Carbonate Beach Research Articles

Hybrid Modeling of Persistent Shoreline Oil Residues on Abu Ali Island, Saudi Arabia: Extent, Degree, and Remediation Implications

Extensive intertidal asphalt pavements and oiled sediment accumulations extend more than 20 km along the northern shoreline of Abu Ali Island, located north of Jubail on the Arabian (Persian) Gulf coast of Saudi Arabia. This shoreline oiling likely originated from two platforms in the Nowruz oil field, which spilled oil from 1983 to 1985; this was one of the largest marine spills in history, with shoreline impacts that were little known. In this study, we used a novel methodology that combined remote sensing analyses with hybrid machine learning–geostatistical modeling of field-collected data to quantify the distribution, extent, and volume of these contaminated sediments to investigate the mechanisms for their persistence and to support the development of remediation plans. After nearly 40 years, approximately 25,000 m3 of contaminated sediments remain, with nearly 50% of these buried underneath clean sediments. The presence of exposed or subsurface carbonate beach rock platforms or ramps clearly influences the ongoing persistence of these asphalt pavements by protecting them from physical energy and sediment mobilization. These rock platforms complicate potential remediation options, with more than 66% of the modeled volume of asphalt pavement estimated to be directly on top of and in contact with carbonate beach rock. The asphalt pavements present persistent ongoing PAH toxicity and continually shed smaller fragments when exposed to wave energy along with localized sheens and liquid oil, presenting a pathway for ongoing chronic exposure of biota.

An ancient analogue of carbonate beach complex: the Upper Triassic limestones of Tran Formation, southwestern Bulgaria

An ancient analogue of carbonate beach complex: the Upper Triassic limestones of Tran Formation, southwestern Bulgaria

Embayment morphometrics, granulometry and carbonate mineralogy of sandy beaches in the Maltese Islands

The beach and shoreface sediments in the Maltese Islands have accumulated in under-filled accommodation space conditions like many temperate and tropical island carbonate beach and shelf environments. Nevertheless, Maltese beach sediments show anomalously low biogenic carbonate content compared to other carbonate beaches. Fourteen carbonate sandy beaches in the Maltese Islands were sampled to examine the relationships between geological controls on both coastal lithology and configurations of bays and beaches and, grain characteristics described by granulometry and mineralogy. The provenance of the medium to coarse-grained beach sand is mainly from the erosion of coastal and shelf Cenozoic limestone that produces lithoclasts of low-magnesium calcite, with an additional 20 ± 10% biogenic grains derived from marine carbonate productivity. The relationships between geological controls and beach sediments are used to establish a general model of sedimentation along the Maltese shelf area. Mean beach sand size is controlled by the degree of embayment, decreasing where bays are narrower and longer, whereas sediment provenance affects granulometry by the contribution of biogenic grains to the coarse and fine grain size fractions. The mineralogy of the biogenic sediment fraction in beaches is mainly high-magnesium calcite and aragonite, with the latter decreasing significantly where coastal clay is found within or close to bays.

A multiproxy analysis of extreme wave deposits in a tropical coastal lagoon in Jamaica, West Indies

The Small Island Developing States (SIDS) of the Caribbean Region are vulnerable to natural hazards including earthquakes, tsunamis and tropical cyclones that can cause widespread devastation. Sedimentary archives of these hazards are often well-preserved in coastal lagoons; however, few studies in the Caribbean have adopted a multiproxy approach to their reconstruction. Here, we present a 1200-year multiproxy record of extreme washover events deposited within a coastal mangrove lagoon on the south coast of Jamaica. Manatee Bay lagoon is a permanent fresh-brackish-water mangrove lagoon separated from the Caribbean Sea by a low-elevation carbonate beach. Fifteen sediment cores recovered along five shore-normal transects contain ostracod-rich authigenic carbonate lake muds interspersed with beds of organic lake mud and mangrove peat. The cores contain evidence of multiple palaeo-washover deposits that are readily distinguished by their sedimentology, geochemistry and microfossil assemblages. Hypersaline conditions dominated the early part of the record (~ 800 to 900 CE), and we infer a freshening of lagoonal waters and the subsequent expansion of the mangrove community following an extreme wave event that occurred some time before ~ 1290 to 1400 CE. We constrain the primary historical-washover deposit to 1810–1924 CE (2σ; 71% probability), a period characterised by extreme tectonic and meteorological events, which include the Great Kingston Earthquake of 1907 and a local episode of enhanced hurricane activity. Whilst the balance of circumstantial evidence indicates that the deposit was probably emplaced during the tsunami generated by the 1907 earthquake, we are currently unable to differentiate between tectonically and meteorologically driven washover events based on their sedimentological characteristics.

