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Rates and Processes from Bega Valley to Arnhem Land

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Rates and Processes from Bega Valley to Arnhem Land Eroding Australia: rates and processes from Bega Valley to Arnhem Land ARJUN M. HEIMSATH1*, JOHN CHAPPELL2 & KEITH FIFIELD3 1School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, USA 2Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia 3Research School of Physical Sciences and Engineering, Australian National University, Canberra, ACT 0200, Australia *Corresponding author (e-mail: [email protected]) Abstract: We report erosion rates determined from in situ produced cosmogenic 10Be 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 Ma21, 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 Ma21, were from the steep, weathering- limited, 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 Ma21, 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 Ma21, similar to the Tyler Pass soil-mantled slopes, whereas the average rate for high, quartzite spurs at Mount Sonder is 1.8 m Ma21. 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 by the erosional processes acting upon them. Erosion characterized by different geomorphological pro- rates determined from in situ cosmogenic nuclides at cesses and landforms. Being a tectonically quiet some sites in Australia are very low (c.1mMa21), continent, Australia might be expected to express, such as from residual hills in the semi-arid zone almost purely, the effect of climate on landscape (Bierman & Caffee 2002). If such rates were wide- evolution, modulated only by lithology and ancient spread, landscapes with local relief of tens to hun- structures. From the temperate eastern highlands dreds of metres would be unlikely to have evolved through the monsoonal north to the arid centre, the under climates similar to those of today. continent presents a broad suite of climatic zones We explore relationships between erosion rate and associated landforms, ranging from soil- and geomorphological form using cosmogenic mantled ranges to stony mesas and inselbergs. nuclide measurements of erosion rates in undis- However, the climate of Australia changed greatly turbed hilly landscapes with local relief ranging during its northward drift over the last few tens of to several hundred metres, in several climatic pro- millions of years (e.g. see Fujioka & Chappell vinces across Australia, including the temperate 2010), and whether the landforms seen today were SE, the monsoonal tropics and the semi-arid centre. formed under climates like those of today depends We determined erosion rates using cosmogenic on their rates of geomorphological change, governed 10Be and 26Al 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. 226 A. M. HEIMSATH ET AL. and slopes, as well as average erosion rates from nuclides in weathered rock at soil base, using samples of stream and river sediments. equation (1) with z ¼ soil thickness. One criterion for steady state is that soil depth is negative- exponentially dependent on curvature (Heimsath Concepts and methods et al. 1997, 1999); another is that the rate of lower- ing at a point remains constant, which can be tested Landforms evolve by surface lowering, caused by by measuring cosmogenic nuclide profiles down the loss or erosion of rock (including weathered rock). sides of emergent bedrock tors, if present (Heimsath In this study, measurements of cosmogenic radio- et al. 2000). nuclides (10Be and 26Al) are used to estimate erosion rates of both soil-mantled and bare rock Rocky slopes. surfaces [reviews of methods used here have been Unless rocky surfaces are eroding smoothly by solution or grain-by-grain loss, sur- given by Nishiizumi et al. (1993), Bierman (1994), Bierman & Steig (1996), Granger et al. face lowering tends to occur by intermittently shed- ding of joint-controlled blocks or exfoliation slabs. (1996), Gosse & Phillips (2001), and Cockburn & Summerfield (2004)]. As shown by Lal (1991), The apparent erosion rate calculated by equation in situ cosmogenic nuclide content is inversely (1) for a surface sample will vary according to the time elapsed since the surface was exposed after related to erosion rate: loss of a prior block: for example, if Nu is the Àmz nuclide content at the upper surface of a block P0e N(z) ¼ (1) when it falls off, the nuclide content Nb of the l þ m 1 surface exposed immediately after it fell is À mL where N(z) is nuclide content (atoms per gram) at Nb ¼ Nu e (2) depth z, P0 is surface (z ¼ 0) production rate of the nuclide of interest at the latitude and altitude of the where L is block thickness. For jointed rocks with sample site, m ¼ r/L where r is rock density and joint spacing L, the nuclide content of most surfaces L is mean free path of cosmic rays, l is the radio- should lie in the range Nb to Nu. Moreover, the mean nuclide decay constant, and 1 is erosion rate. It is nuclide content, N, of a block at the moment of important to note that equation (1) assumes that falling is erosion proceeds smoothly at the grain or small frag- ment scale and at a constant rate. Taking into account (1 À eÀmL) N ¼ N : (3) the effects of latitude, altitude and topographic u m L shielding on P0, equation (1) is widely used to cal- culate rates of erosion and soil production. The Numerical evaluation of stepwise erosion by succes- approaches to soil-mantled and rocky surfaces are, sive block losses shows that the mean value of however, different, and results for both cases may nuclide content in a block closely approximates the be distorted by the effects of climatic shifts, and uniform-rate value in equation (1), for blocks up to are summarized briefly. several metres thick. Provided that block break-up is statistically uniform, the average content of Soil-mantled slopes. Where soil is produced through newly fallen blocks (or sediment derived from weathering of the underlying rock and soil is lost by them) will represent the average erosion rate and, soil transport at the same rate as it is produced, having measured N, the expected range of values erosion equates to soil production. Where transport for a set of random samples from a blocky slope can be characterized as diffusion-like creep, soil can be estimated from equations (2) and (3) together thickness depends on slope curvature and, provided (also see Reinhardt et al. 2007). that factors affecting creep such as soil biota and vegetation cover remain unchanged, the balance Effects of climatic shifts. Major climatic changes, between soil production and transport should be in such as occurred widely and repeatedly throughout steady state and soil thickness at any point should the Quaternary Era, can nullify the assumption of remain constant (Dietrich et al. 1995; Heimsath uniform erosion. For example, slopes that today are et al. 1997, 1999). (It should be noted that, as land soil-mantled may have had negligible soil cover surfaces are lowered, slope curvature may slowly under Late Pleistocene arid or periglacial condi- change and steady state cannot persist indefinitely, tions, in which case the cosmogenic nuclide but is approximated for metre-scale lowering content will be higher than expected under where the radius of slope curvature is large relative steady-state soil cover and the erosion rate calcu- to soil thickness.) When steady-state conditions lated using present soil depth will be falsely low. hold, the rate of erosion/soil production can be Conversely, if slopes that today are bare rock determined from measurements of cosmogenic were previously mantled by soil or sediment, the ERODING AUSTRALIA 227 erosion rate calculated for a bare surface will be too have previously been described in detail whereas high. In short, where these scenarios are likely, results from the remainder are reported here for apparent erosion rates for soil-mantled slopes the first time, some of which will be the subject should be taken as minimum estimates, and those of later, more detailed papers. No major post- for bare rocky surfaces as maximum estimates.
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