Denudation Rates Research Articles

Gneissic tors in the central European upland: Complex Late Pleistocene forms?

Bedrock outcrops punctuating regolith-covered surfaces in the summit/upper slope and hillside positions (tors and crags) have long been a subject of inquiry in geomorphology for their evolutionary trajectories and now also as valuable geoheritage sites. Methodological advances in research, such as DTM-based geomorphological analysis and terrestrial cosmogenic nuclide inventories, create new perspectives to decipher the spatial and temporal context of tor emergence and decay. The shape, structural controls, 10Be inventories and relation to surrounding medium-scale landforms were studied at thirteen conspicuous bedrock outcrops developed in metamorphic and sedimentary lithologies in the Bohemian-Moravian Highland, Czechia. The shapes of studied bedrock outcrops indicate significant litho-structural controls. The general outline is mainly governed by (sub)vertical joints, whereas the inclination of foliation or bedding planes dictates summit and cliff morphology. Some of the tors described in this study developed in exceptionally densely foliated and jointed gneiss, which does not comply with the generally accepted view of tors as massive rock compartments resisting weathering. A series of rock hardness measurements performed at three gneissic tors using Schmidt hammer show statistically significant within-tor variability of R-values, indicating different exposure times to subaerial weathering. The complexity of landform assemblages, within which studied tors/crags are developed, varies from isolated bedrock outcrops surrounded by smooth slopes covered with debris-rich soils to outcrops accompanied by low rock cliffs, debris-covered steps, slope benches, boulder fields and isolated large blocks. The 10Be inventories reveal ages mostly from the Late Pleistocene and rarely from the Holocene. Summit/upper slope tors and hillside crags show considerable variability in maximum denudation rates ranging from 6.4 ± 0.2 to 105.6 ± 4.8 m/Ma. Paired samples from the summits and cliffs of eight outcrops indicate three possible post-emergence evolutionary scenarios with summit downwearing while keeping the outcrop outline, balanced cliff and summit degradation rate, and backwearing as the dominant degradation process.

Duration of the Huangling Dome Activity in the Northern Margin of the South China Continent: Insight From Integration of Zircon (U–Th)/He Dating and Thermal Modeling

AbstractThe Huangling dome, which is one of the largest tectonic domes in the South China continent, was formed between the Late Jurassic and Early Cretaceous, when regional compression was eventually replaced by regional extension. This raises an issue about whether the dome was generated by compression, extension or both. This study integrates single‐grain zircon (U–Th)/He (ZHe) dating with thermal modeling to better constrain the formation process of the dome. The measured ZHe ages of samples collected from the dome and adjacent synclines are all younger than the depositional or crystallization ages of the sampled rocks, and the western and eastern dome sides have much older ZHe mean ages than those of the dome and synclines' cores. They make clear the complex interactions between uplift, denudation, and heating processes during dome formation. For simplicity, we developed one‐dimensional heat‐conduction models that cover a wide range of heating and exhumation scenarios (assuming that uplift rate equals denudation rate) within an interval of 152.1–121.4 Ma. These models were solved and searched for the predictions of the measured ages. The best models have a fast decrease in uplift rate with time and a constant heating rate, indicative of an early interval shaping of the dome in a compressional setting.

Control of inherited structural fabric on the development and exhumation of passive margins – Insights from the Araçuaí Orogen (Brazil)

