Disparate Sandstone Weathering Beneath Lichens, Red Mountain, Arizona

Though the ability of lichens to disaggregate and dissolve the substrate is understood, zones of disparate weathering mechanisms examined in this study were previously unobserved. On a steep (60°) northern (340°N) slope on Red Mountain, Arizona, a 5×5 m study plot was chosen for its maximum lichen coverage (33%) of the same lichen genus Xanthoparmelia in varying growth stages, on a sandstone substrate of consistent lithology. From twenty-two (22) representative lichen thalli, portions were removed to examine sub-thallic rhizine density, and others were resin-inbedded for optical, scanning and backscatter electron microscopy.
In every sample, sandstone beneath and adjacent to the lichen thallus displayed disparate weathering mechanisms. Beneath the center of the lichen cortex, where rhizine density was observed to be the greatest, rhizine penetration disaggregated the sand-stone clasts from the matrix, but little chemical dissolution was apparent. This study found physical weathering predominates beneath the lichen cortex, and chemical weathering predominates at the thallus fringe and beyond the thallus boundary. It was generally found that physical weathering decreased (disaggregation) and chemical weathering increased (dissolution) from the cortex center to the thallus edge. Toward the thallus fringe, minimal clast disaggregation was observed, and substrate dissolution was obvious.     

SPATIAL PATTERNS AND CAUSES OF MARBLE TOMBSTONE WEATHERING IN WESTERN PENNSYLVANIA

A 9600 km² area of southwestern Pennsylvania with widely varying amounts of historic industrial air pollution contains enough old cemeteries (135) to permit high-resolution mapping of Vermont marble tombstone weathering rates. Mean vertical-slab surface recession rates are as much as eight times greater for urban/industrial areas (maximum 3.0 mm [100 yr]^-1) than for rural areas (minimum 0.4 mm [100 yr]^-1). A damage function relates mean weathering rates to long-term numbers of employees in nearby polluting industries, and shows that even small industrial operations significantly augmented local rates of weathering within several kilometers. Marble stones near short-smokestack industries are significantly more weathered than are those near tall-smokestack, coal-fired electrical power plants. The comparatively high mean rate of marble weathering in small town cemeteries (1.1 mm [100 yr]^-1) suggests that recent acidic precipitation may be dissolving the stones throughout the study region or that pollution from home coal and oil furnaces was high. [Key words: marble tombstone, weathering, industrial air pollution, Pennsylvania.]     

Assessment of tafoni distribution and environmental factors on a sandstone djinn block above Petra,Jordan

Through the measurement of tafoni dimensions on an isolated, and unrecorded ‘djinn block’ in Petra's Ordovician Disi Sandstone, minimum surface recession were prevalent on eastern and western faces ranging from 10 to 127 mm/millennia. Moderate tafoni development was identified on northern aspects ranging from 105 to 110 mm/millennia. While on southern vertical faces, the greatest development ranged from 120 to 220 mm/millennia. Solar flux was correlated to the measured tafoni cell dimensions and revealed a minimal recession rate (and depth) of approximately 10–45 mm/millennia above which northern influences increased deterioration ‘moderately’ up to 110 mm/millennia, and southern influences had the greatest exacerbating effect with a maximum recessional rate of 220 mm/millennia – producing the greatest recession of nearly one inch (22 mm) each century. It is speculated that weathering accelerated on shaded (northern) faces through more frequent and longer spans of wetting and drying cycles, in addition to the effects of lichen and cyanobacteria overgrowth. While on regularly sunlit surfaces (southern), increased solar flux (up to ∼3000 MJ/m²/year) accelerated deterioration though more rapid, and extreme heating and cooling cycles. As rarely documented, tafoni on eastern faces were the smallest, and they were only slightly larger on western faces. Slightly increased heating from afternoon temperatures, in tandem with the incidence of frontal precipitation is attributed to the only slightly larger tafoni found on western faces (than on eastern aspects).     

