Sedimentary Landscapes Research Articles

True substrates from a Mississippian wetland: windows into the biogeomorphology of Visean tetrapod habitats (Tyne Limestone Formation, UK)

Records of Carboniferous biogeomorphology and tetrapod distribution are drawn predominantly from Pennsylvanian strata, with Mississippian counterparts more sparsely known. In this paper we describe trace fossils and vegetation-induced sedimentary structures from a Mississippian (Visean) fluvio-deltaic succession (Yoredale Group, UK). These structures are archived on true substrates and as sampled topography, providing a high-resolution glimpse into an early tetrapod habitat. A diverse ichnoassemblage comprises 18 invertebrate ichnospecies in addition to what are likely Britain's earliest tetrapod trackways. Vegetation-induced sedimentary structures include upturned laminae, scour-and-mound bedding and mudstone-filled hollows, which developed as plants mediated sedimentation and erosion. The distribution of both the trace fossil and VISS record is controlled by the abundance of true substrates. We contend that sedimentary landscapes that were primarily stabilized by vegetation promoted true substrate preservation, such that a complex interplay between biogeomorphology and palaeoenvironmental records exists in lower Carboniferous terrestrial successions. This case study demonstrates that an improved consideration of true substrate distribution within sedimentary successions can strengthen understanding of faunal habitats and biogeomorphic interaction.

Implications of active Gavilgarh/Salbardi fault on the evolution of the late Quaternary landscape in Central India

The morphometric indices (MI) have been calculated along the Gavilgarh/Salbardi Fault (G/SF) to investigate active deformation, tectonic uplift, and basin tilt to find out its impact on the late Quaternary landscape. To explore the tectonic intensity, we computed MI such as stream-length gradient index (SL), hypsometric integral (HI), transverse topographic symmetry factor (T), asymmetry factor (Af), basin elongation ratio (Re), mountain front sinuosity (Smf) and valley floor width to valley height ratio (Vf). The necessary field investigations were carried out on the evolution of the Quaternary landscape to ascertain the various morphometric indices. The evaluated values of SL, HI, Re, Smf, and Vf show the high uplift of the northern block exists as opposed to the southern block, and the tilt and asymmetric character of the basins are shown by T and Af values. The presence of sedimentary landscapes like Quaternary alluvium, triangular facets, natural levees, and boulder-pebbly beds, indicates that the area has recently experienced tectonic activity. The assessed values of MI with supporting field evidence proposed that the northern block is uplifted and the basins are tilted because of the effect of G/SF.

The lasting legacy of glacial landscape dynamics: Capturing the transport of boulder armor and hillslope retreat with geochronology in the Flint Hills of Kansas

We present new geochronological data derived from hillslope boulder armor in the Flint Hills in northeastern KS, United States, that provides insights into the rates and timing of lateral retreat in this landscape. Our results show that the surfaces of these limestone boulders date back to the Pleistocene era, well within the last glacial period. We also found that there is a significant increase in the ages of hillslope armor with increasing distance downslope from the modern limestone bench, the source of the boulders. Based on the age-distance relationship of the boulders, we estimate the rate of lateral retreat in this landscape to be 0.02 mm/yr, which falls between the geometrically estimated retreat rates based on calculated denudation rates of the Flint Hills region. We propose that the cooler temperatures and higher effective moisture due to less efficient evapotranspiration during the late Pleistocene period resulted in more effective freeze-thaw and transport processes, such as creep due to soil expansion and contraction. The production and transport of new boulder armor would then have effectively ceased once the climate transitioned to warmer conditions during the Holocene. Our findings suggest that the boulder armor we observe on the soil mantled hillslopes today are relict features from before the LGM-Holocene transition. These results provide important insights into the long-term evolution of these ubiquitous layered sedimentary landscapes.

