Physical Erosion Rates Research Articles

Effects of physical erosion on chemical denudation rates: A numerical modeling study of soil-mantled hillslopes

Many biogeochemical and Earth surface processes depend critically on chemical weathering. The immediate products of chemical weathering are present as solutes and secondary minerals in groundwater, soils, and streams, and form the nutritional foundation for terrestrial biogeochemistry. Chemical weathering also contributes to physical erosion by weakening bedrock and producing easily erodible regolith, and as the primary long-term sink for atmospheric CO 2 it modulates Earth's long-term climate via the greenhouse effect. Long-term chemical denudation rates on soil-mantled hillslopes can be estimated from cosmogenic radionuclide (CRN) concentrations in soil-borne quartz and the enrichment of a chemically inert tracer in soil relative to its parent bedrock, a technique that inherently assumes steady physical erosion over the timescale of CRN accumulation. We present a numerical model that computes changes in soil mineralogy and CRN concentrations under time-varying physical erosion rates, and we use this model to assess the accuracy of the CRN-based technique for estimating chemical denudation rates in non-steady conditions. Our modeling results suggest that CRN-based estimates of chemical denudation rates closely resemble actual chemical denudation rates averaged over the timescale of CRN accumulation, even during large-amplitude and long-period oscillations in physical erosion rates. For example, this model predicts that when physical erosion rates fluctuate sinusoidally by 50% of their mean over any period in time, CRN-based estimates of chemical denudation rates should differ from actual chemical denudation rates by less than 15%. Our model also implies that chemical denudation rates should approach zero both when physical erosion rates approach zero (because soluble minerals become depleted in the soil) and when physical erosion rates approach the maximum soil production rate (because soil thickness approaches zero). Modeled chemical denudation rates thus reach a maximum at intermediate physical erosion rates. If this relationship holds in nature, it implies that in rapidly eroding regions, further increases in physical erosion rates (e.g., due to increases in tectonic uplift rates) may not necessarily lead to faster chemical denudation on soil-mantled hillslopes.

Elevated weathering rates in the Rocky Mountains during the Early Eocene Climatic Optimum

Silicate weathering reactions remove carbon dioxide from the atmosphere and store it in carbonate minerals. During the high atmospheric carbon dioxide conditions of the Early Eocene Climatic Optimum, rates of chemical weathering, physical erosion and denudation in the western USA were equivalent to the highest recorded rates in the non-glacial Quaternary. During the chemical weathering of silicate minerals, atmospheric carbon dioxide is incorporated into carbonate minerals and buried. As the rate of silicate weathering is thought to increase in response to increasing atmospheric CO2 concentrations, this represents an important negative feedback mechanism1. Quaternary records of weathering reflect a narrow range of pCO2 (180–300 p.p.m.v.)2; therefore, the extent of this feedback has been difficult to predict for increasing concentrations of atmospheric CO2. However, high CO2 levels of up to 1,125 p.p.m.v. have been suggested for the Early Eocene Climatic Optimum (52 to 50 million years ago)3,4,5. Here, we combine 40Ar/39Ar ages6 and the measured volumes of river-derived sediments and sodium-bearing evaporites to determine rates of physical erosion and chemical weathering in the Green River Basin, western United State of America, during the Early Eocene Climatic Optimum7. We find physical erosion rates of 420±79 t km2 yr−1 and chemical weathering rates of 62.5±21.9 t km2 yr−1. The calculated denudation rates of 175±30 m Myr−1 rival the highest documented non-glacial Quaternary rates for crystalline bedrock8. We suggest that elevated atmospheric CO2 levels during the Early Eocene epoch led to enhanced silicate dissolution rates9, and thus to increased production of loose rock material and higher rates of physical weathering and denudation.

Another Look at the Chemical Relationships in the Dissolved Phase of Complex River Systems

Large rivers are a major pathway for the erosion products of continents to reach the oceans. The riverine transport of dissolved and particulate materials is generally related to a large number of interactions involving climate, hydrological, physico-chemical and biological aspects. Consequently, the investigation of large rivers allows the erosion processes at a global scale to be addressed, with information about biogeochemical cycles of the elements, weathering rates, physical erosion rates and CO2 consumption by the acid degradation of continental rocks. Today, good databases exist for the major dissolved ions in the world’s largest rivers. Since concentration of ions in river waters has to be considered in a compositional context, it is necessary to study the implications of considering the simplex, with its proper geometry, as the natural sample space. Using the additive (alr) or the isometric (ilr) log-ratio transformations, a composition can be represented as a real vector; but only in the second case can these coordinates be mapped onto orthogonal axes.

