Chemical Erosion Rates Research Articles

Organismal Responses to Coastal Acidification Informed by Interrelating Erosion, Roundness and Growth of Gastropod Shells.

urrent understanding of how calcifying organisms respond to externally forced oceanic and coastal acidification (OCA) is largely based on short-term, controlled laboratory or mesocosm experiments. Studies on organismal responses to acidification (reduced carbonate saturation and pH) in the wild, where animals simultaneously interact with a range of biotic and abiotic circumstances, are limited in scope and interpretation. The present study aimed to better understand how gastropod shell attributes and their interrelations can inform about responses to coastal acidification. We investigated shell chemical erosion, shell roundness, and growth rate of Planaxis sulcatus snails, which are locally exposed to acidified and non-acidified rocky intertidal water. We tested a new approach to quantifying shell erosion based on the spiral suture length (EI, erosion index) and found that shell erosion mirrored field acidification conditions. Exposure to acidification caused shells to become rounder (width/length). Field growth rate, determined from apertural margin extension of marked and later recaptured snails, was strongly negatively related to both shell erosion and shell roundness. Since different shell attributes are indicative of different relationships-shell erosion is an extrinsic passive marker of acidification, and shell roundness and growth rate are intrinsic performance responders-analyzing their interrelations can imply causation, enhance predictive power, and bolster interpretation confidence. This study contributes to the methodology and interpretation of findings of trait-based field investigations to understand organismal responses to coastal acidification.

Mineralogical and Elemental Trends in Regolith on Historically Managed Sites in the southeastern United States Piedmont

AbstractThe deep regolith of the southeastern United States has undergone rapid erosion in the last two centuries due to intensive agricultural practices, which has altered the landscape and its inherent fertility. Parent material, landscape position, and land use are important factors in controlling the mineral and elemental composition of soil profiles. Independent quantitative X-ray diffraction (QXRD) and whole-rock chemical analysis of eight weathering profiles agreed well and allow mineral reaction pathways to be constrained as particles are conveyed in the subsurface. QXRD analysis of saprolite, argillic, and soil A-horizons in the profiles highlights the imprint of bedrock on the regolith, which includes Neoproterozoic meta-tonalitic to meta-granodioritic and Paleozoic meta-granitic to biotite- and amphibolite-gneissic lithologies. Also, aeolian input slightly influenced A-horizon composition. The clay mineral assemblage is dominated by kaolinite, but profiles differ in the amount of interstratified clay minerals, halloysite, hematite, goethite, and gibbsite. Rare-earth element totals vary between 30 and 1048 ppm and are generally correlated positively with clay and clay mineral content. Eu and Ce anomalies reflect parent rocks and subsequent hydrolysis and redox history, with trends depending upon landscape position and clay content in the weathering profile. Weathering profiles on a high-order interfluve and those that were actively cultivated have thick argillic horizons (as defined by clay mineral abundance) and are depleted in alkali and alkaline-earth elements. Profiles proximally developed on old-field pine and never-cultivated hardwood forest land do not show large differences in mineral composition trends, whereas profiles on old-field sites with ongoing cultivation exhibit assemblages enriched in clay minerals and (oxyhydr)oxides. Old-field pine sites that were historically eroded by previous cultivation tend to have shallower and thinner argillic horizons, which may well impact critical-zone processes involving gas and water fluxes. This study highlights that mineral compositions of deep regolith, saprolite, and shallow soil horizons are dependent on local geomorphology (i.e. watershed- and hillshed-orders). Quantifying soil and regolith compositional trends across the landscape is a prerequisite for determining rates of chemical and physical erosion on human and geologic time scales.