Cosmogenic Be and Al exposure ages of tors and erratics, Cairngorm Mountains, Scotland: Timescales for the development of a classic landscape of selective linear glacial erosion

The occurrence of tors within glaciated regions has been widely cited as evidence for the preservation of relic pre-Quaternary landscapes beneath protective covers of non-erosive dry-based ice. Here, we test for the preservation of pre-Quaternary landscapes with cosmogenic surface exposure dating of tors. Numerous granite tors are present on summit plateaus in the Cairngorm Mountains of Scotland where they were covered by local ice caps many times during the Pleistocene. Cosmogenic ¹⁰Be and ²⁶Al data together with geomorphic relationships reveal that these landforms are more dynamic and younger than previously suspected. Many Cairngorm tors have been bulldozed and toppled along horizontal joints by ice motion, leaving event surfaces on tor remnants and erratics that can be dated with cosmogenic nuclides. As the surfaces have been subject to episodic burial by ice, an exposure model based upon ice and marine sediment core proxies for local glacial cover is necessary to interpret the cosmogenic nuclide data. Exposure ages and weathering characteristics of tors are closely correlated. Glacially modified tors and boulder erratics with slightly weathered surfaces have ¹⁰Be exposure ages of about 15 to 43 ka. Nuclide inheritance is present in many of these surfaces. Correction for inheritance indicates that the eastern Cairngorms were deglaciated at 15.6 ± 0.9 ka. Glacially modified tors with moderate to advanced weathering features have ¹⁰Be exposure ages of 19 to 92 ka. These surfaces were only slightly modified during the last glacial cycle and gained much of their exposure during the interstadial of marine Oxygen Isotope Stage 5 or earlier. Tors lacking evidence of glacial modification and exhibiting advanced weathering have ¹⁰Be exposure ages between 52 and 297 ka. Nuclide concentrations in these surfaces are probably controlled by bedrock erosion rates instead of discrete glacial events. Maximum erosion rates estimated from ¹⁰Be range from 2.8 to 12.0 mm/ka, with an error weighted mean of 4.1 ± 0.2 mm/ka. Three of these surfaces yield model exposure-plus-burial ages of 295_− 71^+ 84, 520_− 141^+ 178, and 626_− 85^+ 102 ka. A vertical cosmogenic nuclide profile across the oldest sampled tor indicates a long-term emergence rate of 31 ± 2 mm/ka. These findings show that dry-based ice caps are capable of substantially eroding tors by entraining blocks previously detached by weathering processes. Bedrock surfaces and erratic boulders in such settings are likely to have nuclide inheritance and may yield erroneous (too old) exposure ages. While many Cairngorm tors have survived multiple glacial cycles, rates of regolith stripping and bedrock erosion are too high to permit the widespread preservation of pre-Quaternary rock surfaces.     

Sensitivity analysis of pediment development through numerical simulation and selected geospatial query

Dozens of references recognizing pediment landforms in widely varying lithologic, climatic, and tectonic settings suggest a ubiquity in pediment forming processes on mountain piedmonts worldwide. Previous modeling work illustrates the development of a unique range in arid/semiarid piedmont slope (< 0.2 or 11.3°) and regolith thickness (2–4 m) that defines pediments, despite varying the initial conditions and domain characteristics (initial regolith thickness, slope, distance from basin to crest, topographic perturbations, and boundary conditions) and process rates (fluvial sediment transport efficiency and weathering rates). This paper expands upon the sensitivity analysis through numerical simulation of pediment development in the presence of spatially varying rock type, various base level histories, various styles of sediment transport, and various rainfall rates to determine how pediment development might be restricted in certain environments. This work suggests that in landscapes characterized by soil and vegetation types that favor incisive fluvial sediment transport styles coupled with incisive base level conditions, pediment development will be disrupted by the roughening of sediment mantled surfaces, thereby creating spatial variability in topography, regolith thickness, and bedrock weathering rates. Base level incision rates that exceed the integrated sediment flux along a hillslope derived from upslope weathering and sediment transport on the order of 10^− 3 m y^− 1 restrict pediment development by fostering piedmont incision and/or wholesale removal (stripping) of regolith mantles prior to footslope pediment development. Simulations illustrate an insensitivity to alternating layers of sandstone and shale 3–15 m thick oriented in various geometric configurations (vertical, horizontal, and dip-slope) and generating different regolith hydrologic properties and exhibiting weathering rate variations up to 3-fold. Higher fluxes and residence times of subsurface groundwater in more humid environments, as well as dissolution-type weathering, lead to a thickening of regolith mantles on erosional piedmonts on the order of 10¹ m and an elimination of pediment morphology. An initial test of the model sensitivity analysis in arid/semiarid environments, for which field reconnaissance and detailed geomorphic mapping indicate the presence of pediments controlled by climatic conditions (soil hydrologic properties, vegetation characteristics, and bedrock weathering style) that are known and constant, supports our modeling results that pediments are more prevalent in hydrologically-open basins.     