The Araçuaí Orogen, in eastern Brazil, was formed during the Neoproterozoic – Cambrian amalgamation of West Gondwana. During the Mesozoic – Cenozoic opening of the South Atlantic Ocean, and the associated divergent tectonics, the orogen developed as basement to the passive margin of South America and was progressively covered by thick offshore sedimentary basins, particularly the Espírito Santo, Mucuri, and Cumuruxatiba basins, in which hydrocarbon systems have been exploited. Our understanding of the Araçuaí Orogen's passive margin evolution, erosion, and sediment transfer to these basins ultimately depends on constraining the onshore exhumation in response to Mesozoic – Cenozoic events. Here, new and previously published data from apatite fission-track (AFT) analyses and inverse thermal history modelling of (Pre)Cambrian basement rocks from the Araçuaí Orogen resolve three discrete basement cooling and associated erosional exhumation episodes. In the Pre-Rift phase, Jurassic – Hauterivian erosion of the Araçuaí Orogen is most likely related to the adjoining intra-continental West Gondwana flexural subsidence, which increased hillslope and river erosion power. In the Rift and Transitional phases, Barremian – Albian accelerated phase of erosion is associated with the uplift of the Atlantic rift shoulders and the establishment of an oceanic base-level. In the Drift phase, reactivations in response to far-field stresses likely triggered a Late Cretaceous – Paleocene rapid erosion event. The rates at which these events unfolded vary spatially and are controlled by inherited structures. The Araçuaí Orogen experienced slower denudation rates in areas closer to the São Francisco Craton, which suggests that the tectonic reactivation and related surface uplift during the Mesozoic–Cenozoic is in first-order controlled by lithospheric rigidity. Furthermore, the structural framework of the Paramirim and Pirapora aulacogens and NE-oriented shear zones in the orogen’s southeast facilitated later reactivations. From the spatial pattern of denudation/exhumation of the Araçuaí Orogen during the Mesozoic – Cenozoic, we draw inferences on the tectonic development of the offshore basins regarding their hydrocarbon potentials.

The story of a summit nucleus: hillslope boulders and their effect on erosional patterns and landscape morphology in the Chilean Coastal Cordillera

Abstract. 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.

Controls on sediment residence times in an Alpine river catchment inferred from uranium isotopes

Quantifying the timescale of sedimentary processes in river basins, such as erosion, transport and deposition, and their relationships with geomorphic parameters is important in the context of ongoing climate change. In this study, we combine uranium activity ratios (234U/238U) for fine-grained (<63 μm) fluvial sediments from the Var River basin (Southern French Alps) and inferred sediment residence times, to catchment-scale spatial analysis of geomorphic parameters with the aim to investigate the factors that influence the timescale of hillslope storage and alluvial transport.Our results show that the U isotope composition of sediments and inferred sediment residence time are mostly controlled by geomorphic parameters such as catchment elevation, slope and curvature, rather than catchment lithology. We show that sediment residence times are shorter in steep, high elevation terranes, but markedly increase wherever catchment-averaged slopes <30°, reflecting the transition from bedrock-dominated to soil-covered landscapes. Independent estimates of sediment residence times using spatial data for soil thickness and 10Be-derived denudation rates show good agreement with those calculated from U isotopes. This observation validates the suitability of U isotopes to quantify sediment residence times in river catchments and for investigating how geomorphic processes control the duration of hillslope storage and fluvial transport. Furthermore, these results show that the application of U isotopes to sedimentary deposits provides robust information on the relationship between climate variability and catchment erosion, allowing us to assess the role of intrinsic vs extrinsic controls on sediment routing at the millennial timescale.

High-temporal-resolution of lithium isotopes in Yangtze River headwater: Hydrological control on weathering in high-relief catchments

How climate change regulates silicate weathering in tectonically active areas remains clear. To evaluate the roles of temperature and hydrology in continental-scale silicate weathering in high-relief catchments, we applied a high temporal resolution of lithium isotopes in the Yalong River, which drains the high-relief borders of the eastern Tibetan Plateau. The dissolved δ7Li values range from +12.2‰ to +13.7‰ in the non-monsoon season and are higher and significantly vary from +13.5‰ to +19.4‰ in the monsoon season. The negative correlation between dissolved δ7Li and the Li/Na ratio is attributed to the formation of various proportions of δ7Li-low secondary minerals during weathering. From non-monsoon to monsoon season, the weathering intensity decreases with increasing secondary minerals formation and the weathering transforms from a supply limited to a kinetically limited weathering regime, indicated by a negative correlation between dissolved δ7Li value and SWR/D ratio (SWR = silicate weathering rate and D = total denudation rate). No correlations between temperature and dissolved δ7Li values were observed, and SWR suggested that temperature is not the direct control factor of silicate weathering in high-relief areas. The dissolved δ7Li values display positive correlations with discharge, physical erosion rates (PERs), and SWR. This positive correlations was attributed to an increase in the PER which caused the formation of more secondary minerals with increasing discharge. These results indicate the rapid temporal variability of riverine Li isotopes and chemical weathering process in response to changes in hydrology rather than temperature. Combined with the compiled PER, SWR, and Li isotopes at various altitudes, we further suggest that weathering in high-altitude catchments is more sensitive to hydrological changes than weathering in low-altitude catchments. These results highlight the key role of the hydrologic cycle (runoff and discharge) and the geomorphic regime in controlling global silicate weathering.