Source Region Geochemistry From Unmixing Downstream Sedimentary Elemental Compositions

AbstractThe geochemistry of river sediments is routinely used to obtain information about geologic and environmental processes occurring upstream. For example, downstream samples are used to constrain chemical weathering and physical erosion rates upstream, as well as the locations of mineral deposits or contaminant sources. Previous work has shown that, by assuming conservative mixing, the geochemistry of downstream samples can be reliably predicted given a known source region geochemistry. In this study, we tackle the inverse problem and “unmix” the composition of downstream river sediments to produce geochemical maps of drainage basins (i.e., source regions). The scheme is tested in a case study of rivers draining the Cairngorms, UK. The elemental geochemistry of the μm fraction of 67 samples gathered from the beds of channels in this region is used to invert for concentrations of major and trace elements upstream. A smoothed inverse problem is solved using the Nelder‐Mead optimization algorithm. Predictions of source region geochemistry are assessed by comparing the spatial distribution of 22 elements of different affinities (e.g., Be, Li, Mg, Ca, Rb, U, V) using independent geochemical survey data. The inverse approach makes reliable predictions of the major and trace element concentration in first order river sediments. We suggest this scheme could be a novel means to generate geochemical baselines across drainage basins and within river channels.

Denudation rates of granitic regolith along climatic gradient in Eastern China

In order to assess the impact of the monsoonal climate on denudation, physical erosion, and chemical weathering processes, we measured cosmogenic nuclides 10Be, 26Al and immobile element Zr in five granitic regolith profiles across Northeast to South China. The 10Be and 26Al concentrations are 2.98 × 104–25.20 × 104 atoms g−1 and 2.18 × 105–16.62 × 105 atoms g−1, respectively. Compiling previous depth profile data from Leymon and Beacon Heights, we find that the 26Al/10Be ratios systematically increase or decrease with the depth. Our numerical model shows that muons contributions can produce higher 26Al/10Be ratios (greater than 6.75) in both steady-state and non-steady state scenarios, which can result in underestimated burial age (initial 26Al/10Be ratio) and exaggerated denudation rates (due to non-negligible production from muons) for the application of dual-nuclide in Earth Science. The denudation rates, chemical weathering rates and physical erosion rates derived from cosmogenic nuclides and Zr are 28–45, 9–31 and 6–22 m Ma−1, respectively. Chemical weathering rates, excluding the JLN profile, comprise a large proportion of the denudation rates (greater than 50%). Despite the wide range of mean annual precipitation (MAP) and mean annual temperature (MAT), the denudation rates vary without an apparent gradient from northeast to south. However, the physical erosion and chemical weathering results show good relationships with climate, suggesting the limitation type changed from kinetic-limited in the north, to supply-limited in the south. Chemical weathering indices (chemical index of alteration, CIA and weathering intensity factor, WIF) also indicate that the extent of chemical weathering strengthens from northeast to south and increases with chemical weathering rates, but decreases with physical erosion rates.

The Importance of Hillslope Scale in Responses of Chemical Erosion Rate to Changes in Tectonics and Climate

AbstractChemical erosion is of wide interest due to its influence on topography, nutrient supply to streams and soils, sediment composition, and Earth's climate. While controls on chemical erosion rate have been studied extensively in steady‐state models, few studies have explored the controls on chemical erosion rate during transient responses to external perturbations. Here we develop a numerical model for the coevolution of soil‐mantled topography, soil thickness, and soil mineralogy, and we use it to simulate responses to step changes in rates of rock uplift, soil production, soil transport, and mineral dissolution. These simulations suggest that tectonic and climatic perturbations can generate responses in soil chemical erosion rate that differ in speed, magnitude, and spatial pattern and that climatic and tectonic perturbations may impart distinct signatures on hillslope mass fluxes, soil chemistry, and sediment composition. The response time of chemical erosion rate is dominantly controlled by hillslope length and is secondarily modulated by rates of rock uplift, soil production, transport, and mineral dissolution. This strong dependence on drainage density implies that a landscape's chemical erosion response should depend on the relative efficiencies of river incision and soil transport and thus may be mediated by climatic and biological factors. The simulations further suggest that the timescale of the hillslope response may be long relative to that of river channel profiles, implying that chemical erosion response times may be limited more by the sluggishness of the hillslopes than by the rate of signal propagation through river channel profiles.

