Active Dunes Research Articles

Drought coincident with aeolian activity in a Great Lakes coastal dune setting during the Algoma Phase (3.1–2.4 ka), southwest Michigan

Aeolian studies of Lake Michigan’s coastal dunes have yet to elucidate what factors control their episodic activity over the past 5000 years. High lake levels exposing sand along with increased storminess is generally accepted for high perched dunes. This hypothesis, however, remains poorly tested for low perched dunes along the southeast Lake Michigan coastline. Here, small lakes in the lee of dune complexes contain aeolian sand and various biological proxies. Age and sedimentation rate models from Gilligan Lake cores guide analysis of aeolian sand, charcoal, pollen, and diatoms at high resolution (1 cm [10.4 yr/cm]) during the high-water Algoma Phase (3.1–2.4 ka) of the upper Great Lakes. The diatoms record a transition from a deep, more acidic lake to a shallower, more alkaline lake with fewer wetlands. This transition is accompanied by a stepped increase in the amount of aeolian sand. There is a weak correlation (R2 0.5, p < 0.01) between increasing abundances of charcoal chunks and sand. Peaks of sand follow peaks in charcoal threads and sheets, suggesting fire played a role in removing vegetation, presaging the landscape for increases in storminess. Arboreal pollen records a transition from a mesic forest Fagus-Acer-Quercus-Abies assemblage suggestive of moister conditions to one richer in mesic hardwoods tolerant of drier conditions. Together, the environmental proxy data record a shallowing lake concomitant with increasing aeolian sand, suggesting that drought-like conditions along the coastline conditioned the landscape for renewed aeolian activity. Once initiated, increased storminess and shoreline erosion maintained dune activity through increased sediment supply.

Resurfaced paleodunes from the Botucatu erg amid Cretaceous Paraná volcanics

Extensive exposure of Cretaceous paleodunes partly covered by basaltic rocks is a rare surficial structure in the geomorphology of continents. The Paraná Basin offers the opportunity in southernmost Brazil to study the relationships between the major Botucatu paleoerg-turned into the Guarani Aquifer (1.2 million km2) and the overlying, first flows from the Paraná Volcanic Province (1 million km2). We describe the Inhanduí Paleodune Field (IPDF; 15,000 km2) preserved by the first two (up to six) flows of the Paraná Volcanic Province. The field was exposed by erosion of volcanic overburden in Fronteira Oeste of southernmost Brazil. A plan view of the field is examined in the extensive grasslands and rice crops of the pampas. Observational geology from satellite images was followed by field description, gammaspectrometry, petrography and whole-rock geochemistry. Results show that bedforms include thousands of varied individual and compound paleodunes, including crescentic (barchane), linear, circle (blowout), and complex forms. Cerro do Jarau was considered an impact crater, and is here characterized as a compound paleodune. The paleoerg sand was silicified and deformed by hot water from the Guarani aquifer and the explosion of vapor in IPDF. The active dunes were flooded with tholeiitic lavas; heating by residual heat from volcanism (raised geothermal gradient) led to sand injection into overlying volcanic rocks. Silicification accompanied and followed the injection process. The Botucatu Formation includes the paleodunes and was the source bed of the sand injectites (Novo Hamburgo Complex) into the overlying volcanic group. We thus uncovered the processes associated with the flooding of a major, active erg by lavas from a large volcanic province. Hyperdry climate prevailed in the sandy Botucatu desert and continued in the rocky Serra Geral desert. The studied geological relationships are unique in the geological records; their understanding fosters the correct description of geological evolution of other terrains where some of the parameters are present.

The main periods and environmental controls of coastal dune development along the west coast of the Korean Peninsula during the mid to late Holocene

This study traces coastal sand dune development processes in East Asia during the Holocene to understand climate and environmental changes. We used optically stimulated luminescence (OSL) dating to examine periods of activation and stabilization of dunes along the western coast of the Korean Peninsula. The results from the Taean Chollipo, Seocheon Dasari, and Gochang Sabanri dunes were then compared with those from previous studies to establish a broad understanding of coastal dune development in this region. Sedimentation rates were calculated for the three dunes, and their development periods were classified through statistical analysis that included data from previous studies. We defined five main periods of late Holocene dune development (<0.1 ka, 0.2–0.5 ka, 1.2–2.0 ka, 2.5–3.2 ka, and 5.5–6.5 ka). These correlated to periods of sea level decrease and global cooling determined by ice rafted debris records. We concluded that the development of coastal dunes in western Korea was influenced by sea level decreases, and global cooling events were enhanced by the East Asian winter monsoon. In addition, human activity increases also contributed to the development of the dunes. This study showed how coastal sand dunes can be evaluated as important indicators of global climate change. These results improved our understanding of how paleoclimate changes affected environmental conditions in East Asia and how coastal dune development periods can be used to reconstruct paleo-environmental conditions.