A reactive diffusion model describing transformation of bedrock to saprolite

Despite large ranges in rates of erosion and regolith production globally, much continental surface is regolith-mantled. This implies either that erosion rate is low relative to regolith formation over extensive regions, or that the rates of erosion and weathering advance (regolith formation) are in balance. Recent observations for many catchments that weathering-derived solute fluxes vary with the physical erosion rate of the catchment provide evidence that erosion and weathering advance rates may be balanced or at least coupled. Few theoretical models have been advanced to explain such observations, however, because weathering models generally consist of either fully parameterized reactive transport models that do not attempt to model geomorphological evolution, or geomorphologically parameterized models of landscape evolution that incorporate little geochemistry. Implementation of a full reactive transport code into a landscape evolution model is not currently tractable. We therefore present a reactive diffusion model that captures the essence of the chemical transformation of rock to saprolite for eventual incorporation into such models. Our model granite (albite + FeO-bearing phase + quartz) transforms to saprolite (goethite +kaolinite + quartz). The FeO component transforms to goethite deeper than albite transforms to kaolinite. Between these two reaction fronts, porosity decreases relative to initial porosity. Without erosion, the depth of the weathering fronts vary as ∼ t and the weathering fluxes of aqueous Na or Si, which depend upon the porosity in the saprolite, decrease as 1 t . These results are in good agreement with the assumption of local chemical equilibrium. Analysis of the local equilibrium case for a further simplified two-mineral granite model shows that with constant erosion rate, the weathering rock is comprised of an unaltered-porosity (unoxidized Fe) zone, a reduced-porosity (partly oxidized Fe) zone, and a constant porosity (fully oxidized Fe) zone. This model documents three regimes: kinetic (weathering advance rate depends on reaction rate), transition (weathering advance rate depends on reaction and erosion rates), and local equilibrium (weathering advance rate depends only on erosion rate). The first and last of these have been previously called the weathering- and transport-limited regimes respectively. The time needed to reach steady state in the kinetic regime (70–3300yr) is comparable to geological estimates for weathering-limited landscapes. The analytical solutions document that diffusion-limited weathering advance rates in the presence of constant erosion can be consistent with constant weathering advance rates, thus providing a theoretical basis for coupling geochemical mineral reactions with erosion in landscape evolution models.

Riverine behaviour of uranium and lithium isotopes in an actively glaciated basaltic terrain

This study presents U and Li isotope and major and trace element data for the dissolved load, suspended particulates and bedload for Icelandic rivers draining predominantly basaltic catchments. Physical erosion rates range from 920 to 2084 t/km 2/yr, with the higher values associated with glacier-fed rivers. Chemical erosion rates range from 45 to 91 t/km 2/yr, with lower rates being associated with glacier-fed rivers. Uranium activity ratios, ( 234U/ 238U), are close to secular equilibrium in the suspended and bedloads, but all dissolved load samples show values greater than unity, ranging from 1.13 to 2.41. The highest ( 234U/ 238U) values are found in glacier-fed rivers, and can be attributed to α-recoil effects, as grinding by glaciers locally enhances rates of physical weathering. Activity ratios in glacial rivers decrease with distance from the glacial source due to input from non-glacial tributaries which have high levels of dissolved uranium and lower activity ratios. In contrast, in non-glacial rivers, uranium activity ratios increase with distance downstream due to continued weathering in soils and of bedrock. The δ 7Li value of the suspended load is always lower than that of the bedload due to preferential retention of 6Li in secondary minerals during weathering. In turn, the δ 7Li value of the dissolved load is always greater than that of the bedload, ranging from 17.0 to 43.7‰. δ 7Li for the dissolved load decreases with increasing levels of dissolved silicon, and the saturation index of secondary minerals, which suggests that δ 7Li decreases with increasing chemical weathering. There is no correlation between δ 7Li values and uranium activity ratios for the dissolved load for non-glacial rivers, but for glacier-fed rivers there is an increase in δ 7Li with increasing ( 234U/ 238U), suggesting that where physical comminution of mineral grains by glaciers is high, chemical weathering is suppressed, and vice-versa.

