Origin and development of tafoni in Tunnel Spring Tuff,Crystal Peak,Utah, USA

Bedding‐parallel tafoni are well developed over much of the surface of the Tunnel Spring Tuff (Oligocene) exposed in 300‐m‐high Crystal Peak, an inselberg. The Tunnel Spring Tuff is a crudely stratified, non‐welded rhyolite ash‐flow tuff with > 30 per cent porosity. Clasts of Palaeozoic dolomite, limestone and quartzite make up 10 per cent of the tuff. The tafoni are remarkable because of their size (up to 20 m wide but rarely wider than 4 m), shape of the openings (spherical, arch‐like or crescent‐shaped) and abundance (up to 50 per cent of an outcrop face). They are actively forming today. Calcite is abundant (10 to 40 per cent by weight) in tafoni as an efflorescence in spalling flakes of tuff on their roofs and walls. Halite and gypsum generally make up less than 0·01 per cent of the efflorescence. The absence of corroded quartz and feldspar grains in spall fragments indicates that chemical weathering is unimportant in development of the tafoni. Calcite, aragonite, halite and gypsum dust from modern salt pans less than 20 km from Crystal Peak are potential sources of salt in the tuff, but the prevailing winds are in the wrong direction for significant amounts of these evaporite minerals to reach the inselberg. Calcite is the only evaporite mineral present in the tafoni in more than trace amounts, and this mineral is readily available within the tuff itself as a result of rock weathering. We propose that meteoric water containing carbonic acid infiltrates the tuff, dissolves carbonate clasts, and migrates to the steep flanks (>20°) of the peak through abundant megapores and micropores. There it evaporates and precipitates calcite. Crystallization pressure spalls off grains and sheets as the physical manifestation of salt weathering. The quasi‐uniform spacing of tafoni suggests that a self‐organization process is active in the water flow. Copyright © 2000 John Wiley & Sons, Ltd.     

Simulating the growth of tafoni

Throughout the world, large caves in rocks (tafoni) are found, which originate from salt weathering. The mechanisms that control their development are poorly understood. The growth of tafoni has been studied with a model that describes how a rock surface, containing a small pit, disintegrates by salt crystallization during wetting/drying cycles. In the model the rock is mapped on a grid. The migration and crystallization of salts are simulated explicitly in the drying phase of a cycle. At the end of each wetting/drying cycle the amount of salt deposited at the grid nodes is evaluated and the shape of the rock surface is adjusted by removing nodes. The length of the drying period in a single cycle proved to be the key parameter. For short drying periods the amount of crystallized salt at the surface is proportional to the drying rate. Therefore, for short drying periods most salts are deposited at the more exterior parts of the rock surface. As a result, most damage will develop at these parts of the surface, which results in smooth surfaces. Due to the characteristics of the drying process for long drying periods, most salts accumulate at regions with low evaporation rates, which are the sheltered parts of the rock surface. These parts are not exposed to the wind or the sun. As a result, the pit grows and a tafone develops. Copyright © 2004 John Wiley & Sons, Ltd.     

Tafoni development in a cryotic environment: an example from Northern Victoria Land,Antarctica

Tafoni are a type of cavernous weathering widespread around the world. Despite the extensive distribution of the tafoni, their genesis is not clear and is still a matter of debate, also because they occur in such different climatic conditions and on so many different types of substrate. Geomorphological characterization of more than 60 tafoni in three different Antarctic sites (two coastal and one inland) between 74 and 76° S with sampling of weathering products and salt occurrences are described together with thermal data (on different surfaces) and wind speed recorded in different periods of the year in a selected tafone close to the Italian Antarctic station. The aim of this present study is to provide further information to help understand the processes involved in the growth of tafoni in a cryotic environment, and the relationship of these processes to climate, with particular attention to the thermal regime and the role of wind. The new data presented in this paper suggest that there is no single key factor that drives the tafoni development, although thermal stress seems the most efficient process, particularly if we consider the short‐term fluctuations. The data also confirm that other thermal processes, such as freezing–thawing cycles and thermal shock, are not really effective for the development of tafoni in this area. The wind speed measured within the tafoni is half that recorded outside, thus favouring snow accumulation within the tafoni and therefore promoting salt crystallization. On the other hand, the wind effect on the thermal regime within the tafoni seems negligible. While both salt weathering and thermal stress appear active in this cryotic environment, these are azonal processes and are therefore active in other climatic areas where tafoni are widespread (such as the Mediterranean region). Copyright © 2007 John Wiley & Sons, Ltd.     

