Bedrock Islands Research Articles

Island hydrogeology in the tropics: Constraining a 3D variable-density groundwater flow and solute transport model with geophysics

Saltwater intrusion is the greatest risk to coastal community water supplies where they are dependent on fresh groundwater as the main source of supply. For small, fractured bedrock island aquifers, the fresh groundwater lens dynamics and transition zone geometry are complex. This study investigated the impacts of projected increases in groundwater pumping on a fresh groundwater lens to evaluate changes to the lens geometry and localized up-coning from the deeper, more saline aquifers beneath a small bedrock island in the tropics. A combination of traditional hydrogeological datasets and an airborne electromagnetic survey were used to develop a three-dimensional density-dependent groundwater flow and solute transport model using the SEAWAT code. This investigation represents one of very few studies that have taken such an approach. The model was calibrated using observed groundwater hydraulic heads and chloride concentration data, and calculated chloride values based on bulk conductivity measurements determined from inverted geophysical data. A staged calibration approach was adopted, firstly assessing the time-average lens extent and geometry, and secondly considering the seasonal groundwater level response. In the calibration, the geophysical data helped constrain the lens geometry in the absence of hydraulic head and chloride data. The calibrated model was used to test scenarios where groundwater pumping rates were increased above the current demand of 452 m3 d−1, showing that the lens is likely to be stable, i.e., its available storage is not expected to contract excessively, for extraction rates of up to 3,000 m3 d−1. The combined use of geophysical data and a numerical modeling approach was advantageous in investigating the lens characteristics. It also demonstrated how these techniques can be used together to evaluate coastal water resources and to manage water supply risks for coastal communities. The study demonstrated that the freshwater lens can likely support the freshwater demands of the remote community, and is a preferred option compared with high-cost and more complicated options such as seawater desalination and managed aquifer recharge.

Microplastic pollution in the groundwater under a bedrock island in the South China sea

Groundwater is the only freshwater resource on islands. Research on microplastic pollution in groundwater on islands is scarce. This study is the first to explore microplastic pollution in the groundwater under a bedrock island (Dawanshan Island) located in the South China Sea. The influence of hydrogeological factors on the distribution, source, and ageing features of microplastics in the groundwater were investigated. Despite the small scale of industrial and agricultural activities on the island, the amount of microplastics in the groundwater ranged from 34 to 64 particles/L, with over 80% of the microplastics being polyester fibres with diameters smaller than 2 mm, which is comparable to those in coastal cities. These microplastics were originated from inland plastic usage, rather than from the surrounding sea, which was confirmed by the lack of seawater intrusion on the island. Owing to the low permeability of granite, microplastics were mainly distributed in the water of the loose layer of porous sediment, and their quantity decreased with depth. In addition, the abundance of microplastics in pore groundwater increased with an increase in the velocity of groundwater flow. The severity of microplastic pollution in the groundwater increased with an increase and decrease in the content of total dissolved solids and dissolved oxygen, respectively. The microplastics originated from plastic waste disposed of on the island, rather than from seawater intrusion. Also, through groundwater infiltration into exposed soil at recharge areas, artificial wells at residential areas, and water exchange with surface water at valley areas. Microplastics buried in the groundwater aged faster along the migration path of the groundwater. These microplastics threaten the safety of people and plants on the island through exposure resulting from the extraction of groundwater for irrigation, while they endanger marine life through submarine groundwater discharge.