Measuring Ripple and Dune Migration in Coprates Chasma, Valles Marineris: A Source to Sink Aeolian System on Mars?

AbstractActive aeolian systems are present across the martian surface, with active dune fields commonly found in depressions such as craters and valleys. There are many dune fields within the equatorial region of Mars, including Valles Marineris, but the effects of topography on wind regimes and consequently dune migration in Valles Marineris is poorly understood. We investigated both the ripple and dune migration in a dune field in Coprates Chasma using High Resolution Science Experiment Images (HiRISE) and Context Camera (CTX) images. Migration rates of dune brinks and ripples were measured over varying time scales, between Earth years 2007–2014. The dunes here are some of the tallest on Mars, with heights of up to 182 m. The dune brinks are migrating eastwards at a rate of 0.1–0.3 m/EY through the valley due to topographical influences on the local winds. Potential sediment sources for the dune field were identified and investigated by studying thermal inertia and mineralogy. The topographic slope‐related katabatic winds travel down the valley walls and converge with the dominant winds traveling through the center of the valley, causing overall eastwards dune migration. Topography is likely the dominant control on the local wind regime; slope winds travel down the sides of the valley walls and are funneled through the center of the valley. These local winds subsequently facilitate the migration of the large dunes in Coprates Chasma, thus expanding our understanding of local winds in the Martian environment.

Spatial and temporal changes in vegetation and desertification (1982–2018) and their responses to climate change in the Ulan Buh Desert, Northwest China

To combat desertification, an understanding of its current development is necessary, including that of causative factors associated with climate change and increasing human activity. Whereas, there is an ongoing debate surrounding impact factors for change in vegetation and desertification in arid regions. The present study assesses the spatiotemporal characteristics of vegetation change in the Ulan Buh Desert, northwest China. Driving factors for desertification were discussed using the Global Inventory Monitoring and Modeling System Normalized Difference Vegetation Index (GIMMS NDVI) and SPOT Vegetation NDVI datasets, high-resolution satellite images, and meteorological and socioeconomic data in the period 1982–2018. The results show a trend of overall greening, with increasing trends at rates between 0.0008/year and 0.0013/year indicated by the two NDVI datasets and with a corresponding reduction in dune activity. The spatial variations of two NDVI datasets verified the significantly greening trend in the northern and southern areas of the desert. Human activity was observed to have impacted vegetation dynamics. It is predicted that the implementation of ecological projects will contribute to the restoration of vegetation during a warmer, wetter climate trend. However, the reduction of wind strength played a dominant role in vegetation greenness and dune stability in the Ulan Buh Desert. The present study revealed the spatiotemporal characteristics of vegetation variations and the driving factors for desertification in arid regions.

Temporal–spatial changes in the belowground bud bank in interdune lowlands of an active sand dune ecosystem in northeastern China

Abstract Aims The belowground bud bank plays an important role in vegetation restoration of sand dune ecosystems in semi-arid regions. However, few studies have focused on the temporal–spatial changes of belowground bud banks in interdune lowlands. Methods The size and composition of belowground bud bank in five interdune lowlands with different sizes were investigated for one growing season to determine the temporal and spatial changes in belowground bud bank. Important Findings Total bud bank density was the highest in the medium-sized interdune lowland as was tiller bud density. The density of stem-base buds exhibited an opposite trend while rhizome bud density did not change with interdune lowland size. There was a significant seasonal change in the bud bank size. The total bud density peaked in August and was the lowest in October. A similar trend was found for rhizome bud density, whereas the density of stem-base buds showed an opposite trend, and tiller bud density did not change significantly during the growing season. We conclude that the belowground bud bank density is changed with interdune lowland size and season. These results contribute to the understanding of adaptive strategies of plants growing in active dune ecosystems and provide pointers for adopting effective measures to restore and conserve dune vegetation in semi-arid regions.

