The evolution of Late Quaternary alluvial landscapes in northern Oman
Keywords: alluvial fans
palaeoenvironmental change
quaternary
Arabia
geochronology
geomorphology
climate change
luminescence dating

Dryland fans predominately preserve signals of Late Quaternary (350-0 ka) climatic variability within their stratigraphy because of hydrological changes in their feeding catchments. Their morpho-dynamics are also the product of various other non-climatic controls, such as catchment morphology, geology and depositional setting. The mountain front fans of the Hajar Mountains, south-east Arabia, have been shown to aggrade under more sustained streamflow during periods of insolation maxima which coincide with intensified monsoon rainfall, as well as their diverse morphologies indicating complex, non-climatic controls on their evolution. Better understandings of the ways in which landscapes respond to changes in boundary conditions is important in the context of contemporary climate change, as well as understanding long-term human-environment interactions. However, previous palaeoenvironmental and morphological studies have been concentrated in the very north and south of the range.
Today, the Hajar is also environmentally distinct, relative to other regions of the Arabian Peninsula, in that it receives high amounts of rainfall and is a centre of rich biological endemism. The growing body of dated palaeoenvironmental sites available from alluvial fans and other landforms in the Hajar suggest that the region has remained more hydrologically active than surrounding lowlands during more arid periods in the Late Quaternary. Despite this, there has been no prior attempt to synthesise these environmental data for the Arabian Peninsula and compare palaeoenvironmental heterogeneity between different regional settings.
This thesis addresses these knowledge gaps by 1) providing new, dated records of fan aggradation in the central Hajar; 2) conducting a systematic survey of fan and catchment morphology across the Hajar range, and 3) constructing the first database which critically assesses geochronological data for the entire Arabian Peninsula over the last 20 ka.
The fans of the central Hajar record aggradation in line with orbital (primarily precessional) forcing over the last 300 ka. Taken together with alluvial records from the wider region, Hajar fan aggradation provides the longest and most continuous terrestrial record of environmental and climatic change in south-east Arabia. Fan morphologies are strongly controlled by their diverse catchment morphologies and lithologies, with smaller, steeper catchments comprised of less erodible rocks producing smaller, steeper fans. Fans in more complex depositional settings, such as coastlines, have weaker relationships with catchment morphology due to greater external influences. The new data also show that aggradation continued to occur during more arid periods, primarily within catchment settings, but to a lesser degree under more ephemeral flow conditions. Combined with other palaeoenvironmental archives from the Hajar, these findings support hypotheses of more continuous hydrological activity in mountain catchments despite reductions in monsoon rainfall. Therefore, the Hajar, and other uplands in Arabia, appear to be more resilient to regional aridity than lowlands. These findings have important implications for interpreting increasingly complex palaeoenvironmental and archaeological data from south-east Arabia which seemingly conflict with regional-scale palaeoclimate archives.