Sediment transport trend and its influencing factors in coastal bedrock island sea areas-a case study of Chudao island, China

Coastal bedrock islands sea areas have a unique natural environment, frequent human activities, and complex sedimentary dynamic processes. In this paper, we select the Chudao Island sea area off the coast of Shandong Peninsula, China, as a typical research area to investigate the sediment transport trends and influencing factors by means of high-precision bathymetric survey, high-density sediment sampling, grain-size trend analysis and hydrodynamic numerical modeling. Results and analysis indicate that the grain size parameters including mean grain-size, sorting coefficient and skewness are zonal distributed, roughly parallel to the isobaths. While the overall sediment transport trend is from island shore to sea, with several convergence centers near the loop centers of bottom flow and at the edge of the agriculture area. The near-bottom flow velocity is primary factor that controlling the significance of sediment transport trend, while the flow decides the general patterns of sediment transport trend and sediment distribution. Submarine topography can either directly transport sediments down its slope, or indirectly affect the direction of sediment transport by constraining the near-bottom flow from shallow to deep waters. Besides the natural factors of bottom flow and submarine topography, human activities represented by aquaculture also affect the sediment transport trend in coastal bedrock island sea areas. First, the increased sedimentation rate caused by organic matters and the diffusion of scallop fragments may cause sediment coarsening. Second, the artificial aquaculture facilities can reduce flow velocity and therefore hinder the initiation, suspension and transport of sediment near the aquaculture areas. Our methods and findings provide high-resolution details to insight into the sediment transport trends to improve the understanding of the modern sediment dynamics in small-scale coastal bedrock island sea areas and provide reference for corresponding engineering and agriculture activities.

Anthropocene trajectories of high alpine vegetation on Mont-Blanc nunataks

ABSTRACT Climate warming causes dramatic glacier retreat and intense vegetation changes in alpine regions. High-elevation nunataks, that is bedrock islands protruding from glaciers with uppermost flowering plants, are no exception. Yet the consequences of climate change on nunatak vegetation remain relatively unexplored. Here, we report findings from a re-visit of historical plant surveys carried out on six nunataks situated between 2180 m a.s.l. and 3509 m a.s.l. amidst the glaciers of the Mont-Blanc range (France). We compared vegetation surveys conducted in 2020 to those made 150 years before, and performed remote sensing analyses to depict changes in vegetation productivity during recent decades. We report an increase in plant species richness for the lowest and least isolated nunataks, which contributed to a strong signal of vegetation greening over the last 35 years. This trend was due to the upward migration of competitive species, but also due to species colonization from neighboring high alpine areas into recently unglaciated microsites. We also highlight striking ecological trajectories that have been little discussed so far, such as stable vegetation composition in the highest and most isolated nunataks, an increase of plant species associated with non-permanent snow-cover and water run-off, an increase of graminoids at lower elevations, and of phanerophytes at higher elevations. We argue that high alpine vegetation changes are not driven by the sole upward migration of lowland competitive species, and that careful monitoring of ongoing ecological changes over broad elevation gradients is necessary to better understand the rapid transformation of high alpine landscapes.

Predicting groundwater potential assessment in water-deficient islands based on convolutional neural networks

Due to the special hydrogeological features of the sea islands, the distribution of groundwater is difficult to assess. In order to improve the accuracy of the predicting groundwater potential assessment(GPA) in islands region. We tried to use the convolutional neural networks(CNN) and took lithology, aspect, slope, water density, vegetation fraction, soil humidity and land surface temperature as the remote sensing assessment indicators. The island area of Pearl River Estuary, China was selected as the study area. The groundwater situation of Wailingding Island was taken as the sample data of the total area, and the map of this island was divided into 24 × 22 grids. The levels of GPA of each small grid was determined by hydrogeological maps with results of geophysical method and wells or springs data. Meanwhile, the corresponding data were enlarged and the sample label data set was made. Using the CNN model of learning, training and testing, analysis of groundwater potential and each coupling correlation between remote sensing indicators constantly. After 1500 times of training, loss of model dropped to 0.3113, the accuracy of model was 96.96%. A good 5 levels classification prediction of GPA model was received. The AUC of ROC curve and significance level (P) of the CNN model were 0.855 and 0.001 respectively, which were better than the results of the classic GRSFAI model. The results showed that the prediction of GPA based on CNN model can effectively assess the groundwater distribution levels in the Pearl River Estuary island area, which can provide a certain reference value or a GPA model for the other water-deficient islands. Moreover, the obtained sample set of groundwater volume distribution of bedrock islands can be used as a valuable data source for further in-depth research.