Differentiated Neogene bauxitization of volcanic rocks (western Cameroon): Morpho-geological constraints on chemical erosion

Lateritic weathering of Miocene volcanic rocks from western Cameroon highlands formed duricrusted bauxitic profiles. Two weathering profiles on ca. 14 Ma basalt and ca. 16 Ma trachyte were studied using geochemical mass balance functions. Less mobile elements Ti and Zr were used as references to quantify volumetric change (strain, ε), element transfer rate (τ) and geochemical mass transfers during the bauxitization process of basalt and trachyte. Conversion of parent rocks to kaolinite and goethite rich saprolites evolved to Al-Fe rich bauxites, mostly composed of gibbsite and iron oxy-hydroxides (goethite and hematite). However, formation of Al-Fe bauxitic profiles required higher Si leaching on trachyte than on basalt. Our results document that chemical weathering of a larger thickness of trachyte than basalt has been required to form a unit meter of weathering profile, implying differential rates of rock chemical erosion and topographic decay of landscapes. Rates of chemical erosion and formation of lateritic weathering profiles in western Cameroon have been mostly controlled by drainage conditions and volcanic rocks composition (mostly SiO2 content differences), that also resulted in contrasted landscapes evolution during the Neogene.

Chemical Weathering in Small Mountainous Rivers of Southern Italy and Northern Spain

Since the seminal work of Milliman and Syvitski (J Geol 100:525–544, 1992), there has been interest in evaluating the significance of small mountainous river (SMRs) systems and their role in the transport of both solutes, and especially sediments, to the world ocean. Although some data exist from portions of the Earth’s mountainous regions, the majority of this work has been focused in the western Pacific Ocean and Caribbean regions. During those previous studies, workers have sought to evaluate the interconnection between physical erosion and chemical erosion rates. We report herein on the riverine geochemistry of five rivers draining northern Spain and six rivers draining southern Italy. The geochemistry of these rivers is dominated by calcium carbonate weathering and input from either evaporite dissolution or marine aerosols, or both. Silicate mineral weathering is also occurring but it is not the dominant process. Using previously tabulated annual total suspended load data, we have calculated both physical and chemical erosion yields from seven of the eleven rivers under investigation where complete data sets are available. The physical yields are much higher in the Italian rivers, while chemical yields of all rivers are in a similar range. Our work adds new information on SMRs from geographical regions that have not previously been evaluated within this global context.

Temporal variation of chemical weathering rate, source shifting and relationship with physical erosion in small mountainous rivers, Taiwan

Temporal variation of the chemical weathering rate (CWR) versus physical erosion rate (PER) and the shift of the sources of dissolved solids during rainstorms are explored in two small mountainous rivers, Northern Taiwan. The ionic hydrochemistry of streamwater, rainwater, soil water, and groundwater were collected to quantitatively demonstrate the sources of the dissolved solids and the coupling effect between CWR and PER during typhoon and non-typhoon periods. Results indicate that silicate cation weathering (CWRSC) varies between 33 and 37 t km−2 yr−1, which is ~7 times the global mean, and the annual PER is ~81–166 t km−2 yr−1, similar to the global mean of ~137 t km−2 yr−1. The rainstorm periods only account for 1% of a year but are responsible for 10% of the CWRSC and over 80% of the sediment flux, highlighting the importance of storm events. Silicate cation concentrations present a simple dilution process during rainstorms indicates the high connectivity between hillslopes and streams during rainstorms is established and forces the soil water into streams rapidly. Besides, the coupling between CWRSC and PER becomes less relevant during rainstorms. The coupling between CWRSC and PER is non-linear with flow regime dynamics, and the estimates of CWRSC derived from non-typhoon day are as largely biased as one order of magnitude. High-resolution water chemistry data on the variations in ion production and physical erosion is more valuable than previously thought for characterizing the CWR and PER of active mountain belts.

Hydrochemistry applied to assess the chemical weathering and soil removal rates in the Sorocaba River basin, São Paulo State

ABSTRACT: Chemical weathering and soil removal rates are responsible for the Earth’s landscape, composition of surface and groundwater, producing the soils and buffering the composition of the atmosphere. This study aimed to assess the chemical weathering and soil removal rates in the Sorocaba River basin, Sao Paulo State, Brazil, allowing answering the questions about the dynamics of fluvial transport of dissolved and suspended solids, the chemical weathering processes and associated atmospheric/soil CO2 consumption, and the relationship between chemical weathering and soil erosion rates. The annual specific flux of total suspended solids and total dissolved solids were 49.59 and 60.97 t/km2/yr. The chemical weathering process dominant in the Sorocaba River basin was the monosiallitization (RE = 2.4), with an associated atmospheric/soil CO2 consumption of 2.3 × 105 mol/km2/yr. The chemical weathering and soil removal rates were 7.2 and 29.8 m/Myr, respectively, indicating a soil thickness reduction. Finally, the soil removal rate in the Sorocaba River basin is almost 3-fold higher than the Cenozoic soil removal rates, being this difference related to the current land use which increased the soil removal processes.