The relationship between riverine U-series disequilibria and erosion rates in a basaltic terrain

U-series isotopes have been measured in the dissolved phase, suspended load and bedload of the main rivers draining basaltic catchments in Iceland. For the dissolved phase, ( 234U/ 238U) and ( 238U/ 230Th) range between 1.08 and 2.2, and 7.4 and 516, respectively. For the suspended load and bedload, ( 234U/ 238U) and ( 238U/ 230Th) range from 0.97 to 1.09 and from 0.93 to 1.05, respectively. Chemical erosion rates, calculated from dissolved major elements, range between 13 and 333 t km − 2 yr − 1 . Physical erosion rates have also been estimated, from existing data, and range between 21 and 4864 t/km 2/yr, with an average of 519 t km − 2 yr − 1 . U-series disequilibria indicate that weathering in Iceland operates at close to steady-state conditions. A model of continuous weathering indicates a maximum weathering timescale of 10 kyr, with an average rate of uranium release into water of 1.6 · 10 − 4 yr − 1 , which is significant when compared to granitic terrains located at similar latitudes and to tropical basaltic terrains. All river waters display ( 234U/ 238U) greater than secular equilibrium, consistent with the effects of alpha-recoil. The same dissolved phase ( 234U/ 238U) exhibit a negative trend with physical erosion rates, explained by the dominant effect of close-to-congruent chemical weathering of hyaloclastites in the younger basaltic terrains. Therefore, chemical erosion rate and mineral weathering susceptibility play a major role in determining 234U– 238U disequilibria in basaltic river waters. Comparison of global data for river basins in which weathering was recently strongly limited indicates a negative correlation between silicate weathering rates estimated with major elements and the age of weathering estimated with U-series disequilibria. This strongly suggests a key role of time and soil thickness on the chemical erosion of silicates.

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.

Cosmogenic nuclide methods for measuring long-term rates of physical erosion and chemical weathering

Understanding the evolution of geochemical and geomorphic systems requires measurements of long-term rates of physical erosion and chemical weathering. Erosion and weathering rates have traditionally been estimated from measurements of sediment and solute fluxes in streams. However, modern sediment and solute fluxes are often decoupled from long-term rates of erosion and weathering, due to storage or re-mobilization of sediment and solutes upstream from the sampling point. Recently, cosmogenic nuclides such as 10Be and 26Al have become important new tools for measuring long-term rates of physical erosion and chemical weathering. Cosmogenic nuclides can be used to infer the total denudation flux (the sum of the rates of physical erosion and chemical weathering) in actively eroding terrain. Here we review recent work showing how this total denudation flux can be partitioned into its physical and chemical components, using the enrichment of insoluble tracers (such as Zr) in regolith relative to parent rock. By combining cosmogenic nuclide measurements with the bulk elemental composition of rock and soil, geochemists can measure rates of physical erosion and chemical weathering over 1000- to 10,000-year time scales.

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.

Erosion rates over millennial and decadal timescales at Caspar Creek and Redwood Creek, Northern California Coast Ranges

AbstractComparing millennial‐scale denudation rates from cosmogenic nuclides with decadal‐scale sediment yields can shed light on erosional processes and on the effects of land use on sediment delivery to streams. Detailed measurements of sediment fluxes in the Northern California Coast Ranges at Caspar Creek and Redwood Creek have provided estimates of physical erosion rates since 1963 and 1971, respectively. We used cosmogenic 10Be to measure millennial‐scale denudation rates averaged over 1400–8700 years at six catchments in Caspar Creek and four catchments in Redwood Creek. Our 10Be measurements at Caspar Creek imply denudation rates that are nearly spatially uniform across the entire catchment and average 0·09 ± 0·02 mm a−1. These millennial‐scale rates implied by cosmogenic 10Be are faster than physical erosion rates of 0·005 ± 0·001 mm a−1 to 0·046 ± 0·007 mm a−1 inferred from sediment flux measurements over the past few decades in the same catchments. At Redwood Creek, our cosmogenic 10Be measurements imply millennial‐scale denudation rates that vary across the catchment from 0·14 ± 0·03 mm a−1 to 0·44 ± 0·09 mm a−1, in contrast to physical erosion rates ranging from 0·038 ± 0·011 mm a−1 to 0·48 ± 0·09 mm a−1 derived from sediment flux measurements made over the past few decades at the same catchments. The decadal‐scale and millennial‐scale measurements tend to differ most at the smallest tributaries, but differ by less than a factor of three for the Caspar Creek and Redwood Creek catchments as a whole. These measurements suggest that denudation rates at Caspar Creek are slower than rock uplift rates of 0·3–0·4 mm a−1, implying that Caspar Creek is not in topographic steady state. Copyright © 2005 John Wiley & Sons, Ltd.