Seasonal transition of hydrological processes in a slow‐moving landslide in a snowy region

A comprehensive understanding of seasonal hydrological dynamics is required to describe the influence of pore‐water pressure on the stability of landslides in snowy regions. This study reports on the results of continuous meteorological and hydrological observations over 2 years on a landslide body comprising Neogene sedimentary rocks in northern Japan, where a thick (3–5 m) seasonal snowpack covers the land surface. Monitoring of the volumetric water content in shallow unsaturated zones (<0.8 m depth) and pore‐water pressure in saturated bedrock at depths of 2.0 and 5.2 m revealed clear seasonality in hydrological responses to rainfall and meltwater supply. During snow‐free periods, both the shallow soil moisture and deep pore‐water pressure responded rapidly to intense rainwater infiltration. In contrast, during snowmelt, the deep pore pressure fluctuated in accordance with the daily cycle of meltwater input, without notable changes in shallow moisture conditions. During occasional foehn events that cause intense snow melting in midwinter, meltwater flows preferentially through the layered snowpack, converging to produce a localized water supply at the ground surface. This episodically triggers a significant rise in pore‐water pressure. The seasonal differences in hydrological responses were characterized by a set of newly proposed indices for the magnitude and quickness of increases in the pressure head near the sliding surface. Under snow‐covered conditions, the magnitude of the pressure increase tends to be suppressed, probably owing to a reduction in infiltration caused by a seasonal decrease in the permeability of surface soils, and effective pore‐water drainage through the highly conductive colluvial layer. Deep groundwater flow within bedrock remained in a steady upwelling state, enhanced by increasing moisture in shallow soils under snow cover, reflecting the convergence of subsurface water from surrounding hillslopes.     

Climate–surface–pore‐water interactions on a salt crusted playa: implications for crust pattern and surface roughness development measured using terrestrial laser scanning

Sodium accumulating playas (also termed sodic or natric playas) are typically covered by polygonal crusts with different pattern characteristics, but little is known about the short‐term (hours) dynamics of these patterns or how pore water may respond to or drive changing salt crust patterning and surface roughness. It is important to understand these interactions because playa‐crust surface pore‐water and roughness both influence wind erosion and dust emission through controlling erodibility and erosivity. Here we present the first high resolution (10^?3 m; hours) co‐located measurements of changing moisture and salt crust topography using terrestrial laser scanning (TLS) and infra‐red imagery for Sua Pan, Botswana. Maximum nocturnal moisture pattern change was found on the crests of ridged surfaces during periods of low temperature and high relative humidity. These peaks experienced non‐elastic expansion overnight, of up to 30 mm and up to an average of 1.5 mm/night during the 39 day measurement period. Continuous crusts however showed little nocturnal change in moisture or elevation. The dynamic nature of salt crusts and the complex feedback patterns identified emphasize how processes both above and below the surface may govern the response of playa surfaces to microclimate diurnal cycles. © 2015 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.     

