Aeolian Accretion Research Articles

New Model of Coastal Evolution in the Ria de Vigo (NW Spain) from MIS2 to Present Day Based on the Aeolian Sedimentary Record

Galician Rias are fluvial valleys that were flooded during the last marine transgression in the Atlantic margin. The study of fossil dunes in the Cies Islands, a small archipelago in the mouth of the one of the rias (Ria de Vigo), allowed us to reconstruct the coastal evolution from the end of the Late Pleistocene to the present day. During this period, sea-level was 100 metres below the present one and the shoreline located about 5–10 kilometres away. About 15,000 years ago, sea-level rise began, radically modifying the coastline. This started with a gradual advance of large dune fields on both sides of the valley. The aeolian accretion continued until the Late Holocene, finishing when the sea reached its present level.

The Usumacinta–Grijalva beach-ridge plain in southern Mexico: a high-resolution archive of river discharge and precipitation

Abstract. The beach-ridge sequence of the Usumacinta–Grijalva delta borders a 300 km long section of the southern Gulf of Mexico coast. With around 500 beach ridges formed in the last 6500 years, the sequence is unsurpassed in the world in terms of numbers of individual ridges preserved, continuity of the record, and temporal resolution. We mapped and dated the most extensively accreted part of the sequence, linking six phases of accretion to river mouth reconfigurations and constraining their ages with 14C and OSL dating. The geomorphological and sedimentological reconstruction relied on lidar data, coring transects, GPR measurements, grain-size analyses, and chemical fingerprinting of volcanic glass and pumice encountered within the beach and dune deposits. We demonstrate that the beach-ridge complex was formed under ample long-term fluvial sediment supply and shorter-term wave- and aeolian-modulated sediment reworking. The abundance of fluvially supplied sand is explained by the presence of easily weatherable Los Chocoyos ignimbrites from the ca. 84 ka eruption of the Atitlán volcano (Guatemala) in the catchment of the Usumacinta River. Autocyclic processes seem responsible for the formation of ridge–swale couplets. Fluctuations in their periodicity (ranging from 6–19 years) are governed by progradation rate, and are therefore not indicative of sea level fluctuations or variability in storm activity. The fine sandy beach ridges are mainly swash built. Ridge elevation, however, is strongly influenced by aeolian accretion during the time the ridge is located next to the beach. Beach-ridge elevation is negatively correlated with progradation rate, which we relate to the variability in sediment supply to the coastal zone, reflecting decadal-scale precipitation changes within the river catchment. In the southern Mexican delta plain, the coastal beach ridges therefore appear to be excellent recorders of hinterland precipitation.

Development of large nebkhas along an accreting macrotidal coastline, Northern France

Abstract Very large isolated nebkhas, up to 4 m high and 14 m wide, have developed on the upper beach of a several hundred meters wide foreshore on the French coast of the Dover Strait. This macrotidal shoreline is characterized by abundant sediment supply from the shoreface related to the onshore welding of a nearshore sand bank. Continuing beach accretion provided the conditions for rapid seaward development of incipient foredunes. The nebkhas developed on the uppermost beach, slightly above the highest astronomical tide limit, forming hemispheric to oval-shaped mounds only reached by spring tides and/or storm surges. They do not form an incipient foredune zone, but mounds that grew vertically and remained in the form of isolated huge nebkhas disconnected from the incipient foredune zone by a swale. The nebkhas consist of scattered landforms that do not impede aeolian accretion landward in the incipient foredune zone where the maximum accretion rates were recorded. The distribution of such large coastal nebkhas is probably limited in beach environments since the descriptions of similar examples of aeolian coastal landforms are virtually absent in the scientific literature, suggesting that they are presumably restricted to low gradient macrotidal beaches associated with an excess of sand supply.

