Coastal Barrier Dune Research Articles

Development and preservation of transgressive sandy versus rocky shorelines: Observations from the SE African shelf

Relict rocky shorelines are seemingly rarely preserved in the geological record, often underrepresented in the literature and the reasons for their scarcity, as well as the conditions needed for preservation are not well understood. On the “Wild Coast” shelf along the subtropical east coast of South Africa a series of submerged shorelines are present. High-resolution multibeam bathymetry, side scan sonar, ultra-high-resolution seismic profiling and legacy seismic reflection data show several fluvial channels incised into the shelf, associated with the Last Glacial Maximum (LGM) lowstand. Palaeo-shorelines are preserved at −85, −75, −60 and −40 m. The −85 and −75 m shorelines reflect sediment abundance and depositional features preserved as lagoonal depressions, lithified barrier-spits and coastal barrier dunes with limited bedrock influence. The shallower shorelines at −60 and −40 m occur on a steeper shelf gradient and reflect the development of rocky headlands, palaeo-cliffs and rock shore platforms. Their occurrence implies increasing bedrock control, erosion and sediment bypass during transgression.The preservation of rocky shoreline signatures on the inner shelf is aided by competency of the bedrock lithologies, in addition to their depth relative to sea-level stillstands and rapid rises in sea level associated with meltwater pulses (MWPs) (notably the −60 m shorelines and MWP1B). MWPs may have helped facilitate the overstepping and eventual preservation of these shoreline landforms on the seabed. Preservation of the −40 m shoreline is considered to correlate with accelerated rates in sea- level rise prior to MWP1C. The preservation of the −85 and −75 m aeolianite shorelines are not linked to meltwater pulses, however, their substantial sediment volume precluded total loss through transgressive erosion and this, coupled with early cementation, promoted their partial preservation. Such deposits rarely survive subaerial exposure but could be preserved if covered by marine sediments. Conversely, the existence of well-preserved, Pleistocene-age rocky shorelines on the shelf points to their high potential for inclusion into the longer-term geological record under subsequent high or low sedimentation. The formation of distinctive rocky shorelines, however, may reflect alternating slow and fast rates of sea-level rise.

State changes: insights from the U.S. Long Term Ecological Research Network

AbstractUnderstanding the complex and unpredictable ways ecosystems are changing and predicting the state of ecosystems and the services they will provide in the future requires coordinated, long‐term research. This paper is a product of a U.S. National Science Foundation funded Long Term Ecological Research (LTER) network synthesis effort that addressed anticipated changes in future populations and communities. Each LTER site described what their site would look like in 50 or 100 yr based on long‐term patterns and responses to global change drivers in each ecosystem. Common themes emerged and predictions were grouped into state change, connectivity, resilience, time lags, and cascading effects. Here, we report on the “state change” theme, which includes examples from the Georgia Coastal (coastal marsh), Konza Prairie (mesic grassland), Luquillo (tropical forest), Sevilleta (arid grassland), and Virginia Coastal (coastal grassland) sites. Ecological thresholds (the point at which small changes in an environmental driver can produce an abrupt and persistent state change in an ecosystem quality, property, or phenomenon) were most commonly predicted. For example, in coastal ecosystems, sea‐level rise and climate change could convert salt marsh to mangroves and coastal barrier dunes to shrub thicket. Reduced fire frequency has converted grassland to shrubland in mesic prairie, whereas overgrazing combined with drought drive shrub encroachment in arid grasslands. Lastly, tropical cloud forests are susceptible to climate‐induced changes in cloud base altitude leading to shifts in species distributions. Overall, these examples reveal that state change is a likely outcome of global environmental change across a diverse range of ecosystems and highlight the need for long‐term studies to sort out the causes and consequences of state change. The diversity of sites within the LTER network facilitates the emergence of overarching concepts about state changes as an important driver of ecosystem structure, function, services, and futures.

