Parabolic Dunes Research Articles

Using calibrated surface roughness dating to estimate coastal dune ages at K'gari (Fraser Island) and the Cooloola Sand Mass, Australia

AbstractHere we present a novel application of landscape smoothing with time to generate a detailed chronology of a large and complex dune field. K'gari (Fraser Island) and the Cooloola Sand Mass (CSM) dune fields host thousands of emplaced (relict) and active onlapping parabolic dunes that span 800 000 years in age. While the dune fields have a dating framework, their sheer size (~1930 km2) makes high‐resolution dating of the entire system infeasible. Leveraging newly acquired (n = 8) and previously published (n = 20) optically stimulated luminescence (OSL) ages from K'gari and the CSM, we estimate the age of Holocene dunes by building a surface roughness (σC)–age relationship model. In this study, we defineσCas the standard deviation of topographic curvature for a dune area and we demonstrate an exponential relationship (r2 = 0.942, RMSE = 0.892 ka) betweenσCand timing of dune emplacement on the CSM. This relationship is validated using ages from K'gari. We calculateσCutilizing a 5 m digital elevation model and apply our model to predict the ages of 726 individually delineated Holocene dunes. The timing of dune emplacement events is assessed by plotting cumulative probability density functions derived from both measured and predicted dune ages. We demonstrate that both dune fields had four major phases of dune emplacement, peaking at <0.5, ~1.5, ~4, and ~8.5 ka. We observe that our predicted dune ages did not create or remove major events when compared to the OSL‐dated sequence, but instead reinforced these patterns. Our study highlights thatσC–age modelling can be an easily applied relative or absolute dating tool for dune fields globally. This systematic approach can fill in chronological gaps using only high‐resolution elevation data (3–20 m resolution) and a limited set of dune ages.

High-precision remote sensing mapping of aeolian sand landforms based on deep learning algorithms

Abstract It is significant to adopt deep learning algorithms and higher-resolution remote sensing images in mapping large-scale and high-precision of aeolian landform. In this study, the western part of Horqin Sandy Land was taken as the study area. Based on the data collected from 2,786 verification points located in sandy land and remote sensing images of high-spectral and spatial resolution Sentinel-1, Sentinel-2, and GDEM (V3), this article made a research on data of large-scale and high-precision mapping classification of this area between 2015 and 2020 by using convolutional neural network deep learning algorithm. The results showed that the types of aeolian sandy landform in the west of Horqin Sandy Land mainly include longitudinal dune, flat sandy land, mild undulating sand land, nest-shaped land, parabolic dune, barchan dune, and dune chain, with an area of 1735.62, 51.32, 251.38, 902.07, 49.57, and 101.63 km2. Among them, longitudinal dune, barchan dune, and dune chain have the largest area, while parabolic dunes and flat sand land are smaller. Between 2015 and 2020, the area of aeolian landforms was reduced by 89.27 km2 and transformed into an oasis from a desert. This study adopted remote sensing data by high-resolution Sentinel and GDEM (V3) and convolutional neural network deep learning algorithm to map the aeolian landforms effectively. The precision of aeolian landform classification and Kappa coefficient in the western part of Horqin Sandy Land is as high as 95.51% and 0.8961. Combined with Sentinel-1, Sentinel-2, and GDEM (V3), the deep learning algorithm based on the convolution neural network can timely and effectively monitor the changes of sand dunes, which can be used for large-scale aeolian landforms.

Coriolis effect recorded in Late Pleistocene Marine Isotope Stage 5e Bahamian aeolianites