Land-Use Classifying and Identification of the Production-Living-Ecological Space of Island Villages—A Case Study of Islands in the Western Sea Area of Guangdong Province

Accurately identifying the rural production-living-ecological space (PLES) of different islands can help reveal their distinct natural resources and land-use situations, which is significant for the sorted management, subarea utilization, and protection of islands. At present, studies on the PLES of island villages are deficient. For instance, the existing land-use classification system is incomplete; the PLES is poorly identified; and the dominant function of multiple land-use types based on different island geomorphology types is insufficiently investigated. Therefore, a case study was conducted on the island villages of the western sea area of Guangdong Province, based on remote sensing, spatial analysis, and land classification, with field research and the relevant data. In this study, before establishing the PLES system, the islands were classified, including six bedrock islands, 10 sedimentary islands, and one volcanic island. When the PLES system of the island villages was classified, the ecological and utilized areas of the intertidal zone and neritic region should be combined with the island–continent part, and the distinct industrial types should be emphasized, before forming 22 secondary types of PLES. Furthermore, it is found that each island generally has its own dominant space and land-use type. Ecological space (ES) dominates the bedrock islands, and production space (PS) is prominent for sedimentary islands and volcanic islands. Forestland, aquaculture pond, and dryland are the prominent land-use types for bedrock islands, sedimentary islands, and volcanic islands, respectively. The rural residential lands are the main component of living space (LS) in all islands, and the most urban residential lands are distributed on the bedrock islands. The main driving factors for the formation and distribution of island rural PLES are the altitudinal gradient and geomorphic characteristics. The research shows that the main problems of PLES are that the intertidal zones are threatened by aquaculture ponds at various levels, and the development of LS in these islands is generally backward.

A transdisciplinary engagement with Australian Aboriginal water and the hydrology of a small bedrock island

ABSTRACT Yolŋu Aboriginal people’s knowledge about water (“gapu”) and its governance has strong cultural significance and meaning in East Arnhem Land Aboriginal worlds in northern Australia. This study used transdisciplinary research methods to explore the ways in which Yolŋu Aboriginal gapu and Western science hydrological knowledge can work together and contribute towards water management on Milingimbi Island, a small, resource-constrained, bedrock island. Transcending disciplinary boundaries is distinctly different to an interdisciplinary, socio-hydrological perspective, which can pose a risk to hybridizing Aboriginal knowledge and Western science. Community engagement activities and workshops were conducted as part of a three-year research project to bring together the incommensurable knowledge communities. A participatory three-dimensional mapping exercise created a shared space, facilitating an open-dialogue exchange of insight and knowledge among Aboriginal knowledge authorities, hydrologists, public servants, and academic researchers. This paper prompts readers to reconsider the ways water can be perceived and conserved in a decolonizing way.

Groundwater recharge to a structurally complex aquifer system on the island of Tobago (Republic of Trinidad and Tobago)

Trinidad and Tobago is a developing two-island nation in the Lesser Antilles of the Caribbean. Tobago is the smaller island and has small highly heterogeneous aquifers composed of igneous and metamorphic crystalline rock with strong structural controls on the spatial distribution of permeability. Hydrogeologic analyses of the water budget and groundwater production suggest that portions of the island are underlain by prolific fractured-rock aquifer systems. This study quantifies the amount and spatial distribution of recharge, as well as the fraction of recharge captured by groundwater pumping, using historical data, new field data, remote sensing data, multiple storage quantification methods and stable isotope analysis. Despite extensive freshwater withdrawals, groundwater production reaches only ~10% of annual groundwater recharge. Groundwater capture zones are created using a first-order hydrologic balance approach and with backward particle tracking in a steady-state groundwater model. Both approaches to generating capture zones suggest that many wells capture water from outside their topographic watershed. The location of sustainable, high yield, fresh potable groundwater wells less than 1 km from the coast, that have fractured bedrock intakes well below sea level, supports the concept of a rigorous and active groundwater flow system. Understanding the hydrogeology of small bedrock island aquifers is critical to evaluating groundwater resources, especially in the Caribbean where there is strong seasonality in precipitation, finite surface-water storage and increases in potable water demand.