Erosion and weathering in carbonate regions reveal climatic and tectonic drivers of carbonate landscape evolution

Abstract. Carbonate rocks are highly reactive and can have higher ratios of chemical weathering to total denudation relative to most other rock types. Their chemical reactivity affects the first-order morphology of carbonate-dominated landscapes and their climate sensitivity to weathering. However, there have been few efforts to quantify the partitioning of denudation into mechanical erosion and chemical weathering in carbonate landscapes such that their sensitivity to changing climatic and tectonic conditions remains elusive. Here, we compile bedrock and catchment-averaged cosmogenic calcite–36Cl denudation rates and compare them to weathering rates derived from stream water chemistry from the same regions. Local bedrock denudation and weathering rates are comparable, ∼20–40 mm ka−1, whereas catchment-averaged denudation rates are ∼2.7 times higher. The discrepancy between bedrock and catchment-averaged denudation is 5 times lower compared to silicate-rich rocks, illustrating that elevated weathering rates make denudation more spatially uniform in carbonate-dominated landscapes. Catchment-averaged denudation rates correlate well with topographic relief and hillslope gradients, and moderate correlations with runoff can be explained by concurrent increases in weathering rates. Comparing denudation rates with weathering rates shows that mechanical erosion processes contribute ∼50 % of denudation in southern France and ∼70 % in Greece and Israel. Our results indicate that the partitioning between largely slope-independent chemical weathering and slope-dependent mechanical erosion varies based on climate and tectonics and impacts the landscape morphology. This leads us to propose a conceptual model whereby in humid, slowly uplifting regions, carbonates are associated with low-lying, flat topography because slope-independent chemical weathering dominates denudation. In contrast, in arid climates with rapid rock uplift rates, carbonate rocks form steep mountains that facilitate rapid, slope-dependent mechanical erosion required to compensate for inefficient chemical weathering and runoff loss to groundwater systems. This result suggests that carbonates represent an end member for interactions between climate, tectonics, and lithology.

River incision, 10Be production and transport in a source-to-sink sediment system (Var catchment, SW Alps)

Abstract. Detrital 10Be from continental river sands or submarine sediments has been extensively used to determine the average long-term denudation rates of terrestrial catchments, based on the assumption that the rate of cosmogenic nuclide production by the interaction of source rocks with cosmic radiation balances out the loss of these nuclides by surface denudation. However, the 10Be signal recorded in sediments may be affected at the source by the response time of mountainous catchments to high-frequency forcings. In addition, transient sediment storage in piedmonts, alluvial plains and lakes or near the coast may also induce a difference between the erosive signal and its record in the sedimentary sink. Consequently, a significant part of the signal recorded in shallow-water sediments can be lost, as deep marine sediments may simultaneously record a signal coming from newly eroded source rocks along with one coming from the destabilization of previously deposited sediments. In this paper, we use the landscape evolution model Badlands to simulate erosion, deposition and detrital 10Be transfer from a source-to-sink sedimentary system (the Var River catchment, southern French Alps) over the last 100 kyr. We first compare model-based denudation rates with the ones that would be extracted from the 10Be record of local continental sediments (equivalent to river sands) and from sediments deposited offshore over time in order to examine if this record provides an accurate estimate of continental denudation rates. Then, we examine which conditions (precipitation rate, flexure, ice cover) satisfy published measured river incision rates and 10Be concentration in submarine sediments. Our results, based on the Var catchment cosmic ray exposure dating and modelling indicate that, while river sands do accurately estimate the average denudation rate of continental catchments, this is much less the case for deep submarine sediments. We find that deep-sea sediments have a different and often much smoother 10Be signature than continental ones and record a significant time lag with respect to imposed precipitation rate changes, representing the geomorphological response of the margin. A model which allows us to fit both measured 10Be concentration in marine sediments and river incision rates on land involves an increase in precipitation rates from 0.3 to 0.7 m yr−1 after 20 ka, suggesting more intense precipitation starting at the end of the Last Glacial Maximum.

Denudation of the Golan Heights basaltic terrain using in-situ 36Cl and OSL dating

While erosion rates of carbonates along the Dead Sea fault (DSF) and its margins have been determined in several studies, information about denudation of basalts along the DSF are absent. Here we report concentrations of in situ cosmogenic 36Cl in bedrock and sediment samples combined with OSL ages of terraces from the Meshushim basin, a typical basaltic basin in the Golan Heights, northern Dead Sea Rift (DSR).Denudation rates of exposed and soil-covered bedrock basalts, range from 12.3 ± 1.4 mm/ka to 176.1 ± 47.4 mm/ka, well within the range of DSR values, regardless of lithology and climate. In all locations where exposed and soil-covered bedrock were samples next to each other, the soil-covered basalt erodes faster. This is regardless of notable variations in average annual temperature and precipitation. This means that bedrock outcrops were not covered and exposed very frequently, and that chemical weathering at the bedrock-soil interface is a significant erosional factor.Sediments in the Meshushim basin are stored in soil pockets and blankets on the upper plateau, colluvial material on slopes, and alluvial terraces. The difference in 36Cl concentrations between soil-covered and exposed bedrock indicates coverage duration of ca. 3–6 ka. The residence time estimate of only several thousands of years for sediments stored in alluvial terraces is 170 ± 20 y to 4000 ± 290 y. Nowhere in the Meshushim drainage system did we find evidence for storage of sediment longer than a few thousands of years.36Cl concentrations in colluvial and alluvial sediments are similar to those measured in soil-covered basalts. This similarity suggests that sub-surface weathering of basalts is the major source for sediments in upper and central segments of the basin, concurrent with the higher denudation rates of buried bedrock. However, very low 36Cl concentrations measured in samples collected in the lower parts of the basin indicate involvement of landslides which instantaneously expose previously shielded sediments that were incorporated into the drainage system. The effect of landslides is also manifested by the high basin scale denudation rates calculated from alluvial samples in various locations, as well as the geomorphological profile of the Meshushim basin. Sediment yield calculated from 36Cl concentrations measured in this study suggest temporal changes in denudation rates over the Holocene and up to the last ∼60 ka, with higher past denudation rates and sediment yield even in most of the Holocene related to present rates and yields.Overall, the results of this study show that the fact that the Golan lithology is basaltic does not result in different denudation process and rate from other locations investigated along the Dead Sea Rift, which are mostly of carbonate lithology and that relief and climate are stronger parameters than lithology in controlling denudation.

Six years of fluvial response to a large dam removal on the Carmel River, California, USA

AbstractMeasuring river response to dam removal affords a rare, important opportunity to study fluvial response to sediment pulses on a large field scale. We present a before–after/control–impact study of the Carmel River, California, measuring fluvial geomorphic and grain‐size evolution over 8 years, six of which postdated removal of a 32 m‐high dam (one of the largest dams removed worldwide) and included 11 flow events exceeding the 2‐year flood magnitude. We find that the reservoir‐sediment pulse following dam removal was relatively small (97 000 ± 24 000 t over 4 years), owing to deliberate reservoir‐sediment stabilization. Scaled to the size of the Carmel River watershed and compared against long‐term bedrock denudation rates, the post‐dam‐removal sediment release was slightly less than the annualized long‐term sediment export from this basin. New sediment transited &gt;30 km to the river mouth in less than 2 years, assisted by floods 2 and 4 years after dam removal. The sediment pulse fined the downstream riverbed while causing mostly low‐magnitude bed‐elevation changes: commonly 0.5 to 1 m or smaller, occurring as discontinuous sediment patches or interstitial deposits, aside from the filling and subsequent partial scour of deep pools. There was no major geomorphic reset downstream from the dam site. Geomorphic changes were driven almost entirely by flow rather than by the modest increase in sediment supply, in contrast to recent examples from other large dam removals. The relatively minor disturbance caused by dam removal on the Carmel River is likely analogous to many future dam removals: a relatively small sediment pulse after deliberate limitation of reservoir‐sediment erosion, and with an upstream dam remaining in place. Thus, a large dam removal need not lead to major downstream impacts.