SURFACE EXPOSURE DATING: REVIEW AND CRITICAL EVALUATION

The past decade has seen the development and application of over a dozen new methods for quantitative age-determinations of geomorphic surfaces. Some surface exposure dating methods are numerical, including the accumulation of cosmogenic radionuclides ¹⁰Be, ¹⁴C, ²⁶Al, ³⁶Cl, and ⁴¹Ca, accumulation of cosmogenic stable nuclides ³He and ²¹Ne, ¹⁴C dating of organic matter encapsulated in rock coatings, and dendrogeomorphology. Calendar ages are obtained by dendrogeomorphological analysis. Calibrated ages can be obtained by analysis of rock-varnish chemistry, lichenometry, weathering, and soils. Various methods can be used in combination to overcome individual limitations. Whereas conventional methods provide age control on stratigraphic profiles, surface-exposure dating methods are especially suitable for geographic problems, such as analyzing not only temporal, but also spatial variations in the rates of geomorphic processes. [Key words: geomorphology, process, geochronology, cosmogenic nuclides, Quaternary, surface exposure dating, rock varnish, weathering, soils.]     

Geomorphology's role in the study of weathering of cultural stone

Great monumental places—Petra, Giza, Angkor, Stonehenge, Tikal, Macchu Picchu, Rapa Nui, to name a few—are links to our cultural past. They evoke a sense of wonderment for their aesthetic fascination if not for their seeming permanence over both cultural and physical landscapes. However, as with natural landforms, human constructs are subject to weathering and erosion. Indeed, many of our cultural resources suffer from serious deterioration, some natural, some enhanced by human impact. Groups from the United Nations to local civic and tourism assemblies are deeply interested in maintaining and preserving such cultural resources, from simple rock art to great temples. Geomorphologists trained in interacting systems, process and response to thresholds, rates of change over time, and spatial variation of weathering processes and effects are able to offer insight into how deterioration occurs and what can be done to ameliorate the impact.Review of recent literature and case studies presented here demonstrate methodological and theoretical advances that have resulted from the study of cultural stone weathering. Because the stone was carved at a known date to a “baseline” or zero-datum level, some of the simplest methods (e.g., assessing surface weathering features or measuring surface recession in the field) provide useful data on weathering rates and processes. Such data are difficult or impossible to obtain in “natural” settings. Cultural stone weathering studies demonstrate the importance of biotic and saline weathering agents and the significance of weathering factors such as exposure (microclimate) and human impact. More sophisticated methods confirm these observations, but also reveal discrepancies between field and laboratory studies. This brings up two important caveats for conservators and geomorphologists. For the conservator, are laboratory and natural setting studies really analogous and useful for assessing stone damage? For the geomorphologist, does cultural stone data have any real relevance to the natural environment? These are questions for future research and debate. In any event, cultural stone weathering studies have been productive for both geomorphologists and conservators. Continued collaboration and communication between the geomorphic, historic preservation, archaeological, and engineering research communities are encouraged.     