Fresh groundwater lens dynamics of a small bedrock island in the tropics, Northern Australia

Fresh groundwater lens dynamics and transition zone geometries on small tropical islands have been investigated in a wide range of geological environments. However, the understanding of multi-aquifer fractured bedrock systems and how they respond to episodic wet season rainfall recharge is still limited. This study used a comprehensive suite of isotopic tracers, hydrogeological and near-surface geophysical methods to characterise the multi-layer aquifer system on Milingimbi, a small bedrock island located in the tropics of the Arafura Sea, Northern Australia. Near-surface geophysics was used to determine the subsurface structure of the aquifer, including the spatial extent and thickness of the fresh groundwater lens and the shape of the transition zone. Pore water chloride profiles, hydrochemistry, and δ18O and δ2H data supported the geophysics results. The lens was found to be 40 m thick on average, with an up to 70 m thick transition zone underneath. Water level time series data over 4 years showed that there was a strong tidal signal observed in the groundwater wells screened in the deeper aquifer and that the aquifer system showed a dynamic response to the wet season rainfall. Time series chloride and δ18O and δ2H rainfall and production bore data suggest that there was a freshening (i.e. lower salinity input) of the lens as a result of recharge during the wet season, which occurs up to 6 months after the event. Groundwater residence time indicators showed that the mean residence time in the lens was at least 25 years and the rate of recharge to the system was up to 200 mm y−1. The comprehensive data set, which is rather unique in its wide range of methods that were applied, resulted in a hydrogeological conceptual model of the multi-layer bedrock aquifer system of the island that also provides insight into the fresh groundwater lens and the transition zone geometry of similar island aquifer systems.

Long-term morphodynamic evolution of the Pearl River Delta from the perspective of energy flux and dissipation changes

The Pearl River Delta (PRD) is an extremely complex, large-scale estuary system in China. In this study, a long-term process-based morphodynamic model was developed to investigate the characteristics of variations in energy flux and dissipation and their evolution alongside the hydrodynamic and topographic evolution of the PRD since the Holocene transgression maximum. The results show that the evolution of the dynamics and topography of the PRD are largely influenced by the geological and geomorphological structures of the region. Energy flux and dissipation in the inner estuary exhibited a patchy distribution, with areas of high energy flux appearing in deep troughs, near headlands, and in gorges containing bedrock islands, and areas of relatively small energy flux appearing in the flow shadow zones of the islands, tidal fluctuation confluence area, and shoals. Corresponding to the energy flux distribution, sediment accumulation also exhibited a high degree of spatial and temporal variability. The evolution of the coastline of the PRD is not driven from north to south, or from the river outlet to the downstream area. Rather, multiple deposition bodies simultaneously developed in the north and south without overlapping. With the deposition of sediment in the estuary, the isoamplitude lines of tidal waves were gradually extrapolated and the spatial distribution gradually shifted from the northwest to the southwest. This indicates the mutual transformation between the estuarine river and tidal power, and also reflects the development characteristics of the river system.

Volumetric Change Detection in Bedrock Coastal Cliffs Using Terrestrial Laser Scanning and UAS-Based SfM.