Biogenic origin of coastal honeycomb weathering

Honeycomb weathering occurs in two environments in Late Cretaceous and Eocene sandstone outcrops along the coastlines of south‐west Oregon and north‐west Washington, USA, and south‐west British Columbia, Canada. At these sites honeycomb weathering is found on subhorizontal rock surfaces in the intertidal zone, and on steep faces in the salt spray zone above the mean high tide level. In both environments, cavity development is initiated by salt weathering. In the intertidal zone, cavity shapes and sizes are primarily controlled by wetting/drying cycles, and the rate of development greatly diminishes when cavities reach a critical size where the amount of seawater left by receding tides is so great that evaporation no longer produces saturated solutions. Encrustations of algae or barnacles may also inhibit cavity enlargement. In the supratidal spray zone, honeycomb weathering results from a dynamic balance between the corrosive action of salt and the protective effects of endolithic microbes. Subtle environmental shifts may cause honeycomb cavity patterns to continue to develop, to become stable, or to coalesce to produce a barren surface. Cavity patterns produced by complex interactions between inorganic processes and biologic activity provide a geological model of ‘self‐organization’. Surface hardening is not a factor in honeycomb formation at these study sites. Salt weathering in coastal environments is an intermittently active process that requires particular wind and tidal conditions to provide a supply of salt water, and temperature and humidity conditions that cause evaporation. Under these conditions, salt residues may be detectable in honeycomb‐weathered rock, but absent at other times. Honeycomb weathering can form in only a few decades, but erosion rates are retarded in areas of the rock that contain cavity patterns relative to adjacent non‐honeycombed surfaces. Copyright © 2010 John Wiley & Sons, Ltd.     

Tafoni in the El Chorro area,Andalucia, southern Spain

Using a combination of field, laboratory and micromorphological evidence, this study examines tafoni (singular, tafone) in the El Chorro area of Andalucia, southern Spain, and makes inferences concerning the processes responsible for their formation. Twenty-five tafoni were randomly selected for field examination. The morphology of these cavernous rock domes is characterized by a helmet-shaped outer roof and an arched-shaped cavern, often with a partially overhanging visor; measurements of height, width and depth of the caverns revealed marked variations in size. The presence or absence of lichen cover, surface varnish, overhanging visor, cavern backwall stripes, rock flaking, weathering pits and cavern floor sediments was also noted. Surface hardness values, obtained using a Schmidt hammer, are relatively low but significantly higher on the outer roof of the tafoni than on the inner cavern walls. Analysis of sediment samples collected from the cavern backwalls and floors indicates predominantly sandy textures, alkaline pH values and some base cation enrichment. Micromorphological analysis of thin sections, prepared from undisturbed blocks, reveals large quantities of pore-filling cement, consisting mainly of calcite, mineral grains affected by weathering and pseudomorphic replacement, and dark, rounded nodules with a metallic appearance. In terms of their formation, different processes appear to act on different parts of the landform. On the outer roof surfaces, case hardening, resulting from near-surface cementation and surface varnish development, is dominant. On the inner cavern surfaces, however, core softening, resulting from granular disintegration and flaking, dominates. Exfoliation weathering, running water and wind deflation also appear to play an important role in tafone formation. A phased model of tafone evolution is proposed whereby the features pass through four phases of development–initiation, enlargement, amalgamation and degradation; in the study area there are examples of tafoni in each of these phases. Much of the evidence suggests that the tafoni are actively developing under current environmental conditions. © 1997 by John Wiley & Sons, Ltd.     

The role of rock surface hardness and internal moisture in tafoni development in sandstone

The development of cavernous weathering features such as tafoni remains poorly understood. In particular, the roles played by internal moisture and case hardening remain unclear. In this study, Electric Resistivity Tomography (ERT) has been used to map moisture distribution within inner walls of tafoni developed in sandstone, and an Equotip device used to measure rock surface hardness as a proxy measure of the degree of weathering and case hardening. Seven large tafoni in the Golden Gate Highlands National Park (South Africa), varying in size and degree of development have been monitored. A dynamic relationship between surface hardness, degree of weathering and internal moisture regimes has been found. We propose a new conceptual model which illustrates the complex interaction between case hardening and internal moisture and suggests a new direction for cavernous weathering research. Copyright © 2011 John Wiley & Sons, Ltd.     

Proposing a formula for evaporation measurement from salt water resources

Evaporation involves the change in state of a liquid to a vapour. The evaporation rate from salt‐water resources depends mostly on saturated vapour pressure above its surface. On the other hand, the saturated vapour pressure is affected by the ion activity coefficient, which stems from the chemical salt concentration of water. Thus, an increase in concentration of water results in a reduction of saturated vapour pressure. In order to acquire the actual rate of evaporation from salt‐water resources, a uniform set of evaporation pans with different but specified salt concentrations were used, in a meteorological station under the same conditions. The difference in evaporation rate of each pan can only stem from the difference in chemical salt concentration and, indeed, the molar fraction of water in each saline solution. Therefore, by applying the water molar fraction in the pressure term of fresh‐water evaporation measurement formulas, these equations were developed further for determination of evaporation rate from salt‐water resources. The proposed formulas using very simple terms seem to be suitable for determination of evaporation rate from any water (typically saline, semi‐saline and fresh water) with a satisfactory precision. Copyright © 2007 John Wiley & Sons, Ltd.     

