Week 3 (Feb 12): Erosion and Sediment Transport Discussion Leader: Ariel Deutsch The papers this week explore the topics of erosion and sediment transport, with a major theme revolving around climate-tectonic interactions. Sweeney, K.E., Roering, J.J., Ellis, C., 2015. Experimental evidence for hillslope control of landscape scale. Science 349, 51 53. This study derived laboratory experiments to test the landscape evolution theory, which states that climate sets the scale of landscape dissection through the competition between diffusive and advective processes. Convex hillslopes are produced through diffusive processes (e.g., creep, rain splash, biotic disturbance) and concave valleys are formed from advective processes, where sediment is transported by debris flow or flowing water. Specifically, these lab experiments examined whether the competition between hillslope transport and valley incision determines the spatial scale of landscapes. Lab experiments provide an important intermediate between numerical modeling and field tests. The article utilizes two main equations, derived from the principle of conservation of mass: Fig. 3. Steady-state topography and hillslope morphology. (A to E) Hillshades of experimental topography for (A) 0% drip, (B) 18% drip, (C) 33% drip, (D) 66% drip, and (E) 100% drip overlain with channel networks (blue) and locations of hillslope profiles (red). Topography is 475.5 mm wide in plan-view. (F) Elevation profiles of hillslopes marked by red lines in (A) to (E). Vertical and horizontal length scales are equal. (1) and (2), where dz/dt = rate of elevation change, U = uplift rate, D = hillslope diffusivity, K = stream power constant, P = precipitation rate, A = drainage area, S = slope, m and n are constants, Pe = the Peclet number (the strength of hillslope transport relative to channel processes), and L is the horizontal length scale. From Eq. 1, the authors created an experimental design based on three important components: base-level fall (uplift), surface runoff (advection), and sediment disturbance via rainsplash (diffusion). Two rainfall systems were attached above a substrate of crystalline silica that was mixed with water for sediment cohesion; a mister that resulted only in surface runoff and a drip box that resulted in rainsplash and craters that also caused sediment transport. Each experimental trial was varied by the amount of drip, and fans were used to randomize the location of drip fall. Each experiment ran until a flux steady state was reached, where the spatially averaged erosion rate equaled the base-level fall rate. Fig. 3 illustrates the results of the experiments. Landscapes with less drip showed an increase in surface runoff transport and were more densely dissected. As the percentage of drip increased, the landscapes transitioned to smoother topography, with large regions that were not channelized, as well as wider and more broadly-spaced valleys. Because hillslopes transects increased in length and curvature with an increase in rainsplash (Fig. 3F), the researchers confirmed that their methods were appropriate in modeling adjustments in hillslope transport efficiency. Because both D and K change in the derived experiments, P e was calculated to determine how diffusive and advective processes contribute to the observed landscape transition. Overall, P e increased with the percentage of drip (which resulted in a higher drainage density and shorter slopes), suggesting that an increase in hillslope transport efficiency is dominantly controlled by an increase in rainsplash frequency. In conclusion, the authors demonstrate that hillslope transport exerts a first-order control on landscape scale with their experiments. 1
Von Blanckenburg, F., 2005. The control mechanisms of erosion and weathering at basin scale from cosmogenic nuclides in river sediment. Earth and Planet. Sci. Lett. 237, 462 479. Cosmogenic nuclides can be produced in situ when secondary cosmic rays reach the Earth s surface and can be used to measure spatially averaged denudation rates. The nuclides are generally produced within the upper 1 m of the surface, so concentrations reflect a denudation history (both physical erosion and chemical weathering processes) of material passing through this depth interval. 10 Be is a preferred nuclide because it is almost non-existent in the rock before being produced via cosmic ray interaction with the surface. When using cosmogenic nuclides to derive denudation rates, you are operating under the assumptions that denudation is steady and has been occurring for a period of time greater than the denudation time scale. Averaging time scales, which increase as denudation rates decrease, correspond to the time it takes to erode the upper ~0.6 m of bedrock or ~1.0 m of soil. Overall, cosmogenic nuclides have been shown to be relatively insensitive to shortterm change of erosion rates, except when anthropogenic-induced soil erosion is substantial. Cosmogenic nuclide-derived denudation rates average over 1 100 ky timescales, filling the gap between shorter timescales of hydrologic and land use processes derived from river load gauging and longer timescales observed with zircon and apatite fission track