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Along a climate gradient in the Chilean coastal mountains, we investigated denudation rates using the meteoric cosmogenic nuclide 10Be and its ratio to stable 9Be, and chemical depletion fractions (CDFs) in bulk soil samples. We find that the fraction of 9Be released from bedrock is a sensitive indicator of weathering, similar to CDF. Meteoric 10Be decreases exponentially with depth, reflecting the reactive nature of this tracer. We also measured denudation rates by the well‐understood in situ cosmogenic 10Be system on quartz. Assuming that both systems record the same denudation rate we calculated the depositional flux of meteoric 10Be for each study site. The flux agrees to that derived from atmospheric models in the mediterranean and humid areas. In contrast, in the arid and semi‐arid areas, the calculated flux agrees with a precipitation‐derived flux, indicating delivery of 10Be to be affected by small‐scale climatic variations not reflected by current atmospheric models. Plain Language Summary: To identify how geologic and climatic conditions control how fast soil is removed from the Earth's surface (denudation), we need to measure these rates over long periods, like thousands of years. A common method makes use of a rare radioactive nuclide, cosmogenic 10Be, formed by cosmic rays in quartz. However, if rocks do not contain quartz, an alternative system is provided by meteoric cosmogenic 10Be precipitated from the atmosphere and stable 9Be that is released during rock weathering. From the ratio 10Be to 9Be we can estimate denudation rates if we know the fraction of 9Be released by weathering and how much meteoric 10Be is deposited from the atmosphere per unit time and area. In a comparison with the quartz‐based method we explored these two unknowns on soil samples from four locations in Chile with different climates, all from granitic rock. We discovered two key points: (a) The amount of stable 9Be indicates soil alteration, which depends on water infiltration. (b) In arid climate, the deposition of meteoric 10Be is limited by rainfall, whereas in areas of sufficient yearly rainfall the 10Be deposition reflects large‐scale atmospheric distribution and is predicted by combined global cosmogenic nuclide production and climate models. Key Points: The ratio of meteoric cosmogenic 10Be to stable 9Be was used to determine soil denudation ratesIn regolith profiles, both the infiltration depth of 10Be and the fraction of 9Be released by weathering depends on precipitationAtmospheric models reflect the flux of meteoric 10Be well in humid areas; in arid areas the flux strongly depends on local precipitation [ABSTRACT FROM AUTHOR]

About 75% of the terrestrial land surface of our planet is covered by sedimentary rocks, butabout 80% of this area is composed of fine-grained shale or carbonate lithologies. When alsoincluding mafic rocks, a large global terrestrial area is inaccessible to the in situ 10Becosmogenic nuclide method, as it requires the presence of sand-sized quartz minerals. Hence,in order to quantitatively constrain the erosion and weathering rates of these areas andto derive globally meaningful solid and solute fluxes, new methods are required.A recently developed tracer to study Earth surface erosion and weathering is themeteoric 10Be/9Be ratio1, from which meteoric denudation rates (Dmet) can becalculated. Meteoric cosmogenic 10Be is deposited from the atmosphere onto Earthsurface, where it is adsorbed to any fine-grained solid material or, depending on pH,is present in the dissolved form, whereas stable 9Be is a trace metal released byweathering. We show the broad versatility of this new approach by its application to twogeomorphically contrasting settings. In the transport-limited setting of the middle Europeanuplands of the Slavkov Forest, Czech Republic, small (<1 km2) neighboring catchments ofsimilar size, climate, slope, and vegetation are underlain by mafic (amphibolite), ultramafic(serpentinite), and felsic (granitic) rocks. Quartz is present in all these catchments in thebedrock or in quartz veins, making a comparison with in situ cosmogenic-deriveddenudation rates (Dinsitu)possible. Due to acid rain deposition in the 20th century,pH values of stream waters vary from acidic to alkaline, depending on the acidbuffering capability of individual lithologies. By balancing the depositional 10Be fluxwith the exported dissolved and particulate 10Be flux, we show that steady stateattained before industrial soil acidification in all three catchments, suggesting anegligible loss of dissolved Be due to acidic conditions. This is supported by the overallagreement of Dmet with Dinsituin the studied catchments that range from 0.04 to 0.08mm/yr2. In the supply-limited setting of the high and steep mountainous island of Taiwan, amongthe World´s highest denudation rates prevail due to high uplift rates and frequent typhoonattacks. We studied Taiwan´s longest river, the Zhuoshui River (3000 km2), whereheadwaters are dominated by slate with Dmet ranging from 4-8 mm/yr, agreeing withpublished long-term exhumation rates. In the mid-lower sandstone-dominated reaches, lowerDmet agree with published Dinsitu, being in the order of 1-2 mm/yr. We suggest that agradual dilution of the sediment contribution from the rapidly-eroding slate in the headwaterswith sediment from slowly-eroding sandstone reaches is responsible for the downstreamdecrease in Dmet. In both these settings representing geomorphic end members, denudation rates calculatedfrom the 10Be/9Be ratio of the particle-bound Be are very similar to independent denudationrate estimates and prevailing lithological and geomorphological conditions. This agreementevidences the versatile potential of the meteoric 10Be/9Be proxy to quantify Earth surfaceprocesses in a wide range of landscapes, lithologies, hydrological conditions and sedimenttransport processes. 1 von Blanckenburg, F., et al. EPSL, 2012. 2 Dannhaus, N. et al., GCA, 2018. [ABSTRACT FROM AUTHOR]

