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

The recent surge of applications using terrestrial cosmogenic nuclides (TCNs) to calculate catchment-averaged erosion rates from isotopic concentrations in fluvial sediment, and the prospect of coupling TCN production functions with numerical surface process models (SPMs), necessitate a fast and accurate algorithm for the calculation of topographic shielding. Topographic shielding refers to the proportion of the incoming cosmic radiation that is shielded by the surrounding topography, the scaling factor being defined as the ratio of the unshielded (total minus shielded) to the total (or maximum) cosmic ray flux (i.e. the flux received by a horizontal, unobstructed surface). Topography contributes to the reduction of TCN production by obstructing a certain proportion of the incoming flux and by modifying the angle of incidence. Available algorithms calculate the proportion of obstructed radiation by dividing the horizon as seen by the sample (a grid cell in the case of a DEM), into arc segments (usually of equal length) for which the average obstruction heights expressed as zenith angles are calculated. The use of these methods is feasible only when dealing with a small number of isolated samples, since the identification of obstructions when dealing with an entire area is computationally very intensive. This paper describes a method that uses a relief shadow modelling technique to identify those areas of a DEM that are under shadow (i.e. shielded), and thus to account for the obstructed radiation. This method produces results that are very similar to those obtained using a direct implementation of available methods (maximum difference between results of c. 0·1). The method based on relief shadow modelling is also faster than a direct implementation of any available method and can be readily implemented in any GIS system with raster capabilities.

In four rivers in western Scotland for which there is a wellconstrained record of relative base-level fall, the rate of postglacial bedrock erosion is quantifi ed by measuring the concentration of in situ cosmogenic 10 Be on strath terraces downstream of headwardretreating knickpoints. Along-channel gradients in 10 Be exposure age show two distinct trends: upstream younging and constant age, which we interpret as diagnostic of knickpoint retreat and diffusive transport-limited incision, respectively. We show that bedrock channel incision and regional formation of strath terraces began shortly after deglaciation (ca. 11.5 ka), and that knickpoint retreat rates peaked in the early to mid-Holocene. Erosion rates have since decreased by two orders of magnitude, converging in the late Holocene to low rates independent of stream power per unit channel area. We infer this regional slowing in postglacial knickpoint retreat to be the result of the depletion of paraglacial sediment supply over the Holocene, leading to a defi ciency in "tools" for bedrock erosion. Our results imply that episodes of major fl uvial erosion may be in tune with glacial cycles, and that sediment depletion following glacial-interglacial transitions may be an important cause of bedrock erosion rate variations in rivers draining glaciated landscapes.

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The Cape Mountains of southern Africa exhibit an alpine-like topography in conjunction with some of the lowest denudation rates in the world. This presents an exception to the often-cited coupling of topography and denudation rates and suggests that steep slopes alone are not sufficient to incite the high denudation rates with which they are commonly associated. Within the Cape Mountains, slope angles are often in excess of 30° and relief frequently exceeds 1 km, yet 10Be-based catchment-averaged denudation rates vary between 2.32 ± 0.29 m/m.y. and 7.95 ± 0.90 m/m.y. We attribute the maintenance of rugged topography and suppression of denudation rates primarily to the presence of physically robust and chemically inert quartzites that constitute the backbone of the mountains. 10Be-based bedrock denudation rates on the interfluves of the mountains vary between 1.98 ± 0.23 m/m.y. and 4.61 ± 0.53 m/m.y. The close agreement between the rates of catchment-averaged and interfluve denudation indicates topography in steady state. These low denudation rates, in conjunction with the suggestion of geomorphic stability, are in agreement with the low denudation rates (/m.y.) estimated for southern Africa during the late Cenozoic by means of cosmogenic nuclide, thermochronology, and offshore sedimentation analyses. Accumulatively, these data suggest that the coastal hinterland of the subcontinent may have experienced relative tectonic stability throughout the Cenozoic.

Abstract. We present a database of cosmogenic radionuclide and luminescence measurements in fluvial sediment. With support from the Australian National Data Service (ANDS) we have built infrastructure for hosting and maintaining the data at the University of Wollongong and making this available to the research community via an Open Geospatial Consortium (OGC)-compliant web service. The cosmogenic radionuclide (CRN) part of the database consists of 10Be and 26Al measurements in modern fluvial sediment samples from across the globe, along with ancillary geospatial vector and raster layers, including sample site, basin outline, digital elevation model, gradient raster, flow-direction and flow-accumulation rasters, atmospheric pressure raster, and CRN production scaling and topographic shielding factor rasters. Sample metadata are comprehensive and include all necessary information for the recalculation of denudation rates using CAIRN, an open-source program for calculating basin-wide denudation rates from 10Be and 26Al data. Further all data have been recalculated and harmonised using the same program. The luminescence part of the database consists of thermoluminescence (TL) and optically stimulated luminescence (OSL) measurements in fluvial sediment samples from stratigraphic sections and sediment cores from across the Australian continent and includes ancillary vector and raster geospatial data. The database can be interrogated and downloaded via a custom-built web map service. More advanced interrogation and exporting to various data formats, including the ESRI Shapefile and Google Earth's KML, is also possible via the Web Feature Service (WFS) capability running on the OCTOPUS server. Use of open standards also ensures that data layers are visible to other OGC-compliant data-sharing services. OCTOPUS and its associated data curation framework provide the opportunity for researchers to reuse previously published but otherwise unusable CRN and luminescence data. This delivers the potential to harness old but valuable data that would otherwise be lost to the research community. OCTOPUS can be accessed at https://earth.uow.edu.au (last access: 28 November 2018). The individual data collections can also be accessed via the following DOIs: https://doi.org/10.4225/48/5a8367feac9b2 (CRN International), https://doi.org/10.4225/48/5a836cdfac9b5 (CRN Australia), and https://doi.org/10.4225/48/5a836db1ac9b6 (OSL & TL Australia).

The century-long debate over the origins of inner gorges that were repeatedly covered by Quaternary glaciers hinges upon whether the gorges are fluvial forms eroded by subaerial rivers, or subglacial forms cut beneath ice. Here we apply cosmogenic nuclide exposure dating to seven inner gorges along B500 km of the former Fennoscandian ice sheet margin in combination with a new deglaciation map. We show that the timing of exposure matches the advent of ice-free conditions, strongly suggesting that gorges were cut by channelized subglacial meltwater while simultaneously being shielded from cosmic rays by overlying ice. Given the exceptional hydraulic efficiency required for meltwater channels to erode bedrock and evacuate debris, we deduce that inner gorges are the product of ice sheets undergoing intense surface melting. The lack of postglacial river erosion in our seven gorges implicates subglacial meltwater as a key driver of valley deepening on the Baltic Shield over multiple glacial cycles.

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