Transport and Weathering of Large Limestone Blocks on Hillslopes in Heterolithic Sedimentary Landscapes

AbstractGeomorphic transport laws that describe the movement of material across landscapes generally hide the role of lithological and climatic boundary conditions behind proportionality terms. The geomorphic community aims to characterize the role of these boundary conditions and include them in mechanistic transport laws. A more detailed mechanistic understanding is needed for hillslopes formed in heterolithic rock that weathers into weak regolith and hard blocks, such as in hogbacks. The properties of such blocks may determine both the process and rate of their transport downslope. We focused on soil mantled hillslopes in the Flint Hills of Kansas in the United States, where we studied the role of block shape and size under a limestone cliff that breaks up approximately into equal amounts of cubic and tile‐shaped clasts. Based on previous studies on the transport of rock clasts, we hypothesized that cubic and tile‐shaped blocks should be transported differently, leading to different distributions of size and orientation with increasing distance from the cliff. Block shape does have significant influence over how soil collects and is stored on the hillslope near blocks. Yet, few cubic blocks appear to be transported through tumbling in contrast with assumptions in recent modeling work. We found complex relationships between block size and distance from the cliff face and we propose that this is due to weathering via fragmentation. This process produces discrete smaller fragments from a larger parent block. We suggest that at least in Kansas both cubic and tile‐shaped blocks are transported primarily by creep processes.

New Grain Size and Petromagnetic Records from Lacustrine Sediments in the South of Southwestern East Siberia: Application for the Reconstruction of the Regional Environment in the Late Glacial and Holocene

Abstract —Bottom sediments of lakes are regarded as one of the most informative continental facies used to reconstruct environmental changes in lake catchments. They usually contain a high-resolution and continuous record of geologic events, the knowledge of which is important for understanding the evolution of sedimentary processes and landscapes under the influence of climatic changes and the impact of short-lived catastrophic processes. The southwestern Baikal region (the Tunka rift and its East Sayan mountain framing) is replete with lakes of different geneses and morphologies. Recent fieldworks have allowed a multiproxy study of the lacustrine sediments of several lakes of the Oka and Tunka regions. The results demonstrate that there were no glaciers in the foothill plain of the Tunka rift valley earlier than 14,800 cal yr BP. On the Oka plateau, within the East Sayan high-mountain zone, deglaciation started somewhat earlier than 13,800 cal yr BP. The grain size, petromagnetic, and physical properties of the lacustrine sediments point to a general weak hydrodynamics throughout the 13,800 yr sedimentation history in Lake Khikushka. However, a gradual reduction in fine fractions from base to top of the lake section suggests a gradual drawdown of the water level following the change of the water source from glacial melt waters to atmospheric precipitation. Biogenic silt accumulation in Lake Khikushka started parallel with the onset of the Holocene ~11,700 yr BP, suggesting a quick response of the natural environment of the lake basin to major global climatic changes. The short-term episodes of enhanced hydrodynamics in Lake Khikushka ~9000, 8200, 6500, and 2000 yr BP confirm the high sensitivity of the lake geosystem to climatic changes, not only on a millennial but also on a shorter-time scale. The characteristics of the sediments of Lake Engarginskoe are evident of lacustrine-alluvial sedimentation in the reservoir in the Early and Middle Holocene. New data on the grain size and petromagnetic parameters of the lacustrine sediments from two lakes located within different climatic belts of East Sayan show their high potential for a wide range of paleoecological reconstructions.

Evaluation of basin morphometric indices and tectonic implications in sedimentary landscape, Central India: A remote sensing and GIS approach

Evaluation of basin morphometric indices and tectonic implications in sedimentary landscape, Central India: A remote sensing and GIS approach

Nicolaus Steno on solutes and solvents in time-related structural changes of muscles, fossils, landscapes and crystals, his galilean heritage