Spatial characterization of salt accumulation in early stage limestone weathering using probe permeametry

This research characterizes the weathering of natural building stone using an unsteady‐state portable probe permeameter. Variations between the permeability properties of fresh rock and the same rocks after the early stages of a salt weathering simulation are used to examine the effects of salt accumulation on spatial variations in surface rock permeability properties in two limestones from Spain. The Fraga and Tudela limestones are from the Ebro basin and are of Miocene age. Both stone types figure largely in the architectural heritage of Spain and, in common with many other building limestones, they are prone to physical damage from salt crystallization in pore spaces. To examine feedbacks associated with salt accumulation during the early stages of this weathering process, samples of the two stone types were subjected to simulated salt weathering under laboratory conditions using magnesium sulphate and sodium chloride at concentrations of 5% and 15%. Permeability mapping and statistical analysis (aspatial statistics and spatial prediction) before and after salt accumulation are used to assess changes in the spatial variability of permeability and to correlate these changes with salt movement, porosity change, potential rock deterioration and textural characteristics. Statistical analyses of small‐scale permeability measurements are used to evaluate the drivers for decay and hence aid the prediction of the weathering behaviour of the two limestones. Copyright © 2010 John Wiley & Sons, Ltd.     

Mechanisms and pathways of lateral flow on aspen‐forested,Luvisolic soils,Western Boreal Plains,Alberta, Canada

Rainfall simulation experiments by Redding and Devito ( 2008 , Hydrological Processes 23: 4287–4300) on two adjacent plots of contrasting antecedent soil moisture storage on an aspen‐forested hillslope on the Boreal Plain showed that lateral flow generation occurred only once large soil storage capacity was saturated combined with a minimum event precipitation of 15–20 mm. This paper extends the results of Redding and Devito ( 2008 , Hydrological Processes 23: 4287–4300) with detailed analysis of pore pressure, soil moisture and tracer data from the rainfall simulation experiments, which is used to identify lateral flow generation mechanisms and flow pathways. Lateral flow was not generated until soils were wet into the fine textured C horizon. Lateral flow occurred dominantly through the clay‐rich Bt horizon by way of root channels. Lateral flow during the largest event was dominated by event water, and precipitation intensity was critical in lateral flow generation. Lateral flow was initiated as preferential flow near the soil surface into root channels, followed by development of a perched water table at depth, which also interacted with preferential flow pathways to move water laterally by the transmissivity feedback mechanism. The results indicate that lateral flow generated by rainfall on these hillslopes is uncommon because of the generally high available soil moisture storage capacity and the low probability of rainfall events of sufficient magnitude and intensity. Copyright © 2010 John Wiley & Sons, Ltd.     

Investigations into the relationship between changes in internal moisture regimes and rock surface deterioration in cavernous sandstone features

Cavernous features commonly develop in sandstone, but their development over time remains enigmatic. It has been suggested that moisture movements within the rock mass play a key role in the location, development and dynamics of cavernous features. In this research the role of internal moisture movement is tested through monitoring moisture and surface deterioration dynamics in April 2008 and April 2009 within two large cavernous features (mega‐tafoni) in the Golden Gate Highlands National Park, South Africa (GGHNP). Data are presented from surveys of internal moisture (using electrical resistivity tomography, ERT), surface moisture (using a Protimeter) and deterioration (using surface hardness as recorded with an Equotip as a proxy for surface deterioration) across five 2.45 m long transects. In addition a high resolution temperature record is presented to assess the influence of micro‐climates within the caverns. The results indicate consistency in the gross spatial pattern of moisture flow within the rock mass over a one year period, but significant changes in mean moisture contents and in the fine detail of moisture patterning. Some noticeably weakened areas had developed within the central parts of the cavernous features, often linked to wetter subsurface conditions, supporting the theory that ‘core softening’ is a main driver of cavernous feature formation. However, in some areas surface hardening is also found to be associated with wetter subsurface conditions, supporting the theory that ‘case hardening’ is a main driver of cavernous feature formation. In addition, the presence of well‐established biofilms suggests an even more complex interaction between moisture, surface development and biological activity. A model is presented therefore which integrates this paradox by proposing a non‐linear relationship between moisture dynamics, facilitation of biofilm formation, and deterioration within cavernous features. Copyright © 2013 John Wiley & Sons, Ltd.     