Hydraulic dysfunction due to root-exposure-initiated water stress is responsible for the mortality of Salix gordejevii shrubs on the windward slopes of active sand dunes

Sand-fixing shrub Salix gordejevii plays an important role in sand dune stabilization but often shows severe decline and mortality on the windward slopes of active dunes. The objective of this study was to explore the environmental drivers and physiological mechanisms behind its decline and mortality on the windward slopes of active sand dunes. The xylem hydraulics, sap flow, water relations and non-structural carbohydrate reserves were measured for S. gordejevii plants that respectively inhabited interdune, leeward and windward positions of active dunes with contrasting soil water regimes. Root exposure caused by wind erosion, rather than low soil moisture, predisposed windward S. gordejevii plants to stronger water stress. Correspondingly, the windward plants showed reduced transpirational water use and exhibited a suite of acclimative adjustments in leaf traits favoring water conservation. Despite these adjustments, windward S. gordejevii plants still suffered from higher risks of hydraulic failure and branch mortality. No evidence of carbon depletion in windward S. gordejevii plants was found although the soluble sugar to starch ratios changed significantly. Our findings suggest that wind-erosion-induced root exposure triggers water stress and eventually causes the mortality of windward S. gordejevii plants, during which increased risk of hydraulic dysfunction is pronounced.

Seasonal control on coastal dune morphostratigraphy under a monsoon climate, Mui Ne dunefield, SE Vietnam

Having developed in the later Quaternary Period, the Mui Ne dunefield, representing the upper-most portion of a major coastal barrier complex in SE Vietnam, formed by wind regimes associated with the Asian monsoon climate. The barrier complex is 100 km long and higher than 150 m in elevation but despite its large dimensions, research on its geomorphological and geological characterization is limited. Several active dune fields extend 2–10 km landwards from the coast while the rest of the barrier surface is stabilized. We applied ground-penetrating radar (GPR) and quartz optically-stimulated luminescence (OSL) dating to the transverse dunes of the Mui Ne dunefield to characterize the morphostratigraphy and recent aeolian sedimentation history. Currently the dunes have a relief of typically 5–10 m and a wavelength of 50–100 m. They are driven by the northeasterly winter monsoon winds and reversed with minor relief in response to the southwesterly summer monsoon. GPR profiles and OSL ages define three morphostratigraphic units: the modern, Holocene, and Pleistocene dune units. The GPR data of the modern dune unit is dominated by a series of steep reflections that dip exclusively towards the west reflecting the winter monsoon and is punctuated by reactivation surfaces. Evidence for the wind reversal is not preserved in the sediment successions but is represented by reactivation surfaces. Thus a subunit bounded by the reactivation surfaces defines annual deposition. The preservation potential of a subunit varies with the migration rate of the transverse dune, and in the fastest migrating dunes, approximately one subunit was preserved every year (1.1 ± 0.6 subunit/yr). The modern dunes are generally younger than 100 years and have migrated WSW at net rates of 0.6–2.5 m/yr. The Holocene dune unit is as thick as the modern unit although it spans a period, ten times longer, reflecting the lower preservation potential of the transverse dunes as sedimentary records, due to erosion associated with their migration. The rate of dune migration suggests that an aeolian contribution of coastal sands has occurred following the sea level highstand of the Middle Holocene, accounting for the extent of the active dunefields. This mode of the barrier accumulation is considered as an analogue for earlier highstand events. However, the dunefields only cover a fraction of the entire barrier. It is thus inferred that barrier formation occurred over a long period, probably encompassing several glacial-interglacial cycles. Sand accumulation during the lowstand, either by transport of sand from the subaerially exposed shelf and/or reworking of sand emplaced during the prior highstand, may contribute significantly to the barrier development.

Use of Filter Cake Powder for Enhancing Soil Stability of Active Sand Dunes

Filter cake powder (FCP) is a residual material and the main solid waste of sugar production from sugarcane. This material contains high concentrations of polysaccharides and fats. FCP has previously been shown to act as a soil improver. Active sand dunes (ASD) in arid and semiarid regions, cover agriculture fields, and cause significant damage to field crops and livelihood. Therefore, it is necessary to find a treatment which would improve ADS soil stability. Biological soil crusts (biocrusts) are communities of living organisms on the soil surface in arid and semi-arid ecosystems. It was clearly shown that metabolic polysaccharides secreted by biocrust cyanobacteria, glue the soil particles to aggregates which form the crust layer, thereby stabilizing the soil surface. The aim of this study was to investigate the effect of FCP on ASD stability. We hypothesized that the addition of FCP to the ASD surface would enhance soil polysaccharide content, and that this would lead to soil stabilization improvement. The degree of soil stability was determined using penetration resistance soil biocrust (PRSB) measurements. It was found that FCP treatment increased PRSB significantly when compared with control. These results suggest that FCP can be used as a soil stability enhancing agent in ASD.