In Part One of the Specimen of Elements of Myology, a book in three parts published in Florence 1667, Nicolaus Steno described the changes taking place between muscle relaxation and contraction in a two-stage geometrical model based on anatomical observations in man and animals. The ‘new myology’ was rejected by G.A. Borelli in 1680 and outright ridiculed by J. Bernoulli in 1694. The anatomical correctness and predictive value was rightfully acknowledged only towards the Millennium. In Part Two, the Canis …, Steno gave a detailed anatomical description of a giant shark’s head with focus on the likeness of its teeth and the so-called tongue stones, or “glossopetrae”, dug from the ground. Steno conjectured that remnants from sharks living in the past had become fossils due to chemical processes through interaction with the surrounding sediments at the bottom of the sea, presuming that the finding areas had been sea-covered. From studies in Part Three of reproductive organs in mammals, viviparous ray-fish and shark, he concluded that the so-called female testicles in women and mammals are analogous with ovaries of oviparous animals and should therefore be named accordingly. Two years later in the Prodromus De Solido intra Solidum, Steno described the transformation over time of sedimentary landscapes in Tuscany, and how crystals grow by accretion to the surface of entities derived from limpid sea-water or freshwater in caves. These are studies of time-related transformations of solids in organic and inorganic materials. However, such processes could not be documented by visual observation, since changes go too quickly in muscles, in the case of the landscapes because the transformations took place in the past. Thus, Steno and contemporaries put forward hypotheses on such hidden processes that were only gradually corroborated when fitting into a cluster of evidence. His considerations on crystal growth may have been triggered by an interest as a physician to know how saliva, gall and kidney stones are formed. Likewise, considerations on sharks’ replacement of their teeth could extend knowledge on dentition to bring a better cure for those who complain of being toothless. He emphasized the importance of mathematical methods to describe processes in the human body and cited from Galileo, Discourse on Bodies in Water (1612), in which physics outweighs Aristotelian rules to explain the interaction of solids and solvents. Likewise, Steno’s ‘New Myology’ was a showdown against an Aristotelean physical dogma from Physics VII: everything which moves is moved by another, which excluded fibre shortening in muscles and blinded researchers on muscle contraction for generations after Steno.

Unified Model of Sediment Transport Threshold and Rate Across Weak and Intense Subaqueous Bedload, Windblown Sand, and Windblown Snow

AbstractNonsuspended sediment transport (NST) refers to the sediment transport regime in which the flow turbulence is unable to support the weight of transported grains. It occurs in fluvial environments (i.e., driven by a stream of liquid) and in aeolian environments (i.e., wind‐blown) and plays a key role in shaping sedimentary landscapes of planetary bodies. NST is a highly fluctuating physical process because of turbulence, surface inhomogeneities, and variations of grain size and shape and packing geometry. Furthermore, the energy of transported grains varies strongly due to variations of their flow exposure duration since their entrainment from the bed. In spite of such variability, we here propose a deterministic model that represents the entire grain motion, including grains that roll and/or slide along the bed, by a periodic saltation motion with rebound laws that describe an average rebound of a grain after colliding with the bed. The model simultaneously captures laboratory and field measurements and discrete element method (DEM)‐based numerical simulations of the threshold and rate of equilibrium NST within a factor of about 2, unifying weak and intense transport conditions in oil, water, and air (oil only for threshold). The model parameters have not been adjusted to these measurements but determined from independent data sets. Recent DEM‐based numerical simulations (Comola, Gaume, et al., 2019; https://doi.org/10.1029/2019GL082195) suggest that equilibrium aeolian NST on Earth is insensitive to the strength of cohesive bonds between bed grains. Consistently, the model captures cohesive windblown sand and windblown snow conditions despite not explicitly accounting for cohesion.

Estuarine biofilm patterns: Modern analogues for Precambrian self‐organization

Abstract This field and laboratory study examines whether regularly patterned biofilms on present‐day intertidal flats are equivalent to microbially induced bedforms found in geological records dating back to the onset of life on Earth. Algal mats of filamentous Vaucheria species, functionally similar to microbial biofilms, cover the topographic highs of regularly spaced ridge–runnel bedforms. As regular patterning is typically associated with self‐organization processes, indicators of self‐organization are tested and found to support this hypothesis. The measurements suggest that biofilm‐induced sediment trapping and biostabilization enhance bedform relief, strength and multi‐year persistence. This demonstrates the importance of primitive organisms for sedimentary landscape development. Algal‐covered ridges consist of wavy‐crinkly laminated sedimentary deposits that resemble the layered structure of fossil stromatolites and microbially induced sedimentary structures. In addition to layering, both the morphological pattern and the suggested formation mechanism of the recent bedforms are strikingly similar to microbialite strata found in rock records from the Precambrian onwards. This implies that self‐organization was an important morphological process in times when biofilms were the predominant sessile ecosystem. These findings furthermore emphasize that self‐organization dynamics, such as critical transitions invoking ecosystem emergence or collapse, might have been captured in fossil microbialites, influencing their laminae. This notion may be important for paleoenvironmental reconstructions based on such strata. © 2019 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd

The Physics of Sediment Transport Initiation, Cessation, and Entrainment Across Aeolian and Fluvial Environments

AbstractPredicting the morphodynamics of sedimentary landscapes due to fluvial and aeolian flows requires answering the following questions: Is the flow strong enough to initiate sediment transport, is the flow strong enough to sustain sediment transport once initiated, and how much sediment is transported by the flow in the saturated state (i.e., what is the transport capacity)? In the geomorphological and related literature, the widespread consensus has been that the initiation, cessation, and capacity of fluvial transport, and the initiation of aeolian transport, are controlled by fluid entrainment of bed sediment caused by flow forces overcoming local resisting forces, whereas aeolian transport cessation and capacity are controlled by impact entrainment caused by the impacts of transported particles with the bed. Here the physics of sediment transport initiation, cessation, and capacity is reviewed with emphasis on recent consensus‐challenging developments in sediment transport experiments, two‐phase flow modeling, and the incorporation of granular physics' concepts. Highlighted are the similarities between dense granular flows and sediment transport, such as a superslow granular motion known as creeping (which occurs for arbitrarily weak driving flows) and system‐spanning force networks that resist bed sediment entrainment; the roles of the magnitude and duration of turbulent fluctuation events in fluid entrainment; the traditionally overlooked role of particle‐bed impacts in triggering entrainment events in fluvial transport; and the common physical underpinning of transport thresholds across aeolian and fluvial environments. This sheds a new light on the well‐known Shields diagram, where measurements of fluid entrainment thresholds could actually correspond to entrainment‐independent cessation thresholds.

Sedimentary Landscape of Mun Valley Prehistoric Site: Construction History of Moated Site of Ban Non Wat, Northeast Thailand

This research examines the sedimentological evidence of human occupation on different cultural layers at the prehistoric archaeological sites in northeast Thailand. This study focuses on the sedimentological characters of stratigraphic layers identified at the prehistoric occupation sites of Ban Non Wat and Nong Hua Raet, to demonstrate the capacity of such analyses to elucidate the modification of sediments by past anthropogenic activity, and eventually to contribute to an enhanced understanding of the behaviour of ancient people. The primary intention of this paper is, therefore, to point out potential uses of particle analysis in identifying human–landscape interventions, testing whether meaningful differentiation is possible, and if not, whether this may nevertheless be used to understand the sedimentological relationships between different features. The study finds that although there is relatively little differentiation between sediments across the archaeological site, some insight is possible in identifying relationships between the natural sediments of the floodplain, the channels associated with the archaeological sites, and the archaeological sediments themselves. It is, for example, now possible to raise new questions regarding the construction history of the sites, the history of human behaviour at these sites, socio-spatial relationships between paleo-social activity and natural resources, and fine-scale landscape associations between sites.

Barren Meandering Streams in the Modern Toiyabe Basin of Nevada, U.S.A., and Their Relevance To the Study of the Pre-vegetation Rock Record

Abstract Modern meandering streams devoid of plant life have long been ignored despite their relevance to the study of sedimentary landscapes in the long span of Earth's history that preceded the greening of continents. We present a sedimentologic and remote-sensing analysis of the McLeod Springs Wash, a small-scale, endorheic, and barren fluvial fan in the Toiyabe Basin of Nevada, USA. Fluvial channels therein undergo avulsion at the sub-decadal scale, and follow a morphodynamic cycle from low-sinuosity, higher-gradient, and sediment-starved streams into meandering, lower-gradient channels prone to overspilling. Sedimentation is purely clastic and, unlike in other arid-climate endorheic basins, water recharge maintains positive hydrologic budgets year-round such that any contribution to bank strength due to precipitation of evaporites is ruled out. Likewise, the unvegetated nature of the meandering channels excludes any direct biotic forcing on sedimentation, leaving an equilibrium between discharge, gradient, and bank strength aided by cohesive mud fractions as the main mechanism responsible for channel meandering. Point bars compose a volumetrically significant fraction of the McLeod Springs Wash's stratigraphic record, and consist of sub-meter-thick, fining-upward inclined ripple strata floored by intra-formational conglomerate. Their sedimentary record remains non-diagnostic unless morphodynamic analyses based on accretion and paleoflow vectors are integrated. Interpreting river planform from these deposits is complicated by swift channel avulsion and migration relative to alluvial aggradation, which make the preservation of entire channel-bar complexes exceptional. We conclude that a significant bias may exist against the interpretation and volumetric estimation of the pre-vegetation meandering-fluvial rock record, and that the application of classic point-bar models based on inclined heterolithic stratification to Ordovician and older rocks is intrinsically flawed.