The interaction between the dunes systems and the lower estuary at the Bushmans River Mouth, Eastern Cape over the past 60 years

The morphology of estuary mouths along the Eastern Cape coast changes from year to year and season to season due to natural actions of migrating aeolian dunes, floods and tidal action. However, the Bushmans River Mouth has changed dramatically over the last 60 to 70years, perceivably in an irreversible manner. An account of these changes has been documented from studies of aerial photographs dating back to the 1940s. A study of the dynamics and functions of the coastal dune system on the east bank of the Bushmans River Mouth was conducted in 2003. Surveys of the dunefield describe the sedimentological processes leading to the build-up and migration of a mobile dune. It was found that the factors affecting aeolian accretion at the site are topography, river mouth configuration, dune types and the presence of vegetation as well as the seasonal wind regime. A vegetation survey along transects on the east bank dunefield showed a successional change of plant communities from salt marsh, through dune pioneer, open scrub, closed scrub to a scrub-thicket community. Proposals for the management of the estuary/river mouth/dunefield system have been promulgated to enhance the future recreational use of the estuary mouth area.

MORPHODYNAMICS AND GEOLOGICAL LEGACY OF BERM SCARPS ALONG NON-TIDAL (UKRAINE) AND MICROTIDAL (THE BAHAMAS) COASTS

Berm scarps are erosional features characteristic of all coastal accumulation forms. Their profiles range from steep to overhanging (ψ>90) and evolve through slope adjustment and burial by swash and aeolian accretion. Examples from the mixed siliciclastic-bioclastic beaches of the non-tidal Black Sea coast of Ukraine and the microtidal oolitic-bioclastic carbonate islands of the Bahama Archipelago demonstrate a wide range of scarp morphologies. Truncations of berm strata and density lag at the scarp base accentuate their recognition in the field and in geophysical (georadar) records. Although ephemeral as surface features, berm scarps may act as nucleation sites for aeolian aggradation, thereby generating continuous coast-parallel dune ridges. Accumulation of organic debris promotes colonization of incipient foredunes by plants and provides chronological control of erosional events. Therefore, paleo-berm scarps serve as important geological indicators of past storm activity and aid in constraining sea-level position.

Sediment supply to beaches: Cross‐shore sand transport on the lower shoreface

AbstractMany beaches have been built by an onshore supply of sand from the shoreface, and future long‐term coastal evolution critically depends on cross‐shore sediment exchange between the upper and the lower shorefaces. Even so, cross‐shore sediment supply remains poorly known in quantitative terms and this reduces confidence in predictions of long‐term shoreline change. In this paper, field measurements of suspended sediment load and cross‐shore transport on the lower shoreface are used to derive a model for sediment supply from the lower to the upper shoreface at large spatial and temporal scales. Data collection took place at five different field sites that exhibit a wide range of wave conditions and sediment characteristics. Data analysis shows that both suspended sediment load and cross‐shore sediment transport scale with the grain‐related mobility number which ranged up to ψ ≈ 1000 in the measurements while the effect of orbital velocity skewness is more limited. A 1 year long simulation of sediment transfers between the lower and the upper shorefaces on a natural beach compares well with transport rates estimated from long‐term bar migration patterns and aeolian accretion on the same beach.

A note on contrasting morphodynamics of two beach systems with different backshores, Tetouan coast, northwest Morocco: the role of grain size and human-altered dune morphology

El Mrini, A., Anthony, E.J.,Taaouati, M., Nachite, D. and Maanan, M., 2013. A note oncontrastingmorphodynamics of two beach systems with different backshores, Tetouan coast, northwest Morocco: the role of grain size and humanaltered dune morphology. In: Conley, D.C., Masselink, G., Russell, P.E. and O'Hare, T.J. (eds.)The Mediterranean coast of Tetouan, in Morroco, comprises headland-bound beach-dune systems bordering small coastal plains that have been considerably altered by socio-economic activities and coastal infrastructure essentially related to tourism development. To analyze the response of the beach-dune systems to high-energy winds and storms, three successive surveys covering storm and fair-weather conditions were conducted in February and March 2008 in two sites with contrasting grain-size and morphological characteristics. Ksar Rimal beach is characterized by coarse to very coarse sand and the finer-grained dunes on the backshore have been largely flattened, while Cabo Negro beach exhibits fine sand but the aeolian dunes associated with this beach are still partly intact. The results show that morphological changes are larger and faster on Ksar Rima beach (modally intermediate to reflective) than on Cabo Negro beach (modally intermediate to dissipative). The former beach has a greater tendency to erosion during storms but also shows rapid recovery expressed essentially through berm cut and fill. Mild aeolian accretion seaward of a recently constructed wall on the upper beach also occurred under the influence of strong offshore winds. The changes on Cabo Negro beach were relatively mild and were dominated by dune reworking. Sand transported from the dunes to the beach was reworked into a large swash bar, prelude to landward sand recycling towards the source dunes.

