer from source to sink across Earth's surface. Within sediment routing systems, measurable sedimentological elements can help unlock changes in sediment supply and paleohydraulics; key indicators of changing environmental boundary conditions. One key metric is the spatial change grain size stored within fluvial deposits, which has been shown to be a useful indicator of sediment flux changes and also river sediment carrying capacity; two factors which can be directly attributed to climate change. Catchment-fan systems are short sediment routing systems mostly driven by fluvial processes and may represent the simplest and least complex source to sink system we can study. A sediment pulse produced by a sudden change in climate is likely to translate rapidly into a deposit downstream on an alluvial fan. In arid climates such as Death Valley in the United States, coupling between rainfall and alluvial fan sedimentology/geomorphology may be pronounced, and any climate signatures may be detectable from field observation of grain size changes in time and space. In this thesis I begin by outlining the theory and key problems facing the study of landscape sensitivity to climate, followed by four research chapters that attempt to characterise the sedimentary record of alluvial fans and their climatic component. Each of the research chapters employs a different approach to studying alluvial fans in Death Valley, beginning with the first chapter, which aims to explore the application of self-similar solutions for grain size fining as a means of detecting changes in long term grain size mobility since the late Pleistocene. The second research chapter considers the importance of landscape thresholds and rainfall variability in controlling long term stratigraphic architectures, utilising a newly developed iteration of a 1D numerical catchment-fan model. The third research chapter approaches the problem of conducting fieldwork using remote-sensing techniques, capitalising on recent innovations in web-based GIS platforms, namely Google Earth Engine. Using a new application to gather spectral profiles in Landsat 8 imagery I deploy a recently developed technique to detect changing fan surfaces ages, which could greatly assist chronostratigraphy in arid landscapes. The final research chapter utilises remote-sensed channel geometries and field-derived grain size measurements to reconstruct catchment outlet discharges since the last glaciation. This chapter was inspired by the power of individual storm events to mobilise extremely coarse material in the region. I finish this thesis with a final chapter that synthesises these findings and discusses both the importance of this research but also the potential future work that could be conducted to utilise the pertinent and unique climate-sensitivity of catchment-fan systems.
Supervisor: Whittaker, Alexander Sponsor: NERC
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral