High Denudation Rates Research Articles

Pyrrhotite as a tracer for denudation of the Taiwan orogen

The use of pyrrhotite as an index mineral to delineate metamorphic isograds has long been proposed. However, details of the occurrence of pyrrhotite in different metamorphic facies and its implications have rarely been explored. Here, by simple characterization of magnetic properties and mineral identification, we demonstrate that pyrrhotite is widely distributed in low‐grade metamorphic terranes of Taiwan's Central Range and in sediments derived therefrom. By coupling the distribution of pyrrhotite in rocks with that in dated sediment strata, we have studied the denudation history of Taiwan's orogen from a source‐to‐sink perspective. We suggest that pyrrhotite is a potential tracer for studying surface processes in orogens with high denudation rates similar to that in Taiwan.

Chemical weathering response to tectonic forcing: A soils perspective from the San Gabriel Mountains, California

What controls the chemical weathering of soils in tectonically active landscapes? Recent field and modeling studies suggest that tectonic forcing and associated increases in erosion rates may either promote or hinder soil chemical weathering. These competing trajectories are dependent on two primary controls: the availability of fresh minerals and their residence time on the landsurface. Here, we explore rates and extents of soil weathering in the San Gabriel Mountains of California, where previous work has measured clear tectonic fingerprints on rates of long term exhumation, hillslope erosion and landscape morphology. We quantify chemical weathering across this landscape by elemental analysis of soils, saprolites and bedrock on six sites that bracket the low-gradient hillslopes of the relict upland plateau and the high-gradient hillslopes at the margins of the tectonically-driven incising landscape. Average chemical depletion fractions, which measure weathering losses from soil relative to unweathered parent material, decrease with increasing elevation and decreasing temperature, reflecting a combination of climate influence and potential dust inputs from the Mojave Desert. Weathering fluxes from non-dust-affected sites with similar elevations, climates and lithology correlate with both erosion rates and hillslope gradient. On low-gradient hillslopes (<25°), weathering rates increase with increasing erosion rates, reflecting the influence of mineral supply. However, on high-gradient hillslopes (>25°), weathering intensities and rates both decrease as erosion rates increase and soils thin. At the highest denudation rates (>300tkm−2y−1), saprolite production is outpaced, and soils are produced directly from fractured rock. These patterns are consistent with those predicted by a previously published model for denudation–weathering relationships based on mineral weathering kinetics. Variable weathering extents in soils indicate that weathering in the SGM is largely kinetically limited. This study is the first to quantify decreases in both rates and extents of soil chemical weathering with increasing erosion rates, and suggests tectonic uplift in rapidly eroding and incising landscapes may not stimulate increased silicate weathering.

Magnetic properties as tracers for source-to-sink dispersal of sediments: A case study in the Taiwan Strait

Different lithologies between Taiwan and southeastern China lead to diverse mineralogical composition for weathering products derived from the two shores of the Taiwan Strait. Pyrrhotite and magnetite are respectively the dominant magnetic minerals associated with fluvial sediments from western Taiwan and southeastern China. While magnetite commonly co-exists with pyrrhotite in sediments sourced from Taiwan, pyrrhotite has not been found in sediments sourced from mainland China. Associated with such a distinction are vast differences in magnetic properties, including magnetic susceptibility (χ), SIRM, HIRM and the S-ratio, which can be used to study the provenances of sediments in the Taiwan Strait and adjoining marginal seas. Based on any two of these parameters, the magnetic characteristics of much of the Taiwan Strait sediment can be explained using a two-endmember mixing model. Source-to-sink dispersal of sediments in the Taiwan Strait can then be traced from the distribution of these parameters. The results not only corroborate an earlier study based on radionuclides and particle size distribution ( Huh et al., 2011) but reveal more diagnostic details. Besides spatial distribution based on a large number (216) of surface sediments, we also analyzed temporal variation of magnetic properties in six well-dated cores collected at key sites along the sediment source-to-sink pathways. From profiles of these parameters in cores from the middle of the northern Taiwan Strait, it is calculated that sediment supply from Taiwan has increased substantially in the past five decades, which may very well be related to accelerated land use and increased frequency of intense rainfalls in Taiwan during the same period. The approach described in this work may be extended to other source-to-sink systems around the world and through time, especially the mountainous islands fringing the Pacific and Indian Oceans in southeastern Asia. As with Taiwan, these islands have high denudation rates and pyrrhotite is in all likelihood a mineral characteristic of their metamorphic terrains.

Cosmogenic 10Be-derived denudation rates of the Eastern and Southern European Alps

Denudation rates from cosmogenic 10Be measured in quartz from recent river sediment have previously been used in the Central Alps to argue that rock uplift occurs through isostatic response to erosion in the absence of ongoing convergence. We present new basin-averaged denudation rates from large rivers in the Eastern and Southern European Alps together with a detailed topographic analysis in order to infer the forces driving erosion. Denudation rates in the Eastern and Southern Alps of 170–1,400 mm ky−1 are within a similar range to those in the Central Alps for similar lithologies. However, these denudation rates vary considerably with lithology, and their variability generally increases with steeper landscapes, where correlations with topographic metrics also become poorer. Tertiary igneous rocks are associated with steep hillslopes and channels and low denudation rates, whereas pre-Alpine gneisses usually exhibit steep hillslopes and higher denudation rates. Molasse, flysch, and schists display lower mean basin slopes and channel gradients, and, despite their high erodibility, low erosion rates. Exceptionally low denudation rates are also measured in Permian rhyolite, which has high mean basin slopes. We invoke geomorphic inheritance as a major factor controlling erosion, such that large erosive glaciers in the late Quaternary cold periods were more effective in priming landscapes in the Central Alps for erosion than in the interior Eastern Alps. However, the difference in tectonic evolution of the Eastern and Central Alps potentially adds to differences in their geomorphic response; their deep structures differ significantly and, unlike the Central Alps, the Eastern Alps are affected by ongoing tectonic influx due to the slow motion and rotation of Adria. The result is a complex pattern of high mountain erosion in the Eastern Alps, which has evolved from one confined to the narrow belt of the Tauern Window in late Tertiary time to one affecting the entire underthrust basement, orogenic lid, and parts of the Southern Alps today.

A influência litológica nas taxas de denudação geoquímica do médio Espinhaço Meridional - MG

O presente trabalho visa investigar a influência da litologia nas atuais taxas de denudação geoquímica na média Serra do Espinhaço Meridional. As taxas de denudação correspondem ao volume de material proveniente da alteração das ro chas retirado de uma determinada área em um dado período de tempo (Souch, 2004). Estas taxas são intensamente controladas pelo substrato geológico, notadamente em bacias hidrográficas de menor extensão. O método adotado baseia-se na coleta de amostras das águas superficiais de dezesseis sub-bacias hidrográficas - que apresentam baixa interferência antrópica e estão sobre substrato geológico predominantemente quartzítico - e na medição da vazão dos canais amostrados. Análises químicas de águas superficiais, coletadas nos períodos úmido e seco do ano, foram utilizadas no cálculo da taxa de denudação geoquímica, que, de modo geral, apresentou-se baixa devido a alta resistência dos quartzitos - litologia predominante. Os resultados demonstraram que, apesar da relativa homogeneidade litológica, a área apresenta uma denudação geoquímica diferencial, onde: (i) bacias que drenam rochas carbonáticas possuem taxas mais elevadas de denudação, confirmando a alta solubilidade desse material, (ii) bacias que drenam unidades quartzíticas apresentam taxas baixas de denudação, principalmente aquelas sobre as formações Galho do Miguel e Sopa-Brumadinho.

Sediment production and transport from in situ-produced cosmogenic 10Be and river loads in the Napo River basin, an upper Amazon tributary of Ecuador and Peru

Abstract Cosmogenic nuclide-based denudation rates and published erosion rates from recent river gauging in the Napo River basin (Peruvian Amazonia) are used to decipher erosion and sedimentation processes along a 600 km long transect from the headwaters to the lowlands. The sediment-producing headwaters to the Napo floodplain are the volcanically active Ecuadorian Andes, which discharge sediment at a cosmogenic nuclide-based denudation rate of 0.49 ± 0.12 mm/yr. This denudation rate was calculated from an average 10Be nuclide concentration of 2.2 ± 0.5 × 104 at/g(Qz) that was measured in bedload-derived quartz. Within the Napo lowlands, a significant drop in trunk stream 10Be nuclide concentrations relative to the Andean hinterland is recorded, with an average concentration of 1.2 ± 0.5 × 104 at/g(Qz). This nuclide concentration represents a mixture between the 10Be nuclide concentration of eroded floodplain deposits, and that of sediment eroded from the Andean hinterland that is now carried in the trunk stream. Evidence for addition of sediment from the floodplain to the trunk stream is provided by published decadal-scale sediment flux measurements from gauging stations operated in the Napo basin, from which an increase from 12 × 106 t/yr at the outflow of the Andes to ∼47 × 106 t/yr at the confluence with the Solimoes (upper Amazon River) is recorded. Therefore, approximately 35 × 106 t of floodplain sediment are added annually to the active Napo trunk stream. Combined with our nuclide concentration measurements, we can estimate that the eroded floodplain deposits yield a nuclide concentration of ∼0.9 × 104 at/g(Qz) only. Under steady state surface erosion conditions, this nuclide concentration would translate to a denudation rate of the floodplain of ∼0.47 mm/yr. However, we have no geomorphologic explanation for this high denudation rate within the low relief floodplain and thus suggest that this low-nuclide concentrated sediment is Andean-derived and would have been deposited in the floodplain at a time when erosion rates of the Andes were elevated. Therefore, the recently eroded floodplain sediment provides an Andean “paleo denudation rate” of 1.2 mm/yr that was calculated for high Andean production rates. A likely period for elevated erosion rates is the LGM, where climate and vegetation cover of the Andes differed from that of the Holocene. A possible cause for the erosion of the floodplain is the tectonic uplift of the Eastern Andes, which progressively shifts the Napo River northwards. Hence, the river cuts into ancient lowland sediment, which is admixed to the Andean sediment carried in the main Napo River.

Evidence for subaqueously resedimented sulphate evaporites on Mars

This article documents the clastic nature of sulphate evaporite beds in the Tithonium Chasma located in the Valles Marineris region of Mars. These beds form a stratified succession characterised by very thick interbedded channel-fill breccia bodies. We infer that the bouldery channel-fills were deposited by voluminous mass-flow processes occurring in a relatively deep subaqueous environment. The redeposition of the coarse-grained evaporite would have responded to phases of high denudation rates in rapidly uplifting hinterlands. Tectonic activity also caused the diapiric uprise and exhumation of evaporite diapirs within the Valles Marineris chasmata, where the apparently young and well development karstic landforms probably formed during the late Amazonian age. These new data strongly suggest the deposition of both primary and resedimented evaporites in a marginal basin area, which effectively restricted ocean access through the proposed “proto-Valles Marineris Strait”. The associated ocean may be the “Ocean Borealis” of Late Noachian–Early Hesperian age.

フィッション・トラック熱年代に基づいた六甲地域の冷却・削剥史

Quantitatively estimating denudation is generally difficult because it essentially involves the removal and loss of materials in situ. The denudation rate of mountainous areas in Japan has commonly been studied from the volume of sediment in a basin or catchment. Nonetheless, the availability of these methods is constrained spatially by upstream area and temporally by depositional age. In the last few decades, thermochronometric methods that evaluate thermal history using radiometric-dating methods have been used to evaluate the denudation and tectonic history of orogenic belts around the world. The advantages of thermochronometric methods are that we can calculate the denudation rate at each sampling point and that combining multiple methods and/or target minerals enables us to calculate denudation rates in multiple periods. However, thermochronometric methods have been applied to areas with extraordinarily high denudation rates in island arc areas such as Japan. Thus, the effectiveness of thermochronometric methods for estimating denudation rates in island arc areas has not been demonstrated. We applied apatite and zircon fission-track thermochronometry to granitic rock samples collected from outcrops and a borehole to estimate the tectonic history of the Rokko area, southwest Japan. Previous studies suggested that the Rokko Mountains have been uplifted by active faulting along their northern and southern margins during the Rokko movements, a Quaternary tectonic movement in the Kinki district. However, the tectonic history of the Rokko area prior to 1 Ma has not been revealed due to a lack of prevalent geologic markers. We estimated average denudation rates in various periods based on apatite and zircon FT ages and previously reported radiometric ages. We obtained denudation rates at about 0.04-0.10 mm/yr after about 30 Ma, 0.05-0.7 mm/yr during 50-30 Ma, 0.7-4.0 mm/yr during 70-50 Ma. The denudation rate after the deposition of the Kobe Group (36.9-30.4 Ma) is estimated to be in the 0.01-0.1 mm/yr order, while bedrock uplift rate after about 1 Ma is estimated to be about 0.5 mm/yr. Thus, the tectonic activity of the Rokko Mountains area prior to 1 Ma has been relatively low.

Topographic and lithologic control on catchment-wide denudation rates derived from cosmogenic 10Be in two mountain ranges at the margin of NE Tibet

Fault-bounded mountain ranges with along-strike variations in mean slope angle and local relief offer the opportunity to investigate how denudation depends on these topographic parameters. We determined catchment-wide denudation rates along two actively growing, fault-bounded mountain ranges (the Yumu and western Longshou Shan) from 10Be concentrations in quartz from stream sediments. Both ranges expose (1) low-grade metamorphic Paleozoic rocks in their center, and (2) unmetamorphosed Cretaceous red beds near the tips of the range-bounding thrust faults. Field observations document debris flows, shallow landsliding, and surface wash as dominant mechanisms of mass transport on the hillslopes. Denudation rates obtained for the Paleozoic metasediments range from ∼100 to ∼300mmka−1 and correlate linearly with mean slope angle and local relief. In contrast, Cretaceous sediments exhibit mainly higher and more variable denudation rates between ∼30 and ∼500mmka−1, although slope and relief values are lower for these catchments. The higher denudation rates in the Cretaceous red beds are interpreted as resulting from their lower mechanical strength as compared to the resistant Paleozoic rocks. This important lithologic control on denudation demonstrates that uniform catchment lithologies and rock strength are essential for deciphering the effect of topographic indices on the rate of denudation.

Paraglacial slope adjustment since the end of the Last Glacial Maximum and its long-lasting effects on secondary mass wasting processes: Hauser Kaibling, Austria

This study indicates the possible long-lasting impacts of deep-seated gravitational slope deformation (DGSD) on erosional and mass movement processes. DGSD-features are common large-scale landforms in the crystalline basement rocks of the Central Alps of Austria. Inactive forms are related to paraglacial (nonglacial processes directly conditioned by former glaciation and deglaciation) rock slope adjustment processes acting after glacier retreat towards the end of the Late Glacial period. The near-surface metamorphic bedrock of DGSD-features is generally disintegrated. Such rock type favours erosion and the formation of small landslides. Field data, laboratory analyses, aerial photographs, and GIS-based terrain analyses are used to describe the different geomorphic effects of an extremely disintegrated rock slope. The studied DGSD Hauser Kaibling (13.6 km 2) is located in central Austria. During the Last Glacial Maximum the adjacent valley glacier reached an altitude of approximately 1900 m a.s.l. Glacier retreat in combination with the unfavourable tectono-metamorphic setting caused gravity-induced stresses to act. Subsequent sediment build-up in the main valley restabilised the slope. Linear erosion cut a number of gullies into the lower part of the DGSD. Calculated mean erosional denudation rates at incised valley sections range from 0.5 to 2.6 mm a − 1 since deglaciation. Smaller active landslides on the sides of the steep gullies draining the DGSD contribute to high denudation rates. Active landsliding is favoured by: (i) fragmented rock, (ii) steep slopes, (iii) supply of surface and subsurface water by rain and snowmelt, and (iv) human impact. Debris flows and debris-charged flood events occurred frequently in the past. During such events, material was transported from the unstable slopes to the populated alluvial fans. Thus, knowledge about the spatial distribution and the stage of development of DGSDs (active and currently inactive) provides an important basis for risk management measures.

Inter- and Intra-Catchment Variations in Proglacial Geomorphology: An Example From Franz Josef Glacier and Fox Glacier, New Zealand

Proglacial outwash plains, or “sandar,” can be recognized to be a part of a geomorphic, sedimentary, and hydrological system. At a global scale, glacial meltwater regimes and hence proglacial fluvial systems are strongly determined by glacier basal water conditions and glacier behavior. At a catchment scale it is necessary to consider that proglacial fluvial sedimentation can have a range of frequency and magnitude regimes. This paper presents geomorphological and sedimentological data from Franz Josef Glacier and Fox Glacier sandar, which have adjacent catchments. We determine that glaciofluvial facies are the most abundant sediment-landform association at both sites. However, we also observe considerable intra-catchment variability with respect to the magnitude-frequency regime of fluvial deposition and the relative importance of fluvial processes for sandur character. Franz Josef Glacier sandur is relatively high relief and superficially composed of boulder bedforms that are laterally and longitudinally extensive. It has a sedimentology dominated by massive, poorly sorted sediments containing outsized clasts. Franz Josef Glacier sandur thus has a character consistent with formation by episodic high-magnitude fluvial flows, i.e. jökulhlaups. In contrast, Fox Glacier sandur is of low cross-section relief and comprises two distinct components: an aggrading braided river and paraglacial debris fan deposits. With the exception of the contemporary ice margin, Fox Glacier sandur is of significantly finer-grained material than that at Franz Josef Glacier. We suggest that the contemporary Fox Glacier sandur contains widespread evidence that refutes a hypothesis of high-magnitude episodic events. Additionally, contemporary paraglacial inputs from recently deglaciated valley walls at Fox Glacier are far more important to sandur sedimentation than water or sediment from Fox Glacier. These results present a conceptual model of the predominant contemporary land-forming processes within a glaciated tectonically active region with exceptionally high denudation rates. Intra-catchment variability has important implications for predicting sediment fluxes in response to hydro-climatic forcing.

Direct and indirect evaluation of denudation rates in Central Italy

This paper presents the main results of a series of geomorphological studies, aimed to the quantitative direct and indirect evaluation of denudation processes in Central Italy. These studies, which started in some sample areas since long ago, were further deepened and extended to a wider number of drainage basins, representative of a variety of morphostructural conditions. The complex cause/effect relationships among factors influencing both the morphogenesis and the plano-altimetric configurations of drainage basins resulted in a large variability in the values of the denudation index (Tu), which ranged from 200 to 2500 t/km 2/year. Very high values (Tu > 10,000 t/km 2/year) were obtained for small catchments belonging to the wider basins of Ombrone River and Tevere River, where fast morphological modifications occur due to the action of surface running waters and gravity. In the drainage basins affected by the most powerful denudation processes direct measurements of denudation were performed: several monitoring stations were equipped with iron stakes and disks to record the local modifications of the topographic surface with time. Whereas some of these stations have been working continuously since 1993, some others have been running only periodically. Direct measurements pertinent to periods longer than a decade indicated that mean annual values of denudation range between 1 and 2.5 cm/year for slopes affected by sharp-edged and/or rounded edged badlands. Higher denudation rates (3–4 cm/year) were obtained for shorter observation periods; still higher rates were recorded for short periods at the gully bottom or in coincidence with mass movements before bench marks were turned up or damaged. Denudation processes are likely to occur discontinuously. They show a step-like trend with time and are triggered by rainfall events distributed over a number of consecutive days, though not greatly intense. Particularly, their amount should overcome the threshold of 60 mm per 5 consecutive days. Below this threshold the surface runoff does not seem to reach the power necessary to induce significant modifications of the topographic surface in the study area.

Limits to quantifying climate driven changes in denudation rates with cosmogenic radionuclides

The hypothesis that climate change significantly modifies the denudation history of an orogen can be tested by reconstructing catchment-wide denudation histories. Cosmogenic radionuclides measured in sediment deposits of known age (e.g. a sequence of fluvial terraces) offer one approach for quantifying catchment-wide paleo-denudation rates. We present a numerical model, and two case studies, that evaluate the utility of this technique for reconstructing different denudation histories. The concentration of cosmogenic beryllium-10 ( 10Be) is calculated for a sequence of sediments produced from climate driven variations in the mean denudation rate of a drainage basin at sea level and high latitude. The model accounts for the nucleonic and muonic production, and decay of 10Be as sediment is eroded and stored in deposits of different ages. Two scenarios for different climate driven denudation histories are considered: (1) A sinusoidal variation in the input denudation rate with free parameters of periodicity (23, 41, 100 kyr), amplitude (0.1, 0.5, 1.0), and mean denudation rate (0.01, 0.1, 1.0 mm yr − 1 ). (2) A climate driven denudation history based on oscillations of a global benthic foraminiferal oxygen isotope (δ 18O) record over the last 2 Myr. Results from both scenarios suggest cosmogenic radionuclides measured in a system with high denudation rates (> ∼ 0.5 mm yr − 1 ), longer periodicity (100 kyr), and low amplitude have the highest potential for reconstructing climate driven paleo-denudation rates. Unfortunately, the high denudation rates preferred in the theoretical consideration are not suitable for paleo-denudation rate determination due to overprinting of nuclides accumulated during denudation by post-depositional decay and irradiation. Hence, a compromise between the capability to determine changes in paleo-denudation rates and the determination of true denudation rates has to be accepted. Application of the model to previous data sets from northwest Europe and southwest North America produce a good agreement, but highlight the need for future work to consider how geomorphic processes (e.g. landsliding) respond to climate change and influence catchment-wide denudation rates. In addition, other climate proxies than the benthic foraminiferal δ 18O record should be used for specific sample locations.