Tectonic and climatic controls on silicate weathering

Understanding the controls on chemical weathering, especially of silicate minerals, remains a major challenge, despite its importance in controlling the evolution of the Earth's surface. In particular, it has proved hard to distinguish the temperature sensitivity of silicate weathering rates from other factors. Here we present a new compilation of chemical and physical erosion rates in small catchments and show that silicate weathering rates are not governed by any single parameter but require consideration in multiple dimensions. The overall variation in silicate weathering rates with physical erosion rates, rainfall, and temperature can be quantitatively described by a parameterization based on considering their limiting relationships. At lower erosion rates mineral supply limits weathering. At higher erosion rates there is abundant material but kinetic and therefore climatic factors limit weathering. A predictive model describing the field data based on transport and weathering (kinetically) limited scenarios yields theoretically sensible values for fitted parameters. In the transport-limited case, the supply of silicate cations from weathering is directly proportional to the supply of material by erosion, consistent with complete leaching of cations from fresh regolith. In the kinetically limited case, weathering scales directly with runoff, as the square root of erosion rate, and with an activation energy of 74 ± 29 kJ/mol, consistent with expected values in the Earth's surface settings.

Mobility of U-series nuclides during basalt weathering: An example from the Deccan Traps (India)

U-series disequilibria have been measured by TIMS and MC-ICP-MS in dissolved phases and suspended sediments of the main basaltic rivers from the Deccan Trap region (India). For dissolved phases ( 234U/ 238U), ( 230Th/ 238U), and ( 226Ra/ 238U) range between 1.11 and 1.28, between 0.03×10 −2 and 1.35×10 −2, and between 0.02 and 0.16 respectively. For suspended particles, ( 234U/ 238U), ( 230Th/ 238U), and ( 226Ra/ 238U) range from 0.95 to 1.1, from 1.23 to 1.59 and from 0.1 to 0.47 respectively. 230Th– 238U and 234U– 238U systems show direct links with indices of weathering. The data also indicate possible redistributions between the solid and aqueous phases and indicate that erosion processes do not presently operate at steady-state because present-day physical erosion rates are higher than expected (100–400 mm/ka versus 40 mm/ka for a steady-state process) for the Narmada and the Tapti basins. Models assuming either discrete or continuous particle leaching yield timescales for chemical erosion of 40–90 ka. In contrast with basins located in Northern latitudes, erosion in the Deccan basins does not appear to have been significantly disturbed during the last glaciation. 234U– 238U disequilibria in the dissolved phase of large rivers world-wide are also explored and two key factors are highlighted in this study: the dissolution of carbonates, releasing dissolved U at secular equilibrium and, the silicate weathering rate estimated at the scale of the watershed. In particular, a positive correlation is found between dissolved ( 234U/ 238U) and basaltic weathering rates for the Deccan that could reflect a key role of physical erosion since, for silicate monolithological basins, chemical erosion rates are primarily controlled by physical erosion rates. For basins with mixed lithology, dissolved organic matter also plays a key role in the preferential release of 234U from silicate minerals. ( 234U/ 238U) ratios in rivers therefore reflect a mixture between U released by carbonate dissolution and 234U preferentially released from silicate minerals located in high physical erosion and peat areas.

Late Quaternary climatic control on erosion and weathering in the eastern Tibetan Plateau and the Mekong Basin

High-resolution siliciclastic grain size and bulk mineralogy combined with clay mineralogy, rubidium, strontium, and neodymium isotopes of Core MD01-2393 collected off the Mekong River estuary in the southwestern South China Sea reveals a monsoon-controlled chemical weathering and physical erosion history during the last 190,000 yr in the eastern Tibetan Plateau and the Mekong Basin. The ranges of isotopic composition are limited throughout sedimentary records: 87Sr/ 86Sr = 0.7206–0.7240 and ε Nd(0) = −11.1 to −12.1. These values match well to those of Mekong River sediments and they are considered to reflect this source region. Smectites/(illite + chlorite) and smectites/kaolinite ratios are used as indices of chemical weathering rates, whereas the bulk kaolinite/quartz ratio is used as an index of physical erosion rates in the eastern Tibetan Plateau and the Mekong Basin. Furthermore, the 2.5–6.5 μm/15–55 μm siliciclastic grain size population ratio represents the intensity of sediment discharge of the Mekong River and, in turn, the East Asian summer monsoon intensity. Strengthened chemical weathering corresponds to increased sediment discharge and weakened physical erosion during interglacial periods. In contrast, weakened chemical weathering associated with reduced sediment discharge and intensified physical erosion during glacial periods. Such strong glacial–interglacial correlations between chemical weathering/erosion and sediment discharge imply the monsoon-controlled weathering and erosion.