data. Previous work has documented a correlation between basin relief and erosion rate, in which regions with greater relief (used as a proxy for hillslope, curvature, and rock uplift) had greater denudation rates (e.g., Ahnert, 1970). When denudation rates show a strong dependency on relief, the landscape may be adjusting to tectonic change. Uniform denudation rates in a landscape may be explained by a geomorphic steady state. And finally, a weak correlation between relief and denudation rate may be explained by either a landscape that is approaching a geomorphic steady state or a landscape that is steady state but has been perturbed by short-term overprinting of the topography by glaciation. These different scenarios illustrate how active tectonic forcing exerts a dominant control over denudation. Blackenburg discusses how measurements of cosmogenic nuclides allow for the comparison of denudation rates across different climate regimes. Fig. 8 illustrates that weathering is vaguely if at all correlated with precipitation (Fig. 7a) or temperature (Fig. 8b). There is, however, a strong correlation between physical erosion and chemical weathering (Fig. 8d). The observed scatter reflects climate-dependent variations, which are elucidated in Fig. 8c, where the chemical depletion fraction (relative role of chemical weathering in denudation) is shown to correlate with precipitation levels. This shows that once the dominating effects of physical erosion (likely related with tectonic forcing) are removed, weak trends suggest that precipitation and temperature aid chemical weathering. The author concludes by discussing the application of cosmogenic nuclides to calculate paleo-denudation rates and future studies that can explore the relationship between topography and denudation on all spatial scales. Fig. 8. (a) Chemical weathering rate versus mean annual precipitation; (b) Chemical weathering rate versus mean annual temperature; (c) Chemical depletion fraction (ratio of chemical weathering rate to total denudation rate) versus mean annual precipitation; (d) Chemical weathering rate versus physical erosion rate. 2
Willenbring, J.K., Codliean, A.T., McElroy, B., 2013. Earth is (mostly) flat: Apportionment of the flux of continental sediment over millennial time scales. Geology 41, 343 346. Cosmogenic nuclides can be used to calculate average sediment fluxes over long periods of time. The purpose of this study was to measure production of continental sediments at time scales of 1 ky 1 my using these geochemical tracers, and compare the calculated rate to rates determined from both fluvial suspended sediment loads from large rivers and continental sediment and rock volumes. The authors studied 990 river basins with areas between 1 and 10,000 km 2, measured 10 Be data concentrations in sediments to derive mean denudation rates, and measured topographic metrics for discrete drainage basins with outlets at the sampled 10 Be sites. The authors calculated erosion rates ranging from <0.5 to >6,000 mm/ky, and derived a function for denudation rate (D, mm/ky): D = 11.9e 0.0065S, where S = slope (m/km). The intercept represents the average erosion rate of ~50% of Earth s surface. This equation predicts that the denudation rate increases exponentially with slope, and that there is substantial variation in denudation rates across Earth as a function of basin-scale slope. The derived function, however, breaks down for slope values <~200 m/km (or ~10 ). There are other environmental factors that affect erosion, reflected in the scatter in the collected data, but the authors highlight the fact that mean basin slope explains >50% of the global variance of denudation rates at cosmogenic nuclide time scales. The authors predict an annual global sediment production rate (including both chemical and physical erosion) of 5.5 Gt (Fig. 3), by summing the relation between basin slopes and average denudation rates. When closed basins are excluded from this calculation, the prediction lowers, suggesting that ~20% of the produced sediment does not discharge into the oceans. Fig. 3. A: Slope (x-axis) versus denudation rate (right axis), land area (red), and total sediment production rate (blue). Dashed curves mark area and sediment production with endorheic basins removed. B:Sensitivity analysis exploring contribution to total global sediment flux of areas with slopes <10 m/km, 20 m/km, and 100 m/km as function of their average denudation rates; using values predicted by text Equation1(yellow circles), these areas contribute ~40%, 53%, and 81% of total sediment flux, respectively. Fig. 3 illustrates the inverse relationship between slope and sediment production. Overall, the sediment production per unit area is substantially greater in mountainous regions than in lowland areas. However, although higher slope regions erode more quickly, they cover a relatively small portion of the surface. Lower slope areas, although they erode more slowly, cover enough surface area to outweigh the production rate differential between steep and lowland areas. Previous researchers (Milliman and Syvitski, 1992) suggested that small mountain rivers contribute the most sediment to the ocean, but Willenburg