Predicting the thickness and aeolian fraction of soils in upland watersheds of the Mojave Desert

Soil thickness and soil texture, and their spatial variability at hillslope scales, are important inputs for hydrologic models. Recent studies have demonstrated that mechanistic models can accurately predict the spatial distribution of soil thickness in areas where soil thickness varies gradually along hillslope profiles. To date, however, these models have not been tested in arid regions, where soils are often thin and highly variable at small scales (i.e., 1–10m) and where the influence of aeolian accretion on soil thickness and soil texture may be significant. In addition, most of the data available for the fraction of the soil derived from aeolian deposition (which can serve as a proxy for soil texture) originate from soils developed on stable depositional landforms; data for aeolian fractions from upland soils are scarce. In this study, we developed and tested a numerical model for the prediction of thickness and aeolian fraction of soils in arid upland areas. The model assumes a long-term balance between the rates of soil production from bedrock, aeolian accretion, and soil erosion. Rates of soil production are quantified in the model using the exponential and humped soil production functions, and soil erosion rates are quantified using the nonlinear depth- and slope-dependent transport functions, assuming that only the upper soil horizons (A+B) are capable of transport. The rate of aeolian deposition is assumed to be spatially uniform. We calibrated the three free parameters of the model at a site in the Mojave Desert with a granite lithology using a combination of 1) field-measured data for soil thickness and 2) estimation of the aeolian fraction in the soil using laboratory analysis of immobile element concentrations. We validated the model results in three nearby sites using a combination of measured soil thickness in soil pits and the presence/absence of soil, and laboratory analysis of the aeolian fraction of soil. The study area is characterized by generally thin soils (<1m) and extreme variability in soil thickness at small scales. The model does a relatively poor job of predicting the soil thickness on a point-by-point basis due to the extreme small-scale variability, but the model does a much better job at predicting the average soil thickness at the scale of tens of meters and greater. The model predicts soil thickness with a root-mean-square error of 15–32cm in regions characterized by poorly- to moderately-developed calcic soils, but fails to accurately predict soil thickness at a site characterized by well-developed soils (carbonate stages II–IV) and low-relief topography. The model predicts the aeolian fraction and content in soil with root-mean-square errors of 10–13% and 8cm, respectively. The prediction of aeolian content captures the overall pattern of increasing trend in aeolian content towards the lower portions of slopes. Our results show that the model can simultaneously predict the spatial variation of thickness and aeolian fraction/content of soils even in arid, dust-rich environments where soils are thin and spatial variability is extreme.

Effects of beach raking and sand fences on dune dimensions and morphology

Raking beaches to remove wrack (natural and human litter) to enhance recreational use, and using sand fences to build and stabilize dunes are common practices on developed shores, but their effects on the landscape are not well understood. Managers in Avalon, NJ ceased using sand fences and raking in a portion of beach in 1991, providing the opportunity to evaluate evolution of natural and managed dune systems over a 19-year period. Topographic and vegetation characteristics were evaluated on 15 transects within 5 sites in a managed area, where raking and sand fences are still used, and at 15 transects within 5 sites in a naturally evolving (unmanaged) area. Raking and deploying fences in the managed area restricted the size of the cross-shore zone in which aeolian accretion occurred but resulted in greater dune crest heights than in the unmanaged area. Crest heights of the foredune in the managed area ranged from 4.45m to 5.27m. Crest heights in the unmanaged area ranged from 3.02m to 4.27m. Dune volume and beach volume were greater on average in the unmanaged area, and the dune was wider (mean of 72m versus 40m) and had an additional ridge. The unmanaged area had 35% more species and contained a greater percentage of all species encountered in the study area than the managed area (88% compared to 65%). More species occurred in the swales than any other of the cross-shore habitats, and most of these species were observed in the deeper, wider swales in the unmanaged area.Removing wrack from the beach prevents incipient dunes from forming, thus reducing the number of dune crests and size of swales that create topographic variability and increase the number and types of habitats for flora and fauna. Suspension of raking can allow the dunes to build seaward while producing a greater variety and number of habitats, but seaward growth does not mean that the height of the dune will increase substantially, and fences may be needed to build an initial dune ridge for flood protection. Thereafter, a policy of not raking in specially designated zones would be a good compromise between the need to accommodate nature and beach recreation.