Three-dimensional (3D) morphological changes in rocky coasts need to be precisely measured for protecting coastal areas and evaluating the associated sediment dynamics, although volumetric measurements of bedrock erosion in rocky coasts have been limited due to the lack of appropriate measurement methods. Here we carried out repeat surveys of the 3D measurements of a small coastal island using terrestrial laser scanning (TLS) and structure-from-motion (SfM) photogrammetry with an unmanned aerial system (UAS) for 5 years. The UAS-SfM approach measures the entire shape of the island, whereas the TLS measurement enables to obtain more accurate morphological data at a scale of centimeters on the land side. The multitemporal TLS-derived data were first aligned in timeline by the iterative closest point (ICP) method and they were used as positionally correct references. The UAS-SfM data were then aligned to each of the TLS-derived data by ICP to improve its positional accuracy. The changed areas for each period was then extracted from the aligned UAS-derived point clouds and were converted to 3D mesh polygons, enabling a differential volume estimate (DVE). The DVE for each period was revealed to be from 3.1 to 77.2 m3/month. These changes are rapid enough to force the coastal bedrock island to disappear in 30 years. The temporal variations in the DVE is roughly associated with those in the frequency of high tidal waves.

Application of remote sensing fuzzy assessment method in groundwater potential in Wailingding Island

To solve the problems related to the assessment of groundwater potential in small-scale areas or small arid bedrock island areas in the field of groundwater potential assessment via remote sensing technology based on fuzzy assessment methodology, we used maps, DEM data, GF-1 satellite data, and so on, employing Wailingding Island in the Pearl River estuary as the study area. Then, we used a lithology index, relief index, slope index, vegetation fraction index, land temperature index, water density index, and soil humidity index as extraction indices. Next, we assessed the groundwater potential of this island. We obtained the results of the level charts of groundwater potential assessment in this area based on the RS fuzzy assessment method. The results were verified according to the water flow rate of the wells. Finally, we determined the distribution of groundwater in the area. This study is significant for guiding the exploitation and utilization of groundwater in small islands.

Beach Ridges as Favored Locales for Human Settlement on Pacific Islands

Coastal beach ridges have been favored locales for human habitations on Pacific islands since initial human settlement. Their smooth crests shaped by surf action form sandy substrates that differ in morphology from eolian dunes. Island beach ridges that were accreted to island cores during post‐mid‐Holocene drawdown in regional hydro‐isostatic sea level include coastal spits that partially enclose backbeach wetlands, tombolos that tie previously separate islets together, and cheniers that form elongate elevated tracts on accretionary coastal plains. Some beach ridges were accreted directly to the flanks of bedrock islands, but others grew at varying distances from ancestral bedrock shorelines to trap remnants of offshore lagoons as lakes or swamps and marshes between the beach ridges and exposures of island bedrock. Composite chenier plains formed by the successive lateral accretion of multiple beach ridges over time are prominent locally as broad coastal lowlands. Longshore sand transport driven by prevailing trade winds influences beach ridge evolution but beach dynamics reflected by beach sand textures confirm the dominance of onshore sand movement from wave attack for the morphology of the beach ridges.

Barrier shoreline evolution constrained by shoreface sediment reservoir and substrate control: The Miquelon-Langlade Barrier, NW Atlantic

ABSTRACT Billy, J., Robin, N., Certain, R., Hein, C. and Berne, S., 2013. Barrier shoreline evolution constrained by shoreface sediment reservoir and substrate control: the Miquelon-Langlade Barrier, NW Atlantic. The Saint-Pierre-et-Miquelon Archipelago (France) is located in the NW Atlantic Ocean, proximal to the Cabot Straight outlet of the Gulf of Saint-Lawrence, and 50 km south of Newfoundland (Canada). The Miquelon-Langlade Barrier is a 12-km-long, 100–2500-m-wide, north-south–oriented isthmus connecting two bedrock islands (Miquelon to the north; Langlade to the south). This study aims to improve our understanding of shoreface-shoreline sediment exchange processes by comparing medium-term (1949–2011) shoreline changes, determined from aerial photographs and differential GPS data, with total shoreface sediment reservoir volumes estimated using seismic along the west coast of the Miquelon-Langlade Barrier. Spatial variability between the northern and southern sectors of the study site are seen both in...