Rock control on microweathering of bedrock surfaces in a periglacial environment

Microweathering of ice-smoothed bedrock surfaces was investigated in the Røldal area of Hardanger Plateau (60°), southern Norway. Postglacial rates of weathering were determined from surface lowering using quartz veins as reference surfaces. Weathering processes are inferred from assessment of weathering rind formation, surface hardness, and the preservation of small-scale glacial erosional features.Surface lowering rates for a range of metamorphic rocks vary from 0.05 to 2.20 mm ka^− 1 and are broadly comparable with those obtained from crystalline rocks in other periglacial environments. The mean rate of surface lowering at 0.55 mm ka^− 1 is low and demonstrates the relatively small impact of microweathering on postglacial landscape evolution. Variations in bedrock microweathering can be explained by lithological variation. Amphibolite and mica-rich bedrock surfaces experience greater denudation and weakening, least weathering rind formation, and abundant preservation of glacial striae, despite greater surface lowering. Conversely, quartz-rich bedrock surfaces are most resistant to denudation and weakening, but have greater weathering rind formation and fewer preserved striae. Postglacial microweathering is achieved primarily through granular disintegration involving detachment and removal of mineral grains and weakening from increased porosity. Granular decomposition is manifest in the formation of weathering rinds. Analysis of interactions between weathering indices indicates that rind accumulation is limited by microerosion.A conceptual model is proposed that illustrates the temporal interrelationships between in situ and erosional facets of microweathering in two contrasting mineral assemblages. The model proposes that cyclic processes of in situ disintegration, decomposition, and erosion are at work. The relative balance between these processes varies with lithology so that in more resistant quartz-rich rocks the net effect is minimal surface lowering and accumulation of weathering rind. In weaker, amphibolitic and micaceous rocks, the net effect is greater surface lowering and minimal accumulation of weathering rind. The results of the research demonstrate the important influence of rock properties, notably mineral composition, in postglacial microweathering of crystalline bedrock in a periglacial environment.     

Combining cosmogenic nuclides and major elements from moraine soil profiles to improve weathering rate estimates

Previous studies of chemical weathering rates for soil developed on glacial moraines generally assumed little or no physical erosion of the soil surface. In this study, we investigate the influence of physical erosion on soil profile weathering rate calculations. The calculation of chemical weathering rates is based on the assumption that soil profiles represent the integrated amount of weathering since the time of moraine deposition. The weathering rate of a surface subjected to denudation is the sum of the weathering loss from the existing soil profile added to the weathering loss in the material removed by denudation, divided by the deposition age. In this study, the amount of weathered material removed since moraine deposition is calculated using the denudation rate estimated from cosmogenic nuclide data and the deposition age of the moraine. Weathering rates accounting for denudation since moraine deposition are compared to weathering rates based on the assumption of no physical erosion and on the assumption of steady-state denudation for the Type Pinedale moraine ( 21 ka) and the Bull Lake-age moraine ( 140 ka) in the Fremont Lake Area (Wind River Mountains, Wyoming, USA). The total weathering rates accounting for denudation are 8.15 ± 1.05 g_(oxide) m^− 2 y^− 1 for the Type Pinedale moraine and 4.78 ± 0.89 g_(oxide) m^− 2 y^− 1 for the Bull Lake-age moraine, which are  2 to 4 times higher, respectively, than weathering rates based on the assumption of no physical erosion. The weathering rates based on denudation since moraine deposition are comparable or smaller than weathering rates assuming steady-state denudation. We find the assumption of steady-state denudation is not valid in depositional landscapes with young deposition ages or slow denudation rates. The decrease in weathering rates over time between the Type Pinedale and Bull Lake-age soils that is observed in the case of no physical erosion is decreased when the influence of denudation on the total weathering rates is taken into account. Fresh unweathered material with high reactive mineral surface area is continuously provided to the surface layer by denudation diminishing the effect of decreasing weathering rate over time.     

