Rock uplift and erosional denudation of orogenic belts have long been the most important geologic processes that serve to shape continental surfaces, but the rate of geomorphic change resulting from these natural phenomena has now been outstripped by human activities associated with agriculture, construction, and mining. Although humans are now the most important geomorphic agent on the planet's surface, natural and anthropogenic processes serve to modify quite different parts of Earth's landscape. In order to better understand the impact of humans on continental erosion, we have examined both long-term and short-term data on rates of sediment transfer in response to glacio-fl uvial and anthropogenic processes. Phanerozoic rates of subaerial denudation inferred from preserved volumes of sedimentary rock require a mean continental erosion rate on the order of 16 m per million years (m/m.y.), resulting in the accumulation of ~5 gigatons of sediment per year (Gt/yr). Erosion irregularly increased over the ~542 m.y. span of Phanerozoic time to a Pliocene value of 53 m/m.y. (16 Gt/yr). Current estimates of large river sediment loads are similar to this late Neogene value, and require net denudation of ice-free land surfaces at a rate of ~62 m/m.y. (~21 Gt/yr). Consideration of the variation in large river sediment loads and the geomorphology of respective river basin catchments suggests that natural erosion is primarily confi ned to drainage headwaters; ~83% of the global river sediment fl ux is derived from the highest 10% of Earth's surface. Subaerial erosion as a result of human activity, primarily through agricultural practices, has resulted in a sharp increase in net rates of continental denudation; although less well constrained than estimates based on surviving rock volumes or current river loads, available data suggest that present farmland denudation is proceeding at a rate of ~600 m/ m.y. (~75 Gt/yr), and is largely confi ned to the lower elevations of Earth's land surface, primarily along passive continental margins; ~83% of cropland erosion occurs over the lower 65% of Earth's surface. The conspicuous disparity between natural sediment fl uxes suggested by data on rock volumes and river loads (~21 Gt/yr) and anthropogenic fl uxes inferred from measured and modeled cropland soil losses (75 Gt/yr) is readily resolved by data on thicknesses and ages of alluvial sediment that has been deposited immediately downslope from eroding croplands over the history of human agriculture. Accumulation of postsettlement alluvium on higher-order tributary channels and fl oodplains (mean rate ~12,600 m/m.y.) is the most important geomorphic process in terms of the erosion and deposition of sediment that is currently shaping the landscape of Earth. It far exceeds even the impact of Pleistocene continental glaciers or the current impact of alpine erosion by glacial and/ or fl uvial processes. Conversely, available data suggest that since 1961, global cropland area has increased by ~11%, while the global population has approxim...
We present results from a laboratory experiment documenting the evolution of a sinuous channel form via sedimentation from 24 turbidity currents having constant initial conditions. The initial channel had a sinuosity of 1.32, a wavelength of 1.95, an amplitude of 0.39 m, and three bends. All currents had a densimetric Froude number of 0.53 and an initial height equal to the channel relief at the start of the experiment. Large superelevation of currents was observed at bend apexes. This superelevation was 85%-142% greater than the value predicted by a balance of centrifugal and pressure-gradient forces. An additional contribution to the superelevation was the runup of the current onto the outer banks of bends. This runup height is described by a balance between kinetic and potential energy. Runup resulted in deposition of coarse particles on levee crests that were indistinguishable from those deposited on the channel bottom. Deposit thickness and composition showed a strong cross-channel asymmetry. Thicker, coarser, steeper levees grew on the outer banks relative to the inner banks of bends. Zones of fl ow separation were observed downstream from bend apexes along inner banks and affected sedimentation patterns. Sedimentation from currents caused the channel to aggrade with almost no change in planform.However, channel relief decreased throughout the experiment because deposition on the channel bottom always exceeded deposition at levee crests. The fi rst bend served as a fi lter for the properties of the channelized current, bringing discharge at the channel entrance into agreement with the channel cross-sectional area. Excess discharge exited the channel at this fi ltering bend and was lost to the overbank surface.
Sinuous ridges are important landforms on the surface of Mars that show promise for quantifying ancient martian surface hydrology. Morphological similarity of these ridges to river channels in planform led to a hypothesis that ridges are topographically inverted river channels, or "inverted channels", formed due to an erosion-resistant channel-filling material that preserved a snapshot of the channel geometry in inverted relief due to differential erosion. An alternative depositinversion hypothesis proposes that ridges represent exhumed river-channel belts, with geometries that reflect the lateral migration and vertical aggradation of rivers over significant geologic time, rather than the original channel geometry. To investigate these hypotheses we studied sinuous ridges within the Cretaceous Cedar Mountain Formation near Green River, Utah, USA. Ridges in Utah extend for hundreds of meters, are up to 120 meters wide, and stand up to 39 meters above the surrounding plain. Ridges are capped by sandstone bodies 3-10 meters thick that contain dune-and bar-scale inclined stratification, which we interpret as eroded remnants of channel ACCEPTED MANUSCRIPT 2 4/18/2019 12:45 PM belts that record the migration and aggradation of single-thread, sand-bedded rivers, rather than channel fills that can preserve the original channel geometry. Caprocks overlie mudstones and thinner sandstone beds that are interpreted as floodplain deposits, and in cases additional channel-belt sandstones are present lower in the ridge stratigraphy. Apparent networks from branching ridges typically represent discrete sandstone bodies that cross at different stratigraphic levels rather than a coeval river network. Ridge-forming sandstone bodies also have been narrowed during exhumation by cliff retreat and bisected by fluvial erosion. Using a large compilation of channel-belt geometries on Earth and our measurements of ridges in Utah, we propose that caprock thickness is the most reliable indicator of paleo-channel geometry, and can be used to reconstruct river depth and discharge. In contrast, channel lateral migration and caprock erosion during exhumation make ridge breadth an uncertain proxy for channel width. An example in Aeolis Dorsa, Mars, illustrates that river discharge estimates based solely on caprock width may differ significantly from estimates based on caprock thickness. Overall, our study suggests that sinuous ridges are not inverted channel fills, but rather reflect exhumation of a thick stratigraphic package of stacked channel belts and overbank deposits formed from depositional rivers over significant geologic time.
Earth is (mostly) flat: apportionment of the flux of continental sediment over Earth is (mostly) flat: apportionment of the flux of continental sediment over millennial time scales millennial time scales
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