Chemical weathering in high‐sediment‐yielding watersheds, New Zealand

We have determined the chemical erosion yields for fifteen watersheds in New Zealand, ranging in size from 12.2 to 2928 km2. These rates, coupled with previously measured physical erosion yields, allow us to compare these two modes of landscape denudation. The physical erosion yields are some of the highest measured in the world. Although in most instances the chemical erosion yields are only a small fraction of the total erosion yields, the absolute values are very high. Our data strongly support the notion that chemical erosion rates are greatly influenced by the yield of physical erosion and that the rapid production of fresh surfaces as a result of high physical erosion rates and subsequent denudation is critical to the high chemical erosion yields observed.

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.

Cosmogenic nuclide evidence for low weathering and denudation in the wet, tropical highlands of Sri Lanka

Some of the lowest weathering and erosion rates in any mountain range in the world have been measured using cosmogenic nuclides in the steep, humid, tropical highlands of Sri Lanka. The total preanthropogenic denudation rates were measured in creek sediments and soil samples from unperturbed rain forest sites, bedrock from mountain crests, and bedrock from inselbergs. Denudation rates are in the range of 5–30 t km−2 yr−1 (2–11 mm ky−1). These rates average denudation over the last 50–250 ky. Weathering exports in rivers draining the mountainous Central Highlands show that silicate weathering rates are also low, varying from 5 to 20 t km−2 yr−1 today (2–7 mm ky−1), but they represent a significant fraction of the total denudation. All these observations run contrary to the conventional geomorphologic and geochemical wisdom that would predict rapid erosion for highlands of high relief, temperatures, and precipitation. We speculate that the high relief in Sri Lanka represents the remnant of a geomorphic block that was uplifted during rifting at 130 Ma or even earlier and that was reduced to the interior of the island by rapid receding of escarpments after continental breakup. It is possible that throughout this history, hillslopes, where not exposing bare bedrock, were protected by thick weathered profiles. Such clay‐rich layers would inhibit silicate weathering by shielding bedrock from weathering agents. In the absence of landscape rejuvenation, physical erosion rates are low, and fresh mineral surfaces are not being supplied. The observation that wet, steep, tropical highlands can have low rates of rock weathering and erosion has some potentially profound implications for the long‐term controls of atmospheric CO2 budgets: High temperature and precipitation, which are much invoked though controversial agents for silicate dissolution and CO2 drawdown, become ineffective in promoting weathering in areas that are not tectonically active.

Sharp decrease in long-term chemical weathering rates along an altitudinal transect

We used cosmogenic nuclide and geochemical mass balance methods to measure long-term rates of chemical weathering and physical erosion across a steep climatic gradient in the Santa Rosa Mountains, Nevada. Our study sites are distributed along a 2 km ridgeline transect that spans 2090 to 2750 m in altitude, and encompasses marked contrasts in both vegetative cover and snow depth, but is underlain by a single, roughly uniform, granodiorite bedrock. Cosmogenic nuclides in colluvial soils reveal that denudation rates vary by less than a factor of 1.4 (104–144 t/km 2/yr) along this transect. Bulk elemental analyses indicate that, relative to the parent rock, soils are less intensively weathered with increasing altitude, and show little evidence of weathering-related mass losses near the top of the ridge. Chemical weathering rates decrease rapidly with increasing altitude, both in absolute terms (from 24 to 0 t/km 2/yr) and as a fraction of total denudation rates (from 20 to 0%). Thus these results indicate an increasing dominance of physical erosion with altitude. The observed decrease in chemical weathering rates is greater than one would predict from the decrease in mean annual temperature using simple weathering kinetics, suggesting that weathering rates along our transect may also be affected by the progressive decline in vegetative cover and increase in snow depth with increasing altitude. These results, considered together with weathering rate measurements for a wide range of climates in the Sierra Nevada, USA, suggest that chemical weathering rates may be particularly sensitive to differences in climate at higher-altitude sites. Consistent with this hypothesis, chemical weathering rates fall virtually to zero at the highest sites on our transect, suggesting that sparsely vegetated, high-altitude crystalline terrain may often be characterized by extremely slow silicate weathering rates.