Weathering Pits as Indicators of the Relative Age of Granite Surfaces in the Cairngorm Mountains, Scotland

Weathering pits 1–140 cm deep occur on granite surfaces in the Cairngorms associated with a range of landforms, including tors, glacially exposed slabs, large erratics and blockfields. Pit depth is positively correlated with cosmogenic exposure age, and both measures show consistent relationships on individual rock landforms. Rates of pit deepening are non‐linear and a best fit is provided by the sigmoidal function D = b1 + exp(b2+b3/t). The deepest pits occur on unmodified tor summits, where ¹⁰Be exposure ages indicate that surfaces have been exposed to weathering for a minimum of 52–297 ka. Glacially exposed surfaces with pits 10–46 cm deep have given ¹⁰Be exposure durations of 21–79 ka, indicating exposure by glacial erosion before the last glacial cycle. The combination of cosmogenic exposure ages with weathering pit depths greatly extends the area over which inferences can be made regarding the ages of granite surfaces in the Cairngorms. Well‐developed weathering pits on glacially exposed surfaces in other granite areas are potential indicators of glacial erosion before the Last Glacial Maximum.     

藏南普莫雍错流域水体离子组成与空间分布及其环境意义(英文)

The chemistry of major cations (Mg²⁺, Ca²⁺, Na⁺, and K⁺) and anions (HCO₃ ⁻, SO₄ ²⁻, and Cl⁻) in the water of Lake Pumayum Co and its inflow river was studied, revealing the obvious ionic difference among various inflow rivers and the lake. The chemical type of the lake water was Mg²⁺-Ca²⁺-HCO₃ ⁻-SO₄ ²⁺, but the major ions of the main inflow rivers were Ca²⁺-Mg²⁺-HCO₃ ⁻. In the lake inlet of Jiaqu River, the main inflow river, there was significant variance of water chemistry within the depth less than 2 m. However, it was almost homogeneous at other area of the lake. Therefore, with the evidence of distribution of water chemistry and oxygen isotope of lake water, a conclusion can be outlined that Jiaqu River had a distinct effect on the hydrochemistry of the water on the submerged delta, whereas this is not the case for other rivers. The Gibbs plot revealed that the dominant mechanism responsible for controlling chemical compositions of the lake water was rocks weathering in the drainage area. Ion ratios and ternary plots further explored the main processes controlling the water chemistry of the catchment, i.e., carbonate weathering, pyrite weathering, and silicate weathering. The different hydrochemistry characteristics between river water and lake water may result from the CaCO₃ precipitation. The findings will benefit the explanation of the environmental significance of carbonate in paleolimnological studies in the lake.     

Quaternary bedrock erosion and landscape evolution in the Sør Rondane Mountains, East Antarctica: Reevaluating rates and processes

Rates and processes of rock weathering, soil formation, and mountain erosion during the Quaternary were evaluated in an inland Antarctic cold desert. The fieldwork involved investigations of weathering features and soil profiles for different stages after deglaciation. Laboratory analyses addressed chemistry of rock coatings and soils, as well as ¹⁰Be and ²⁶Al exposure ages of the bedrock. Less resistant gneiss bedrock exposed over 1 Ma shows stone pavements underlain by in situ produced silty soils thinner than 40 cm and rich in sulfates, which reflect the active layer thickness, the absence of cryoturbation, and the predominance of salt weathering. During the same exposure period, more resistant granite bedrock has undergone long-lasting cavernous weathering that produces rootless mushroom-like boulders with a strongly Fe-oxidized coating. The red coating protects the upper surface from weathering while very slow microcracking progresses by the growth of sulfates. Geomorphological evidence and cosmogenic exposure ages combine to provide contrasting average erosion rates. No erosion during the Quaternary is suggested by a striated roche moutonnée exposed more than 2 Ma ago. Differential erosion between granite and gneiss suggests a significant lowering rate of desert pavements in excess of 10 m Ma^− 1. The landscape has been (on the whole) stable, but the erosion rate varies spatially according to microclimate, geology, and surface composition.     

