Comparing erosion rates across timescales and processes: insights from the Western Southern Alps of New Zealand

Understanding the landscape evolution and potential geohazards of mountain landscapes requires quantifying the rates at which they uplift and erode, and the relative importance of different erosional processes. However, each of the available techniques to quantify rates of landscape change provides only a partial account of mountain erosion, given their inherent methodological biases and measuring timescales. Therefore, reconciling erosion and uplift rates estimated with different methods can be challenging, but can also provide insights into changes in erosion rates and dominant erosional processes.

Here, we present a combination of new and compiled data from the western Southern Alps of New Zealand (WSA), one of the fastest-eroding ranges on Earth, which is believed to be in steady state. We present new data on: (a) 20 in-situ ¹⁰Be catchment-averaged denudation rates, which mostly range between ~0.6-9 mm/yr and represent erosion integrated over the last 275 years on average; (b) 17 ¹⁰Be concentrations from recent landslide deposits, which together with drone photogrammetry of landslide scars, provide information about landslide recurrence intervals on millennial timescales; (c) the grain size distributions of sediment supplied from hillslopes and transported by rivers, which allows us to convert published suspended sediment load estimates (0.2-6.7 mm/yr, recorded over the 1960s-1990s) into total sediment flux estimates. Based on (b) and published landslide frequency-area data, we estimate landslide erosion rates on millennial timescales, and compare these to erosion rate estimates from (a), (c); and (d) modern published landslide erosion rates (1.8-18 mm/yr, 1948-1986), (e) published millennial soil erosion rates (up to 2.5 mm/yr), (f) compiled Late Quaternary fault throw rates (up to 12 mm/yr), and (g) recalculated thermochronological exhumation rates (up to ~6-9 mm/yr, Myr timescales).

This comprehensive data set allows us to examine: (i) the proportion of total erosion driven by landslides versus other erosional processes; (ii) how modern denudation rates and landslide erosion rates compare to long-term erosion rate, rock uplift, and exhumation rate estimates; and (iii) the potential fluctuations of erosion rates and processes over seismic cycles and Holocene climate change.