Towards quantifying the negative feedback regulation of peatland evaporation to drought

The frequency and intensity of drought is projected to increase within the boreal region under future climatic conditions. Peatlands are widely considered to regulate water loss under drought conditions, increasing surface resistance (r_s) and reducing evaporative losses. This maintains peat moisture content, increasing the resilience of these globally important carbon stores. However, the magnitude and form of this important negative feedback response remains uncertain. To address this, we monitored the response of r_s to drought within four peat cores under controlled meteorological conditions. When the water‐table was dropped to a depth of 0.30 m and the humidity reduced to ≤40%, a step shift in r_s from ~50 s m^‐1 up to 1000 s m^‐1 was observed within burned and unburned peat, which virtually shuts down evaporation, limiting water loss. We show that measured near‐surface tension cannot be used to directly calculate this transition in peat surface resistance. However, empirical relationships that account for strong vertical variations in tension through the near‐surface and/or disequilibrium between pore air and near‐surface pore water pressure provide the potential to incorporate this negative feedback response into peatland ecohydrological models. Further observations are necessary to examine this response under dynamic atmospheric conditions. We suggest that the link between surface temperature and evaporation provides potential to further examine this feedback in either burned peatlands or peatlands with a low vascular vegetation cover. Copyright © 2013 John Wiley & Sons, Ltd.     

Rock‐protecting seaweed? Experimental evidence of bioprotection in the intertidal zone

Biogeomorphological processes are an important component of dynamic intertidal systems. On rocky shores, the direct contribution of microorganisms, plants and animals to weathering and erosion is well known. There is also increasing evidence that organisms can alter rock breakdown indirectly, by moderating temperature and moisture regimes at the rock–air interface. These influences have been purported to represent mechanisms of bioprotection, by buffering microclimatic fluctuations associated with weathering processes such as wetting and drying and salt crystallization. However, virtually nothing has been done to test whether microclimatic buffering translates to differences in actual rock breakdown rates. Here we report a preliminary laboratory experiment to assess how an artificial canopy (chosen to represent seaweed) affects mechanical rock breakdown. Using a simplified and accelerated thermal regime based on field data from a rocky shore platform in southern England, UK, we find that breakdown (mineral debris release) of mudstone covered with a canopy is reduced by as much as 79% relative to bare rock after around 100 thermal cycles. Reduction in rock surface hardness (measured using an Equotip device) was also greater for bare rock (17%) compared to covered rock (10%) over this period. Measurements of salt crystal formation indicate that the mechanism driving these differences was a reduction in the frequency of crystallization events, via moisture retention and shading of the rock surface. Copyright © 2015 John Wiley & Sons, Ltd.     

Building sandstone surface modification by biofilm and iron precipitation: emerging block‐scale heterogeneity and system response

Stone surfaces are sensitive to their environment. This means that they will often respond to exposure conditions by manifesting a change in surface characteristics. Such changes can be more than simply aesthetic, creating surface/subsurface heterogeneity in stone at the block scale, promoting stress gradients to be set up as surface response to, for example, temperature fluctuations, can diverge from subsurface response. This paper reports preliminary experiments investigating the potential of biofilms and iron precipitation as surface‐modifiers on stone, exploring the idea of block‐scale surface‐to‐depth heterogeneity, and investigating how physical alteration in the surface and near‐surface zone can have implications for subsurface response and potentially for long‐term decay patterns. Salt weathering simulations on fresh and surface‐modified stone suggest that even subtle surface modification can have significant implications for moisture uptake and retention, salt concentration and distribution from surface to depth, over the period of the experimental run. The accumulation of salt may increase the retention of moisture, by modifying vapour pressure differentials and the rate of evaporation. Temperature fluctuation experiments suggest that the presence of a biofilm can have an impact on energy transfer processes that occur at the stone surface (for example, buffering against temperature fluctuation), affecting surface‐to‐depth stress gradients. Ultimately, fresh and surface‐modified blocks mask different kinds of system, which respond to inputs differently because of different storage mechanisms, encouraging divergent behaviour between fresh and surface‐modified stone over time. Copyright © 2014 John Wiley & Sons, Ltd.     