Stratigraphic development of an Upper Jurassic deep marine syn‐rift succession,Inner Moray Firth Basin,Scotland

The stratigraphic development of an Upper Jurassic syn‐rift succession exposed at outcrop in the Inner Moray Firth Basin has been investigated using high‐resolution biostratigraphy and sedimentology. A continuous 970 m thick section, exposed in the hangingwall of the Helmsdale Fault was logged in detail. The succession spans 8 Ma and contains eight lithofacies types, which indicate deposition in a deep marine setting. Boulder beds contain large, angular clasts, with bed thicknesses typically >2 m and poor sorting suggesting deposition by debris flows. An inverse clast stratigraphy is observed; the oldest boulder beds contain sandstone clasts of Upper Old Red Sandstone (ORS) with younger debris flows containing clasts of Middle ORS calcareous siltstone. A marked change from siliciclastic to carbonate dominated sedimentation occurred during the Early Tithonian, interpreted primarily as a result of change in lithologies in the footwall catchment from sandstone to calcareous siltstone, which reduced supply of siliciclastic sediment. Secondary factors are identified as increased aridity in the Early Tithonian, which reduced sand supply from the hinterland and a third‐order Early Tithonian eustatic sea‐level rise, which trapped coarser clastic sediment within the hinterland. Biostratigraphy allows calculation of variations in sedimentation rates with recognition of: (1) an early rift phase characterised by sandy turbidite deposition, when sedimentation rates averaged 0.08 m/ky, (2) a rift climax phase from the Early Kimmeridgian where sedimentation rates increased steadily to a maximum of 0.64 m/ky in the Early Tithonian, with strata dominated by boulder scale clast‐supported debris flows and (3) a late stage of rifting from the mid Tithonian, where sedimentation rates decreased to 0.07 m/ky. Overall sedimentation rates are comparable to those of other deep marine rift basins. Unroofing a resistant lithology on the footwall of a rift has important implications for siliciclastic sediment supply in rift basins.     

Newly Discovered Submicron-Scale Weathering in Quartz: Geographical Implications*

Quartz is regarded as one of the minerals most resistant to chemical weathering. Nevertheless, quartz does weather under certain conditions. Several geographic phenomena, including downstream sorting of fluvial sediments, the distribution of loess, and the absence of silica in tropical soils, may be directly or indirectly related to quartz weathering. Weathering may impact the use of quartz in geographic applications of geochronology and paleoenvironmental analysis. This paper defines the molecular-scale mechanisms of chemical weathering in quartz, and presents the first examination of quartz weathering in the terrestrial environment using high resolution transmission electron microscopy (HRTEM). HRTEM images illustrate crystalline disintegration, or amorphization, of exterior surfaces and internal fractures of quartz sand grains collected from tills of the Eastern Sierra Nevada, California. Of several alternative mechanisms, in situ hydration of the crystalline matrix is the most plausible for quartz amorphization. Scale and mineral structure appear to be prominent factors in quartz amorphization; microenvironment and weathering rates may also be significant, but data are insufficient to make more precise generalizations.     

Surface temperature differences between minerals in crystalline rocks: Implications for granular disaggregation of granites through thermal fatigue

Thermal expansion differences between minerals within rocks under insolation have previously been assumed to drive breakdown by means of granular disaggregation. However, there have been no definitive demonstrations of the efficacy of this weathering mechanism. Different surface temperatures between minerals should magnify thermal expansion differences, and thus subject adjacent minerals to repeated stresses that might cause breakdown through fatigue failure. This work confirms the existence of surface temperature differences between minerals in granitic rocks under simulated short-term temperature fluctuations so as to discriminate their potential for initiating granular disaggregation. The influence of colour, as a surrogate for albedo, and crystal size, as a function of thermal mass are specifically identified because of their ease of quantification. Four rock types with a range of these properties were examined, and subjected to repeated short-term temperature cycles by radiative heating and cooling under laboratory conditions. Results show that while albedo is the main control for overall and individual maximum temperatures, crystal size is the main factor controlling higher temperature differences between minerals. Thus, stones with large differences of mineral sizes can undergo magnified stresses due to thermal expansion differences.     