Sedimentological, mineralogical and geochemical investigation of Essaouira’s coastal dune system: potential origin and provenance

Quaternary coastal sand dunes occur frequently along the Atlantic coast of Morocco and result from the combined effect of river discharge, littoral drift, winds, vegetation, and other meteorological conditions. In this paper, we investigate the textural and compositional parameters of the coastal dune sands of Essaouira across the Ksob riversides, as well as their provenance. Morphologically, they occur as present-day active dunes, Holocene fossil dunes and Pleistocene aeolianite deposits. To investigate the grain provenance, we have opted for laboratory analyses including grain size, mineralogy, microtextures, and micromorphology of quartz grains. Sands from Holocene fossil and active dunes revealed very fine to medium mean grain size and a moderate sorting, while those of the Pleistocene aeolianites showed fine to very fine mean grain size and very poor sorting. Studied dunes showed the dominance of grain shapes varying from sub-rounded to fresh angular. Regarding mineralogy, calcite bioclasts are prevalent in sands of the right bank of the river, while quartz is more abundant in dunes located at the left bank. In the active dunes of the left bank, other carbonates were also detected in bioclasts such as aragonite, dolomite, and magnesian calcite, suggesting a marine provenance. The siliciclastic grains of the studied generations of aeolian dunes are likely originating from Triassic, Jurassic, and Quaternary units crossed by the Ksob River. In fact, the results revealed the presence of minerals such as albite, plagioclase, microcline, and orthoclase which could have been sourced from Triassic basalts and gypsum which is often associated to Jurassic evaporitic rocks. This study allowed to suggest the probable origin of the sands that form these dunes based on the detailed study of their grains, which has not been studied on previous research works of the area. In addition, this research is of particular relevance as it fits into the current research trend related to understanding the evolution of Plio-Quaternary coastal dune systems in the world and could be applicable to other dune systems of the Atlantic coast of Morocco in the future.

Etched linear dunefields of the Nullarbor Plain; A record of Pliocene-Pleistocene wind patterns across southern Australia

The southern Nullarbor Plain is covered by low amplitude (generally 1–2 m) parallel ridges and swales with variable wavelength (300–900 m). The ridges are stony; the swales hold thin deposits of unconsolidated silt and clay. The ridges and swales have previously been attributed to joint control but are not parallel to joints exposed in cave roofs. Instead, they have a striking similarity to central Australian linear sand dunes: they are parallel, regularly spaced and most importantly, fork in a consistent direction. They are here explained as the etched footprints of an extensive linear dune system that once covered much of the southern Nullarbor. Rainwater collecting in the troughs between the sand dunes dissolved the underlying limestone to produce the swales. Recognition of this dune field fills a major gap in the distribution of linear dunes across Australia. The dunes have subsequently disappeared, apart from red aeolian sand in dolines in the southeastern part of the Nullarbor; most of the sand was probably blown offshore.The ridge and swales are oriented predominantly north-south in the west, and northeast-southwest and northwest-southeast in the centre and east, due to deposition by northerly and westerly winds respectively, as shown by directions of forking. The north-south Nullarbor dunes are probably older and may have been deposited in the arid climate of the late Neogene; this would make them the oldest linear dunes in Australia, remnants of an early phase of linear dune activity that has been removed or overprinted elsewhere. The change in wind direction shown by the dune orientation could represent northward migration of sub-tropical high-pressure systems during the late Neogene.