Effects of Combined Salt-Damage Resistant Agent on the Shrinkage, Chloride Penetration and Chemical Erosion of Mortar

Reinforced concrete structures located on coastal landfill frequently adjoin sea-water environment, and are exposed to sea water and humid environment during construction. Particularly, in the case of large-scale structures like dams, their drying shrinkage is accompanied by fatal cracking, and thus chlorine ion penetration becomes easier. The present study develops a salt damage-resistant agent (SRA) to which aluminum salts, oligomer condensate, and amino alcohol derivatives with the alkyl group are applied as binding inducers. SRA performs the roles of reducing the drying shrinkage of cement composites, binding chlorine ions, and preventing erosion by sulfate ions. This study tests and evaluates its resistance to degradation factors that may occur to structures constructed on coastal landfill and so on. As a result of evaluating shrinkage cracking properties by performing the restrained shrinkage cracking test, SRC showed the shrinkage reduction compared with BSC. As for the performance of resistance to chlorine ion and the chemical sulfate erosion rate, SRC showed the highest resistance performance, followed by BSC and OPC, regardless of the concentration of aqueous solutions for immersion. In addition, as for the rate of mortar weight change by sulfate erosion, the SRA-intermixed SRC mixture showed a weight reduction rate at the level of 1/3 of BSC and 1/6 of OPC, respectively.

Chemical erosion rates in the upper Blue Nile Basin and related atmospheric CO2 consumption

Silicate weathering of basaltic rocks constitutes a non-negligible sink of atmospheric CO2 but the role it plays in the regulation of past and future global climate is still matter of debate. In this study, silicate weathering rates for various sub-basins of the Ethiopian Traps, emplaced 30 million years ago, and the corresponding atmospheric CO2 consumption rates are evaluated. For this, major and trace elements were measured in the dissolved phases and in the sedimentary particles carried and deposited by the main rivers flowing through this steep region. Lithium isotopes and major elements were also measured in the extracted clay fractions in order to infer complementary information on weathering processes in this region. Clay δ7Li values correlate positively with Mg/Ti ratios, and are best explained by varying ratios of leaching versus clay formation rate.Although located in a region annually submitted to monsoon, average silicate weathering rate (16.1 tons/km2/year) and CO2 consumption rate (0.65 × 1012 mol/year) are estimated to be low when compared to other basaltic regions such as the Deccan Traps, and volcanically active islands of the tropical zone. This is surprising since the concentrations of Total Dissolved Solids of the Ethiopian rivers are among the highest ones. With a 2D rainfall model that takes into account the detailed topography of the region, annual occurrence of the Monsoon, and monitoring station data, we show that runoff intensity is a key parameter that explains this difference. We determine that, at present, the weathering of the Ethiopian Traps plays a negligible role in the carbon cycle. However, simple calculations, which integrate recent knowledge on African climate variations and on weathering controls, illustrate that during the African Humid Period (14–8 kyr), a significant increase of Monsoon precipitation may have resulted in much higher weathering rates and related CO2 consumption (0.91–1.5 × 1012 mol/year). This study therefore evidences the potential importance of this region in the past, and the need to quantify more precisely the variations of the monsoon intensity and its impact on tropical watersheds for reconstructing past CO2 levels.

Alternating coarse- and fine-grained sedimentation in Precambrian deep-water ramp (Apiúna Formation, SE of Brazil): Tectonic and climate control or sea level variations?