Past atmospheric Pb deposition in Lake Qinghai,northeastern Tibetan Plateau

Two short sediment cores were recovered from sub-basins of Lake Qinghai, China and were analyzed for concentrations of Pb and 16 other elements to determine historic, regional atmospheric Pb deposition on the Tibetan Plateau. Core chronologies, dating back to the eighteenth century, were established using activities of ²¹⁰Pb and ¹³⁷Cs. The 17 elements were divided into three principal components. Variations in concentrations of PC1 elements (Al, Cr, Cu, Fe, K, Mn, Ni, and Ti) demonstrate different patterns between the two cores, and are attributed to different sediment sources in the two sub-basins. PC2 elements (Ba, Ca, Na, and Sr) may be associated with the degree of catchment weathering and/or water chemistry. Four elements (Pb, Zn, P, and Co) are related to both PC1 and PC2, and reflect a mixture of natural and anthropogenic sources. The PC3 element is Mg in the north sub-basin, and is perhaps related to aragonite precipitation and/or increased farming. Elevated Pb concentrations in uppermost sediments of both cores signify a recent regional/global increase in anthropogenic Pb release into the environment. After subtracting lithogenic Pb, derived from rock weathering and/or dust and normalized to the background immobile element Ti, results suggest that excess, anthropogenic Pb is transferred to the lake and its sediments predominantly via the atmosphere. This anthropogenic atmospheric Pb is comparable in magnitude and displays similar variation patterns in the two cores, reflecting regional atmospheric deposition and local erosion. The average anthropogenic Pb deposition rate in Lake Qinghai since the 1960s has been ~12.2 ± 3.5 mg/m²/a, comparable with atmospheric Pb fluxes reported for sites elsewhere in the northern hemisphere.     

Boulder weathering and erosion associated with a wildfire, Sierra Ancha Mountains, Arizona

An April–May 2000 “Coon Creek Fire” burned 37.5 km² of the Sierra Ancha Mountains, 32.3 km miles north of Globe, AZ—including 25 sandstone and 19 diorite boulders surveyed in 1989 and resurveyed after the burn, after the summer 2000 monsoon season, and after the winter 2001 season. When viewed from the perspective of cumulative eroded area, both sandstone and diorite displayed bimodal patterns with 79% of sandstone boulder area and 93% of diorite boulder area undergoing either no fire-induced erosion or fire-induced erosion >76 mm. When stretched over cumulative boulder areas, erosion due to the fire averaged >26 mm for sandstone and >42 mm for diorite. Post-fire erosion from thunderstorm summer rains averaged <1 mm for 5 diorite and 1 mm for 10 sandstone boulders. While only a single diorite boulder eroded an average of 1.2 mm after the winter, winter erosion removed an average of 5.5 mm from 14 sandstone boulders. Thus, fire appears to increase a rock's susceptibility to post-fire weathering and erosion processes, as predicted by Goudie et al. [Earth Surf. Process. Landf. 17 (1992) 605]. In contrast to experimental research indicating the importance of size in fire weathering, no statistically significant relationship exists between erosion and boulder height or boulder surface area—a result similar to Zimmerman et al. [Quat. Res. 42 (1994) 255]. These data exclude 12 original sites and 85 boulders at sites impacted by the fire that could not be relocated, with a reasonable cause for the lack of relocation being boulder obliteration by the fire. Given evidence from ¹⁰Be and ²⁶Al cosmogenic nuclides [Earth Planet. Sci. Lett. 186 (2001) 269] supporting the importance of boulders in controlling evolution of nonglaciated, bouldered landscapes [Geol. Soc. Amer. Bull. 76 (1965) 1165], fire obliteration of boulders could be an important process driving drainage evolution in nonglaciated mountains.     