Roles of aboveground vegetation, soil properties, and disturbance in determining belowground bud bank in sand dune ecosystems

The processes whereby vegetation characteristics, soil properties, and disturbance events interact to determine the characteristics of plant bud bank are of considerable significance with respect to population recruitment and vegetation restoration in sand dune ecosystems. However, gaining an understanding of these processes represents a major challenge. In this study, we examined variations in the bud bank demographics of dune vegetation and the role of vegetation, soil properties, and disturbance in determining the size and composition of bud bank during the stabilization processes from active to fixed sand dunes. We investigated bud bank size and composition, vegetation characteristics, and soil properties in the interdune lowlands of active and fixed sand dunes in Inner Mongolia, China. We found that sand dune fixation increased bud bank density. Soil properties were determined to be the primary factors affecting bud bank in the interdune lowlands of an active dune, whereas vegetation density was the most important factor affecting bud bank in the interdune lowlands of a fixed sand dune. Among the different bud types, we found that rhizome and root-sprouting buds were dependent to a greater extent on soil moisture content in the interdune lowlands of active dunes, while in the interdune lowlands of fixed dunes, rhizome and root-sprouting buds had a greater dependence on soil total carbon. In contrast, tiller buds tended to be more related to vegetation density, whereas no evidence showed there was a prominent influence of any factor on collar buds. The mechanisms (vegetation, stress, and disturbance) affecting bud bank demographics are modified by sand dune fixation. The responses of different bud types to environmental factors differ in the interdune lowlands of both active and fixed sand dunes.

On the formation of desert loess

AbstractSequences of quartz-rich coarse (20−63 μm) silt occur in many low- and midlatitude unglaciated arid and semiarid areas and have been termed “desert loess.” The processes by which these deposits are generated have been debated for decades. All hypotheses to explain their origin seek to provide mechanisms for the generation of silt-sized material without glacial grinding, which is the main process involved in the production of coarse silt at high latitudes. Possible mechanisms for the formation of coarse silt in arid regions include derivation from preexisting siltstones, mechanical weathering of silicate rocks, and abrasion of sand grains in active dune environments during intense transport events. Examination of the characteristics of desert loess and field and laboratory experiments to assess the role of dune areas as a source of coarse silt by abrasion and/or resuspension of residual fines suggests that many loess sequences are dominated by locally derived coarse silt. Improvements in the characterization of desert loess particle size, mineralogy, and geochemistry are needed, however, to identify sources and sinks of coarse silt, especially when combined with climatic back-trajectory analysis. Properly scaled experiments and modeling of particle collisions will also help to better quantify the effectiveness of abrasion in the generation of coarse silt in support of field observations.

The conflict between urban planning and the preservation of sand dunes – The case of the Big Dune in the city of Ashdod, Israel

The town of Ashdod in Israel has historical roots from the Bronze Age. During the long history, Ashdod was located on a low hill of aeolianite (lithified sand dune) a couple of kilometers east of the sand dunes, while the sand dunes were active because of strong impact of the residents of the town.Modern Ashdod was founded in 1956 as a port city and grew rapidly to become the 6th biggest city of Israel. The city was built on the sand dunes north-west of the ancient town and was divided into seventeen neighborhoods that were built from 1956 until today. Neighborhood 14 was not built until recently because of a bare and active big dune that is located in the northern part of it and the strong pressure made by public and environmental NGOs to preserve it. During the last few years planners have suggested several plans to preserve the dune and to build around it in such a way that the dune will continue to be accessible to the public.This paper explains how the Big Dune was formed and why it is the only active dune in the area. Eight aerial photographs taken in 38 years were used to calculate the rate of advance of the Big Dune and the potential hazard of this dune to neighborhood 14 and the neighborhood north of the dune. We suggest some solutions to solve this problem and how to reduce the rate of advance. Leaving a shifting sand dune in the city is a risk that should be avoided.

An 800 kyr record of dune emplacement in relationship to high sea level forcing, Cooloola Sand Mass, Queensland, Australia

The Cooloola Sand Mass is a large coastal dune field situated in southeast Queensland, Australia and is part of a much larger system of coastal dune fields, including the world's largest sand Island, Fraser Island, as well as Moreton Island and Stradbroke Island. Cooloola is characterised by a sequence of onlapping, parabolic dune units that have been emplaced episodically over the Pleistocene and Holocene. The tectonically stable coastline of SE Queensland is an ideal area to study the driving mechanisms of coastal dune development, as sea level variability is driven primarily by glacial eustasy. Geomorphic and chronostratigraphic analyses have identified seven major periods of dune activity with the earliest phase of deposition occurring ca. 800 ka. Subsequent periods of dune emplacement date to about ca. 150 ka, 110 ka, 10–6 ka, 5–3.5 ka, ca. 2 ka and 0.4–0.2 ka.The Holocene dune activity coincides with the last stages of post-glacial marine transgression and suggests these large parabolic dunes formed in response to rising sea levels. It is likely that this pattern is consistent through the Pleistocene, and the MIS 5 interglacial period is also recorded. For earlier events the large absolute errors on the ages make interpretation difficult. There is a notable absence of OSL ages dating to the Last Glacial Maximum and no ages that are unambiguously associated with lowstand periods. This is most likely because the coast would have been 60 km offshore of its current position during lowstands, limiting sediment supply and wind energy at the modern coast. These results provide strong evidence that coastal dune fields in SE Australia are predominantly emplaced during sea-level rise, as originally proposed in the Cooper-Thom model.