Luminescence chronology of late Holocene extreme hydrological events in the upper Penner River basin, South India

AbstractField stratigraphy, sedimentology and optically stimulated luminescence (OSL) dating have been used to reconstruct the southwest monsoon variability in the semi‐arid region of southern India during the late Holocene. Facies architecture and OSL dating of the water‐lain sediment suggest prevalence of a weak hydrological regime around 3 ka. Following this, a progressive strengthening of monsoon occurred till 2 ka. After 2 ka and until 1 ka fluvial activity was nearly dormant, indicating weakening of the monsoon. Presence of high‐magnitude flood deposits, overbank sedimentation and pedogenesis during 1–0.6 ka indicate intensification of the southwest monsoon in the basin. The onset of aridity was associated with episodic storm surge events that are manifested in the pond sedimentation and localised aeolian accretion. This phase is bracketed between 0.5 ka and 0.2 ka. A renewed phase of monsoonal activity was observed in the form of floodplain aggradation between 180 and 90 years ago. In the past 70 years no significant change in the monsoon performance has been observed. Copyright © 2007 John Wiley &amp; Sons, Ltd.

Genesis of pedons with discontinuous argillic horizons in the Holocene loess mantle of the southern Pampean landscape, Argentina

Soil development in the plain landscape of the southern Argentinean Pampa is related to pulses of aeolian accretion of calcareous loess during the Holocene epoch. Such plain relief is associated with landform stability that favors pedogenesis. In some sectors of the Holocene loess mantle, detailed soil surveys show a great variability of soil morphology in short distances (<7 m), such that pedons with Bt horizon (Ap-Bt-C-2Ckm) coexist with pedons with an AC horizon (Ap-AC-C-2Ckm) in a plain landscape, within identical loess parent material over a tosca layer (2Ckm-calcrete-petrocalcic horizon), and in a similar pedoclimate. This article studies the origin of this spatial variation. Loess parent materials directly overlie the relic tosca layer, exhumed after erosion of preexisting soils of the Late Pleistocene. The contrast in soil morphology between the petrocalcic horizon and the overlying Holocene soils reflects the effect of polygenesis. The complex soil spatial distribution pattern over the tosca layer appears unrelated to its paleomicrotopography, because soils with Bt horizons are identified in positive and depressed microlandforms of the tosca. The absence of Bt horizons might be caused by formerly intense biological activity related to a stable pattern of two natural vegetation covers or a surface paleomicrotopography that supported distinct vegetation types depending on the soil moisture in each paleomicrolandform.

Luminescence chronology of aeolian deposition during the Late Quaternary on the southern margin of Thar Desert, India

Aeolian deposits constitute the uppermost lithostratigraphic unit in the southern margin of the Thar desert. They occur as poorly organized dunes resting on the upper part of a major fluvial sequence. Considerable evidence exists to show that the southern margin witnessed fluvial aggradation before the onset of dune accretion. Transition from a dominantly fluvial phase to aeolian deposition was gradational. Blue-green light stimulated luminescence chronometry of six fossil dunes indicates that the dune building activity started around 26 ka and continued episodically until around 5 ka . Evidence of a phase of landscape stability is seen corresponding to the interval 11– 9 ka . This is also reflected by the presence of Stone Age artifacts (microliths) and fluvially reworked aeolian sand, and corresponds to the strengthening of the southwest monsoon in the region. A progressive northward shift through time in the parameters controlling dune building activity is seen. Aeolian accretion in the southern part (Mahi basin) ended around 10 ka , whereas in the northern Sabarmati it continued until around 5 ka and in the case of core regions of Thar, it continued until 0.6 ka . The southern margins of the Thar Desert thus have not experienced any major aeolian deposition since 5 ka .

