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The uplift of the Shillong Plateau, in northeast India between the Bengal floodplain and the Himalaya Mountains, has had a significant impact on regional precipitation patterns, strain partitioning, and the path of the Brahmaputra River. Today, the plateau receives the highest measured yearly rainfall in the world and is tectonically active, having hosted one of the strongest intra-plate earthquakes ever recorded. Despite the unique tectonic and climatic setting of this prominent landscape feature, its exhumation and surface uplift history are poorly constrained. We collected 14 detrital river sand and 3 bedrock samples from the southern margin of the Shillong Plateau to measure erosion rates using the terrestrial cosmogenic nuclide 10Be. The calculated bedrock erosion rates range from 2.0 to 5.6 m My−1, whereas catchment average erosion rates from detrital river sands range from 48 to 214 m My−1. These rates are surprisingly low in the context of steep, tectonically active slopes and extreme rainfall. Moreover, the highest among these rates, which occur on the low-relief plateau surface, appear to have been affected by anthropogenic land-use change. To determine the onset of surface uplift, we coupled the catchment averaged erosion rates with topographic analyses of the plateau's southern margin. We interpolated an inclined, pre-incision surface from minimally eroded remnants along the valley interfluves and calculated the eroded volume of the valleys carved beneath the surface. The missing volume was then divided by the volume flux derived from the erosion rates to obtain the onset of uplift. The results of this calculation, ranging from 3.0 to 5.0 Ma for individual valleys, are in agreement with several lines of stratigraphic evidence from the Brahmaputra and Bengal basin that constrain the onset of topographic uplift, specifically the onset of flexural loading and the transgression from deltaic to marine deposition. Ultimately, our data corroborate the hypothesis that surface uplift was decoupled from the onset of rapid exhumation, which occurred several millions of years earlier.

We analyze the source-to-sink variations of in situ 10Be, 26Al and 21Ne concentrations in modern sediment of the Po river catchment, from Alpine, Apennine, floodplain, and delta samples, in order to investigate how the cosmogenic record of orogenic erosion is transmitted across a fast-subsiding foreland basin. The in situ 10Be concentrations in the analyzed samples range from ∼ 0.8 × 10 4 at / g QTZ to ∼ 6.5 × 10 4 at / g QTZ . The 10Be-derived denudation rates range from 0.1 to 1.5 mm/yr in the Alpine source areas and from 0.3 to 0.5 mm/yr in the Apenninic source areas. The highest 10Be-derived denudation rates are found in the western Central Alps (1.5 mm/yr). From these data, we constrain a sediment flux leaving the Alpine and the Apenninic source areas (>27 Mt/yr and ca. 5 Mt/yr, respectively) that is notably higher than the estimates of sediment export provided by gauging (∼10 Mt/yr at the Po delta). We observe a high variability in 10Be concentrations and 10Be-derived denudation rates in the source areas. In the Po Plain, little variability is observed, and at the same time, the area-weighed 10Be concentration of ( 2.29 ± 1.57 ) × 10 4 at / g QTZ (±1 SD of the dataset) from both the Alps and the Apennines is poorly modified (by tributary input) in sediment of the Po Plain ( ( 2.68 ± 0.78 , ± 1 SD ) × 10 4 at / g QTZ ). The buffering effect of the Po floodplain largely removes scatter in 10Be signals. We test for several potential perturbations of the cosmogenic nuclide record during source to sink transfer in the Po basin. We find that sediment trapping in deep glacial lakes or behind dams does not significantly change the 10Be-mountain record. For example, similar 10Be concentrations are measured upstream and downstream of the postglacial Lake Maggiore, suggesting that denudation rates prior to lake formation were similar to today's. On the scale of the entire basin, the 10Be concentration of basins with major dams is similar to those without major dams. A potential modification of the 10Be-mountain record during sediment storage in the subsiding Po Plain can be excluded as measured 26Al/10Be ratios do not show the addition of deeply buried material. A barely resolvable excess 21Ne signal in the Po plain indicates recycling of previously eroded sediment rather than accumulation of cosmogenic nuclides during surficial floodplain transport. Our results demonstrate that the cosmogenic record of mountain erosion is effectively transmitted from the source areas to the sediment sink, even across a strongly subsiding foreland basin. We show that this record is poorly influenced by a range of potential geological and anthropogenic sources of bias, and is largely independent from upstream sediment interception and sediment storage in the floodplain.