The response of a dune succession from Lençóis Maranhenses, NE Brazil, to climate changes between MIS 3 and MIS 2

In tropical Southern Hemisphere South America, late Quaternary marine core and speleothem records provide different proxies and accurate chronologies correlating millennial-scale intervals of increased precipitation with southward shifts of the Intertropical Convergence Zone (ITCZ). However, the climatic effect of these intervals on coastal landforms is poorly understood. Here we investigated the response of abrupt and long-lasting climate events during the Marine Isotope Stages 3 and 2 in a 13.8-m thick sand succession located in a large-scale coastal eolian system, the Lençóis Maranhenses Dunefield, NE Brazil, where winds and precipitation are anti-phased and controlled by the single action of the ITCZ. A chronology of the sediment overburden determined by optically stimulated luminescence (OSL) dating of 14 sediment samples obtained at 1-m intervals showed ages between 132.2 ± 7.0 and 12.9 ± 0.6 thousand years ago in stratigraphic order. A multi-proxy approach based on grain size, surface grain texture, heavy minerals, thermoluminescence (TL) sensitivity, inorganic geochemistry, reflectance, and magnetic parameters from 268 sediment samples collected at 5-cm intervals indicated periods of dune building and stabilization. Significant stabilization periods are synchronous with Heinrich Stadials 6, 4, 3, 2, and 1, and Greenland Stadial 4, within age model uncertainties. Heavy mineral analysis indicated a steady sand source to the dune succession, while TL sensitivity analysis of quartz grains showed the input allochthonous sediments during stadials. The preservation of peak dune activity during the Last Glacial Maximum is attributed to the enduring precipitation brought by the subsequent Heinrich Stadial 1 event.

Contrasts in xylem hydraulics and water use underlie the sorting of different sand-fixing shrub species to early and late stages of dune stabilization

Dominating sand-fixing shrubs play crucial roles in the stabilization and rehabilitation of sand dunes. Different sand-fixing shrub species often separate along the temporal-spatial environmental gradient during the sand dune stabilization process; however, the physiological mechanisms underlying such a separation remain poorly understood, which limits our ability to identify the causes of land desertification and the cruxes of rehabilitation. We investigated xylem hydraulics and water use characteristics of four important shrub species used for sand dune fixation projects in northern China that show distinct preferences to different stages of dune stabilization, i.e. two species succeed in active dunes and the other two in fixed dunes. The major aim was to examine the roles of xylem hydraulics, water use and the coordination of these two aspects in determining the habitat preferences of sand-fixing shrubs along the process of dune stabilization in water-limited environments. The two active-dune species consistently exhibited higher stem hydraulic conductivity but lower resistance to drought-induced xylem embolism than the two fixed-dune species, which reflects contrasting requirements to shrub hydraulic functionality in sand dunes of the two successional stages that differ substantially in soil water regimes. In coordination with contrasts in hydraulics, they also diverged clearly in water use strategies with the fixed-dune shrubs showing more conservative water use. Our results highlight the critical roles that hydraulics and water utilization play in determining the adaptation of dominating sand-fixing shrub species to their respective environments shaped by the plant-soil interactions during sand dune vegetation development.