Are the depositional processes and architecture of Precambrian deep-water deposits the same of the Phanerozoic time? Few examples of Precambrian deep-water deposits exist in the literature. This paper describes the Apiúna Formation, a Neoproterozoic deep-water succession, c. 4500 m thick, located in SE Brazil. Detailed facies analysis, six large logs, 2000–3150 m thick, and a geological map have been applied to define types and geometrical distribution of three fine-grained and four coarse-grained architectural elements. Fine-grained elements (mudstone and fine-grained sandstone) constitute more than 70% of the thickness of the Apiúna Formation; these enclose coarse-grained elements, constituted of lenticular sandstone or pebbly sandstone bodies. Isolated coarse-grained elements display parallel palaeoflow directions suggesting multi-point source of the sediment. Due to this last aspect and the mud-dominance, the Apiúna Formation is interpreted as fine-grained ramp depositional system characterised by episodes of coarse-grained sedimentation. Nevertheless, this unit diverges from the Phanerozoic models for having (1) a wide range of grain-size, (2) small channel-levee systems, (3) immature clastic material and (4) irregular distribution of the architectural elements. In Apiúna sedimentary succession, five phases of coarse-grained sedimentation alternate to six phases of fine-grained sedimentation. In unvegetated earth, different rates of chemical weathering and mechanical erosion of the soils, controlled by climate and tectonic activity, can explain these variations of the clastic input. In rainy and warm climate conditions, Precambrian soils on silicate rocks formed thick clayey profiles. During quiescent or low tectonic activity, a dynamic equilibrium between weathering and erosion by runoffs on unprotected by rooted-vegetation soils produced high quantity of fine-grained material. During high tectonic activity and drier climate conditions, clayey soils did not form or were very reduced and mechanical erosion of the silicate rocks produced coarse-grained sediment. Thereby, high input of fine-grained deposits can be considered a peculiar characteristic of Precambrian deep-water systems.

Contributions of solar-wind induced potential sputtering to the lunar surface erosion rate and it’s exosphere

Abstract Sputtering of lunar regolith by solar-wind protons and heavy ions with kinetic energies of about 1 keV/amu is an important erosive process that affects the lunar surface and exosphere. It plays an important role in changing the chemical composition and thickness of the surface layer, and in introducing material into the exosphere. Kinetic sputtering is well modeled and understood, but understanding of mechanisms of potential sputtering has lagged behind. In this study we differentiate the contributions of potential sputtering from the standard (kinetic) sputtering in changing the chemical composition and erosion rate of the lunar surface. Also we study the contribution of potential sputtering in developing the lunar exosphere. Our results show that potential sputtering enhances the total characteristic sputtering erosion rate by about 44%, and reduces sputtering time scales by the same amount. Potential sputtering also introduces more material into the lunar exosphere.

Development of Artificial Reefs Using Environmentally Safe Ceramic Material

To develop artificial reefs, that are environmentally friendly and inexpensive, prototype artificial reefs made of ceramic material have been studied. The properties of the reefs (chemical constituents, surface texture, water absorbability, mechanical strength, erosion rate, and sustainability) were examined. The reefs were compared with natural coral rubbles. The effects of the reefs on nutrient availability, toxic chemical diffusion, and the propagation of plankton (bacteria, diatom) and other benthic organisms were examined, for which the reefs were set in aquaria for a long time. It was shown that the reefs were nontoxic, pH-neutral, mechanically strong, and sustainable in a hostile shallow sea environment. This study suggests that artificial reefs made from the ceramic material are not hazardous to small fish.

Responses of chemical erosion rates to transient perturbations in physical erosion rates, and implications for relationships between chemical and physical erosion rates in regolith-mantled hillslopes

Understanding the relationship between chemical erosion rates (W) and physical erosion rates (E) is of wide interest due to their roles in driving landscape evolution, supplying nutrients to soils and streams, and modulating the global carbon cycle. Measured relationships between W and E vary around the globe, with some regions exhibiting positive correlations between W and E, some negative correlations, and others no correlation within uncertainty. Here we use a numerical model for mineral weathering in well-mixed ridgetop regolith to explore how complex W–E relationships can be generated by simple transient perturbations in E. We show that a Gaussian perturbation in E can produce positive or negative responses in W, and can result in a variety of hysteresis loops – clockwise, counterclockwise, or figure-eight – in plots of W against E. The nature of the transient response depends on the shape of the steady-state W–E relationship, which is set by regolith mineralogy, and the ratio of E to the maximum possible regolith production rate. The response time of W is controlled by the response time of soluble mineral concentrations at low E, where soluble mineral concentrations are low, and by the response time of regolith thickness at high E, where regolith thickness is low. These complex W–E relationships arise in the absence of variations in climate and lithology, which suggests that transients may account for some of the observed differences in W–E relationships among field sites, even among sites that share the same climate and lithology.