Complex relationships between salt type and rock properties in a durability experiment of multiple salt–rock treatments

A laboratory salt weathering experiment was performed using five salts to attack eight types of rocks to determine the relative significance of rock durability and salt aggressivity to salt crystallization damage. The influence of individual rock properties on the salt susceptibility of the rocks was also evaluated. To study the relation between pore characteristics, salt uptake, and damage, the pre‐ and post‐experiment pore size distributions of the rocks were also examined. It is observed that both salt type and rock properties influenced the damage pattern. The durability ranking of the rocks became significantly altered with the salt type while the variation in salt efficacy ranking with rock type was less pronounced. Of the five salts used, sodium chloride and aluminium sulfate were invariably ineffective with all rock types while sodium carbonate, sodium sulfate, and magnesium sulfate, were markedly more effective in damaging most types of rock used. Of the rock properties investigated, the microporosity (of pores smaller than 0·05 or 0·1 µm) showed the most significant influence on deterioration of the rocks associated with salt crystallization, whereas microporosity of pores smaller than 5 µm played a more important role in salt uptake. Pore size distribution was thus the key factor controlling salt uptake and damage. Rocks with a large number of pores (<5 µm) and a high proportion of pores (<0·05 or 0·1 µm) were particularly susceptible to salt crystallization damage. However, anomalies arose that could not be explained in terms of rock properties or salt efficacy alone. Overall, the relative influences of salt type/efficacy and rock type/properties on salt damage propensity were not clear enough to draw a reasonable conclusion. Salt crystallization damage appears to be influenced by the individual interactions between salts and rocks, which could explain the anomalous results. Copyright © 2009 John Wiley & Sons, Ltd.     

Impact of soil moisture heterogeneity length scale and gradients on daytime coupled land‐cloudy boundary layer interactions

Using a coupled large‐eddy simulation–land surface model framework, the impact of two‐dimensional soil moisture heterogeneity on the cloudy boundary layer under varied free‐atmosphere stabilities is investigated. Specifically, the impacts of soil moisture heterogeneity length scale and heterogeneity in terms of soil moisture gradients on micrometeorological states, surface fluxes, boundary layer characteristics, and cloud development are examined. The results show that mesoscale circulations due to surface heterogeneity in soil moisture play an important role in transferring water vapour within the boundary layer and in regulating cloud distribution at the entrainment zone, which, in turn, provides feedbacks on boundary layer/surface energy budgets. The initial domain‐averaged soil moisture is identical for all homogenous and heterogeneous cases; however, the soil moisture heterogeneity in gradient and length scale between dry and wet regions has a significant impact on the estimates of near‐surface micrometeorological properties and surface fluxes, which further affect the boundary layer states and characteristics. Both liquid water potential temperature and liquid water mixing ratio increase with an increasing soil moisture gradient, whereas the amount of specific humidity decreases. Heterogeneity length scale and free atmosphere stability also amplify these impacts on the boundary layer structure and cloud formation. In a low atmospheric stability condition that potentially allows for a deeper boundary layer and a higher entrainment rate, cloud base height and cloud thickness significantly increase as the soil moisture gradient and length scale increase. Analysis to differentiate the influences of surface heterogeneity type (i.e. length scale vs gradient) shows that in general soil moisture gradient provides a larger impact than heterogeneity length scale, although the heterogeneity length scale is large enough to initiate circulation features responsible for differences in the coupled system between homogeneous and heterogeneous soil moisture cases. Copyright © 2012 John Wiley & Sons, Ltd.     