Le vent et l'eau dans la construction et le démantèlement du plateau des Lœss en Chine (Role of wind and water in the growth and dismantling of the Chinese loess plateau)

Abstract. - The thick loessic sequence of central China is a product of the accumulation of aeolian dust over the last 2.5 millions years marked by climatic contrasts in both temperature and precipitation. In the present monsoonal rainfall regime, degradation of the vegetation cover, due to intensive land use, increases runoff and water infiltration and results in intense gullying and landsliding. The long-term tendency towards landform growth by aeolian accretion changes towards its dismantling by water erosion.

Fibrous clay minerals in the soils of Namaqualand, South Africa: characteristics and formation

Palygorskite- or sepiolite-containing soils occur at elevations up to 1000 m, in gently sloping landscapes formed by aeolian sands or by strongly eroded pedisediments overlying the Namaqualand Metamorphic Complex. The unconsolidated sands are in various stages of calcretization. Most of the soils are sandy, shallow, non-saline, with a pH ranging between 7.2 and 8.2. Carbonate contents are low, but increase towards the calcretized horizons. Most of the carbonates appear in the form of nodules. In the clay fractions, palygorskite or sepiolite dominate and are accompanied by illite and non-expanding 14 Å minerals, or mixed-layer minerals. DTA, TGA and chemical analysis indicate that the sepiolite is Fe-rich and contains organic material trapped within its channels. The highest concentrations of the fibrous clay minerals commonly occur in the bottom horizons. It is postulated that they formed pedogenically from silica released by silicates and Mg derived from Mg-rich carbonates introduced by aeolian accretion. This hypothesis is supported by a decrease in Mg Ca of the carbonates with soil depth. Another possible Mg source considered is sea-spray. Soil-water extract compositions are not compatible with fibrous clay mineral stability. Electron micrographs of sepiolite clays suggest alteration into sheet clay minerals.

Effect of source width and tidal elevation changes on aeolian transport on an estuarine beach

ABSTRACTData from a moderate energy, meso‐tidal beach on the east side of Delaware Bay, New Jersey, USA, revealed the significance of both beach width as a source for aeolian transport and the effect of tidal rise on source width. Wind speeds averaged over 17·1 min, recorded 6 m above the crest of a 0·5 m high dune, ranged from 11·6 to 12·7 m s−1 during the experiment. The highest observed rate of transport on the beach was 0·0085 kg m−1 s−1, monitored at rising low tide when the average wind speed was 11·6 m s−1 across 0·35 mm diameter surface sediments. The wind direction was oblique to the shoreline, creating a source width of 34 m. The reduction in the width of the beach as a source for aeolian transport during rising tide was approximately arithmetic, whereas the reduction in volume of sediment trapped was exponential. Aeolian transport effectively ceased when source width was less than 8 m. Wind conditions, moisture content of the surface sediments and presence of binding salts did not appear to vary dramatically, and no coarse grained lag deposit formed on the surface of the beach. The decrease in rate of sediment trapped through time in the tidal cycle is attributed to differences in source width. Sediment deposited in the litter behind the active beach by strong winds during the rising tide was eroded during the high water period by the high waves and storm surge generated by these winds, and net losses of sediment were observed despite initial aeolian accretion.