Interactions Between Tidal Flows and Ooid Shoals, Northern Bahamas

Abstract Oolitic strata are common through the geologic record. Ooids generally form in high-energy environments, but it remains unclear how ooids remain in such systems and form geomorphic features such as bars and shoals. Integrating remote sensing, hydrodynamic, bathymetric, granulometric, and field observations of modern tidal systems in the Bahamas provides insight into this fundamental question. Oolitic tidal sands in the northern Abacos display a geomorphic pattern in which bedrock islands restrict and focus tidal flow down a main channel. Within the channel, a shallow shoal separates an ebb-dominated subchannel from a flood-dominated subchannel. Tidal velocities in these subchannels can exceed 1 m/s, enough to transport the oolitic sediments. Hydrodynamics and bathymetry in these subchannels produce a net circular hydrodynamic pattern around the shoal (the spin cycle), allowing the sands to remain in motion without being transported out of the ooid factory. This general pattern is apparent in several other ooid shoal complexes. This concept provides integrated insights into the physical influences impacting the formation, suspension, transport, and deposition of ooids and the resulting geomorphic forms. These results represent first steps toward developing more comprehensive and predictive analogs of spatial heterogeneity in ancient tidally dominated oolitic shoals.

Seismic stratigraphy of Lago Puyehue (Chilean Lake District): new views on its deglacial and Holocene evolution

Prior to the collection of a series of sediment cores, a high- and very-high-resolution reflection seismic survey was carried out on Lago Puyehue, Lake District, South-Central Chile. The data reveal a complex bathymetry and basin structure, with three sub-basins separated by bathymetric ridges, bedrock islands and interconnected channels. The sedimentary infill reaches a thickness of >200 m. It can be sub-divided into five seismic-stratigraphic units, which are interpreted as: moraine, ice-contact or outwash deposits (Unit I), glacio-lacustrine sediments rapidly deposited in a proglacial or subglacial lake at the onset of deglaciation (Unit II), lacustrine fan deposits fed by sediment-laden meltwater streams in a proglacial lake (Unit III), distal deposits of fluvially derived sediment in an open, post-glacial lake (Unit IV) and authigenic lacustrine sediments, predominantly of biogenic origin, that accumulated in an open, post-glacial lake (Unit V). This facies succession is very similar to that observed in other glacial lakes, and minor differences are attributed to an overall higher depositional energy and higher terrigenous input caused by the strong seismic and volcanic activity in the region combined with heavy precipitation. A long sediment core (PU-II core) penetrates part of Unit V and its base is dated as 17,915 cal. yr. BP. Extrapolation of average sedimentation rates yields an age of ca. 24,750 cal. yr. BP for the base of Unit V, and of ca. 28,000 cal. yr. BP for the base of Unit IV or for the onset of open-water conditions. This is in contrast with previous glacial-history reconstructions based on terrestrial records, which date the complete deglaciation of the basin as ca. 14,600 cal. yr. BP. This discrepancy cannot be easily explained and highlights the need for more lacustrine records from this region.