Empirical and dynamical models of production and biomass of benthic algae in lakes

This work presents new empirical and dynamical models for benthic algae in lakes. The models were developed within the framework of a more comprehensive lake ecosystem model, LakeWeb, which also accounts for phytoplankton, bacterioplankton, two types of zooplankton (herbivorous and predatory), macrophytes, prey fish and predatory fish. The new dynamic model provides seasonal variations (the calculation time is 1 week). It is meant to account for all factors regulating the production and biomass of benthic algae for lakes in general. This work also presents and uses a new data-base established by us from published sources. Many of the lakes included in this study are situated in the former Soviet Union. They were investigated during the Soviet period and the data and results have up until now been largely unknown in the West. We present empirical models for benthic algae, and show that the biomass of benthic algae in whole lakes can be estimated from the ratio between the lake area above the Secchi depth to the total lake area and the primary production of phytoplankton. We also present several critical tests of the dynamical model. The dynamical and empirical models give corresponding results over a wide limnological domain. We provide algorithms for (1) the production rate of benthic algae (2) the elimination rate (related to the turnover time of benthic algae), (3) the rate of benthic algae consumption by zoobenthos, and (4) the rate of physical erosion of benthic algae. Our results indicate that the production of benthic algae is highly dependent on lake morphometry and sediment character, as well as water clarity, and less dependent on nutrient (phosphorus) concentrations in water and sediments. This work provides new quantitative support to such conclusions and also a useful model for predictions of production and biomass of benthic algae.

Lead isotopic systematics of major river sediments: a new estimate of the Pb isotopic composition of the Upper Continental Crust

The lead isotopic composition of river sediments is reported in the present work for the Earth's major river basins, from old cratonic to young orogenic areas and from subarctic to tropical climates. Sediment samples from these large river basins provide a useful tool to calculate the average upper crustal composition because they are large-scale integrated samples of the weathering products of the present-day Upper Continental Crust (UCC). Two different and complementary calculations were done to estimate the average lead isotopic composition of the UCC. The first, based on the flux weighted average of particulate lead delivered by the rivers, gave values of 19.07, 15.74 and 39.35 for 206Pb/ 204Pb, 207Pb/ 204Pb and 208Pb/ 204Pb ratios, respectively. To avoid over-estimating the contribution of orogenic areas, which produces a bias (because the flux of particulate lead depends strongly on the physical erosion rate), a second calculation was done by averaging with drainage areas of each river basin. This gave values of 18.93, 15.71 and 39.03 for 206Pb/ 204Pb, 207Pb/ 204Pb and 208Pb/ 204Pb ratios, respectively. These direct calculations of the lead isotopic composition of the UCC are similar and are in agreement with previous estimates made using an indirect approach.

Long-term rates of chemical weathering and physical erosion from cosmogenic nuclides and geochemical mass balance

Quantifying long-term rates of chemical weathering and physical erosion is important for understanding the long-term evolution of soils, landscapes, and Earth's climate. Here we describe how long-term chemical weathering rates can be measured for actively eroding landscapes using cosmogenic nuclides together with a geochemical mass balance of weathered soil and parent rock. We tested this approach in the Rio Icacos watershed, Puerto Rico, where independent studies have estimated weathering rates over both short and long timescales. Results from the cosmogenic/mass balance method are consistent with three independent sets of weathering rate estimates, thus confirming that this approach yields realistic measurements of long-term weathering rates. This approach can separately quantify weathering rates from saprolite and from overlying soil as components of the total. At Rio Icacos, nearly 50% of Si weathering occurs as rock is converted to saprolite; in contrast, nearly 100% of Al weathering occurs in the soil. Physical erosion rates are measured as part of our mass balance approach, making it particularly useful for studying interrelationships between chemical weathering and physical erosion. Our data show that chemical weathering rates are tightly coupled with physical erosion rates, such that the relationship between climate and chemical weathering rates may be obscured by site-to-site differences in the rate that minerals are supplied to soil by physical erosion of rock. One can normalize for variations in physical erosion rates using the “chemical depletion fraction,” which measures the fraction of total denudation that is accounted for by chemical weathering. This measure of chemical weathering intensity increases with increasing average temperature and precipitation in data from climatically diverse granitic sites, including tropical Rio Icacos and six temperate sites in the Sierra Nevada, California. Hence, across a wide range of climate regimes, analysis of chemical depletion fractions appears to effectively account for site-to-site differences in physical erosion rates, which would otherwise obscure climatic effects on chemical weathering rates. Our results show that by quantifying rates of physical erosion and chemical weathering together, our mass balance approach can be used to determine the relative importance of climatic and nonclimatic factors in regulating long-term chemical weathering rates.