Abstract The windward islands of the Lucayan Archipelago (Bahamas) form an Atlantic Ocean–facing transect spanning >950 km in length and 6° of latitude. The islands' topography is largely constructed from carbonate wind-blown dunes (i.e., aeolianites) deposited during the interglacial phases of the Late Pleistocene and Holocene. New digital elevation data from satellite radar interferometry (TanDEM-X German Earth observation satellite) enables a step change in the ability to map and quantify Bahamian aeolian landforms across the archipelago. A semi-automated mapping approach that leverages object-based image analysis yields a total aeolianite area of ~1674 km2 across Great Abaco, Eleuthera, Cat, San Salvador, Long, Crooked, Acklins, and Mayaguana islands (Bahamas) and the Turks and Caicos Islands. Longitudinal axis measurements from 747 Pleistocene parabolic dunes record increasing consistency of east-west orientation with decreasing latitude. Three U.S. National Data Buoy Center data buoys provided modern wind direction and velocity measurements (n = 730,933 of each) along this transect. Analysis of wind vectors (>P90 [90th percentile], n = 70,095) demonstrates increasing organization of easterlies at southern latitudes and an offset in directionality compared to formational winds of Pleistocene Marine Isotope Stage (MIS) 5e deposits. Southward trends of increasing wind strength and consistency reflect geostrophic flow driven by atmospheric circulation within the Hadley cell and right-hand deflection of the Coriolis effect in the Northern Hemisphere. We propose that the offset in directionality between dune axes and modern wind vectors is related to changes in latitudinal width of the Hadley cell from the Late Pleistocene (MIS 5e) to today. This data set is robust enough to serve as a benchmark against which future atmospheric circulation models can be compared.

Late Pleistocene and Holocene aeolian activity in the Deliblato Sands, Serbia

AbstractThe Deliblato Sands is among the largest uniform dune fields of Europe, with a very pronounced topography reflecting extensive past aeolian events. Although lacking numerical age data, previous researchers have hypothesized various periods of dune formation. Our research goals were to map the main morphological units of the Deliblato Sands, and to provide the first optically stimulated luminescence (OSL) ages for the major dune types. Mapping was carried out using digital elevation models, satellite images, and GPS profiles. Dune development was investigated using OSL. Several tests were performed concerning thermal treatment, signal characteristics, dose recovery, and dose distributions to assess the suitability of sediments for luminescence dating. Based on our results, two dune generations could be identified that differed in morphology and age. Older dune forms are primarily low sand-supply, hairpin-like parabolic dunes that developed from the last glacial maximum until the end of the early Holocene, then became stabilized. Younger, superimposed parabolic dunes record an intensive aeolian signal from the eighteenth and nineteenth centuries. The history of the Deliblato Sands fits with those from other European sand dune areas, and provides further details to understand paleoenvironmental changes in the region.

Blowing the rivers: Regional-scale control of the drainage network by wind in northern Kalahari (Africa)

Understanding the impact of aeolian processes on the regional-scale dynamics of fluvial systems in semi-arid regions is a major issue in the framework of climate change. Although the interactions between aeolian and fluvial processes have been studied on a meter to kilometer scale, few studies have upscaled to the regional scale. Using satellite images and SRTM-1 digital elevation models we document the formation of parabolic dunes and foredunes along the main rivers of the southern Angolan Plateau and northern Kalahari Basin (Africa). We demonstrate that these aeolian features derive from interactions between the morphology of the drainage system, the wind pattern, the vegetation cover and the transport capacity of the rivers. As a retro-action phenomenon, the progradation of parabolic dune fields induces tens-of-kilometers shifting in rivers courses in the northern Kalahari basin. The mechanism proposed has implications in terms of fluvial dynamics, river capture, geometry of clastic deposits in basins or source-to-sink budget.

Continental interior parabolic dunes as a potential proxy for past climates

The global occurrence of parabolic dunes and their reactivation potential make them significant features in the present global landscape. Several studies have assessed the distribution and structure of parabolic dunes and their relationship to local climate factors with the aim to constrain the processes influencing their activation and potential reactivation. However, none of them have addressed the connection between recently active inland parabolic dunes and climatic conditions at a global scale, nor their potential as proxy for past climates. Here we report that the activity of 106 continental interior parabolic dunes from the last millennium is largely controlled by few quantifiable climate variables that are mutually influenced: aridity, type of vegetation, and wind speed. We then compare the mid-Holocene activity of 67 continental dunes with the palaeolevel of 394 lakes to confirm their relationship to past aridification, itself triggered by a northward displacement of the Intertropical Converge Zone. This study provides a quantitative tool to constrain past climates in regions where inland parabolic dunes have shown to be active, especially where other proxies are not available; along with linking continental parabolic dune activity with atmospheric circulation.

