Eroding Australia: rates and processes from Bega Valley to Arnhem Land

We report erosion rates determined from in situ produced cosmogenic Be across a spectrum of Australian climatic zones, from the soil-mantled SE Australian escarpment through semi-arid bedrock ranges of southern and central Australia, to soil-mantled ridges at a monsoonal tropical site near the Arnhem escarpment. Climate has a major effect on the balance between erosion and transport and also on erosion rate: the highest rates, averaging 35 m Ma, were from soil-mantled, transport-limited spurs in the humid temperate region around the base of the SE escarpment; the lowest, averaging about 1.5 m Ma, were from the steep, weatheringlimited, rocky slopes of Kings Canyon and Mt Sonder in semi-arid central Australia. Between these extremes, other factors come into play including rock-type, slope, and recruitment of vegetation. We measured intermediate average erosion rates from rocky slopes in the semi-arid Flinders and MacDonnell ranges, and from soil-mantled sites at both semi-arid Tyler Pass in central Australia and the tropical monsoonal site. At soil-mantled sites in both the SE and tropical north, soil production generally declines exponentially with increasing soil thickness, although at the tropical site this relationship does not persist under thin soil thicknesses and the relationship here is ‘humped’. Results from Tyler Pass show uniform soil thicknesses and soil production rates of about 6.5 m Ma, supporting a longstanding hypothesis that equilibrium, soil-mantled hillslopes erode in concert with stream incision and form convex-up spurs of constant curvature. Moreover, weathering-limited slopes and spurs also occur in the same region: the average erosion rate for rocky sandstone spurs at Glen Helen is 7 m Ma, similar to the Tyler Pass soil-mantled slopes, whereas the average rate for high, quartzite spurs at Mount Sonder is 1.8 m Ma. The extremely low rates measured across bedrock-dominated landscapes suggest that the ridge–valley topography observed today is likely to have been shaped as long ago as the Late Miocene. These rates and processes quantified across different, undisturbed landscapes provide critical data for landscape evolution models. It is widely held that different climatic regions are characterized by different geomorphological processes and landforms. Being a tectonically quiet continent, Australia might be expected to express, almost purely, the effect of climate on landscape evolution, modulated only by lithology and ancient structures. From the temperate eastern highlands through the monsoonal north to the arid centre, the continent presents a broad suite of climatic zones and associated landforms, ranging from soilmantled ranges to stony mesas and inselbergs. However, the climate of Australia changed greatly during its northward drift over the last few tens of millions of years (e.g. see Fujioka & Chappell 2010), and whether the landforms seen today were formed under climates like those of today depends on their rates of geomorphological change, governed by the erosional processes acting upon them. Erosion rates determined from in situ cosmogenic nuclides at some sites in Australia are very low (c. 1 m Ma), such as from residual hills in the semi-arid zone (Bierman & Caffee 2002). If such rates were widespread, landscapes with local relief of tens to hundreds of metres would be unlikely to have evolved under climates similar to those of today. We explore relationships between erosion rate and geomorphological form using cosmogenic nuclide measurements of erosion rates in undisturbed hilly landscapes with local relief ranging to several hundred metres, in several climatic provinces across Australia, including the temperate SE, the monsoonal tropics and the semi-arid centre. We determined erosion rates using cosmogenic Be and Al in samples collected across spurs From: Bishop, P. & Pillans, B. (eds) Australian Landscapes. Geological Society, London, Special Publications, 346, 225–241. DOI: 10.1144/SP346.12 0305-8719/10/$15.00 # The Geological Society of London 2010. and slopes, as well as average erosion rates from samples of stream and river sediments. Concepts and methods Landforms evolve by surface lowering, caused by loss or erosion of rock (including weathered rock). In this study, measurements of cosmogenic radionuclides (Be and Al) are used to estimate erosion rates of both soil-mantled and bare rock surfaces [reviews of methods used here have been given by Nishiizumi et al. (1993), Bierman (1994), Bierman & Steig (1996), Granger et al. (1996), Gosse & Phillips (2001), and Cockburn & Summerfield (2004)]. As shown by Lal (1991), in situ cosmogenic nuclide content is inversely related to erosion rate: N(z) 1⁄4 P0e mz lþ m1 (1) where N(z) is nuclide content (atoms per gram) at depth z, P0 is surface (z 1⁄4 0) production rate of the nuclide of interest at the latitude and altitude of the sample site, m 1⁄4 r/L where r is rock density and L is mean free path of cosmic rays, l is the radionuclide decay constant, and 1 is erosion rate. It is important to note that equation (1) assumes that erosion proceeds smoothly at the grain or small fragment scale and at a constant rate. Taking into account the effects of latitude, altitude and topographic shielding on P0, equation (1) is widely used to calculate rates of erosion and soil production. The approaches to soil-mantled and rocky surfaces are, however, different, and results for both cases may be distorted by the effects of climatic shifts, and are summarized briefly. Soil-mantled slopes. Where soil is produced through weathering of the underlying rock and soil is lost by soil transport at the same rate as it is produced, erosion equates to soil production. Where transport can be characterized as diffusion-like creep, soil thickness depends on slope curvature and, provided that factors affecting creep such as soil biota and vegetation cover remain unchanged, the balance between soil production and transport should be in steady state and soil thickness at any point should remain constant (Dietrich et al. 1995; Heimsath et al. 1997, 1999). (It should be noted that, as land surfaces are lowered, slope curvature may slowly change and steady state cannot persist indefinitely, but is approximated for metre-scale lowering where the radius of slope curvature is large relative to soil thickness.) When steady-state conditions hold, the rate of erosion/soil production can be determined from measurements of cosmogenic nuclides in weathered rock at soil base, using equation (1) with z 1⁄4 soil thickness. One criterion for steady state is that soil depth is negativeexponentially dependent on curvature (Heimsath et al. 1997, 1999); another is that the rate of lowering at a point remains constant, which can be tested by measuring cosmogenic nuclide profiles down the sides of emergent bedrock tors, if present (Heimsath et al. 2000). Rocky slopes. Unless rocky surfaces are eroding smoothly by solution or grain-by-grain loss, surface lowering tends to occur by intermittently shedding of joint-controlled blocks or exfoliation slabs. The apparent erosion rate calculated by equation (1) for a surface sample will vary according to the time elapsed since the surface was exposed after loss of a prior block: for example, if Nu is the nuclide content at the upper surface of a block when it falls off, the nuclide content Nb of the surface exposed immediately after it fell is