Issues with integrating carbonate sand texture data generated by different analytical approaches: A comparison of standard sieve and laser-diffraction methods

Palaeoenvironmental reconstructive work relies on sediment-texture information as a proxy for particle-transport dynamics. As the integration of data from multiple sources is often sought for studies at the regional scale, a body of work exists that addresses compatibility levels of different analytical approaches. Past insights are derived predominantly from siliciclastic deposits. This study instead addresses standard sieve and laser-diffraction methods of texture analysis on a suite of carbonate beach sands from San Salvador Island, Bahamas. While the near-equant grain shapes of siliciclastic sands generate comparable results between methods, our findings show that laser diffraction tends to overestimate the coarser fractions in carbonate sands, given indiscriminate measurement of elongate particle axes. Peloidal grains that have width-to-length ratios on the order of 0.5 can pass through much smaller mesh sizes when sieved than diffraction would suggest. The resulting deviation in analytical outputs significantly influences measures of skewness, sorting, and other derivative metrics, which vary by up to several Wentworth classes between procedures. Meaningful integration of texture data obtained by different methods is ill-advised when dealing with carbonate beach sands. Similar problems are documented for muddy deposits, wherein platy clay minerals favor size-overestimation by laser diffraction and a size underestimation by settling tube technique. An improved understanding of methodological biases resulting from application of different particle-size methods is needed to help facilitate data-reconciliation efforts. This study provides a glimpse of issues facing the integration of carbonate sand-texture information, troublesome given prevalence of irregular grain shapes.

Erosion of Carbonate Beaches on the Northeastern Coast of Cuba

Rodriguez Paneque, R. and Finkl, C.W., 2020. Erosion of carbonate beaches on the northeastern coast of Cuba. Journal of Coastal Research, 36(2), 339–352. Coconut Creek (Florida), ISSN 0749-0208.Beaches on the northeastern coast of Cuba have made this area one of the most important tourist destinations in the Caribbean, despite the fact that many beaches are visibly eroded. In this study, the causes and magnitude of coastal erosion on the northeastern coast of Cuba were evaluated using the Digital Shoreline Analysis System (DSAS 4.3) application, satellite images, and beach profiles. Time frames between 7 and 14 years were considered from 2003 to 2017 using satellite images and between 11 and 18 years using beach profiles. Results of this investigation showed that 56% of northeastern coast beaches tend to erode at a rate of less than 1.2 m/y. An increase in the recurrence of extreme events (hurricanes and tropical storms) during the last 39 years induced more beach erosion. It was furthermore observed that El Nino–Southern Oscillation events enhanced the sedimentary balance of the beaches by returning sand volumes that were transported in a westerly direction to their original shoreline locations. Possibly due to sea-level rise, which may be occurring in response to global warming, beaches on the northeast coast will retreat on average about 0.17 m/y. At this rate of shoreline recession, by the end of the twenty-first century, 14% of the beaches on this littoral will be lost, and the width of 27% of the other beaches will be significantly reduced.

A comparative study of the 2013 typhoon Haiyan overwash sediments from a coastal cave and beach system at Salcedo, Eastern Samar, central Philippines