Modified sediments and subsurface hydrology in natural and recreated salt marshes and implications for delivery of ecosystem services

Recreation or restoration of salt marsh through the deliberate removal of flood defences (managed realignment or de‐embankment) is a common practice across Europe and the USA, with potential to enhance delivery of ecosystem services. However, recent research suggests that physical, chemical and ecological processes may be impaired in recreated sites as a result of the modified morphology, sediment structure and hydrology associated with both the restoration process and historic land use. This paper compares physical sediment properties and subsurface water levels recorded in paired natural and de‐embanked (recreated) salt marshes in SE England. Using a combination of statistical and time‐series modelling, significant differences between the natural and recreated marshes are identified. Sediment properties (bulk density, moisture content and organic content) within each marsh were statistically different and imply that de‐embanked sediments are compacted, which may affect subsurface water movement. Analysis of hydrological time series reveals that the de‐embanked salt marsh is characterized by a damped response to tidal flooding with elevated and less variable water levels. This, combined with analysis of hydrographs and hysteresis patterns over individual tidal cycles, suggests that fast, horizontal near‐surface flows enhanced by the relict land surface may play a greater role in drainage of the de‐embanked salt marsh. The importance of hydrological functioning in governing many important physical and biogeochemical processes in salt marshes suggests any modifications would have significant implications for the delivery of ecosystem services. Copyright © 2014 John Wiley & Sons, Ltd.     

Physical controls on septic leachate movement in the vadose zone at the hillslope scale,Putnam County,New York,USA

The fate and transport of contaminants in the vicinity of septic fields remains poorly understood in many hydrogeomorphological environments. We report hydrometric data from an intensive hillslope‐scale experiment conducted between 29 August and 11 November 1998 at a residential leach field in New York State. The objective of our study was to characterize water flux within the vadose zone, understand the physical controls on the flux, and predict how this ultimately will affect subsurface water quality. Soil‐water flux was calculated using matric potential measurements from a network of 25 tensiometer nests, each nest consisting of three tensiometers installed to depths of 10, 50 and 130 cm. Unsaturated hydraulic conductivity curves were derived at each depth from field‐determined time‐domain reflectometry–tensiometry moisture‐release curves and borehole permeametry measurements. Flownets indicated that a strong upward flux of soil water occurred between rainstorms. Following the onset of (typically convective) rainfall, low near‐surface matric potentials were rapidly converted to near‐saturated and saturated conditions, promoting steep vertical gradients through the near‐surface horizons of the hillslope. Lateral hydraulic gradients were typically 10 times smaller than the vertical gradients. Resultant flow vectors showed that the flux was predominantly vertical through the vadose zone, and that the flux response to precipitation was short‐lived. The flux response was controlled primarily by the shape of the unsaturated hydraulic conductivity curves, which indicated a rapid loss of conductivity below saturation. Thus, soil water had a very high residence time in the vadose zone. The absence of rapid wetting at 130 cm and the delayed and small phreatic zone response to rainfall indicated that water movement through macropores did not occur on this hillslope. These results are consistent with a Cl tracing experiment, which demonstrated that the tracer was retained in the vadose zone for several months after injection to the system. Copyright © 2002 John Wiley & Sons, Ltd.     

Tafoni on coastal slopes,South Devon,U.K.

Cavernous weathering (tafoni development) occurs on coastal slopes in greenschist bedrock at elevations up to 40 m above sea level. The freshly weathered surfaces of the cavern interiors are irregular in morphology, discordant to major rock structure, formed by substantially weakened rock and associated with granular weathering debris. The weathering debris contains soluble elements in proportions similar to those present in seawater, and the penetration of elements associated with sea salts into the weathering surface to estimated depths of at least 0·1–0·2 m is indicated by the presence of chlorine. Scanning electron microscopy and microprobe analyses suggest that rock breakdown occurs principally through limited chemical weathering at grain boundaries. The mechanism for the emplacement of marine salts within sheltered rock surfaces in the tafoni is postulated to be a combination of dry deposition under turbulent atmospheric conditions and wetting by coastal fog.