Demarcation of Coastal Dune Morphology and Dune Patterns using Geospatial Models: A Case Study from Manapad Coastal Stretch, Tamil Nadu, South India

Abstract Coastal sand dunes are the dynamic depositional landform which are formed near the coast, due to aeolian activity. Coastal sand dune acts as a natural barrier against coastal erosion hence its periodic monitoring is essential. Modification of this landform due to anthropogenic activities was observed in the coastal stretch towards the south of Tamil Nadu, even though these dune ecosystems were meant to be conserved and preserved in the coastal zone regulation notification. Present study, focuses on finding key techniques required to demarcate the morphological parameters such as ridge line, crest portion, toe, dune orientation, and its pattern from the open source ALOS World 3D (AW3D) DEM for periodic monitoring of coastal dune ecosystems. Topographic models such as hill shading, slope, aspect, curvature, and hybrid techniques were performed. The outcomes obtained from topographic models were validated with spatial profiling and field survey. From the study it is observed that, hybrid techniques particularly slope overlaid on hill shade, slope, gaussian stretched hill shade image, and spatial profiling are the key techniques to bring out the morphological variations and the parabolic dune patterns, which were hitherto not reported in earlier literatures. Gaussian stretched hill shade image enhanced the micro landforms which were difficult to extract from normal hill shading. The slope image of the study is found to be more informative in exhibiting the minor dunes and patterns compared to shaded relief image. The ridge portion of the dune, crest and limb portion and coastal plains were differentiated based on the slope value ranges. The hybrid image formed by overlaying slope and curvature data on hill-shaded image is more informative in providing most of the parabolic patterns formed by micro and meso scale dunes. Spatial profile could be used as an additional tool to validate the information’s derived from topographic models, wherever there is limitation in field accessibility.

Formation of parabolic dunes on the shore of Poyang Lake in East China

Parabolic dunes can transform from and into other types of dunes in response to environmental changes. The sand hills, including some parabolic dunes, on the shore of Poyang Lake mainly accumulated during the Last Glaciation and are rare inland aeolian landforms in the humid monsoon climate zone in East China. However, the characteristics, distribution and formation of the parabolic dunes in this region are still not clear. Three sand hills with similar scales and wind regimes at Shaling, Jishan and Songmenshan were selected as the research targets. Integrating topographical, hydrological, vegetation and wind data, we found that the development of the parabolic dunes was effectively controlled by both the vegetation cover of the sand hills and the space available downwind of the sand hills. No parabolic dunes were found at Songmenshan and Shaling; the former has too little vegetation cover, favouring the formation of blowouts, and the latter lacks available space since Shaling is surrounded by villages and farmlands. Three types of parabolic dunes that developed sequentially in the downwind direction, namely, type A, type B and type C dunes, are observed at Jishan, and their formation is related to a narrowly unimodal and high-energy wind regime. Different from existing models, type A dunes are found to originate from the wedge-shaped depositional lobes of trough blowouts; type B dunes transform from type A dunes after migration; and type C dunes develop from very low sand drifts. Composite parabolic dunes are formed when the sand supply is plentiful, while simple parabolic dunes appear when the sand supply is limited. The formation of the parabolic dunes occurred approximately 250 years ago, which is close to the documented time when the sand hills were reactivated due to the vegetation being anthropogenically damaged. Therefore, anthropogenic destruction of vegetation is a trigger for producing parabolic dunes in this humid area. This study may shed light on the evolution of aeolian landforms in humid monsoon climate zones.

