Abrupt Increase in the Mid-Holocene Flood Magnitudes Recorded in the Grain Size of Alluvial Fan Surfaces: Death Valley, CA

Abstract:

Catchment-fan systems are natural recorders of environmental changes. Their sedimentary record is a potential archive of past extreme events, particularly in arid to hyper-arid climates, where sediment is transported only during a few, large floods. Here, we quantify the caliber of sediment transported by fan-forming floods from the Late Pleistocene to modern times in Death Valley, California, to test if these deposits record high-frequency climate change. In the field, we measured 25,300 individual clasts across 4 alluvial fans using the Wolman point count method along downstream apex-to-toe transects. Our measurements were equally distributed across 12 mapped and age-correlated fan surfaces that spanned Late Pleistocene, Mid-Holocene, and modern times. Our results demonstrated that the median grain size (D₅₀) of the modern (40.8±5 mm) and Late Pleistocene (33±6 mm) fan surfaces were broadly similar; however, all the Mid-Holocene fan surfaces (D₅₀ = 60.5±11 mm) were systematically coarser than those of the modern by 1.5 times. We also observed this systematic increase in the coarsest-fraction of sediment transport, characterized by the 90^th percentile of the grain-size distribution (D₉₀). The D₉₀ of the mid-Holocene surfaces was 140±23 mm, compared to D₉₀ of 100±16 mm and 97±8 mm for the Late Pleistocene and modern fan surfaces, respectively. We found that measured fan surface slopes were uncorrelated with measured grain size. We combined our field measurements with quantitative palaeohydraulic tools to show that formative discharges during the Mid-Holocene period were statistically different than the modern and Late Pleistocene values, with Mid-Holocene water discharges at least 40% larger than the modern. In contrast, the estimated formative discharges during the Late Pleistocene period were statistically similar to the modern values. In the absence of any tectonic perturbations, our results support the hypothesis that the Death Valley region experienced a shift in hydroclimate during the Mid-Holocene, potentially due to changing westerly storm tracks or a northward shift in the North American Monsoon. These results highlight how the stratigraphic archive of catchment-fan systems can shed new light on past high-frequency climate variability over the last glacial-interglacial cycle.