Quantifying complementary measures of climate-driven sediment dynamics on alluvial fans

The effects of environmental change on eroding landscapes and their sedimentary products remains poorly understood. While sediment routing systems at the Earth’s surface can record changes in past environmental boundary conditions, the extent to which landscapes can buffer signals of climate change—of varying magnitude and timescale— is contentious. Mountain catchments and their alluvial fans offer one way to address this question, as they form accessible sediment routing systems in which source and sink are closely coupled and sediment budgets can be closed. Here we consider the extent to which sediment granulometry in stream-flow-dominated alluvial fans records signals of past environmental change. We focus on well-constrained field examples in Death Valley, California, such as the Hanaupah Canyon Fan, which have experienced climate forcing associated with late Pleistocene glacial-interglacial cycles. Using field-derived measures of grain size, we compare three complementary methods that can be used to reconstruct sediment dynamics on alluvial fans. First, we use a self-similarity analysis of sediment calibre to reconstruct sediment mobility on fans over time. Second, we use a downstream-fining model to evaluate the extent to which different depositional units on the fans may record changing sediment fluxes from source catchments. Third, we adopt a palaeohydrological approach to reconstruct unit discharges, bed shear stresses and instantaneous sediment transport capacities for fans, based on field measures of hydraulic geometry and grain size. We evaluate the extent to which these three methods provide consistent results, and we quantify how grain mobility, water and sediment discharge scale with documented variations in the regional climate. Our work demonstrates the potential for using alluvial-fan sedimentology as an archive of information about palaeo-environmental changes, including quantitative measures of past hydroclimate.