Exploring Sea Country through high-resolution 3D seismic imaging of Australia’s NW shelf: Resolving early coastal landscapes and preservation of underwater cultural heritage

Almost 2 million square km of Australia’s continental shelf was flooded following the termination of the last glacial maximum, and with it the cultural heritage of the first arrival and coastal occupation of Australia beginning some 65,000 years ago. In order to prospect for this missing cultural record, we must first identify submerged coastal landscapes and landforms that likely provided favourable environments for occupation and resource use. However, this task is challenged by the sheer size of the Australian continental margin. To help address this, we use industry 3D seismic datasets, that cover vast areas of Australia’s continental shelf, to map seafloor bathymetry at high resolution (10–25 m). Our study focuses an area of 6500 square km on the mid/outer shelf regions proximal to Barrow Island. The 3D seismic bathymetry revealed a highly complex and geomorphically mature coastal landscape preserved at depths of 70–75 m below sea level, including coastal barrier dunes, lagoonal systems, tidal flats and estuarine channels. Based on the depth of the submerged shorelines and reconstructed sea level curves, the age range of these coastal landforms is constrained to Marine Isotope Stage 3 (57–29 ka), which overlaps with the known onset of occupation at Barrow Island and the wider Carnarvon bioregion and the adjacent Pilbara. Such feature preservation has significant geoheritage value, but also allows for human behavioural ecology modelling and provides targets for future dating and site survey.

Multi-proxy palaeosol evidence for late Quaternary (MIS 4) environmental and climate shifts on the coasts of South Africa

Palaeosols are common along the coastline of southern South Africa as stacked aeolian dune deposits but have rarely been studied. We selected two late Quaternary palaeosols exposed in a marine cliff-face at Koeberg and coastal barrier dune at Goukamma, South Africa in order to improve our understanding of their pedogenesis and palaeoclimate dynamics. Palaeosol-based proxies explored include: elemental geochemistry by X-ray fluorescence spectrophotometry, δ13C and δ18O isotopes, micromorphology and clay mineralogy by x-ray diffraction. Selected physico-chemical soil properties were analyzed by routine laboratory procedures. The palaeosols comprise predominantly loamy sand to sandy clay loam textures, have a high pH (>6.5), and very low electrical conductivity (<0.89 mS cm−1). SiO2 and CaO are the most abundant of all the elements in the cambic and calcic horizons respectively. The low levels of Al in the parent materials most likely invalidated the applicability of chemical weathering indices (CIA) to assess weathering intensity. In the case of chemical index of weathering (CIW), the age and sedimentary settings of the palaeosols overruled the possibility for K metasomatism and illitization by metamorphism. The indices WI-1 and W1-2 developed by Darmody et al. (2005) appear more consistent with depth. The palaeo MAT computed from palaeosol carbonate oxygen isotope is 14 and 11 °C for Koeberg and Goukamma respectively, while the maximum MAP obtained from the cambic horizon (Bw) of the Goukamma coastal barrier is 653 mm y−1. The layering seen in the thin section of the calcic layer at Goukamma indicates deposition, possibly by sedimentary differentiation across a palaeo-slope. Clastic calcite and muscovite mica are the dominant minerals in these palaeosols indicating impeded chemical weathering. Similar to many other parts of the world, the coastlines of South Africa has experienced environmental and climate oscillations in the Quaternary. We conclude that along the southern South Africa coasts, a palaeosol based approach to palaeoenvironmental and palaeoclimate reconstruction in combination with other proxies such as pollen and marine based isotopes can provide insights into the environmental oscillations of the late Quaternary.

Predicting and Monitoring the Evolution of a Coastal Barrier Dune System Postbreaching

O'Shea, M. and Murphy, J., 2013. Predicting and monitoring the evolution of a coastal barrier dune system postbreaching.A study of the morphodynamic evolution of a midbay barrier beach system of Inner Dingle Bay, County Kerry, Ireland is presented. The system has been under observation by the Hydraulics and Maritime Research Centre (HMRC) of University College Cork since 2008 when the Rossbeigh barrier breached. This event had been the culmination of 10 years of intensive erosion. Since breaching occurred, erosion rates have continued to increase on Rossbeigh, while offshore an ebb tidal delta continues to grow. Similar barrier inlet systems are examined in order to identify trends and features that might provide an insight into the future morphology of Dingle Bay. The study utilises field data collection sediment transport analysis and remote sensing to understand the coastal processes driving the systems evolution. The changing orientation of Rossbeigh, the relative stability of the Inch barrier beach, and the variability in ebb tidal delta were the findings of the long-term morphological part of the study. The influence of the ebb tidal delta on wave refraction, emergence of a vegetated gravel ridge in the breach zone, and the role of tidal currents in sediment transport along Rossbeigh are significant findings of the monitoring campaign. These results provide indicators of future evolution of the system. The study concludes that the realignment of the beach will continue on Rossbeigh beach while alongshore tidal currents dominate sediment transport in the breached area.