Properties and genesis of some soils of Guanxi Province, China

Soils representing a south-north cross-section through Guanxi Province in southeastern China were examined in the field and additional characteristics of 5 pedons determined in the laboratory. In the Wuming area (rainfall 1300 mm yr −1) of the central section, three major soil units were identified: soils associated with Permian limestones, soils formed on Early Tertiary dark-red shales (“Wuming Red Beds”) and soils associated with Pleistocene sediments of lateritic origin. The limestone-derived soils are shallow, slightly acid and have mollic epipedons. They are considered Brown Rendzinas in the Chinese classification and Hapludolls in the American system. The shale-derived soils are clayey and highly acid. They are considered Lateritic Red Earth in China and are close to Dystropepts in the American taxonomy. Soils in Pleistocene sediments are deep, clayey, very acid and contain abundant iron oxide (goethite) concretions. Field evidence indicates that the concretions were inherited from the parent materials. Liming of these soils has raised the pH. All soils of the area contain primarily kaolinite accompanied by minor vermiculite and goethite. The soils of the northern section (Liuchou area) are mildly acid and have argillic horizons. Significant silt-sized components suggest some aeolian accretion. They are classified as Yellow Red Earths in China and as Tropudalfs in the American system. The soils of the southern section (Xing-Chao area, rainfall 2200 mm yr −1) are formed on granite. They are sandy, with argillic horizons, and very acid. They are classified among the Yellowish Lateritic Red Earth subgroup in China and as Tropudults in the American system. Climate (including paleoclimate) and parent material were major soil shaping factors in Guanxi. Anthropogenic effects have also been important.

Overwash occurrence consequent on morphodynamic changes following lagoon outlet closure on a coarse clastic barrier

AbstractThe reduction in sediment volume of an ebb‐tide delta, as a consequence of lagoon outlet closure on a coarse clastic barrier in southeast Ireland, initiated a sequence of beachface and barrier changes downdrift. Elimination of sediment supply to the ebb‐delta caused a cessation of downdrift longshore swash bar welding, and led to beach volume reductions which in turn allowed a temporal sequence of beach and nearshore morphodynamic domains to develop. These domains then controlled the sedimentation regime of the barrier adjacent to the old outlet. A temporal sequence of (a) dissipative barrier; (b) reflective barrier; and (c) inner reflective (barrier face)/outer dissipative (subtidal) wave regimes match respectively periods of (a) barrier crest build up by crestal dune development; (b) barrier crest instability (barrier width increasing, barrier height decreasing) due to rhythmic overwash; and (c) a return to barrier crest stability with limited aeolian accretion. Two barriers at different stages in this sequence are discussed.

Sand movement and vegetation in the Apostle Islands, Lake Superior

Recent sand movement was inferred from grain morphology, modal grain size, and grain size distributional skewness of samples from the surfaces of beach–dune landscapes. Negative skewness indicated recent net deflation, and either positive skewness or small modal grain size indicated recent net aeolian accretion. Large grains that were shiny and angular indicated surfaces that could not have been deposited by wind. Net deflation occurred where vegetational cover was below 10%, in foredunes and an interdunal blowout. All other sites in dunal, interdunal, or forested zones showed evidence of net accretion or a balance of accretion and deflation. Net accretion was evident in locations with as little as 15% cover. Relationships between sand movement and vegetational composition could not be separated from relationships with total cover. The sandy foreland of one island was bulldozed during the 1950s. This apparently brought coarser Holocene beach deposits near the surface, which limited the severity of blowout by providing a deflation armor. Ammophila breviligulata has not recolonized the interdunal zone of this island, though it persists in this zone on the other islands. The study of grains in cores may reveal the history of vegetational cover at a site.

Clay Formation from Basic Volcanic Rocks in a Humid Mediterranean Climate

AbstractClay minerals from soils formed on a variety of basic volcanic rocks in the northern Golan Heights, under humid Mediterranean conditions, have been examined mineralogically. The clay from basalt derived soils, mainly Rhodoxeralfs and Haploxeralfs (Red and Brown Mediterranean Soils) contains kaolinite and dioctahedral vermiculite partially interlayered with Al and Fe hydroxy layers. The presence of some illite and quartz in the upper soil horizons is explained by aeolian accretion. A scoria‐derived Rhodoxeralf contains, in addition, some hematite.The clay fraction from Rhodoxeralfs and Haploxeralfs formed on partly palagonitised tuff‐lapilli is composed primarily of b‐axis disordered kaolinite and a poorly crystalline, partly expanding, 2:1 clay mineral, presumed to represent a smectite randomly interlayered with chloritic layers. In the lower horizons of these soils the kaolinite is replaced by a hydrated, spheroidal halloysite. No distinctly amorphous clay minerals were identified. A scheme of an alteration sequence of basic pyroclastics under humid Mediterranean conditions is suggested.