Waxing and waning slacks: The changing ecohydrology of interdunal wetlands/slacks in a Lake Michigan coastal dune complex during rising Lake Michigan-Huron levels

Since 2016 we have studied the largest interdunal wetlands/slack lying within a deflated parabolic dune east of Lake Michigan. Geologic cross-sections show ∼ 15 m of sand and gravel beneath the dunes, creating an aquifer hydraulically connecting Lake Michigan-Huron (MH) with the water table/shallow groundwater influencing the slack. Lake Michigan-Huron (MH) water levels have risen ∼ 1 m from 2016 to 2020, increasing water levels within and around the slack ∼ 1 m. Color-infrared images and vegetation quadrat sampling show water appearing, then significantly expanding with the main slack and upland/dune vegetation transitioning to wetland vegetation in response to this rise. Monitoring well data show slack water levels rise in spring as Lake MH rises. Levels drop as the growing season begins while Lake MH continues to rise through summer. Short-term slack water level increases occur due to local rain events, but significant water level declines follow due to evapotranspiration. Slack water levels begin to rise again in late summer and into fall as the end of the growing season arrives, evapotranspiration decreases, and heavier, more frequent rain events occur. Together, these factors push slack water levels to their highest point of the year while Lake MH levels are decreasing. In late fall–winter, slack water levels drop in concert with Lake MH levels. Climate change effects, increased transpiration from higher temperatures, summer drought, and greater variability in lake level fluctuations, may make it more difficult to maintain wet growing conditions for hydrophytic vegetation. Hence, climate change poses risks to the existence of this imperiled ecosystem.

Ecogeomorphological Transformations of Aeolian Form—The Case of a Parabolic Dune, Poland

The range of natural environmental degradation caused by anthropogenic activity may include geomorphological forms such as dunes resulting from the build-up activity of the wind. In effect, such environmental transformation affects changes connected not only with their relief, but also with the presence and health of diverse plant and animal inhabitants. The subject of the survey was a parabolic dune with asymmetric shape, the sand of which was subjected to exploitation over many years. Terrain data acquired by means of GNSS (Global Navigation Satellite Systems) served to elaborate the present relief of the surveyed dune and to reconstruct its primary relief. These were mainly places where the impacts of human activities were recorded. For this purpose, ordinary kriging (OK) estimation was performed. Simultaneously, satellite data and UAV (Unmanned Aerial Vehicle) imaging were acquired, and subjected to image fusion in order to acquire near infrared bands (NIR), red, green, blue in high spatial resolution. These in turn were applied so as to estimate the condition of the vegetation overplanting the dune and surrounding terrain. The correctness of the modelling was verified by cross-validation (CV), which disclosed low error values. Such values in present and primary relief were, respectively, mean error (ME) at −0.009 and −0.014, root mean square error (RMSE) at 0.564 and 0.304 and root mean square standardised error (RMSSE) at 0.999 and 1.077. Image fusion, with use of pansharpening allowed a colour-infrared composition (CIR) and a Modified Chlorophyll Absorption in Reflectance Index 1 (MCARI1) to be obtained. Their analysis disclosed that vegetation on the dune is characterised by worse health condition as compared with the surrounding area thereof. The proposed approach enabled the environmental condition of the surveyed dune to be analysed, and thereby it allows for a determination of the consequences of further uncontrolled sand recovery without taking into account the historical cartographic materials customarily considered to be the main source of information.

Physiochemical Characteristics, Provenance, and Dynamics of Sand Dunes in the Arid Hexi Corridor

Dynamic changes of aeolian landforms under changing environments in a middle-latitude desert belt is a typical problem of climate change and related landscape response. It need a comprehensive understanding of the formation mechanisms of dune landforms with the supply of material suitable for aeolian transport and favorable conditions of sediment availability and wind regimes in the region. Based on comprehensive evidences from geomorphological, sedimentological, geochemical, and hydrological analysis, this study discussed the dynamical changes of different dune landforms during the past half century and their provenance in the Hexi Corridor, China. The results show that there are two states of sand dunes movement in the Hexi Corridor in the past half century, dynamic migration and basically stable. The crescent-shaped dunes move the fastest, followed by the chains of barchan dunes. Only the top of the pyramid dunes wigwags, while the parabolic dunes and the longitudinal dunes hardly move forward. The moving speed of sand dunes is positively correlated with the wind speed ≥5 m/s at a yearly scale. The grain size of sand dunes in the western Hexi Corridor is coarser than that in the central-eastern part, and also larger than those in other deserts of northern China and of the world. Different motion modes of saltation, suspension, and creeping are identified between aeolian, alluvial/fluvial and gobi sediments. Dune sands are mainly “sediments ofin-siturising” that originated from alluvial/fluvial/lacustrine deposits of ancient rivers, lakes, and aeolian deposits in the erosion zone of the forelands of the Qilian and Beishan Mountains and the north-neighboring deserts. This reveals a significance interaction between wind and water dynamics in the formation and evolution of aeolian landforms in the arid study area. Sufficient transport capacity is evidenced for both the western and eastern parts of the Hexi Corridor, sufficient sand supply and sand availability, however, is the favorable factor for dune formation in the east part but is the limiting factor for the west.