Coastal barrier dune construction during sea-level highstands in MIS 3 and 5a on Tottori coast-line, Japan

The southern coast of Japan Sea exhibits a number of Late Pleistocene coastal barrier dunes, where the sediment record potentially provides important evidence for the palaeoenvironmental history of East Asia. The lack of direct dating of these barrier dunes, however, has veiled relationships between dune deposition, sea level, and other controlling factors. Quartz single aliquot regenerative optically-stimulated luminescence (OSL) dating was applied to the Late Pleistocene barrier dune at the Tottori coast, southern central Japan Sea. We investigated a borehole drilled in a continuous ground-penetrating radar (GPR) profile of the dune and an outcrop excavation, where there has been recognised a sedimentary succession exposed of the Pleistocene dune sand, palaeosol, Daisen–Kurayoshi Pumice (DKP) tephra, palaeosol, and Holocene dune sand, in ascending order. The gradual transition from the Pleistocene dune sand to palaeosol indicates a termination of the Late Pleistocene dune activity. Five OSL ages from the uppermost Pleistocene dune overlap within the period 48.2–51.4 ka, which confirms that both deposition and termination of the dune occurred in early to mid Marine Isotope Stage (MIS) 3. Another OSL age from the lower section was 77.7 ± 6.1 ka, and indicates an earlier depositional stage in late MIS 5a. These reveal that the coastal dune deposition occurred in relation to the sea-level highstands. It would appear that a sea level of c. –50 m relative to the present was the threshold for the dune activation. Morphology and sediment distribution of the modern shelf off the Tottori coast suggest that the sea-level lower than − 50 m leads to increasingly unfavourable conditions for coastal dune activity due to longer sediment transport distances and decreased sediment supply. These results have implications for the Late Pleistocene barrier dune along the southern Japan Sea coast and suggest that other dunes, correlated with Tottori based on the DKP tephra, possibly would also have experienced a similar termination in relation to the sea-level fall from mid MIS 3. Results also demonstrate that the depositional setting of the Late Pleistocene dunes was different from those constructed in the Holocene due to the lower interstadial sea-level highstands and resultant longer distance to the shore during MIS 3 and 5a. As a result, the Pleistocene dune exhibits less pronounced stratification and in finer grain size than the outer Holocene dune ridge, where sand is directly fed from the shore. The grain size of the inner Holocene dune ridge in contrast is almost identical to the Pleistocene dune, suggesting that the inner ridge partly consists of sand reworked from the Pleistocene dune.

Sediment provenance of the Wilderness barrier dunes, southern Cape coast, South Africa

Trace-element geochemistry has been used to discern the provenance of sediments derived from geologically distinct sources found in a range of depositional settings. This study investigates whether this approach can also identify provenance signatures in sediments derived from clastic sedimentary rocks of similar origins, within a complex coastal transport environment. The potential local sediment sources of a coastal barrier dune in South Africa were analysed to assess their relationships with the barrier and whether these have varied with time. Results indicate that the oldest part of the barrier, partly formed during MIS 6 when sea levels were lower than at present, has a closer affinity to fluvial sediment provided by local rivers than either younger samples from the barrier′s seaward flank or the Holocene dunes atop its crest, both formed during sea-level highstands. Constraints on the timing of barrier formation provided by OSL dating proved helpful in discriminating between provenance and weathering signals. Terra Nova, 00, 000–000, 2010

Habitat Preferences of the Australian Endangered Samphire Tecticornia Flabelliformis

Australia’s only federally listed endangered samphire, the fan or bead samphire Tecticornia flabelliformis (Paul G.Wilson) K.A.Sheph. & Paul G.Wilson, is a small deciduous forb up to twenty centimetres high that is generally found growing in monospecific patches on clay pans or sabkhas directly behind coastal barrier dunes or on salt lakes further inland. The habitat requirements that control the distribution of fan samphires are poorly understood.During September 2004, thirty-six soil samples were collected from within and surrounding three stands of fan samphires near Gulf St Vincent, South Australia. Analysis of the samples showed that pH had the strongest association with the presence of fan samphires, with field moist chlorinity also strongly associated. Oven dried chlorinity and percentage moisture were not significantly different between sites with fan samphires and those sites without, although the fan samphires occurred on soils with a wider range for these parameters than the surrounding areas displayed.Soil samples were also analysed for soil particle size. The species appears to prefer locations where the ‘within site’ mean clay content is between 31% and 37%. Field observations of a light-coloured subsurface hard pan under fan samphire stands were noted.