Landscape response to deglaciation in a high relief, monsoon-influenced alpine environment, Langtang Himal, Nepal

Significant glacial fluctuations and rapid paraglacial reworking of glacigenic sediments characterize the Middle and Late Holocene of the Langtang Khola Valley, Central Nepal Himalaya. Geomorphic mapping and beryllium-10 cosmogenic radionuclide (CRN) dating of moraines and paraglacial fans were undertaken to test the existing paraglacial fan, terrace and moraine chronologies. The new dating compares favorably with prior studies that utilized radiocarbon, adding additional support to the assumption that fan and terrace formation are strongly linked to deglaciation. Fan and terrace denudation rates are so rapid in this region, averaging ∼33 mm/yr, that no depositional landforms older than 5 ka are preserved within 250 m of the valley floor. In this region, high rates of denudation during the Late Quaternary are driven by a combination of rapid tectonic uplift, numerous glacial fluctuations and intense weathering driven by an active monsoon climate. Extensive reworking of glacigenic sediments in Langtang during the latter half of the Holocene is consistent with studies completed in other areas of the Himalaya that are strongly influenced by the monsoon.

Effects of large deep‐seated landslides on hillslope morphology, western Southern Alps, New Zealand

Morphometric analysis and air photo interpretation highlight geomorphic imprints of large landslides (i.e., affecting ≥1 km2) on hillslopes in the western Southern Alps (WSA), New Zealand. Large landslides attain kilometer‐scale runout, affect &gt;50% of total basin relief, and in 70% are slope clearing, and thus relief limiting. Landslide terrain shows lower mean local relief, relief variability, slope angles, steepness, and concavity than surrounding terrain. Measuring mean slope angle smoothes out local landslide morphology, masking any relationship between large landslides and possible threshold hillslopes. Large failures also occurred on low‐gradient slopes, indicating persistent low‐frequency/high‐magnitude hillslope adjustment independent of fluvial bedrock incision. At the basin and hillslope scale, slope‐area plots partly constrain the effects of landslides on geomorphic process regimes. Landslide imprints gradually blend with relief characteristics at orogen scale (102 km), while being sensitive to length scales of slope failure, topography, sampling, and digital elevation model resolution. This limits means of automated detection, and underlines the importance of local morphologic contrasts for detecting large landslides in the WSA. Landslide controls on low‐order drainage include divide lowering and shifting, formation of headwater basins and hanging valleys, and stream piracy. Volumes typically mobilized, yet still stored in numerous deposits despite high denudation rates, are &gt;107 m3, and theoretically equal to 102 years of basin‐wide debris production from historic shallow landslides; lack of absolute ages precludes further estimates. Deposit size and mature forest cover indicate residence times of 101–104 years. On these timescales, large landslides require further attention in landscape evolution models of tectonically active orogens.

The control mechanisms of erosion and weathering at basin scale from cosmogenic nuclides in river sediment

The study of a sample of river sediment enables the determination of spatially averaged denudation rates that provide an exceptional perspective on erosion and weathering processes that have taken place within a landscape. These measurements are done with in-situ-produced cosmogenic nuclides (e.g., 10Be, 26Al), mostly in quartz from alluvial sediment. Cosmogenic nuclides are produced when secondary cosmic rays interact with the very uppermost layer of the Earth's surface. They are produced within a characteristic depth scale of about 1 m, which means that the measured concentrations record an integrated denudation history while material passed through this depth interval. Depending on the denudation rate the resulting integration time scales are 103 to 105 years, and one obtains a robust long-term estimate of natural denudation that is relatively insensitive to short-term changes. The last 10 years have seen significant research activity using these methods, and an array of fascinating tectono-geomorphologic and geochemical insights are emerging. Amongst these is the ability to identify the physical and chemical processes with which a landscape responds to tectonic activity or climate change. A compilation of world-wide denudation rates in non-glaciated areas, that however, does not yet include some of the world's most active mountain belts, has resulted in the following findings, some of which have been unexpected: (1) No obvious relationship between precipitation or mean annual temperature and total denudation is apparent. (2) Topographic relief alone does not result in high rates of denudation. (3) Denudation rates are high in areas of landscape rejuvenation; that is triggered and controlled by tectonic activity (faulting, escarpment formation and retreat, rifting, surface uplift). (4) Rates of weathering (using a combination of cosmogenic nuclides and zirconium-normalised cation loss balances) co-vary primarily with physical erosion rates and much less with temperature or precipitation. (5) In some areas of high land use short-term rates (from river load gauging) exceed those from cosmogenic nuclides by several orders of magnitude, which serve to highlight the severity of geomorphic change caused by human action. In the future, the control mechanisms over denudation will be determined on all spatial scales, ranging from the single soil section to entire river basins. The same analysis can be done back through time on well-dated terraces, lake records, and marine sediment cores, which is possible with 10Be for the past 1–2 My. The rates obtained will be used to develop a quantitative understanding of tectonic, geomorphologic, and geochemical landscape processes, which in turn is a prerequisite to design and calibrate models of the response of landscapes to tectonic, climate, and anthropogenic forcing.