et al. propose the steady erosion over large areas of low slopes overpowers the small areas of high slopes. The authors conclude by discussing the implications for the global silicate weathering flux, and that previous studies indicate that silicate weathering rates and denudation are highly correlated. A silicate cation denudation rate of 0.6 t/km/yr was calculated, which translates to 0.72 x 10 8 tco 2 /yr for the ice-free area of Earth. Willenburg et al. discuss that these denudation rates are for quartzbearing landscapes, and should thus be viewed as a minimum since mafic rocks have greater proportions of Ca and Mg to supply for carbonate formation. Willenburg et al. suggest that the real driver of denudation and environmentally or tectonically driven climate change is still unknown. 3
Whipple, K., 2009. The influence of climate on the tectonic evolution of mountain belts. Nature Geoscience 2, 97 104. This paper examines the effect of climate on tectonic evolution of mountain ranges. Previous research suggests that climate influences the internal dynamics of actively deforming collisional mountains, but collecting field data to corroborate this influence has been historically difficult. The author postulates how to derive field tests that may be effective in exploring the links between climate and tectonic evolution of active mountain belts. Whipple explores three orogenic systems: 1) fixed-width systems, 2) frictional critical-taper wedges, and 3) large, hot orogenic systems. The fixed-width system is not a realistic analogue for active collisional mountain belts, but represents an inactive system where there are no interactions between the rate/style of deformation and climate-driven erosion. In this system, topography is isostatically compensated. The mountain range grows in height until a balance is achieved between erosional efflux and tectonic influx. The erosional efficiency of a material determines the rate of erosion, and depends on the rock type, debris size, and climate. Regions with a greater erosional efficiency (i.e., a wetter climate) also have a lower steady-state relief, although no difference in a steady-state rock uplift rate. A transient effect of an increase in erosional efficiency is isostatic rebound caused by the erosional unloading. An asymmetry in erosional efficiency can create a topographic asymmetry, causing increased precipitation on windward slopes and a rainshadow effect on leeward slopes, and can cause a tectonic asymmetry, as seen in the Southern Alps. Although isostatic rebound in response to an increase in erosion has been suggested as evidence for a climatic control over tectonics, Whipple points outs that such a response occurs in a fixed-width system. Therefore evidence of isostatic rebound and an increase in sediment delivery with the onset of glaciation does not demonstrate a link between climate and tectonics, but does demonstrate that climate change can enhance erosional efficiency. Next Whipple discussed friction-dominated narrow mountain belts. The total sediment delivery to adjacent basins does not differ between fixed-width and wedge systems and therefore cannot be used as evidence for or against interactions between climate and tectonics. Importantly, models indicate that a tectonically driven increase in sediment delivery would result in an increase in width and relief of the mountain belt, while a climatically driven increase would result in a decrease in the mountain belt width and relief. Using analytical and numerical models as a starting point, Whipple discusses two important diagnostic criteria for determining climatic influence on tectonics in the field. Firstly, a climatic influence on tectonics that causes an increase in sediment delivery to basins will also result in a retreat of active deformation to the interior of the range and a reduction in the foreland subsidence rate. And secondly, responsive critical-taper wedge systems experience a change in near-surface rock uplift rate that persists beyond the initial isostatic rebound. Finally, the author discusses large, hot orogens, where climate-tectonic feedbacks may be particularly strong. In such cases, an erosional-rheological positive-feedback loop causes upward and outward flow of rock. In summary, it is difficult to resolve whether climate substantially influenced the tectonic evolution of mountain belts. Whipple stresses the importance of field tests to groundtruth model predictions and discusses the difficulties in deriving appropriate lab experiments, explaining that there are no perfect experiments that can isolate the effects of climate on the evolution of mountain belts. For example, isostatic responses to erosion can occur even in inactive orogenic systems and do not readily demonstrate a link between climate and tectonics. In addition, rainshadow effects can cause spatial correlations between intense rainfall and rapid rock-uplift, even in the absence of erosional influence on tectonics. Lastly, the timescales of climate change and mountain building are significantly different. Because of all of these factors, Whipple suggests that future work would benefit from focusing on a relationship between a measurable climate parameter (e.g., rainfall) and erosional efficiency. The author argues that only once a link between precipitation and erosional efficiency has been quantified, can the field progress to meaningful assessments of spatial correlations between climate and deformation rates. 4