Spatio-temporal variability monitoring of the floods in the center-west of the Buenos Aires Province (Argentina) using remote sensing techniques

Floods are recurring events in Buenos Aires Province, mainly caused by high-intensity precipitation, and in some cases, they persist for long periods. The objective of this study is to analyze how the topography of the central-western portion of Buenos Aires Province determines the occurrence and the extent of floods. The presence of shallow lakes characterizes the area which is controlled by the topography. LANDSAT 5 and 8 images corresponding to periods of drought and flooding in the area were compared with topographic profiles and digital elevation models generated from SRTM 3 arc/sec data. The results showed that in those areas where there are aligned dunes, and after significant storm drain engineering works, during the wet periods, the lakes, their overflows, and the subsequent runoff to the east of Buenos Aires Province flow freely, limiting the flood areas. Due to this topography, the lakes became a more permanent feature of the landscape, even during extreme drought events. In contrast, in those areas where parabolic dunes predominate, the landscape is more affected by droughts and floods. During drought events, we observed a small number of shallow lakes, and during strong storms with high-intensity rainfall, the number of lakes increases and large flooded areas generate damage to agricultural fields and neighboring cities, either due to natural causes or improvised and illegal storm drains. Early warning of flood risks and a systematic territorial ordaining would be the key to the management of the area.

Degradation of a transgressive coastal Dunefield by pines plantation and strategies for recuperation (Lagoa Do Peixe National Park, Southern Brazil)

The transgressive dunefield of the Lagoa de Peixe Natural Park was modified drastically due to intensive pine plantation and the leeward development of the associated degraded areas. The present study analyzed the disturbances in the dunefield of the Lagoa do Peixe Natural Park due to pine tree plantation and the impact of the subsequent deforestation program conducted in the region. Aerial photographs, satellite images, and DGPS topographic data collected over a 70-year-long period were retrieved, analyzed, and compared, which allowed the observation of the geomorphological evolution of the dunefield. In addition, a profile of GPR on the inlet (during a period with the channel closed) was analyzed. In 1948, the surface of the sand barrier was occupied by high transverse dunes and low barchan dunes. Pine tree plantation on the inner side fixed the transgressive dunes and, consequently, avoided the filling of the shallow lagoon, although degraded areas were generated on the lee side of the pines. Simultaneous pine plantations in the backshore avoided the aeolian sediment input to the dunefield, generating a large interdune area along with the development of a few parabolic dunes, which resulted in cannibalization of the transgressive dunes. In 2001, pine trees occupied 15.03% of the total area analyzed in the present study, while the degraded area accounted for 10.81% of the total area. Progressive deforestation was performed (ring bound technique for tree gradual death), maintaining three lines of pines in contact with the dunes, to promote autochthonous vegetation growth, thereby preventing the filling of the adjacent lagoon with aeolian sediments. By the year 2018, the pine tree plantation area reduced to 3.25%, the dunefield area was 79.03%, and the extension of the degraded areas had increased and reached 17.71% of the total area. The pine tree plantation and the deforestation for conservation purposes are the main factors explaining the degradation of the dunefield during the period between 1948 and 2018, while regional climatic oscillations contributed as the secondary factor. Although internationally controversial, the present case study demonstrates that the removal of this exotic vegetation, through dune vegetation recovery programs, is often unsuccessful and may generate more degraded areas. However, in the case presented here, it was essential to remove the forest to ensure the dune field geodynamics and, therefore, the base (biotope) of the natural system (maintenance of the lagoon and the dunefield).