Not so fast: promoting invasive species to enhance multifunctionality in a native ecosystem requires strong(er) scrutiny

Since at least the 1980s, ecologists have argued that restoring ecosystem functioning in highly degraded areas is the “acid test” for ecological understanding (Bradshaw 1987). Ecosystem engineers and foundational species are often considered pivotal in the restoration of degraded areas (Suding et al. 2004; Byers et al. 2006), as by definition, they “engineer” biotic structure that serves as habitat. For decades, ecologists have debated when and where we may promote non-native engineers instead of native engineers for restoration. Entering into this long-standing debate, Ramus et al. (2017) reported the results from a field experiment in North Carolina with the Japanese seaweed Gracilaria vermiculophylla and concluded that this and other invasive engineering species should more frequently be considered as candidate species to restore ecosystem function of degraded habitats. Here, we argue that it is premature to suggest we understand the effects of the non-native Gracilaria on the native estuarine system well enough to promote this invader as a lynchpin of restoration efforts. Our argument is fourfold: (1) The net ecosystem effects of Gracilaria remain unknown because Ramus et al. overstated or did not examine the ability of the invasive seaweed to perform key services. (2) The conclusion of enhanced multifunctionality is highly dependent on several subjective, poorly justified decisions regarding the treatment of variables. (3) Contrary to the claim by Ramus et al., the mudflats where Gracilaria resides are not a barren sedimentary landscape without its presence. Finally, (4) Ramus et al. rely on a well-worn “strawman” approach that ignores decades of ecological research. No doubt, there are systems in which non-native engineers benefit local ecosystem functioning, but any recommendation to use a non-native in such a capacity should require careful and thorough evaluation.

河北省崆山白云洞喀斯特景观保护研究

河北省临城崆山白云洞发育在以下古生界寒武系为主的碳酸盐岩地层中，是中国北方罕见的大型喀斯特溶洞。洞穴系统平面上呈近南北向延伸的“P”形，洞域面积约32,550 m2，垂向上可分为三层，上层为大型厅堂状，中层和下层为廊道状，各层洞穴以沿陡倾断裂侵蚀、溶蚀而成的通道相连。白云洞拥有形态各异、成因类型多样的喀斯特景观，其中玉簪对宝瓶、五腿神鹿、网状卷曲石、横天一枝等沉积景观、洞顶和洞壁窝穴等侵蚀景观、金钩倒挂等蚀余景观都具有极高的观赏价值和科学研究价值。然而，白云洞自更新世形成至今，曾经历了多次构造运动破坏，而且随着洞穴环境变化以及近年来旅游人数的大幅度增长，洞穴景观损毁日趋严重。洞体岩石破碎、结构稳定性差、洞内景观劣化等问题已较大程度地制约了当地旅游业发展。因此洞穴景观资源保护和洞穴环境治理是当前面临的重要而紧迫的任务。 Baiyun Cave in Lincheng, Hebei Province formed in the Cambrian-dominated carbonate strata of the Lower Paleozoic Erathem is a rare and unique large karst system in Northern China. The cave system is “P” shaped along the north-south direction with an area of about 32,550 m2. It can be di-vided into three layers in the vertical direction; the caves in the upper layer is a large hall-shaped; the middle and the lower parts are corridors; each part was connected by the eroded-dissolved channels along steep faults. Baiyun Cave has different forms and various types of karst landscape. Many sedimentary landscapes like “hosta to aquarius”, “five-leg deer”, “reticular helictite”, “Heng-tianyizhi” etc. and other erosion landscapes such as the potholes in the roofs and walls of the caves, and eroded remnant landscapes such as “Gold hook upside down”, have excellent ornamental value and scientific research value. However, Baiyun Cave has suffered a number of tectonic movements since its establishment in the Pleistocene. With the environment changes of the cave and the large increase in the number of tourists in recent years, the damage of the cave landscapes has become more and more serious. The problems such as rock crush, poor structural stability and landscape deterioration in the cave and other issues have restricted the development of the local tourism. Therefore, the protection of the cave landscape resource and environmental management of the cave is currently facing an important and urgent challenge.