Texture and Composition of Titan's Equatorial Sand Seas Inferred From Cassini SAR Data: Implications for Aeolian Transport and Dune Morphodynamics

AbstractThe texture, composition, and morphology of dunes observed in the equatorial regions of Titan may reflect present and/or past climatic conditions. Determining the physio‐chemical properties and the morphodynamics of Titan's dunes is therefore essential to understanding of the climatic and geological history of the largest moon of Saturn. We quantitatively derived average surface properties of dune and interdune areas (texture and composition) from modeling of the microwave backscattered signal and Monte Carlo inversion of the despeckled Cassini/Synthetic Aperture Radar data over Titan's three largest sand seas: Belet, Shangri‐La, and Fensal. We present the first analysis of the backscatter functions extracted from despeckled Synthetic Aperture Radar images that cover such a large range in incidence angles, including data from the beginning of the Cassini mission up to its Grand Finale. We show that dunes and interdunes have significantly different physical properties. Dunes are found to be more microwave absorbent than interdunes. Additionally, potential secondary bedforms, such as ripples and avalanches, may have been detected, providing potential evidence for currently active dunes and sediment transport. Our modeling shows that the interdunes have multiscale roughnesses with higher dielectric constants than the dunes which have a low dielectric constant consistent with organic sand. The radar brightness of the interdunes can be explained by the presence of a shallow layer of significantly larger organic grains, possibly nonmobilized by the winds. Together, our findings suggest that Titan's sand seas evolve under the current multidirectional wind regimes with dunes that elongate with their crests aligned in the residual drift direction.

Geographical distribution and habitat characterization of Poa schizantha (Poaceae), a narrow endemic of the coastal sand dunes of the southern Pampas, Argentina

AbstractPoa schizantha is a dioecious, psammophilous grass endemic to Atlantic coastal dunes in Argentina. We quantified the distribution area of this rare endemic species and characterized its habitat in the southern Pampas region. Convex hull and α-hull methods were used to estimate and map its overall geographical range. Ecological characterization was based on location, environmental variables, floristic composition and vegetation cover. In total, 117 quadrats were sampled in spring. The convex hull (7553 ha) was split, after excluding discontinuities, into two polygons: area 1 (968 ha; a novel area for the species) and area 2 (449 ha) separated by 36 km, indicating two geographically distinct population groups. In active coastal dune fields, P. schizantha occurs mainly in interdune slacks, associated with herbaceous, short, grassland-type vegetation and moist sandy soils with low vegetation cover, suggesting high habitat specificity. Based on TWINSPAN classification and ordination analysis, P. schizantha is related to two main vegetation groups with 100% constancy and c. 30% relative importance: subgroups E (dominated by pioneer dune species) and F (associated with hygrophilous and ubiquitous species). Given the extent of occurrence (restricted to &lt;100 km2) and the severely fragmented habitat condition, we propose the species be categorized as Critically Endangered, following IUCN criteria. Actions aimed towards the survival of this species must prioritize population monitoring and conservation of its dune habitat.

Quantifying the geomorphologic and urbanization influence on coastal retreat under sea level rise

In response to increasing greenhouse gases emissions, the global climate is undoubtedly changing. As a consequence of rising temperatures, mean sea level also shows an increasing tendency globally, still, uncertainties in relation to its regional specific trends can be identified. Besides that, uncertainties also remain regarding regional and local coastal response to sea level rise. Coastal geomorphology (topography, bathymetry, and sediment texture) plays a relevant role, especially in defining how sediment exchanges occur in the active zone, thus inducing different morphodynamic readjustments. In this context, this study is focused on projecting the future coastline position for the years 2040 and 2100 along three sectors at Hermenegildo Beach, and on investigating the influence of site-specific geomorphological characteristics, urbanization and the presence of hard coastal protection structures on the coastal response under accelerated rates of sea level rise using a stochastic morpho-kinematic model, the Random Shoreface Translation Model. Model outputs as coastal recession distances were submitted to a Kruskal-Wallis test to verify if there were significant differences in coastal recession 1) amongst the three sectors (standard own topography and bathymetry); 2) due to changes in dune topography only; and 3) due to the presence or absence of hard coastal protection structures at the urbanized sector. Overall, the results indicate that the urbanized area presented the highest recession distance amongst the sectors. Differences in dune heights between the northern and southern dune field sectors at Hermenegildo Beach do not significantly influence the mean coastal retreat. On analyzing mean coastal recession results for the urbanized sector, with and without hard coastal protection structures, we conclude that the presence of urbanization and hard structures on the active dune and beach contributed to a maximum increase of 13.52% in mean coastal recession distance and that it significantly (P < 0.01) affects coastline recession in comparison to that in the case of a non-structured dune field for both the time horizons considered (2040, 2100). The results presented here provide a basis for future planning and management at the area, pointing out to the increased erosion risk caused by the existence of an artificially structured shoreline.