Detection of transience in eroding 1 landscapes

6 Past variations in climate and tectonics have led to spatially and temporally varying 7 erosion rates across many landscapes. In this contribution I examine methods for 8 detecting and quantifying the nature and timing of transience in eroding landscapes. At a 9 single location, cosmogenic nuclides can detect the instantaneous removal of material or 10 acceleration of erosion rates over millennial timescales using paired nuclides. Detection 11 is possible only if one of the nuclides has a significantly shorter half-life than the other. 12 Currently, the only practical way of doing this is to use cosmogenic in-situ C alongside 13 a longer lived nuclide, such as Be. Hillslope information can complement or be used in 14 lieu of cosmogenic information: in soil mantled landscapes, increased erosion rates can 15 be detected for millennia after the increase by comparing relief and ridgetop curvature. 16 This technique will work as long as the final erosion rate is greater than twice the initial 17 rate. On a landscape scale, transience may be detected based upon disequilibria in 18 channel profiles or ridgetops, but transience can be sensitive to the nature of transient 19 forcing. Where forcing is periodic, landscapes display differing behavior if forcing is driven 20 by changes in base level lowering rates versus changes in the efficiency of either channel 21 or hillslope erosion (e.g. driven by climate change). Oscillations in base level lowering 22 lead to basin averaged erosion rates that reflect a long term average erosion rate despite 23 Simon M. Mudd, Detection of transience; revised manuscript for ESPL. 2 strong spatial heterogeneity in local erosion rates. This averaging is reflected in Be 24 concentrations in stream sediments. Changes in hillslope sediment transport coefficients 25 can lead to large fluctuations in basin averaged erosion rates, which again are reflected 26 in Be concentrations. The variability of erosion rates in landscapes where both the 27 sediment transport and channel erodibility coefficients vary is dominated by changes to 28 the hillslope transport coefficient. 29 Introduction 30 Gone are the days when geomorphologists thought of landscapes as experiencing a 31 period of ‘rejuvenation’ followed by a dignified, if not dull, period of gradual adjustment 32 (c.f., Davis, 1899). We now understand the Earth’s crust to be constantly in motion, with 33 faults rupturing and plates buckling under tectonic stresses, all leading to surface 34 deformation (e.g., Kirby and Whipple, 2010). In addition, there is now growing recognition 35 that the mantle also has a role to play, as large scale convection, mantle plumes and 36 diapirs are thought to lead to vertical displacements over geologic timescales (e.g., 37 Rohrman and van der Beek, 1996; Saunders et al., 2007; Braun, 2010; Hartley et al., 38 2011; Moucha and Forte, 2011). In addition, plutonism can lead to density differences 39 that drive uplift (e.g., Braun et al., 2011). Isotope records show beyond doubt that our 40 planet’s climate varies wildly and sometimes abruptly, with ice sheets growing and 41 shrinking (e.g., Dansgaard et al., 1993) and sea levels rising and falling by tens of meters 42 (e.g., Lambeck and Chappell, 2001). 43