A century of change in coastal sedimentary landscapes in the Canary Islands (Spain) — Change, processes, and driving forces

A global understanding of landscape dynamics, through local case studies, is a prime topic in land change science. Consequently, this approach should be urgently applied to changing and heavily transformed environments, such as coastal areas. The aim of this work is to study landscape changes, comparatively, by focusing on three coastal sedimentary landscapes in the Canary Islands (Spain). We examine changes in main land cover types and identify their driving forces, actors and institutions, analyze their inter-relationships, and match each case to a case-specific development model. The three landscapes reveal very different change processes with regard to population density, land cover, and major land change processes (i.e., resource extraction, urbanization, tourism, and nature protection). We found that socioeconomic, political, and natural/spatial driving force types are of highest importance, while the local level is dominant in all three cases. The findings of this study can be used to interpret other similar landscapes worldwide. Driving-force and organizational-level patterns may be expressed in a synthesized manner as we demonstrate in this paper.

On the Fundamental Causes of High Environmental Alkalinity (pH ≥ 9): An Assessment of Its Drivers and Global Distribution

AbstractVery alkaline environments exceeding calcite buffering are globally rare but conspicuous in many sedimentary plains of the World. While the deleterious effects of high alkalinity on soils are well understood, less agreement exists on its causes. We revise these causes to understand these exceptional environments and explain the pervasiveness of calcite buffering elsewhere. We argue that the injection of respired CO2 into stagnant hydrological systems subject to evaporative discharge is the key context for high alkalinization. The evolution of evaporites in nature reaches highly alkaline stages only when excess of (bi)carbonate with respect to divalent cations occurs. In most dry landscapes, evaporating groundwater solutions lose this condition as respired inorganic carbon (recharge zone supply) equilibrates with divalent cations from rocks (whole hydro‐trajectory supply). Groundwater in stagnant landscapes avoids this limitation owing to short/shallow trajectories sustaining (bi)carbonate excess until evaporative discharge zones are reached. Flat sedimentary landscapes that are (i) wet enough to develop stagnation and have shallow water tables but (ii) sufficiently dry to expose them to evaporative concentration should host very alkaline soils. This is confirmed with >9,000 soil profiles from the global WISE database, which shows that profiles with pH ≥ 9 in the top meter are 2·7% globally but 18% in areas with low slope (<0·05%, 25‐km radius, SRTM digital elevation model (SRTM DEM)) and semiarid–subhumid climate (annual precipitation to potential evapotranspiration ratio = 0·2–1, CRU database). Understanding how climate and vegetation change as well as irrigation practices influence hydrological stagnation and evaporative concentration may provide the key to manage very alkaline environments. Copyright © 2017 John Wiley & Sons, Ltd.

Not That Flat: Physical Geography of Rugged Sedimentary Landscapes of the Great Plains by Anthony J. Dzik (review)

Not That Flat: Physical Geography of Rugged Sedimentary Landscapes of the Great Plains by Anthony J. Dzik (review)