Les Sillons: a relict foredune plain

`Les Sillons' (meaning `furrows') is the local name given to an outstanding example of a relict foredune plain on Les Iles de la Madeleine, Quebec. This series of shore-parallel dune ridges was built seaward during a period of post-glacial sea level rise, an indication of moderate to high sediment supply. Sea level has continued to rise; on the seaward side of the complex, wave erosion is now reactivating sand and active dunes are burying relict foredune ridges. Other similar features exist in Atlantic Canada (King 2000), but Les Sillons is the largest, having a maximum width of 2250 m and covering an area of 10.6 [km.sup.2]. The objectives of this article are to describe the setting and morphology of Les Sillons, and to explain its development based on regional Holocene history. Relict Foredune Plains Prograding coastlines are often associated with conditions of falling or stable sea level; however, progradation may also occur with rising sea level if there is a sufficient rate of sediment supply (Thompson and Baedke 1995; Hesp and Short 1999). Repeated construction of new foredune ridges at the back of the beach causes the shoreline to advance, or prograde, seaward. More widely spaced, lower ridges indicate more rapid progradation (Hesp 1984; Shepherd 1987). Examined closely, the `parallel' nature within a relict foredune plain is usually an approximation, as individual ridges are frequently a few degrees off parallel and merge into adjacent ridges. Genesis of foredune ridges is a subject of some controversy (Hesp 1984; Trenhaile 1997). The terms `beach ridge' and `foredune ridge' have been used both interchangeably and with differing meanings (Hesp 1984). The distinction is preferred- foredune ridges are characterized by the establishment of pioneer vegeration and can attain greater heights (swale to crest) than beach ridges. A series of arcuate shore-parallel progradational foredune ridges forms in a low wave energy sediment sink that is protected from significant prevailing longshore currents; an ideal location for these conditions is between two headlands (Komar 1998). In a higher energy coastal environment, the preservation potential of long, continuous ridges is lower; and with prevailing currents a prograding spit is more likely to form, such as Buctouche Spit, N.B. (Ollerhead and Davidson-Arnott 1995) or Long Point, Ontario (Davidson-Arnott and Conliffe Reid 1994). Regional Setting Les lies de la Madeleine are located in the southern Gulf of St. Lawrence (Figure 1). Two effects of the Laurentide ice sheet in the region during the last glacial period were depression of the crust, and provision of large quantities of sediment that were later moved onshore and used in the development of coastal landforms (Loring and Nora 1973). Deglaciation was accompanied by isostatic rebound that varied according to the thickness of ice and the passing of a forebulge, and by concurrent rising eustatic sea level (Quinlan and Beaumont 1981). Thus, the timing and magnitude of relative sea level changes in Atlantic Canada was variable. Evidence of past sea levels on Les Iles de la Madeleine is limited, and a specific sea level curve for the islands has not been produced. However, the general pattern for the southern Gulf of St. Lawrence is that the combination of isostatic rebound and eustatic sea level rise resulted in coastal emergence followed by submergence (Quinlan and Beaumont 1981; Scott et al. 1987; Grant 1989). Observations of modern sea level rise in the region are approximately 0.35 m per century (Shaw and Forbes 1990). [FIGURE 1 OMITTED] Barriers on Les Iles de la Madeleine Les Iles de la Madeleine consist of seven major bedrock islands and six major sandy depositional barriers connected to the islands (Figure 1). Two of the sandy barriers are terminal spits, and the other four comprise two double tombolo systems, separated by shallow lagoons, that extend between six of the islands. …

Cyclone frequency during the last 5000 years at Curacoa Island, north Queensland, Australia

A series of storm deposits at Curacoa Island, a steep bedrock island in the Palm Group, north Queensland, record the passage of cyclonic events. Storm ridge accretion occurs when cyclonic waves and surge combine to transport coarse sediment, mostly broken coral, onto the shore and deposit it above normal tidal levels. Narrow fringing reefs at Curacoa act as a rapidly replenished sediment source, augmented by lithic gravel transported longshore. Beachface, stormberm, ridge platform and washover deposits can accrete during a single event. A series of storm ridges were examined via stratigraphic pits and trenches on two traverses; deposits from 22 events were identified and their ages were determined using 68 radiocarbon dates from coral clasts. Most dates from each storm deposit formed statistically accordant groups. Only 15% of the dates were older than the group ages and were interpreted as reworked specimens. Accordant ages within each group were combined and converted to calibrated radiocarbon ages. Ridge ages decrease uniformly with distance from the oldest ridge, indicating uniform depositional processes over the last 5000 years. A time series of storm deposit ages indicates that cyclone frequency was statistically constant over the last 5000 years. This suggests that cyclone frequency may not have been affected by sea surface temperatures in the region, which appear to have been about 1°C warmer about 5000 years ago (Gagan, M.K., Ayliffe, L., Hopley, D., Cali, J., Mortimer, G., Chappell, J., McCulloch, M., Head, M.J., 1998. Temperature and surface water balance of the mid-Holocene tropical western Pacific. Science 279, 1014–1018).