Concentrations of in-situ-produced cosmogenic 10Be in river sediment are widely used to estimate catchment-average denudation rates. Typically, the 10Be concentrations are measured in the sand fraction of river sediment. However, the grain size of bedload sediment in most bedrock rivers covers a much wider range. Where 10Be concentrations depend on grain size, denudation rate estimates based on the sand fraction alone are potentially biased. To date, knowledge about catchment attributes that may induce grain-size-dependent 10Be concentrations is incomplete or has only been investigated in modelling studies. Here we present an empirical study on the occurrence of grain-size-dependent 10Be concentrations and the potential controls of hillslope angle, precipitation, lithology, and abrasion. We first conducted a study focusing on the sole effect of precipitation in four granitic catchments located on a climate gradient in the Chilean Coastal Cordillera. We found that observed grain size dependencies of 10Be concentrations in the most-arid and most-humid catchments could be explained by the effect of precipitation on both the scouring depth of erosion processes and the depth of the mixed soil layer. Analysis of a global dataset of published 10Be concentrations in different grain sizes (n=73 catchments) – comprising catchments with contrasting hillslope angles, climate, lithology, and catchment size – revealed a similar pattern. Lower 10Be concentrations in coarse grains (defined as "negative grain size dependency") emerge frequently in catchments which likely have thin soil and where deep-seated erosion processes (e.g. landslides) excavate grains over a larger depth interval. These catchments include steep (> 25 ∘) and humid catchments (> 2000 mm yr -1). Furthermore, we found that an additional cause of negative grain size dependencies may emerge in large catchments with weak lithologies and long sediment travel distances (> 2300–7000 m, depending on lithology) where abrasion may lead to a grain size distribution that is not representative for the entire catchment. The results of this study can be used to evaluate whether catchment-average denudation rates are likely to be biased in particular catchments. [ABSTRACT FROM AUTHOR]

While landscapes are broadly sculpted by tectonics and climate, on a catchment scale, sediment size can regulate hillslope denudation rates and thereby influence the location of topographic highs and valleys. In this work, we used in situ 10Be cosmogenic radionuclide analysis to measure the denudation rates of bedrock, boulders, and soil in three granitic landscapes with different climates in Chile. We hypothesize that bedrock and boulders affect differential denudation by denuding more slowly than the surrounding soil; the null hypothesis is that no difference exists between soil and boulder or bedrock denudation rates. To evaluate denudation rates, we present a simple model that assesses differential denudation of boulders and the surrounding soil by evaluating boulder protrusion height against a two-stage erosion model and measured 10Be concentrations of boulder tops. We found that hillslope bedrock and boulders consistently denude more slowly than soil in two out of three of our field sites, which have a humid and a semi-arid climate: denudation rates range from ∼5 to 15 m Myr -1 for bedrock and boulders and from ∼8 to 20 m Myr -1 for soil. Furthermore, across a bedrock ridge at the humid site, denudation rates increase with increasing fracture density. At our lower-sloping field sites, boulders and bedrock appear to be similarly immobile based on similar 10Be concentrations. However, in the site with a Mediterranean climate, steeper slopes allow for higher denudation rates for both soil and boulders (∼40 –140 m Myr -1), while the bedrock denudation rate remains low (∼22 m Myr -1). Our findings suggest that unfractured bedrock patches and large hillslope boulders affect landscape morphology by inducing differential denudation in lower-sloping landscapes. When occurring long enough, such differential denudation should lead to topographic highs and lows controlled by bedrock exposure and hillslope sediment size, which are both a function of fracture density. We further examined our field sites for fracture control on landscape morphology by comparing fracture, fault, and stream orientations, with the hypothesis that bedrock fracturing leaves bedrock more susceptible to denudation. Similar orientations of fractures, faults, and streams further support the idea that tectonically induced bedrock fracturing guides fluvial incision and accelerates denudation by reducing hillslope sediment size. [ABSTRACT FROM AUTHOR]