Rates Of Postglacial rock weathering on glacially scoured outcrops (Abisko–Riksgränsen area, 68°N)

Ice–polished quartz veins, feldspar phenocrysts and quartzite layers were used as reference surfaces to assess the impact of Postglacial rock weathering in Lapland (68°N). Over 3200 measurements were carried out on roches moutonées and glaciofluvially scoured outcrops distributed within three study areas covering 8 km². Inferred weathering rates demonstrate that 10,000 years of Holocene weathering did not significantly modify the geometry of Weichselian rock surfaces. However, rates of general surface lowering range from 1 to 25, depending on the rock type, with average values at 0.2 mm ka⁻¹ for homogeneous crystalline rocks (irrespective of their acidity and grain size), 1 mm ka⁻¹ for biotite–rich crystalline rocks, and 5 mm ka⁻¹ for carbonate sedimentary rocks. Accelerated rates were recorded in weathering pits and along joints with values up to ten times higher than on the rest of the rock surface. Comparisons with cold and temperate areas suggest that solution rates of carbonate rocks are highly dependent on climate conditions, whilst granular disintegration of crystalline rocks operates at the same rate whatever the environment. It probably means that microgelivation is not efficient on ice–polished crystalline outcrops even under harsh climate conditions, and that granular disintegration proceeds under various climates from the same ubiquitous combination of biochemical processes. Last, the weathering state of Late–Weichselian roches moutonées can be usefully compared to that of Preglacial tors of the nearby Kiruna area.     

Rates Of Postglacial rock weathering on glacially scoured outcrops (Abisko–Riksgränsen area, 68°N)

Ice–polished quartz veins, feldspar phenocrysts and quartzite layers were used as reference surfaces to assess the impact of Postglacial rock weathering in Lapland (68°N). Over 3200 measurements were carried out on roches moutonées and glaciofluvially scoured outcrops distributed within three study areas covering 8 km². Inferred weathering rates demonstrate that 10,000 years of Holocene weathering did not significantly modify the geometry of Weichselian rock surfaces. However, rates of general surface lowering range from 1 to 25, depending on the rock type, with average values at 0.2 mm ka⁻¹ for homogeneous crystalline rocks (irrespective of their acidity and grain size), 1 mm ka⁻¹ for biotite–rich crystalline rocks, and 5 mm ka⁻¹ for carbonate sedimentary rocks. Accelerated rates were recorded in weathering pits and along joints with values up to ten times higher than on the rest of the rock surface. Comparisons with cold and temperate areas suggest that solution rates of carbonate rocks are highly dependent on climate conditions, whilst granular disintegration of crystalline rocks operates at the same rate whatever the environment. It probably means that microgelivation is not efficient on ice–polished crystalline outcrops even under harsh climate conditions, and that granular disintegration proceeds under various climates from the same ubiquitous combination of biochemical processes. Last, the weathering state of Late–Weichselian roches moutonées can be usefully compared to that of Preglacial tors of the nearby Kiruna area.     

Spatial variation of early season surface water chemistry in Kärkevagge, Swedish Lapland

This study examines the spatial variability of early season water chemistry in the arctic-alpine valley of Kärkevagge, Sweden. The data demonstrate the spatially heterogeneous nature of water chemistry and the general patterns of chemical weathering in the valley. Water chemistry in this valley is dominated by two anions, bicarbonate and sulfate. Bicarbonate is derived from the dissolution of atmospheric CO₂ and the weathering of carbonate units in the local metamorphic rocks, while the sulfate is derived from the oxidation of pyrite in the Seve-Koli tectonic nappe. Spatial patterns of chemical constituents reflect the broad effects of local geology on surface water chemistry. In particular, they demonstrate the effects that mineral species present in minor amounts have on basin-wide water chemistry. However, solute flux rates derived from water chemistry and discharge demonstrate less variability.