A new angle on streams.

Stream Geometry Climate has clear effects on streams. Theory and observations both show that channel incision rates increase with precipitation, and chemical erosion rates scale with temperature, but how does the surface geometry of rivers depend on those factors? Seybold et al. analyzed nearly 1 million river junctions across the contiguous United States and found that branching angles vary systematically with climatic aridity. Branching angles are smaller in arid regions than in humid ones, where groundwater seepage is more important, and are stronger functions of aridity than of other commonly invoked controls such as topographic gradient or downstream concavity. These findings may aid in characterizing channelization processes in landscapes where relict streams are found, such as on Mars. Geophys. Res. Lett. 10.1002/2016GL072089 (2017).

Soil formation and weathering in a permafrost environment of the Swiss Alps: a multi‐parameter and non‐steady‐state approach

AbstractSpatially discontinuous permafrost conditions frequently occur in the European Alps. How soils under such conditions have evolved and how they may react to climate warming is largely unknown. This study focuses on the comparison of nearby soils that are characterised by the presence or absence of permafrost (active‐layer thickness: 2–3 m) in the alpine (tundra) and subalpine (forest) range of the Eastern Swiss Alps using a multi‐method (geochemical and mineralogical) approach. Moreover, a new non‐steady‐state concept was applied to determine rates of chemical weathering, soil erosion, soil formation, soil denudation, and soil production. Long‐term chemical weathering rates, soil formation and erosion rates were assessed by using immobile elements, fine‐earth stocks and meteoric 10Be. In addition, the weathering index (K + Ca)/Ti, the amount of Fe‐ and Al‐oxyhydroxides and clay minerals characteristics were considered. All methods indicated that the differences between permafrost‐affected and non‐permafrost‐affected soils were small. Furthermore, the soils did not uniformly differ in their weathering behaviour. A tendency towards less intense weathering in soils that were affected by permafrost was noted: at most sites, weathering rates, the proportion of oxyhydroxides and the weathering stage of clay minerals were lower in permafrost soils. In part, erosion rates were higher at the permafrost sites and accounted for 79–97% of the denudation rates. In general, soil formation rates (8.8–86.7 t/km2/yr) were in the expected range for Alpine soils. Independent of permafrost conditions, it seems that the local microenvironment (particularly vegetation and subsequently soil organic matter) has strongly influenced denudation rates. As the climate has varied since the beginning of soil evolution, the conditions for soil formation and weathering were not stable over time. Soil evolution in high Alpine settings is complex owing to, among others, spatio‐temporal variations of permafrost conditions and thus climate. This makes predictions of future behaviour very difficult. Copyright © 2016 John Wiley & Sons, Ltd.

Catchment-scale denudation and chemical erosion rates determined from 10Be and mass balance geochemistry (Mt. Lofty Ranges of South Australia)