An intense terminal epoch of widespread fluvial activity on early Mars: 2. Increased runoff and paleolake development

To explain the much higher denudation rates and valley network development on early Mars (&gt;∼3.6 Gyr ago), most investigators have invoked either steady state warm/wet (Earthlike) or cold/dry (modern Mars) end‐member paleoclimates. Here we discuss evidence that highland gradation was prolonged, but generally slow and possibly ephemeral during the Noachian Period, and that the immature valley networks entrenched during a brief terminal epoch of more erosive fluvial activity in the late Noachian to early Hesperian. Observational support for this interpretation includes (1) late‐stage breaching of some enclosed basins that had previously been extensively modified, but only by internal erosion and deposition; (2) deposition of pristine deltas and fans during a late stage of contributing valley entrenchment; (3) a brief, erosive response to base level decline (which was imparted as fretted terrain developed by a suite of processes unrelated to surface runoff) in fluvial valleys that crosscut the highland‐lowland boundary scarp; and (4) width/contributing area relationships of interior channels within valley networks, which record significant late‐stage runoff production with no evidence of recovery to lower‐flow conditions. This erosion appears to have ended abruptly, as depositional landforms generally were not entrenched with declining base level in crater lakes. A possible planetwide synchronicity and common cause to the late‐stage fluvial activity are possible but remain uncertain. This increased activity of valley networks is offered as a possible explanation for diverse features of highland drainage basins, which were previously cited to support competing warm, wet and cold, dry paleoclimate scenarios.

Time scale and conditions of weathering under tropical climate: Study of the Amazon basin with U-series

The Rio Solimões/Amazonas (Amazon River) and its major tributaries have been analyzed for U-series nuclides. 238U– 234U– 230Th– 226Ra disequilibria have been measured in the dissolved (<0.2 μm) and suspended loads (>0.2 μm) as well as bed sands. U-series disequilibria are closely related to major and trace element compositions and therefore reflect elemental fractionation during chemical weathering. Moreover, while the dissolved load records present-day weathering, suspended particles integrate the erosion history over much longer time scales (>100 ka). Lowland rivers are characterized by long time scales of chemical erosion (⩾100 ka) resulting in a high weathering intensity. Moreover, exchange between suspended particles and the dissolved load may explain the U-series signature for these rivers. By combining U-series and Pb isotopes in suspended particles, we show that erosion in the Rio Madeira basin occurred as a multi-step process, whereby the pristine continental crust was eroded several hundreds of Ma ago to produce sediments that have then been integrated in the Cordillera by crustal shortening and are currently eroded. In contrast, recent erosion of a pristine crust is more likely for the Rio Solimões/Amazonas (<10 ka). The suspended particles of the rivers draining the Andes (Solimões/Amazonas, Madeira) suggest time scales of weathering ranging between 4 and 20 ka. This indicates that suspended particles transported by those rivers are not stored for long periods in the Andean foreland basin and the tropical plain. The sediments delivered to the ocean have resided only a few ka in the Amazon basin (6.3 ± 1 ka for the Rio Amazonas at Óbidos). Nevertheless, a large fraction of the sediments coming out from the Andes are trapped in the foreland basin and may never reach the ocean. Erosion in the Andes is not operating in steady state. U-series systematics shows unambiguously that rivers are exporting a lot more sediments than predicted by steady-state erosion and that is a consequence of soil destruction greater than production. By relating this observation to the short time scales of weathering inferred for the Andes (a few ka), it appears that the erosion regime has been recently perturbed, resulting in high denudation rates. A possible explanation would be the increase in precipitation less than 5 ka proposed by recent paleoclimatic studies. Our results indicate that erosion responds rapidly to high-frequency climatic fluctuations.