Holocene evolution of a barrier-spit complex and the interaction of tidal and wave processes, Inskip Peninsula, SE Queensland, Australia

This paper presents a reconstruction of the Holocene evolution of the Inskip Peninsula in SE Queensland. The peninsula links two major dune fields, the Cooloola Sand Mass to the south and K’gari (Fraser Island) to the north. Geomorphic features of this peninsula include remnant parabolic dunes, numerous beach ridges with foredunes, and a series of spits. Together these features provide insight into Holocene coastal evolution and changing marine conditions. A remnant beach ridge/foredune complex at the northern portion of Inskip may have been connected to K’gari and a river/tidal channel near Rainbow Beach township which separated it from the Cooloola Sand Mass to the south. This channel avulsed northward in the early mid-Holocene (after 8.8 ka) with spit development from the south. This was followed by a phase of beach-ridge/foredune complex development that started by ~6.7 ka. Stratigraphic evidence from the highest and best developed parabolic dunes in the northern portion of Inskip Peninsula indicates dune development from the mid-Holocene beach complex by 4.8 ka. Beach ridges with foredunes continued to prograde but notably declined in size during the late-Holocene. In the latest Holocene (<4.8 ka) many of the late-Holocene beach ridges/foredune complexes have been truncated by a re-orientation of the shoreline and longshore sediment transport has promoted the growth of the modern spit at the northern end of the peninsula. Erosive and longshore processes continue to be highly active because of tidal interactions between Great Sandy Strait and the Coral Sea. This detailed study of Inskip Peninsula’s evolution aids significantly in future coastal management decisions, and provides evidence for World Heritage Area extension for the Cooloola Sand Mass, including the incorporation of Inskip Peninsula itself. It also contributes to the global understanding to coastal evolution in an area of strong wave and tidal interaction.

Late Pleistocene Climate Variation On The Khorat Plateau, Northeastern Thailand Inferred From The Remnants Of Sand Dunes

The preservation of terrestrial dune field in a tropical region is rare and relies significantly on the degree of weathering process, humidity and anthropogenic condition. In this paper, we report the remnants of sand dunes that is uncovered from Thungkula Ronghai (TKR) dune field in the southern part of the Khorat Plateau, northeastern Thailand. We reveal, for the first time, the results of systematic geomorphological, sedimentological and chronological analyzes of barchanoid ridges and parabolic dunes found on terraces of the Mun and the Chi Rivers. Interpretation in a series of 1952 aerial photographs, satellite images coupled with Ground Penetrating Radar (GPR) survey, and optically stimulated luminescence (OSL) dating were applied. As a result, we found remnants of mega-barchanoid ridges and isolated parabolic dunes varying in height from 1-2 m with maximum length of 4 km, locally distributed in between terraces of the Mun and Chi Rivers, the middle to eastern part of TKR. Dune shapes include lobate, en-echelon and elongate partially overlying on crevasse sand splay, meandered scar, paleo-channel, mid-channel bar of the fluvial depositional sequences. Orientation of all dunes is in NW-SE direction reflecting the formation was due to the prevailing NW monsoon wind. Transition from barchanoid ridge to parabolic dune was observed. Preliminary OSL dating reveals the deposition of sand dunes occurred between 45 to 28 ka. This age range can be inferred to a warmer and drier period occurred in Marine Isotope Stage 3 (MIS3) before the Last Glacial Maximum.