Late Quaternary–Recent intraplate topographic uplift of the St. Francois Mountains (Ozark Plateau), Missouri (USA), driven by differential density and erosion

AbstractThe origin of relatively high topography in intraplate settings is elusive. The St. Francois Mountains, a tectonically inactive region with high relief, a Mesoproterozoic granite and rhyolite core, and Paleozoic sedimentary flanks, provide insights into the evolution of intraplate landscapes. We determined 14 in situ10Be exposure ages, 23 10Be catchment-wide denudation rates (CWDRs), and geomorphic indices using a geographic information system, and we modeled surface uplift using an isostatic-flexure model. CWDRs varied from ~5 to 10 ± 1.7 mm/k.y. and averaged 1.5 times greater in streams draining granite and rhyolite than in those draining flanking sedimentary rocks. In situ results from bedrock strath terraces gave Quaternary incision ages and incision rates that were tenfold higher than CWDRs. Major knickpoints (locally called shut-ins), all at 200 ± 70 m elevation, are located at contacts between the igneous and sedimentary rocks. Our exposure and denudation results inform isostatic-flexural modeling, which together support the conclusion that late Quaternary–Recent surface uplift driven by differential density and erosion focused in the igneous core is creating significant topography and relief in this intraplate setting.

Low rates of rock organic carbon oxidation and anthropogenic cycling of rhenium in a slowly denuding landscape

AbstractThe oxidation of petrogenic organic carbon (OCpetro) is a source of carbon dioxide to the atmosphere over geological timescales. The rates of OCpetro oxidation in locations that experience low rates of denudation remain poorly constrained, despite these landscapes dominating Earth's continental surface area. Here, we track OCpetro oxidation using radiocarbon and the trace element rhenium (Re) in the deep weathering profiles, soils and stream waters of the Susquehanna Shale Hills Critical Zone Observatory (PA, USA). In a ridge‐top borehole, radiocarbon measurements reveal the presence of a broad OCpetro weathering front, with a first‐order assessment of ~40% loss occurring over ~6 m. However, the low OCpetro concentration (&lt; 0.05 wt%) and inputs of radiocarbon throughout the deepest parts of the profile complicate the assessment of OCpetro loss. The OCpetro weathering front coincides with a zone of Re depletion (~90% loss), and we estimate that &gt; 80% of Re in the rock is associated with OCpetro, based on Re/Na and Re/S ratios. Using estimates of long‐term denudation rates, the observed OCpetro loss and the Re proxy are equivalent to a low OCpetro oxidation yield of &lt; 1.7 × 10−2 tC km−2 yr−1. This is consistent with the low OCpetro concentrations and low denudation rates at this location. In addition, we find the surface cycle of Re is decoupled from that of deep weathering, with an enrichment of Re in surface soils and elevated Re concentrations in stream water, precipitation, and shallow groundwater. A mass balance model shows that this can be explained by a historical anthropogenic contribution of Re through atmospheric deposition. We estimate that the topsoil Re pool could take decades to centuries to deplete and call for a renewed focus on anthropogenic perturbation of the surface Re cycle in low denudation rate settings.

An astronomical timescale for the Permian-Triassic mass extinction reveals a two-step, million-year-long terrestrial crisis in South China

The Permian-Triassic Mass Extinction (PTME) is the greatest biotic crisis of the Phanerozoic. In terrestrial settings, the PTME appears to have been diachronous and it has been suggested that losses initiated before the marine crisis. We examine organic carbon-isotope (δ13Corg) and geochemical proxies for environmental change in a palaeotropical wetland succession from southwest China. A newly constructed astronomical timescale provides a temporal framework for constraining the timing of the terrestrial PTME. Two major, negative carbon isotope excursions (CIEs) of 5.3‰ and 3.9‰ are observed between the top of the (Permian) Xuanwei Formation and the middle of the (Permian-Triassic) Kayitou Formation respectively. Our cyclostratigraphic model suggests that carbon cycle destabilization lasted ∼0.6 ± 0.1 Myr. We calculate total erosion rates for basaltic landscapes as a proxy for volumes of bare soil resulting from deforestation. Two phases of accelerated erosion saw denudation rates rise over a ∼1 Myr period from ∼150 t/km2/yr in the upper Xuanwei Formation (Permian) to >2000 t/km2/yr at the base of the Dongchuan Formation (Triassic). Calibrating the collapse of terrestrial ecosystems indicates that although the equatorial terrestrial PTME initiated before the marine crisis, it was a protracted process with the final coup-de-grâce not until ∼ 700 ky later. This has a bearing on extinction scenarios in which the terrestrial PTME is a causal factor in marine losses via enhanced nutrient runoff, because the final devastation on land post-dates the much more abrupt marine PTME.