Land-use and topography shape soil and groundwater salinity in central Argentina

Being one of the oldest and most serious environmental problems, soil and groundwater salinization poses critical challenges for the managing of agricultural and natural areas. Together with climate, topography and land-use are main controls dictating salt accumulation patterns at different spatial scales. In this paper, we quantified the response of salt accumulation to the interactive effects of topography (lowland-upland gradients) and vegetation (annual crops, tree plantations, native grasslands) across a sub-humid sedimentary landscape with shallow groundwater in the Inland Pampas of Argentina. We measured salt stocks from the surface down to the water-table through soil coring and their horizontal distribution through electrical-resistivity imaging in eleven fields occupied by annual crops, eucalyptus plantations and grasslands, encompassing water-table depth gradients of 1–6m below the surface. Land-use and topography exerted strong influences on salinity and explained together 82% and 66% of the spatial variability of groundwater salinity and soil salt accumulation (0–2m of depth), respectively. As a single explanatory variable, land-use overwhelmed topography dictating salinity patterns. Tree plantations stored 7–8 times more salts than croplands and grasslands throughout the unsaturated soil profile in areas with shallow water-tables (<3.5-m depth). As groundwater became shallower, its salinity and that of the unsaturated soil above it increased, although the slope of this relationship was significantly higher in tree plantations. Soil salinity profiles and electrical-resistivity imaging showed maximum salinization around the water-table in tree plantations, indicating that groundwater absorption and solute exclusion by tree roots may be the dominant salinization mechanism. Our study highlights the strong influence of land-use on salinization patterns, which can be even stronger than the more widely recognized controls of climate and topography, and proposes some guidelines for a better use of vegetation to manage hydrology in salt-affected areas. A poor comprehension of this influence, as well as its underlying mechanisms, may lead to incorrect diagnosis of salinization and the implementation of ineffective management actions.

Abrupt watercourse formation in a semiarid sedimentary landscape of central Argentina: the roles of forest clearing, rainfall variability and seismic activity

ABSTRACTIn a semiarid sedimentary catchment of central Argentina, we describe a recent process of landscape dissection, abrupt canyon and watercourse formation and soil salinization. We link these adverse hydrological and geomorphological transformations to three potentially important drivers: precipitation increase, seismic activity and deforestation. Remote sensing imagery in the last 48 years showed an exponential increase in the length of streams, with drainage density values tripling to reach 0.22 km km2. During the same period, forest area declined from 47% to 10%, at the expense of agriculture expansion. A 3.4‐fold expansion of surface water bodies and water table level raise of 0.15 m y−1over the last 35 years was observed. Discharge of a new stream at the middle of the basin ranged between 0.25 and 0.45 m3s−1accompanied by a large and stable load of salts (~0.7 g l−1). Nil recharge and large vadose accumulation of salts in dry forests stands contrasted with recharge rates of ~16 mm y−1and salt‐leached profiles under agriculture. Although the process of landscape dissection occurred during decades of higher than average precipitation, extreme rainfall events and seismic activity were not exceptional in that period. Results suggest that the replacement of forests by annual crops played a more important role, reducing evapotranspiration, triggering the onset of groundwater recharge and favouring subsurface through piping/sapping processes. The abrupt landscape dissection shows no signs of stabilization at the present and may only be ameliorated through changes in vegetation that restore the original non‐flow condition of the forest. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Interacting effects of deposit feeding and tidal resuspension on benthic microalgal community structure and spatial patterns

Benthic microalgae (BMA) are important primary producers constituting an essential component of nearshore food webs. BMA also release extracellular polymeric secretions that sta- bilize sediments. Primary objectives of our research were to assess the separate and interactive effects of biotic and abiotic disturbances on structure and spatial patterns of microalgal communi- ties in shallow-water sediments. Using field comparative studies, we characterized the effects of 2 prominent types of disturbance: macroinvertebrate ingestion and tidal resuspension. BMA bio- mass and composition in fecal materials from the enteropneust Balanoglossus aurantiacus and sediments were followed through time using fluorometry and molecular techniques (PCR-DGGE analysis of 18S rDNA and sequencing). We also examined the effects of disturbance on spatial pat- terns of BMA biomass, diversity, and composition over the larger sedimentary landscape using correlative studies. Deposit-feeder ingestion was found to significantly reduce BMA biomass and alter BMA composition, although qualitative changes were comparatively less remarkable. A sig- nificant difference was also found between average biomass before and after tidal immersion. Spatial autocorrelation revealed heterogeneity in BMA distribution during low tide, and that this spatial patterning correlated with B. aurantiacus fecal coils. Samples taken after tidal immersion showed no patchiness, nor was there a correlation between BMA biomass and pre-immersion fecal cover. In intertidal sediments, the qualitative and quantitative impacts of deposit feeding, and the resulting landscape-scale patchiness, appear to be short-lived due to erasure by frequent tidal resuspension.