Fluvial aggradation and incision in the Brazilian tropical semi-arid: Climate-controlled landscape evolution of the São Francisco River

Large rivers are dynamic systems whose evolution depends on both internal and external forcing, particularly tectonics, sea level, and climate. Associating fluvial responses to a specific driver is a complex task that has been debated for a long time. Thus, rivers that flow exclusively under tectonically stable areas and without direct influence of relative sea level changes are suitable targets to understand how large fluvial systems responded to past climate changes. The São Francisco River is one of the largest cratonic rivers across South America, and its late Quaternary sedimentary deposits record the fluvial landscape evolution in a thousand-year timescale. The São Francisco River flows northward over different climate zones, with its upper course in a semi-humid setting, but with most of its watershed under semi-arid conditions. To understand the controls on sediment erosion, transport, and storage from uplands to lowlands, we investigated a 200-km section of the medium course of the São Francisco River in northeast Brazil. Several geomorphological zones were characterized, mapped, and dated by optically stimulated luminescence (OSL). Two zones are represented by degraded terraces with lakes, but no preserved alluvial features: (zone 1) high-level terrace (87.7 ± 12.7 ka) and (zone 2) low-level terrace (65.5 ± 5.3 to 39.3 ± 4.3 ka). Three zones comprise the active confined aggradational plain, with features such as scroll bars and abandoned channels: (zone 3) older meander belt (18.1 ± 1.6 ka); (zone 4) young meander belt (15.5 ± 1.5 to 9.5 ± 1.0 ka), and (zone 5) modern channel belt (0.4 ± 0.1 to 0.3 ± 0.1 ka). Zone 6 comprises an eolian dune field composed of parabolic dunes with two phases of active sedimentation (45.1 ± 5.2 to 25.5 ± 4.4 ka and 14.3 ± 2.6 to 5.2 ± 1.4 ka). Sediment deposition ages allowed the recognition of at least four phases of fluvial aggradation (⁓90 ka; ⁓66 to 39 ka; ⁓18 to 9 ka and ⁓0.3 ka to recent), three phases of incision (⁓85 to 66 ka; ⁓39 to 18 ka and ⁓9 to 1 ka), and two phases of dune field stabilization (⁓25 to 15 ka and ⁓5 ka to recent). Development of the eolian dune fields occurred during drier conditions, when the inland activity of trade winds reworked sediments deposited on the fluvial plain. We interpret the incision events as having been set in motion by an increase of fluvial discharge in the upper catchment area, produced by rainfall intensification due to activity of the South Atlantic Convergence Zone (SACZ). The aggradation and incision phases on the São Francisco River during the last 100 ka are therefore likely controlled by multi-millennial precipitation changes, possibly related to precession cycles. The events of high sedimentation rate in the São Francisco river mouth are partially correlated with incision phases in its middle course. This suggests that sedimentation in plains of large plateau rivers can be decoupled from the coastal area.

Upper Pleistocene parabolic ridges (i.e. ‘chevrons’) from the Bahamas: Storm‐wave sediments or aeolian deposits? A quantitative approach

AbstractThe Upper Pleistocene (Marine Isotope Stage 5e; ca 120 ka) stratigraphic record from the Bahamas comprises large, kilometre‐long parabolic ridges of oolitic composition, that point landward, and have been up to now called ‘chevrons’. A debate about their genesis has led previous researchers to consider two processes of deposition: (i) a catastrophic event involving giant storm‐generated waves produced by specific climatic conditions at the end of Marine Isotope Stage 5e; and (ii) a more uniformitarian process which characterizes ‘chevrons’ as aeolian parabolic dunes because of their similar morphology. Since there are few unequivocal sedimentological criteria to discriminate aeolian from water‐deposited sediments, only a quantitative, multi‐method approach could provide enough evidence to produce a viable diagnosis on the genetic processes involved. Following this reasoning, the quantification of the morphological parameters of ‘chevrons’, a precise study of their sedimentary structures on previously and newly discovered sections, and several statistical grain‐size analyses, all advocate for an aeolian origin. Moreover, when the aforementioned characteristics of ‘chevrons’ are compared with those of storm deposits (for example, washovers) and parabolic dunes occurring elsewhere on Earth, the dissimilarity with the former and the resemblance with the latter is evident. Amino‐acid racemization dating, together with stratigraphic and petrographic investigations, constrain the age of the ‘chevrons’ to the late part of Marine Isotope Stage 5e. Their occurrence during this specific time interval can be explained by both strengthened easterly winds and drier climatic conditions associated with changing vegetation cover. Fixation of the arms by sparse vegetation, coupled with the loose ‘chevron’ nose sediment migrating farther inland, form the peculiar U‐shaped morphology of these ridges.