The constancy of chemical weathering intensity on hillslopes in the arid to semiarid Qilian Mountains, NE Tibetan Plateau

Quantifying the relationship between the chemical weathering and denudation rates of active orogenic belts over a range of climates is the key to addressing the controversy over the uplift-weathering hypothesis. However, studies have focused on warm and humid environments and have not examined cold and arid environments. Here, we present a new dataset of the chemical depletion fraction (CDF: ratio of the chemical weathering rate to the total denudation rate) across the arid to semiarid Qilian Mountains on the northeastern Tibetan Plateau, where the uplift-weathering hypothesis has been proposed. We selected 60 points from 12 catchments in the middle Qilian Mountains. At each point, we collected three samples (soil, saprolite, and bedrock samples) and calculated the CDF values based on their Zr concentrations. We found no clear correlation between the CDF and climatic factors (temperature, precipitation, and normalized difference vegetation index (NDVI)), topographic factors (slope and local relief), and denudation rate. The calculated chemical weathering rates, nevertheless, are positively correlated with precipitation, NDVI, and denudation rate, and negatively correlated with temperature. This result indicates that the Qilian Mountains are under supply-limited conditions, even at high denudation rates (>800 t km−2 y−1). We speculate that low temperatures (<0 °C) could intensify near-surface chemical weathering by promoting the physical breakdown of the bedrock and increasing soil water availability. This mechanism causes a compensation effect maintaining the supply-limited conditions in landscapes with high denudation rates. Combing a worldwide dataset regarding the correlations between CDF and climatic factors and denudation rates, we argue that intensified denudation since the late Cenozoic contributed to global cooling.

Unsteady topography in the eastern Tianshan due to imbalance between denudation and crustal thickening

The Tianshan mountains have complex and variable topography and documenting their growth is important for understanding both intracontinental mountain building and the evolution of the global climate. We investigate whether this topography is in equilibrium with crustal influx (thickening) and sediment outflux (denudation). Based on literature, we estimate that the eastern Tianshan has been subject to a total crustal shortening rate of ∼9.4 mm/a across the Kuitun–Kuche transect, implying ∼1.3 mm/a of crustal thickening and a total crustal influx of ∼9 × 107 m3/a. We measured in-situ cosmogenic 10Be concentrations in modern river sands of 34 catchments to constrain recent (0–6 ka) basin-averaged denudation rates within the range and on its two flanks. Denudation rates range from 0.020 ± 0.002 to 0.53 ± 0.07 mm/a, averaging 0.20 ± 0.04 and 0.11 ± 0.02 mm/a in the north and south, respectively; these rates correspond to respective total sediment outfluxes of (542 ± 69) × 104 and (164 ± 24) × 104 m3/a. To ensure that these values can be compared to Pleistocene tectonic rates, we reconstructed Pleistocene denudation rates in seven of the studied basins. For this, we determined inherited in-situ cosmogenic 10Be concentrations from 11 cosmogenic depth profiles of abandoned fluvial terraces deposited in the Tianshan piedmonts. These data indicate that denudation rates have been relatively steady since the Pleistocene and thus that recent and Pleistocene sediment fluxes can be compared. These results show that crustal thickening outpaced denudation and sediment outflux by a factor of ∼10. Therefore, the Tianshan topography is not in dynamic equilibrium and is growing, even if materials are being subducted into the mantle. Consequently, to sustain this disequilibrium, the range grew laterally. This lateral growth and the inheritance of structures and basins are likely responsible for the complex topography of the range.