Discussion Summary for Week 3 (Feb 12): Erosion and Sediment Transport Discussion Leader: Ariel Deutsch This week we read four articles discussing the processes of erosion and sediment transport on Earth and Mars, with a major theme revolving around climate-tectonic interactions. In a scenario where climate drives tectonics, one could expect increased precipitation and temperatures to result in an increase in erosion. Where tectonics drives climate, high slopes would increase the surface area of rock available to weathering, and the silicate weathering cycle would withdraw CO 2 from the atmosphere. Cosmogenic nuclides are produced in situ when secondary cosmic rays reach the Earth s surface, and can be used to measure spatially averaged denudation rates. Cosmogenic nuclides and their utility are discussed in depth by D. Lal (1991). 10 Be is a preferred nuclide for Earth studies because it is almost nonexistent in the rock before being produced via cosmic ray interaction with the surface, and quartz is the mineral of choice largely because of its abundance in silicate rocks and sediments (Von Blanckenburg, 2005). On Mars, however, quartz is not an ideal mineral given the basaltic composition of the planet. We discussed alternative measurements that may be useful when making analogous measurements on Mars, specifically the utility of Ne, He, and Ar isotopic ratios. Von Blanckenburg presents vague correlations existing between average annual precipitation and chemical weathering, as well as between mean annual temperature and chemical weathering rate, and a tighter correlation between chemical weathering and physical erosion rates. We discussed the implications of these correlations for Mars and the possibility of using chemical weathering rates to reconstruct annual precipitation and temperatures. Willenbring et al. (2013) demonstrated that global denudation on Earth is controlled by the lowsloping areas. Although mountainous regions have higher production of sediment rates per unit area, they only account for a small percentage of denudation on a global scale. The Earth s surface is dominantly covered by low-sloping areas, which erode more slowly but steadily over a large area. This was presented as evidence that, largely, tectonics do no drive climate on Earth. To understand global climatic processes on Earth, future studies must focus on exploring how denudation is controlled in these low-sloping areas. This article raised questions about the global distribution of sloped terrain on Mars. Is the surface of Mars also dominated by these low-sloped terrains? We discussed designing a global sediment study on Mars, and the importance of selecting regions to represent a global sample. Perhaps sampling from large-scale outflow channels would be an appropriate approximation, given their widespread prevalence on the planet suggesting they represent globally relevant processes. We also considered how the topographic landscape changed before, during, and after the Late Heavy Bombardment. How does the landscape reflect the intense period of erosion caused by this time of high rates of impact cratering? The distribution of sloped landscapes has implications for which regions provide the greatest fraction of total denudation on the planet, and for understanding the feedbacks between erosion and climate. Many studies indicate a link between silicate weathering rates and denudation rates. Willenbring et al. (2013) argue mountain building does not contribute much to global CO 2 withdrawal. With Earth s surface dominated by low-sloping areas, tectonics is not a major driver in climate change. Lastly, we discussed how climate can set the scale of landscape dissection through competition between diffusive and advective processes. Sweeney et al. (2015) derived laboratory experiments to test the landscape evolution theory using a mister representing advective processes that resulted only in surface runoff and a drip box representing diffusive processes that resulted in rainsplash and craters that also caused sediment transport. Similarly, computer simulations of landscape dissection have been devised for Mars by Craddock and Howard (2002) that model sediment transport through the competition of advective and diffusive processes, which the researchers use to argue for a warm and wet early Mars. In summary, the interactions between climate and tectonics are not straight-forward, either on Earth or Mars. Interactions occur at different spatial scales, and it is always critical to understand what an average value really represents. It is also important to have an appreciation for what is actually preserved in the rock record, understanding that it represents a biased view. Many outstanding questions on this theme remain, and understanding what controls denudation in low-sloping landscapes, which cover the majority of Earth s surface, is critical in characterizing feedbacks between erosion and climate.