Search

Theory suggests that the response time of alluvial channel long-profiles to perturbations in climate is related to the magnitude of the forcing and the length of the system. Shorter systems may record a higher frequency of forcing compared to longer systems. Empirical field evidence that system length plays a role in the climate periodicity preserved within the sedimentary record is, however, sparse. The Toro Basin in the Eastern Cordillera of NW Argentina provides an opportunity to test these theoretical relationships as this single source-to-sink system contains a range of sediment deposits, located at varying distances from the source. A suite of eight alluvial fan deposits is preserved along the western flanks of the Sierra de Pascha. Farther downstream, a flight of cut-and-fill terraces have been linked to eccentricity-driven (100-kyr) climate cycles since ca. 500 ka. We applied cosmogenic radionuclide (10Be) exposure dating to the fan surfaces to explore (1) how channel responses to external perturbations may or may not propagate downstream, and (2) the differences in landscape response to forcing frequency as a function of channel length. We identified two generations of fan surfaces: the first (G1) records surface activity and abandonment between ca. 800 and 500 ka and the second (G2) within the last 100 kyr. G1 fans record a prolonged phase of net incision, which has been recognised throughout the Central Andes, and was likely triggered by enhanced 100-kyr global glacial cycles following the Mid-Pleistocene Transition. Relative fan surface stability followed, while 100-kyr cut-and-fill cycles occurred downstream, suggesting a disconnect in behaviour between the two channel reaches. G2 fans record higher frequency climate forcing, possibly the result of precessional forcing of climate (ca. 21/40-kyr timescales). The lack of a high-frequency signal farther downstream provides field support for theoretical predictions of a filtering of high-frequency climate forcing with increasing channel length. We show that multiple climate periodicities can be preserved within the sedimentary record of a single basin. Differences in the timing of alluvial fan and fluvial terrace development in the Toro Basin appears to be associated with how channel length affects fluvial response times to climate forcing as well as local controls on net incision, such as tectonic deformation. [ABSTRACT FROM AUTHOR]

The hydrogen isotope composition of leaf wax biomarkers (δ2 H wax) is a valuable tool for reconstructing continental paleohydrology, since it serves as a proxy for the hydrogen isotope composition of precipitation (δ2 H pre). To yield robust palaeohydrological reconstructions using δ2 H wax in marine archives, it is necessary to examine the impacts of regional climate on δ2 H wax and assess the similarity between marine sedimentary δ2 H wax and the source of continental δ2 H wax. Here, we examined an aridity gradient from hyperarid to humid along the Chilean coast. We sampled sediments at the outlets of rivers draining into the Pacific as well as soils within catchments and marine surface sediments adjacent to the outlets of the studied rivers and analyzed the relationship between climatic variables and δ2 H wax values. We found that apparent fractionation between leaf waxes and source water is relatively constant in humid and semiarid regions (average: - 121 ‰). However, it becomes less negative in hyperarid regions (average: - 86 ‰) as a result of evapotranspirative processes affecting soil and leaf water 2 H enrichment. We also observed that along strong aridity gradients, the 2 H enrichment of δ2 H wax follows a non-linear relationship with water content and water flux variables, driven by strong soil evaporation and plant transpiration. Furthermore, our results indicate that δ2 H wax values in marine surface sediments largely reflect δ2 H wax values from the continent, confirming the robustness of marine δ2 H wax records for paleohydrological reconstructions along the Chilean margin. These findings also highlight the importance of considering the effects of hyperaridity in the interpretation of δ2 H wax values and pave the way for more quantitative paleohydrological reconstructions using δ2 H wax. [ABSTRACT FROM AUTHOR]

The hydrogen isotope composition of leaf wax biomarkers (δ2Hwax) is a valuable tool for reconstructing continental paleohydrology, as it serves as a proxy for the hydrogen isotope composition of precipitation (δ2Hpre). To yield robust palaeohydrological reconstructions using δ2Hwax in marine archives, it is necessary to examine the impacts of regional climate on δ2Hwax and assess the similarity between marine sedimentary δ2Hwax and the source of continental δ2Hwax. Here, we examined an aridity gradient from hyperarid to humid along the Chilean coast. We sampled sediments at the outlets of rivers draining into the Pacific, soils within catchments and marine surface sediments adjacent to the outlets of the studied rivers and analyzed the relationship between climatic variables and δ2Hwax values. We find that apparent fractionation between leaf waxes and source water is relatively constant in humid/semiarid regions (average: −121 ‰). However, it becomes less negative in hyperarid regions (average: −86 ‰) as a result of evapotranspirative processes affecting soil and leaf water 2H enrichment. We also observed that along strong aridity gradients, the 2H enrichment of δ2Hwax follows a non-linear relationship with water content and water flux variables, driven by strong soil evaporation and plant transpiration. Furthermore, our results indicated that δ2Hwax values in marine surface sediments largely reflect δ2Hwax values from the continent, confirming the robustness of marine δ2Hwax records for paleohydrological reconstructions along the Chilean margin. These findings also highlight the importance of considering the effects of hyperaridity in the interpretation of δ2Hwax values and pave the way for more quantitative paleohydrological reconstructions using δ2Hwax. [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]

While landscapes are broadly sculpted by tectonics and climate, on a catchment scale, the density of bedrock fractures can influence hillslope denudation rates and dictate the location of topographic highs and valleys. In this work, we used 10Be cosmogenic radionuclide analysis to measure the denudation rates of bedrock, boulders, and soil, in three granitic landscapes with different climates in Chile, with the hypothesis that high fracture density reduces grain size and increases denudation rates. Denudation rates range from 10 to 15 m Myr-1 for bedrock and boulders and from 15 to 20 m Myr-1 for soil in the humid and semi-arid climates, and are higher in the mediterranean climate (~40–140 m Myr-1), likely due to steeper slopes. We found that hillslope bedrock and boulders erode more slowly than the surrounding soil in the diffusively-eroding study sites. Furthermore, across a bedrock ridge in the humid site, bedrock denudation rates increase with fracture density. These findings are consistent with the observation that streams in our field sites follow the orientation of at least one major fault orientation. Our results suggest that tectonically-induced fractures and faults dictate landscape evolution through reducing grain size and thus enhancing differential denudation rates. [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]

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]

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]