The control mechanisms of erosion and weathering at basin scale from cosmogenic nuclides in river sediment

The study of a sample of river sediment enables the determination of spatially averaged denudation rates that provide an exceptional perspective on erosion and weathering processes that have taken place within a landscape. These measurements are done with in-situ produced cosmogenic nuclides (e.g. 10Be, 26Al), mostly in quartz from alluvial sediment. Cosmogenic nuclides are produced when secondary cosmic rays interact with the very uppermost layer of the Earth's surface. They are produced within a characteristic depth scale of about 1 m, which means that the measured concentrations record an integrated denudation history while material passed through this depth interval. Depending on the denudation rate the resulting integration time scales are 10 3 to 10 5 years, and one obtains a robust long-term estimate of natural denudation that is relatively insensitive to short-term changes. The last 10 years have seen significant research activity using these methods, and an array of fascinating tectono-geomorphologic and geochemical insights are emerging. Amongst these is the ability to identify the physical and chemical processes with which a landscape responds to tectonic activity or climate change. A compilation of world-wide denudation rates in non-glaciated areas, that however, does not yet include some of the world's most active mountain belts, has resulted in the following findings, some of which have been unexpected: (1) No obvious relationship between precipitation or mean annual temperature and total denudation is apparent. (2) Topographic relief alone does not result in high rates of denudation. (3) Denudation rates are high in areas of landscape rejuvenation; that is triggered and controlled by tectonic activity (faulting, escarpment formation and retreat, rifting, surface uplift). (4) Rates of weathering (using a combination of cosmogenic nuclides and zirconium-normalised cation loss balances) co-vary primarily with physical erosion rates and much less with temperature or precipitation. (5) In some areas of high land use short-term rates (from river load gauging) exceed those from cosmogenic nuclides by several orders of magnitude, which serves to highlight the severity of geomorphic change caused by human action. In the future, the control mechanisms over denudation will be determined on all spatial scales, ranging from the single soil section to entire river basins. The same analysis can be done back through time on well-dated terraces, lake records, and marine sediment cores, which is possible with 10Be for the past 1–2 My. The rates obtained will be used to develop a quantitative understanding of tectonic, geomorphologic, and geochemical landscape processes, which in turn is a prerequisite to design and calibrate models of the response of landscapes to tectonic, climate, and anthropogenic forcing.

Glaciers show direct linkage between erosion rate and chemical weathering fluxes

Field studies suggest a linkage between high physical erosion rates and rates of chemical denudation. Mechanical erosion by temperate glaciers is commonly an order of magnitude higher than in mountainous fluvial catchments, leading to an expectation that chemical weathering fluxes should also be high from glacierized basins. Yet solute fluxes from glaciers are not found to be higher than non-glacierized catchments. Application of a model of silicate weathering from glaciers based on mineral surface area production and mineralogy shows that solute fluxes are consistent with the low temperatures, dilute water chemistry, and high mineral surface area production in these environments. Low temperatures reduce silicate-weathering rates; this effect explains the difference between silica fluxes from glaciers and from non-glacierized basins. As in laboratory flow-through reactors, glacial solute flux should depend on surface area production and mineral weathering rate constants. The surface area production is significant: a typical glacial erosion rate and grain-size distribution produces on the order of 104 km2 of mineral surface area per square kilometer per year. This new surface area is highly reactive because mineral weathering rates decline with surface age. Application of the “reactor” model yields results largely consistent with measured solute fluxes for the example of Bench Glacier, Alaska. The model underpredicts potassium fluxes, probably due to accelerated initial dissolution of biotite strained by abrasion. The success of the model in predicting other silicate weathering fluxes reflects the far-from-equilibrium conditions in glacial runoff, such that mineral weathering rate constants are not limited by saturation state. In a small data set from glacial catchments, both annual silica fluxes and mean concentrations increase with water discharge. This suggests that mineral surface area increases with water discharge from glaciers, an effect plausibly linked to erosion rates. Those glaciers for which both erosion rate and silica flux data are available support the idea that production of new reactive mineral surface area by glacial erosion drives silicate weathering fluxes.