Meteoric cosmogenic 10Be is a powerful tracer to quantify dates and rates of Earth surface processes over timescales of 103-105 yrs. A prerequisite for its applications is knowledge of the flux at which 10Be, produced in the atmosphere, is delivered to the Earth surface. Four entirely independent approaches are available to quantify this flux: 1) General Circulation Models (GCM) combined with 10Be production functions and aerosol dynamics; 2) 10Be in precipitation collections; 3) 10Be inventories in dated soil profiles; 4) riverine 10Be exported in solid and dissolved forms. We compiled and reprocessed published globally distributed 10Be flux data from each of these methods and compared them with each other after normalization to a common atmospheric production rate. Based on precipitation records, we propose a simple framework to discriminate between two delivery effects on 10Be fluxes. In the additive effect water vapor and 10Be are continuously accumulating during long-distance transport, leading to an increase in 10Be flux with precipitation rate. In the dilution effect, the 10Be flux is delivered from proximal vapor sources, limited by the rate of 10Be introduction from the stratosphere and independent of precipitation rate. Both effects are mostly present in combination, and the relative weight of either effect depends on vapor condensation rate and on the ratio of vapor condensation area to precipitation area. A comparison between precipitation-derived fluxes and GCM-derived fluxes shows that half of the precipitation estimates are >2 times greater than GCM-derived fluxes. By comparison, soil- and GCM-derived fluxes agree within a factor of 2 for more than half (∼57%) of the dataset, and the remaining soil estimates (∼43%) are much lower than GCM-derived fluxes. 71% of 10Be flux estimates from riverine export using 10Be (meteoric)/9Be ratios also agree with GCM-derived fluxes within a factor of 2. We explain the precipitation-derived fluxes that commonly exceed all other estimates by short-term stochasticity in precipitation events that might introduce a measurement time-interval bias towards higher fluxes. This bias is not present over longer-term (103-105 yrs) flux estimates like those from soil profiles. Soil-derived fluxes might still present an underestimation when retention of 10Be in soil is incomplete. We recommend producing more 10Be depositional flux data from soil inventories with full Be retention, as these generate in our view the most relevant estimates for applications of meteoric 10Be on millennial-scale Earth surface processes.

The denudation rate signal of the Bolivian Andes as measured by cosmogenic 10 Be in sediment is preserved in the floodplain of adjacent foreland basins even though these basins store the sediment for thousands of years. This conclusion is drawn from comparing published Andean source area denudation rates with new cosmogenic 10 Be data as measured in the floodplains of the large Beni and Mamore basins. For the entire Beni basin including the sediment-producing Andes and the vast flooded plains of the foreland, the cosmogenic nuclide-derived denudation rate is 0.45 ± 0.06 mm/yr, while that of the Mamore basin is 0.55 ± 0.19 mm/yr. By comparison, the respective Andean source areas erode at averaged rates of 0.37 ± 0.06 mm/yr (upper Beni), and at 0.56 ± 0.09 mm/yr (upper Mamore). We notice a decrease in variability of denudation rate with increasing spatial scale as small-scale processes are averaged out. Sediment mixing within the floodplain damps scatter of nuclide-derived denudation rates picked up in the source area. On the temporal scale, a remarkable agreement between cosmogenic nuclide-derived rates averaging over a few kiloyears with those from fission track dating averaging over millions of years seems to suggest that cosmogenic nuclide-derived denudation rates capture the long-term erosional characteristics of the mountain belt. The sum of these observations suggests that any sample collected along a river traversing a floodplain will yield the denudation rate of the source area. This finding opens the unique possibility of constraining cosmogenic nuclide-derived paleo-sediment budgets for these large basins as the long-term, spatially-averaged denudation rate signal of the sediment-producing area is preserved in sedimentary archives.

The confidence in surface exposure dating and related research, such as erosion rate studies or burial dating, strongly depends on the accuracy and precision of the currently used production rates of in situ-produced cosmogenic nuclides. Reducing the uncertainties of nuclide production rates by more accurate calibrations with independently dated natural rock surfaces is crucial for further improving the quantification of earth surface processes. Here we use surface samples from the 760 ± 2 ka old Bishop Tuff in eastern California to quantify the 10 Be/ 21 Ne and 26 Al/ 21 Ne production rate ratios in quartz. Our determination is based on (1) measured nuclide concentrations of cosmogenic 10 Be, 21 Ne, and 26 Al, (2) a conservative estimate for the erosion of the tuff in the Volcanic Tableland area, which we base on our previously published 21 Ne concentrations [Goethals, M.M., Niedermann, S., Hetzel, R., Fenton, C.R., 2009. Determining the impact of faulting on the rate of erosion in a low-relief landscape: A case study using in situ produced 21 Ne on active normal faults in the Bishop Tuff, California. Geomorphology 103, 401–413] and a conservative estimate for the uncertainty of the 21 Ne production rate, and (3) the assumption of steady-state erosion. Other assumptions, such as the applied scaling procedure, the muon contribution to nuclide production, or the attenuation lengths of neutrons and muons in rock, do not substantially affect the results. Based on 13 samples, the following average production rate ratios and conservative uncertainty estimates are obtained for sea level, high latitude, open sky, and rock surface: 0.249 ± 0.009 or 0.232 ± 0.009 for 10 Be/ 21 Ne using 10 Be half-lives of 1.51 and 1.39 Ma, respectively, and 1.80 ± 0.09 for 26 Al/ 21 Ne (for an 26 Al half-life of 0.72 Ma). The 10 Be/ 21 Ne and the 26 Al/ 21 Ne production ratios are consistent with currently used production rates but the ratios are much more precise than previous determinations. The resulting 26 Al/ 10 Be production ratio of 7.23 ± 0.45 (for a 10 Be half-life of 1.51 Ma) or 7.76 ± 0.49 (for a 10 Be half-life of 1.39 Ma) is high when compared to previously published values. We discuss reasons for this difference, amongst them the possibility that 26 Al analyses in general might be compromised by artefacts affecting the stable Al concentration measurements. When combined with a 10 Be production rate of 5.01 at g − 1 a − 1 for the 1.51 Ma half-life (or 4.61 at g − 1 a − 1 for the 1.39 Ma half-life), our production ratios convert to 21 Ne and 26 Al production rates of 20.1 and 36.2 at g − 1 a − 1 (or 19.9 and 35.7 at g − 1 a − 1 ), respectively.