Typhoon Haiyan crossed the central Philippines on 8 November 2013 and generated storm surge up to 7 m with runup reaching 12 m which deposited overwash sediments inland in many different locations and environments. Collectively, these overwash sediments serve as key modern analogs to compare with geological records of storm events in the region. To date, comparatively few studies have been conducted on sandy storm deposits in carbonate coasts with none to date on storm sediments in coastal caves. Here, we report the grain size and microfossil assemblages of the Typhoon Haiyan overwash sediments from a coastal cave and carbonate beach system in Eastern Samar, central Philippines and infer the source of the Haiyan overwash sediments. We show that overwash sediments inside and immediately outside the cave (beach system) are similar but they can be discriminated from the underlying pre-Haiyan soil through field evidence, grain size, organic matter, and microfossil content. To investigate the source of both the cave and non-cave overwash sediments and in the absence of pre-storm beach sediment samples, we collected post-Haiyan beach sediments in February 2018 when the beach has indicated recovery to its pre-storm condition. We assume that these post-Haiyan beach sediments are representative of pre-storm sediments. The underlying pre-Haiyan soil comprises poorly-sorted, coarse to very coarse sands with higher organic matter content, and a lower microfossil concentration. Also, the pre-Haiyan soil contains foraminifera species similar to those identified in the Haiyan overwash sediments, but rarely containing subtidal foraminifera species. In comparison, the Haiyan overwash sediments share similar attributes with the post-Haiyan coastal sediments, consisting of moderately-sorted gravelly sands predominantly carbonate grains with low organic matter content. The Typhoon Haiyan overwash sediments contain foraminifera species dominated by Amphistegina radiata, Baculogypsina sphaerulata, Calcarina gaudichaudii, Calcarina splengeri suggesting an intertidal environment. Although, lower abundances of benthic species Peneroplis planatus and Quinqueloculina parkeri imply contributions from the subtidal environment. Notably, the Haiyan overwash sediments dominantly contain intertidal foraminifera species with abraded tests similar to foraminifera found in the post-Haiyan beach sediments. The similarity of foraminiferal assemblage and taphonomic condition strongly indicates that the overwash sediments found on the coastal cave were most likely sourced from the beach and reef flat. Our results show overwash sediments in coastal caves are indistinct from those in the nearby beach system indicating the potential of coastal caves to harbor well preserved archives of paleo storm deposits.

Carbonate Beach Sand of Abaiang Atoll, Kiribati: Geochemistry, Biogenic Sources, and Properties

Sandy beaches are critical resources for low-lying Pacific atoll communities, providing protection during storms, and land area for many coastal villages. Information on the nature of atoll beach sediment, its geochemistry and composition, can help to establish priorities to effectively protect the sources of Pacific island beach sediment. To understand sand sources, this study evaluated its physical characteristics including grain-size, geochemistry and composition, from windward and leeward beach profiles around Abaiang Atoll, Kiribati. Beach sand was >99% carbonate, averaging 37% coral fragments, 30% mollusc shells, 12% foraminifera, and 20% calcareous algae. Significant differences were found between reef and lagoonal sites in proportions of coral and mollusc fragments and foraminifera tests, with lagoon beaches having higher mollusc and coral proportions and lower foraminifera relative to reef beaches. This is attributed to high foraminiferal productivity offshore of reef beaches, and taphonomic durability of coral fragments in longshore drift into the lagoon. Mean sediment diameter increased from the upper to lower beaches at all sites, but fine sediment was lacking, attributed to its dissolution by rainfall and groundwater outflow. Geochemical analysis showed a mean of 84% Ca-Mg carbonates, of which 80% was calcium carbonate. There was no significant difference in the mean calcium percentage or calcium carbonate composition of the sediment between lagoon and reef beach sediment sources. Magnesium and magnesium carbonate content were significantly higher at reef sites relative to lagoon sites, attributed to higher proportions of foraminifera. Sediment-producing near shore habitats are critical to village protection through provision of beach sand, and this study shows the need to better conserve and manage coral reefs and habitats such as lagoon seagrass beds, to ensure continued atoll beach sand supply.

Impact of ocean acidification on the carbonate sediment budget of a temperate mixed beach

The production of sediments by carbonate-producing ecosystems is an important input for beach sediment budgets in coastal areas where no terrigenous input occurs. Calcifying organisms are a major source of bioclastic carbonate sediment for coastal systems. Increased levels of CO2 in the atmosphere are leading to an increase in the partial pressure of CO2 on ocean seawater, causing ocean acidification (OA), with direct consequences for the pH of ocean waters. Most studies of OA focus on its impact on marine ecosystems. The impact of OA on carbonate-producing ecosystems could be to reduce the amount of sediments supplied to temperate coastal systems. The aim of this study was to quantify the effect of the predicted OA on the long-term sediment budget of a temperate Mediterranean mixed carbonate beach and dune system. Based on projections of OA we estimated a fall of about 31% in the present bioclastic carbonate sediment deposition rate, with the biggest decreases seen in the dunes (− 46%). OA is also expected to affect the carbonate sediment reservoirs, increasing the dissolution of CaCO3and causing net sediment loss from the system (~ 50,000 t century−1). In the long-term, OA could also play a primary role in the response of these systems to sea-level rise. Indeed, the reduction in the quantity of carbonate sediments provided to the system may affect the speed with which the system is able to adapt to sea-level rise, by increasing wave run-up, and may promote erosion of dunes and subaerial beaches.