Assessment of stability of an exposed microtidal beach, Western Tasmania

Beach erosion is an increasing threat to the natural resources of many countries, and is expected to increase with rising sea-level. West Tasmania has extensive fetch from the circum-polar Antarctic Ocean, also located in the “Roaring Forties” latitudes. Beaches of this coastline previously studied have shown major erosion, attributed to recent relative sea-level rise as they are largely secluded from human impacts. This study assessed Four Mile Beach, located in the central section of the west coast using methods of spatial analysis 1952–2011, beach profile survey and sediment analysis. Results showed only minor erosion of the vegetation line to the north, and stability and progradation in the central and southern extents of the beach. Blowouts showed more major changes, with three large blowouts towards the south becoming parabolic dunes, and one in the north re-vegetating. Dune sediment was mostly rounded fine sand indicative of active aeolian processes, while seaward edge sediment was sub-angular with a 15–20% of carbonate content on most transects, indicative of a sediment supply from offshore. Ongoing coastal stability of this pristine coastline would be facilitated by continued monitoring, management of any potential human impacts, and control of any potential invasive species that may affect sediment budgets.

Morphodynamics of short-lived wetlands of coastal dune slacks, Manawatū, New Zealand

While coastal sand dunes are common in New Zealand, dune slack wetlands are one of the country's most rare and threatened ecosystems, containing a number of rare turf species of seasonally aquatic habitats. To understand changes in such wetlands, historical aerial images were analysed for three study sites along the prograding Manawatū coast and mapped using GIS. Morphometrics, times of origin, turnover and longevity of wetlands were derived. Dune slack wetlands are located amongst depressions in parabolic dune fields, about every 250 m along the coast, have a grand mean of 6130 m2 for area of open water, with a maximum of 8 ha, and are 2.3 times longer than wide, with the long axis mostly oriented 113 degrees from the strandline. They maintain seasonally wet areas for an average of 20–30 years, before being gradually infilled by aeolian sand. New wetlands can recur at the same place as previous wetlands, but adjacent plantations of exotic pines appear to alter their morphodynamics. These results indicate that Manawatū's dune slack wetlands form very dynamic, short-lived ecosystems, which may be internationally unique. They need to be better understood to formulate adequate management initiatives, essential for the perpetuation of the rare plant communities that inhabit them.

Mapping and chronology of coversands and dunes from the Aquitaine basin, southwest France

New stratigraphic (including GPR profiles) and chronological data provide a detailed understanding of the emplacement of aeolian sands in the Aquitaine basin (SW France). The main phase of coversand deposition ranges from 25 ka to 14 ka. The ages cluster around two periods that match Heinrich events (He-2 and He-1) and reflect increased sedimentation in response to aridification and high mean wind speeds. The age distribution is skewed toward recent phases, which likely results from continuous sand recycling. The dunes currently visible in the landscape were formed during the Younger Dryas/very early Holocene and the historical period. The Younger Dryas dunes are inland low parabolic dunes that have been strongly degraded. Only the last phases of the progression of the coastal dunefield are preserved because of coastal erosion. Two depositional phases have been identified, the Dark Ages Cold Period and the Little Ice Age, which correspond to phases of increased storminess already known from the coastal record in Europe. The Dark Ages Cold Period dunes are high parabolic dunes, whereas the Little Ice Age dunes are predominantly barchanoid. The impact of trampling of the dunes by cattle during the latter period, resulting in reduced vegetation cover, and their artificial stabilization by pine forests seems to be the plausible causes of this morphological difference. Overall, the Aquitaine aeolian system is a good example of a mid-latitude coastal to inland system that was intermittently active during the Quaternary, and where the reconfiguration of the coast following sea level fluctuations played an important role in the chronological distribution of sediments.