The luminescence chronology of dune development on the Maputaland coastal plain, southeast Africa

Geomorphological and lithostratigraphic mapping of dune systems on the Maputaland coastal plain of northeastern KwaZulu-Natal Province, South Africa has defined the relative-age relationships between the complex pattern of Quaternary parabolic and hummocky dunes, sand megaridges and the composite coastal barrier dune cordon. Here infra-red-stimulated luminescence (IRSL) ages are presented in the context of regional stratigraphic relationships and serve to bracket the period of accumulation of the main dune systems. The luminescence ages challenge the widely held belief that the north–south aligned dune pattern reflects a coastward-younging sequence. The framework of ages highlights the spatial variations in dune mobilization and long accretion history on some dune forms. Weathered dune and interdune wetland sediments spanning from prior to marine isotope stages (MIS) 11 and up to MIS 7 underlie the coastal plain. At least two generations of decalcified aeolianite of MIS 5 and 4 form the core of the complex coastal barrier dune. On the coastal plain, frequent sand mobilization events during MIS 3–2 resulted in the development of discrete complexes of highly extended, northward directed parabolic dune systems and reworking of the crest of the central sand megaridge. The luminescence-dated dune mobilization history brackets the period of formation of interdune and lacustrine peat and diatomite deposits. During the Holocene marine transgression at least four laterally extensive, complex transverse ridges of coalesced ascending parabolic dunes accreted against the aeolianite core of the coastal barrier. Polyphase dune formation and remobilization is discussed in the context of the regional groundwater and vegetation responses to global climatic changes, wind regimes and glacio-eustatic sea-level fluctuations.

Soil chronosequence development in dunes on the southeast African coastal plain, Maputaland, South Africa

Dunes have accreted on the southeast African coastal plain in coast-parallel patterns of degraded whaleback ridges, sand megaridges and extended parabolic dunes since the Pliocene. In the Maputaland dune field, relative dating is complicated by soil degradation, erosion and vegetation cover. This project assessed alternative relative and numeric dating techniques that can be used to differentiate dune systems and eolian sand bodies. Soil profile characteristics were used to calculate soil development indices (SDI) for soil horizons sampled from hand augered holes and rare exposures. Infrared stimulated luminescence (IRSL) dating of representative dunes and sand units helped define the pedogenesis achieved since deposition. The quartzose dune sands have weathered to form very deep soil profiles. Enhanced SDI values for horizons or sampled intervals down the profile reflect pH decrease, advanced rubification or distinct mottling, clay increase and harder consistency with depth. The youngest Holocene profiles exhibit decreasing horizon index values with depth below the A horizon whereas late Pleistocene profiles display SDI values increasing with depth within the upper 3 m of the profile. Within some Mid- to Late Pleistocene eolian sands greater profile-horizonation and catena complexity manifests as complex soil profiles or distinctly mottled, clay-enriched horizons, at depths of 2–5 m below the surface. The SDI profiles from specific dune systems or stratigraphic units cluster well and effectively characterize soil development in dune sand units of different ages. Depositional age plotted against sampled horizon index values makes it possible to determine generalized rates of soil profile development, and to compare pedogenic processes on the high-rainfall, coastal barrier dunes to those found in the drier interior. Sampled horizon index values distinguish buried soil profiles and highlight possible localized surficial reworking.