Chemical denudation dynamic based on hydrochemical measurements in the area of Drienovecká jaskyňa Cave basin (Slovak karst, Slovakia)

Denudation is a fundamental process for the development of karst relief. It has different intensity in different geological and climatic condition and beside this many other partial factors influence the intensity and dynamic of the process during the year. In karst systems, there is often a positive correlation between water partial pressure of CO2 and denudation. The Drienovecká jaskyňa Cave spring represents typical active fluviokarstic spring and its catchment area is represented by Wetterstein and Waxenec limestones, and Drienovec conglomerates. This catchment study area is located at the contact of the conti-nental and oceanic climate, in terms of altitude at the border between the lowland and moun-tain climate. During three hydrological years we measured main karst water parameters that helped us bring first results on chemical denudation in the catchment area. The water from the spring is of calcium-magnesium-bicarbonate type. Hydrogeochemical analyses in-dicate that it is mostly water from an open hydrogeological structure. Water of the cave spring is oversaturated with respect to calcite, which is documented by the saturation index of calcite, whose values are higher than the solubility product of calcite. During the studied period, the dynamic of the chemical denudation rate is clearly observable with peaks in spring months in year 2014 and 2016. The average value of denudation rate for the entire period is 21.06 mg Ca2+. Factors that influence chemical denudation include, for example, type of carbonate rock, CO2 partial pressure, water and air temperature, chemical compo-sition of the aqueous solution, activity of soil organisms, vegetation and others. Water from melted snow can also play a significant role in changes in water chemistry and thus chemical denudation. URL: https://www.gcass.science.upjs.sk/

Quantifying Permanent Uplift Due To Lithosphere‐Hotspot Interaction

AbstractVertical motions that accompany the passage of the lithosphere over a mantle hotspot can shed light on the nature of the hotspot and its effect on the lithosphere. However, quantifying the temporal vertical and spatial extent, is challenging due to the paucity of evidence in the geological record. Here, we utilize dense seismic and well data covering the intersection of the Great Meteor Hotspot (GMH) track with the U.S. Atlantic continental margin to constrain the surface expression of the hotspot passage under the lithosphere. The continuous sedimentary record of the eastern North American margin during its passage over the hotspot allows determination of the timing, magnitude, width and rate of denudation. We find that a ∼300 km wide region was denuded by up to 850 m between ∼97 and 86 Ma, ∼10 m.y. after the passage of the GMH. Stratigraphic relationships suggest a decaying rock uplift rate with time and no subsequent sagging. The broad, long‐lasting, and delayed uplift was modeled as a surface manifestation of either sub‐lithospheric mantle depletion, permanently eroded base of the continental lithosphere, or intrusions of depleted magma. We consider sub‐lithospheric depletion to be the most likely cause, based on seismic imaging results.

Characteristics, provenance, ore genesis and exploration of the Shimshal Valley placer gold deposits in the Karakoram Block, North Pakistan

The Karakoram Block in North Pakistan formed during the late Permian-Triassic on the southern margin of Asian Plate with a topography characterized by glaciated high mountains. Several rivers and streams draining from these mountain peaks carry detritus with precious and base metals. However, the primary source deposits and their ore genesis remain unknown due to the rugged glaciated topography and inaccessibility for prospecting and in situ sampling. Here, we present a multi-parametric analytical approach including major and trace element characteristics, thermoelectric analysis, EPMA elemental mapping, XRD analysis, and sulfur and lead isotopic studies on the detrital auriferous pyrite from Shimshal Valley placer gold deposits in the Karakoram Block of the north Pakistan to gain insights into the ore genesis, provenance and current erosion or denudation rate of the source primary deposits found in the hinterlands of the Shimshal Valley. The pyrite with varying forms such as {100} + {111} + {210} followed by {210} + {111} and {100} + {210}, which are highly enriched in gold (up to 1700 ppm) as revealed through EPMA analysis. The low to moderate temperature estimated for their formation (100 to 360 °C), pyrite geochemical characteristics, geochemical discrimination diagrams (Fe/S-As, Fe + S-As, As-Co-Ni) and elements contents (Au, Ag, Cu) of the placer gold grains, as well as the high (58.2 %) and low (41.8 %) thermoelectric coefficients of N-type and P-type, suggest that the auriferous pyrite belong to the epithermal type of magmatic-hydrothermal systems. The XnP value (average −48.7 %), γ value (average 62.8 %) in conjunction with the n(S)/n(Fe) values of 1.97–2.11 of pyrite indicate that the placer gold deposits are derived from the middle-upper part of the ore body. The high Mo/Ni (average 5.4) and Co/Ni (average 2.7) ratio combined with detrital zircon UPb geochronology, zircon fertility indicators such as Eu/Eu*, Ce/Nd, (Ce/Nd)/Y, Dy/Yb, (Eu/Eu*)/Y, texture and REE patterns correlate the ore mineralization with the Hunza Plutonic Unit emplaced on the catchment of the valley. The narrow variations in δ34S values, ranging from −5.3 to 3.8 ‰ with an average of −4.7 ‰, suggest that sulfur has a homogeneous magmatic source. The Pb isotopes values range 18.487 to 18.692, 15.713 to 15.745, and 38.964 to 39.416 for 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb, respectively. Our results suggest that metals for ore-forming fluid involved considerable contribution of crustal-derived materials.