The denudation of rocks in mountain belts exposes a range of fresh minerals to the surface of the Earth that are chemically weathered by acidic and oxygenated fluids. The impact of the resulting coupling between denudation and weathering rates fundamentally depends on the types of minerals that are weathering. Whereas silicate weathering sequesters CO2 , the combination of sulfide oxidation and carbonate dissolution emits CO2 to the atmosphere. Here, we combine the concentrations of dissolved major elements in stream waters with 10Be basin-wide denudation rates from 35 small catchments in eastern Tibet to elucidate the importance of lithology in modulating the relationships between denudation rate, chemical weathering pathways, and CO2 consumption or release. Our catchments span 3 orders of magnitude in denudation rate in low-grade flysch, high-grade metapelites, and granitoid rocks. For each stream, we estimate the concentrations of solutes sourced from silicate weathering, carbonate dissolution, and sulfide oxidation using a mixing model. We find that for all lithologies, cation concentrations from silicate weathering are largely independent of denudation rate, but solute concentrations from carbonates and, where present, sulfides increase with increasing denudation rate. With increasing denudation rates, weathering may therefore shift from consuming to releasing CO2 in both (meta)sedimentary and granitoid lithologies. For a given denudation rate, we report dissolved solid concentrations and inferred weathering fluxes in catchments underlain by (meta)sedimentary rock that are 2–10 times higher compared to catchments containing granitoid lithologies, even though climatic and topographic parameters do not vary systematically between these catchments. Thus, varying proportions of exposed (meta)sedimentary and igneous rocks during orogenesis could lead to changes in the sequestration and release of CO2 that are independent of denudation rate. [ABSTRACT FROM AUTHOR]

The intermontane Humahuaca Basin in the Eastern Cordillera of the northwest Argentine Andes lies leeward of an orographic barrier to easterly derived moisture. An average of >2000 mm/yr of rainfall along the eastern flanks of the barrier contrasts with <200 mm/yr in the orogen interior. Paleoenvironmental reconstructions suggest that the basin became disconnected from the foreland during the Miocene-Pliocene by the growth of fault-bounded mountain ranges. Fossil records, sedimentology, and stable isotope data imply that rerouting of the fluvial network by 4.2 Ma and reduced rainfall by ca. 3 Ma were consequences of that range uplift. Here, we present cosmogenic nuclide-derived (10Be) paleodenudation rates from 6 to 2 Ma fluvial deposits collected from the Humahuaca Basin. Despite increased tectonic activity, our 10Be data show a tenfold decrease in denudation rates at ca. 3 Ma, documenting a link between uplift-induced semiarid conditions and decreasing hillslope denudation rates. This new data set thus demonstrates the influence of hydrological change on spatiotemporal denudation patterns in tectonically active mountain areas. [ABSTRACT FROM AUTHOR]

The Labrador Sector of the Laurentide Ice Sheet is characterized by a complex network of ice divides and an extensive landform record outlining two broad and opposing ice flows that are separated by a narrow Horseshoe Intersection Zone (HIZ). This geomorphic system gave rise to contrasting reconstructions, which reflect uncertainties on the temporal evolution of ice divides, the nature and age of the main landform systems and the overall pattern of ice retreat during the last deglaciation. Here, we address these issues through systematic mapping of glacial landforms and ice-flow indicators in a large area of northeastern Quebec and Labrador. The application of cosmogenic (10Be and 26Al) dating to esker-fed glaciomarine deltas and different rock surfaces brings new constraints on the chronological framework and insights on the subglacial thermal regime. Our results outline four main landform assemblages, two of which relating to a major system of opposite ice flows that delineate the position of the eastern Ancestral Labrador ice divide. Our reconstruction shows that the HIZ occurs several tens of km to the west of this divide and that these two features are genetically distinct. The HIZ relates to the late-glacial development of ice streams showing a massive convergent ice flow into Ungava Bay. The upstream extent of this ice flow system shows a strong coupling with the outline of Ungava Bay drainage divide, suggesting that the configuration of the HIZ could be the expression of a topographic control on the late-glacial ice sheet dynamics. Landforms also indicate a significant shift in the deglaciation mode, which evolved from an overall warm-based to an areally-confined cold-based ice retreat near the center of the former ice mass – a change in dynamics that also played a role in the configuration of the HIZ. Overall, areas indicative of cold-based ice conditions are limited to highly elevated terrains, thus ruling out models arguing for an extensive cold-based ice cover over Ungava Bay throughout the last deglaciation. • The Labrador Sector is characterized by a complex network of ice divides and two major glacial geomorphic systems; • The Horseshoe Intersection Zone (HIZ) forms a long and narrow U-shaped sector separating two systems of opposite ice flows; • Extensive mapping and field investigations outline several morpho-sedimentary assemblages related to distinct glacial events ; • Mapping and 10Be dating of landforms north of the HIZ do not show support for cold-based ice cover over Ungava Bay ; • The configuration of the HIZ was controlled by the regional topography and local subglacial conditions. [ABSTRACT FROM AUTHOR]