Evidence for dune reactivation from GPR profiles on the Maputaland coastal plain, South Africa

Abstract The Maputaland area of northeastern South Africa is characterized by extensive dunefields which developed during polyphase reworking of regional aeolian cover sand from the Mid-Pleistocene to the Holocene. Extended parabolic dunes, many preserved only as wind-rift trailing limbs, as well as areas of sinuous crested dunes, hummocky dune systems and the high, composite, accretionary coastal barrier dune cordon are the dominant dune forms. There are few natural sections exposing the stratigraphic succession and unequivocal relative age relationships between dune systems are uncommon. A ground penetrating radar (GPR) survey of dunes and representative aeolian sand stratigraphic units was undertaken in order to investigate the internal structure of the different dune forms and identify stratigraphic relationships between buried sedimentary units. The GPR profiles revealed that the trailing limbs of almost all the parabolic dunes that were surveyed comprise stacked sand units, separated by low-angle reflections interpreted as bounding surfaces, which accumulated through polyphase vertical accretion. Most extended parabolic dunes are aligned north-south and the upper parts of the dunes are characterized by inclined reflections in GPR profile interpreted as large-scale sets of cross-stratification with apparent dips toward the west. A hummocky dune revealed cross-stratified aeolian sand superimposed on a truncated dune form and probably formed through deflation of pre-existing dunes. Using 100 MHz and 200 MHz antennae, it is clear that GPR is capable of imaging very fine sedimentary structures and buried erosional surfaces in the homogeneous aeolian sand of Maputaland. At some of the sites investigated, the buried sand units identified were sampled by hand-augering for infrared-stimulated luminescence dating. The age determinations on these samples suggest that vertical accretion of up to 7 m of sand occurred intermittently over variable time scales up to 25 000 years on some parabolic dune limbs during the Late Pleistocene. In some complex dunefields, adjacent dunes were mobilized at different times, suggesting that remobilization was localized. The implications of the complex internal structure and vertical accretion of extended parabolic dunes are discussed in the context of changes in vegetation cover and water table due to seasonal and short-term cyclical climate variations as well as long-term climate change patterns during the last glacial cycle and the Holocene.

The late Cenozoic evolution of coastal water bodies in Northern Kwazulu-Natal, South Africa

Abstract Three large coastal water bodies (St Lucia, Sibaya and Kosi) of markedly different contemporary morphology occur on the eastern margin of the northern KwaZulu-Natal coastal plain, South Africa. This paper examines their Late Cenozoic evolution and investigates the controls on their evolution that have produced the observed differences in morphology. All three systems origins were due to Mio-Pliocene low sea-level still stands allowing rivers to scour channels into Cretaceous and Palaeocene sedimentary sequences. During the Last Interglacial (oxygen-isotope stage 5e), sea level was approximately 5 m above present level and the morphology of the three systems was similar. An offshore barrier archipelago partially isolated a back-barrier environment that comprised intertidal sand-flats, localised reefs and tidal channels. The area around Bazaruto Island, Mozambique, provides a modern analogy. River rejuvenation and associated incision up to and during the Last Glacial Maximum (16 000–18 000 BP) caused a minimum of −40 m scour which is still reflected in the bathymetry of Lake Sibaya and Kosi. Accumulation of barrier sands partly against pre-existing Interglacial barrier remnants during the Holocene marine transgression (16 000–4500 BP, Ramsay, P.J., 1995. 9000 Years of sea-level change along the South African coastline, Quaternary Int. 27, 1–5.) was responsible for lagoon isolation, although the extent and nature of separation from the sea varies markedly between the systems. The northern part of Lake St. Lucia was closed from the sea during the late Holocene, probably during a sea-level fall from a Holocene high stand of ca. 3 m, at 3000 BP. The Kosi system had a late Pleistocene to early Holocene outlet situated at Bhanga Nek, Lake Nhlange, but this inlet mouth closed approximately at 3000 BP. The present mouth was established by 1600 BP. Lagoonal conditions within Lake Sibaya terminated ca. 5000 BP when coastal barrier dune formation caused complete impoundment and enabled the lake water level to rise to 20 m above mean sea-level. Lake St. Lucia's large catchment has meant that fluvial sediment input has kept pace with the Holocene transgression, whereas the localised drainage associated with Sibaya and Kosi has resulted in a negligible fluvial sediment input allowing for deep lacustrine conditions to develop. Lagoonal segmentation is present to some extent in each system as a result of mobile coastal sediments and their reworking by short-period wind waves. The differential stages of lagoonal segmentation in the three systems can be explained by (a) the presence of cohesive sediment and bedrock around St Lucia which reduce shoreline mobility, and (b) the great water depths in Sibaya compared to Kosi which require longer periods of infilling before segmentation is complete