Global biogeochemical cycles have, as a central component, estimates of physical and chemical erosion rates. These erosion rates are becoming better quantified by the development of a global database of cosmogenic radionuclide 10Be (CRN) analyses of soil, sediment, and outcrops. Here we report the denudation rates for two small catchments (~0.9km2) in the Mt. Lofty Ranges of South Australia as determined from 10Be concentrations from quartz sand from the following landscape elements: 1) dissected plateaux, or summit surfaces (14.10±1.61tkm−2y−1), 2) sandstone outcrops (15.37±1.32tkm−2y−1), 3) zero-order drainages (27.70±1.42tkm−2y−1), and 4) stream sediment which reflect a mix of landscape elements (19.80±1.01tkm−2y−1). Thus, the more slowly eroding plateaux and ridges, when juxtaposed with the more rapidly eroding side-slopes, are leading to increased relief in this landscape.Chemical erosion rates for this landscape are determined by combining cosmogenic denudation rates with the geochemical mass balance of parent rock, soil and saprolite utilizing zirconium immobility and existing mass balance methods. Two different methods were used to correct for chemical weathering and erosion in the saprolite zone that is shielded at depth from CRN production. The corrected values are higher than uncorrected values: total denudation of 33.24 or 29.11tkm−2y−1, and total chemical erosion of 15.64 or 13.68tkm−2y−1. Thus, according to these methods, 32–40% of the denudation is taking place by chemical weathering and erosion in the saprolite below CRN production depth. Compared with other similar areas, the overall denudation and chemical erosion rates are low. In most areas with sub-humid climates and tectonic uplift, physical erosion is much greater than chemical erosion. The low physical erosion rates in these Mt. Lofty Range catchments, in what is a relatively active tectonic setting, are thought to be due to low rainfall intensity during the winter wet season, which inhibits physical erosion such as land-sliding and debris flows.

Integrating Extensive Livestock and Soil Conservation Policies in Mediterranean Mountain Areas for Recovery of Abandoned Lands in the Central Spanish Pyrenees. A Long‐Term Research Assessment

AbstractLand abandonment is a global issue with important implications in Mediterranean mountain areas. Abandoned Mediterranean croplands start a process of secondary succession that is initially colonized by grasslands, shrubs and forest. In Mediterranean mountain areas, the process is very slow, so the shrubs remain for decades, preventing livestock from accessing pastureland. Therefore, farmers have to burn or clear the shrubs in order to provide pasture, a practice that has recently been encouraged by several regional governments in Spain. Data from experimental plots of the Aísa Valley Experimental Station in the Spanish Pyrenees allow to evaluate the effects of burning and clearing shrubs on physical and chemical soil properties, runoff production and soil erosion rates. The results suggested that clearing shrubs returned more positive results than burning, as it improved soil quality and slowed soil erosion while producing slightly higher runoff coefficients, which is very important in Mediterranean environments where water is a scarce resource. Clearing shrubs improves soil characteristics by increasing organic matter and CN ratio, which promote the expansion of herbaceous species with a high pastoral value. The results suggested that the policy of clearing shrubs is suitable for managing abandoned lands in Mediterranean mountain areas. Further studies at catchment scale will be needed to confirm the impact of substituting shrubs for meadows and to understand the connectivity of the flows measured from pedon and slope to the watershed scale. Copyright © 2016 John Wiley & Sons, Ltd.

Testing for supply‐limited and kinetic‐limited chemical erosion in field measurements of regolith production and chemical depletion

AbstractChemical erosion contributes solutes to oceans, influencing atmospheric CO2 and thus global climate via the greenhouse effect. Quantifying how chemical erosion rates vary with climate and tectonics is therefore vital to understanding feedbacks that have maintained Earth's environment within a habitable range over geologic time. If chemical erosion rates are strongly influenced by the availability of fresh minerals for dissolution, then there should be strong connections between climate, which is modulated by chemical erosion, and tectonic uplift, which supplies fresh minerals to Earth's surface. This condition, referred to as supply‐limited chemical erosion, implies strong tectonic control of chemical erosion rates. It differs from kinetic‐limited chemical erosion, in which dissolution kinetics and thus climatic factors are the dominant regulators of chemical erosion rates. Here we present a statistical method for determining whether chemical erosion of silicate‐rich bedrock is supply limited or kinetic limited, as an approach for revealing the relative importance of tectonics and climate in Earth's silicate weathering thermostat. We applied this method to published data sets of mineral supply rates and regolith chemical depletion and were unable to reject the null hypothesis that chemical erosion is supply limited in 8 of 16 cases. In seven of the remaining eight cases, we found behavior that is closer to supply limited than kinetic limited, suggesting that tectonics may often dominate over climate in regulating chemical erosion rates. However, statistical power analysis shows that new measurements across a wider range of supply rates are needed to help quantify feedbacks between climate and tectonics in Earth's long‐term climatic evolution.