Denudation rates across a steep rainfall gradient on Kauai, constrained by cosmogenic nuclides and landslide mapping (Invited)

Climate has long been thought to influence landscape form and dynamics, but climatic effects on erosion rates have been difficult to discern in field measurements, in part because site-to-site variations in climate are often accompanied by variations in non-climatic factors that also affect erosion rates, such as rock uplift rates and lithology. The Hanalei River canyon on Kauai offers an exceptional natural laboratory for studying landscape evolution under spatial variations in climate, as it is home to one of Earth's steepest precipitation gradients and it exhibits minimal variations in lithology. Over a 15 km transect, mean annual rainfall in the Hanalei basin ranges from >11 m/yr at Mt. Wai'ale'ale - one of the wettest places on Earth - to <2 m/yr at the river's outlet to the ocean. Field observations and satellite imagery suggest that soils in the Hanalei basin are intensely weathered and that hillslope erosion proceeds by a combination of soil creep and frequent shallow landsliding. Over the past fifteen years, many studies have inferred basin-wide millennial-scale denudation rates from concentrations of cosmogenic nuclides in detrital sediment under the assumption of steady erosion at the hillslope surface. To assess the effects of non-steady erosion on cosmogenic ³He in detrital olivine in the Hanalei basin, we modeled mineral exposure to cosmogenic radiation on eroding hillslopes in a synthetic Hanalei-like basin. In this model, hillslope mass transport proceeds by a combination of slow, steady erosion at the hillslope surface and intermittent shallow landsliding. Under this erosional scenario, modeled cosmogenic ³He concentrations in detrital olivine largely reflect the background hillslope denudation rate and are largely insensitive to additional erosion by shallow landslides. When interpreted within this framework, our measurements of ³He in detrital olivine imply minimum denudation rates as high as 443 t km^-2 yr^-1 in Hanalei’s wettest, steepest tributary basin and as low as 133 t km^-2 yr^{-1} in a drier, gentler tributary basin 10 km downstream of the headwaters. To supplement these ^3He measurements, we mapped >1000 landslides in repeat images of the Hanalei basin from aerial photography and satellite imagery from 2003-2010. Our initial mapping implies an average landslide scar generation rate of 336 m^2 km^{-2} yr^{-1} over the lower two-thirds of the Hanalei basin, where repeat satellite images overlap and permit comparisons of landslide coverage over time. When combined with empirical relationships relating landslide volume to area and our field measurements of soil density (1.1 g cm^{-3}), this scar generation rate corresponds to a short-term landslide erosion rate of 317 t km^{-2} yr^{-1}. To date, our combined measurements of ^3He and landslide occurrence are consistent with faster total erosion rates in wetter, steeper basins.