Climate-induced denudation of the Eastern Ghat during the Holocene: A multi-proxy study from Chilika Lagoon (India)

A Holocene sediment record from Chilika Lagoon, India, suggests that changes in climate control denudation and weathering rate change. We assessed climate variability in the region and through time using multiproxy data comprising biotic and abiotic proxies. Mangrove and its associate assemblages suggest a backwater estuarine environment between ∼7.8 and 6 ka B.P. Depleted δ13Corg values during ∼7.8 to 5.5 ka B.P. indicate dominance of C3 vegetation, suggesting a warmer climate and strengthened monsoon. Enhanced δ13Corg and absence of pollen/spores are observed in the sediment layers deposited after ∼5 ka B.P., indicating arid climatic conditions with a dominance of C4 photosynthesizing plants and a relative lowering of sea level. Paleodenudation rates estimated using 10Be(meteoric)/9Be ratios varied between 556 ± 35 to 242 ± 15 t.km−2.yr−1. Phases of intense denudation occurred during 7.8–6.7 and 2.2–1 ka B.P., corresponding to Holocene Climatic Optimum and Medieval Climatic Anomaly, respectively. The degree of chemical weathering appears to have gradually increased from the early to late Holocene and correlates well (negatively) with the denudation rate. A gradual increase in the degree of chemical weathering from the early to late Holocene suggests an increase in the residence time of minerals in the weathering zone as the monsoon weakened.

Paleo‐denudation rates suggest variations in runoff drove aggradation during last glacial cycle, Crete, Greece

AbstractFluvial aggradation and incision are often linked to Quaternary climate cycles, but it usually remains unclear whether variations in runoff or sediment supply or both drive channel response to climate variability. Here we quantify sediment supply with paleo‐denudation rates and provide geochronological constraints on aggradation and incision from the Sfakia and Elafonisi alluvial‐fan sequences in Crete, Greece. We report seven optically stimulated luminescence and ten radiocarbon ages, eight10Be and eight36Cl denudation rates from modern channel and terrace sediments. For five samples,10Be and36Cl were measured on the same sample by measuring10Be on chert and36Cl on calcite. Results indicate relatively steady denudation rates throughout the past 80 kyr, but the aggradation and incision history indicates a link with climate shifts. At the Elafonisi fan, we identify four periods of aggradation coinciding with Marine Isotope Stages (MIS) 2, 4, 5a/b, and likely 6, and three periods of incision coinciding with MIS 1, 3, and likely 5e. At the Sfakia fan, rapid aggradation occurred during MIS 2 and 4, followed by incision during MIS 1. Nearby climate and vegetation records show that MIS 2, 4, and 6 stadials were characterized by cold and dry climates with sparse vegetation, whereas forest cover and more humid conditions prevailed during MIS 1, 3, and 5. Our data thus suggest that past changes in climate had little effect on landscape‐wide denudation rates but exerted a strong control on the aggradation–incision behaviour of alluvial channels on Crete. During glacial stages, we attribute aggradation to hillslope sediment release promoted by reduced vegetation cover and decreased runoff; conversely, incision occurred during relatively warm and wet stages due to increased runoff. In this landscape, past hydroclimate variations outcompeted changes in sediment supply as the primary driver of alluvial deposition and incision.