Sand dynamics at the mouth of a rock-bound, tide-dominated estuary

The lowermost Kennebec River is a relatively narrow, deep, bedrock-cut estuary situated along the highly indented, central coast of Maine. A large tidal prism and strong tidal currents control bedload sediment transport at the estuary mouth. Riverine sediment is introduced to the estuary during spring freshets when freshwater discharge may increase by an order of magnitude. During the same time, some of the existing estuarine sediment is exported to the nearshore. A barrier adjacent to the estuary is the product of an abundant riverine sand supply and numerous bedrock ridges, which anchor this sand deposit. A variety of sedimentologic, geomorphic, and hydraulic data collected over a 14-year period document a clockwise, sand-circulation cell that involves the exchange of bedload among the entrance channel to the estuary, adjacent beaches, nearshore, and offshore region. Tidal currents transport sand from the estuary through a subsidiary channel, bordered on both sides by bedrock islands, to an offshore subtidal bar. Northeast storm waves breaking along this outer bar move sand westward and landward toward the beach, leading to the development of large bar complexes. The welding of a bar complex to the western end of Hunnewell Beach occurs every 6–9 years and adds 2–300,000 m3 of sand to the shoreline. Sediment transport calculations along the outer bar are consistent with this volume of sand. The sediment gyre is completed as beach sands are moved eastward along the beach and dumped into the estuarine channel. This movement of sand is produced by wave action and tidal currents that enter the estuary peripherally during the flooding tide. During the ebb cycle, strong tidal currents flowing seaward through the subsidiary channel advect water from along the beach, which enhances sand transport into the estuary. Thirteen years of beach profile surveys demonstrate a good correspondence between bar welding events and short-term (1–4 years) accretionary shoreline changes. However, the volumetric changes of Hunnewell Beach cannot be accounted for by bar welding alone. The apparent movement of sand between Hunnewell Beach and other sand is further evidenced by the 135 year record of historical shoreline changes that reveal up to 200 m of shoreline excursions over a 26 year period. These data suggest that erosional–depositional trends along Hunnewell Beach are complex and the result of the exchange of sand among the adjacent beaches, nearshore zone, Fox and Wood Island tombolos, Kennebec River channel, and offshore region. The effects of major spring floods and strong bedrock influences produce patterns of sedimentation and a distribution of sand bodies at the mouth of Kennebec River that differ markedly from accepted estuarine models that are based on the relative importance of wave and tidal energy. This study demonstrates the need to consider structural controls when using these models.

Acid sulphate soil-landscape relationships in the Pearl River Delta, southern China

Relationships between the development of acid sulphate soil and landscape characteristics were examined in a fluvial-dominated, estuarine-deltaic plain of the South China coast. It was found that the accumulation of pyrite (potential acid sulphate material) has been limited generally by rapid sedimentation rates and insufficient supply of sulphate ion. The occurrence of the pyritic sediments is, however, closely associated with a particular depositional pattern, i.e. “surrounding island deposition”. The acid sulphate soils in the area frequently occur within narrow zones surrounding present-day's rocky hills (previously bedrock islands). This provides a guide for predicting the occurrence of potential acid sulphate soils in the area. Apart from the naturally large input of fluvial sediment, human activities have also accelerated the delta progradation and limited the vertical accretion of the deltaic surface. These effects have had significant impacts on the thickness and occurrence depth of the pyritic layer. With a history of more than 1000 years of land reclamation, the intensive cultivation in the area has turned most of the potential acid sulphate soils into actual acid sulphate soils, however, the soil properties vary markedly with landuse types. The case of the Pearl River Delta is generally in agreement with Pons and van Breemen's (1982) conclusion in that the accumulation of pyrite is inversely related to the rate of sedimentation. Our results show that the existence of bedrock islands in the estuary basin provided zones of slow sedimentation and therefore enhanced pyrite accumulation. On the other hand, human intervention over the past 2000 years has acted to reduce pyrite accumulation both through landward catchment clearing that increased sediment discharge to the estuary and through empolderment and associated land management practices.