Biogenic sediments from coastal ecosystems to beach–dune systems: implications for the adaptation of mixed and carbonate beaches to future sea level rise

Abstract. Coastal ecosystems produce and store carbonate particles, which play a significant role in the carbonate dynamics of coastal areas and may contribute to the sediment budget of adjacent beaches. In the nearshore seabed of temperate zones (e.g. Mediterranean Sea and South Australia), marine biogenic carbonates are mainly produced inside seagrass meadows. This study quantifies the contribution of biogenic sediments, mainly produced in Posidonia oceanica seagrass meadows and secondarily in photophilic algal communities, to the sediment budget of a Mediterranean beach–dune system (San Giovanni beach, western Sardinia, western Mediterranean Sea). A set of geophysical, petrographic and sedimentological data was used to estimate the sediment volume and composition of the beach–dune system as a whole. The San Giovanni beach–dune system contains 3 797 000 ± 404 000 t of sediment, 83 % (3 137 000 ± 404 000 t) of which is located in the coastal wedge, 16 % (619 000 ± 88 000 t) in the dune fields and 1 % (41 000 ± 15 000 t) in the subaerial beach. The sediments are composed of mixed modern bioclastic and relict bioclastic and non-bioclastic grains from various sources. The system receives a large input of modern bioclastic grains, mainly composed of rhodophytes, molluscs and bryozoans, which derive from sediment production of present-day carbonate factories, particularly P. oceanica seagrass meadows. Radiocarbon dating of modern bioclastic grains indicated that they were produced during the last 4.37 kyr. This value was used to estimate the long-term deposition rates of modern bioclastic sediments in the various beach compartments. The total deposition rate of modern bioclastic grains is 46 000 ± 5000 t century−1, mainly deposited in the coastal wedge (39 000 ± 4 000 t century−1) and dunes (7000 ± 1000 t century−1), and 46 000 t represents ∼ 1.2 % of the total beach–dune sediment mass. Carbonate production from coastal ecosystems was estimated to be 132 000∕307 000 t century−1, 28 % (15 % ∕ 34 %) of which is transported to the beach–dune system, thus significantly contributing to the beach sediment budget. The contribution to the beach sediment budget represents a further ecosystem service, which our data can help quantify, provided by P. oceanica. The value of this sediment-supply service is in addition to the other important ecological services provided by seagrass meadows. The dependence of the beach sediment budget on carbonate production associated with coastal ecosystems has several implications for the adaptation of mixed and carbonate beaches to the loss of seagrass meadows due to local impacts and the changes expected to occur over the next few decades in coastal ecosystems following sea level rise.

Effect of Tropical Cyclones on Short-Term Evolution of Carbonate Sandy Beaches on Reunion Island, Indian Ocean

ABSTRACT Mahabot, M.-M.; Pennober, G.; Suanez, S.; Troadec, R., and Delacourt, C., 2017. Effect of tropical cyclones on short-term evolution of carbonate sandy beaches on Reunion Island, Indian Ocean. Carbonate sandy beaches in Reunion Island show various forms of evidence of erosion. Extreme waves associated with tropical cyclones (TCs) play a major role in beach dynamics. The present study analyzes and quantifies back-reef beach response and recovery from forcing generated by TCs Dumile, Felleng, and Bejisa, which occurred in 2013 and 2014. The study focuses on carbonate beaches of Reunion Island from Cap Champagne to the Passe de Trois-Bassins. Morphological and volumetric changes on beaches were analyzed by comparing 19 beach profiles. The results show that TCs are able to cause significant morphosedimentary change on the back-reef beaches of Reunion Island. These changes affect beach topography and involve longshore and cross-shore sediment transport. An alongshore variation in beach response is obse...

Building biogenic beachrock: Visualizing microbially-mediated carbonate cement precipitation using XFM and a strontium tracer

The fate of reef islands is a topic of ongoing debate in the face of climate change-induced sea-level rise and increased cyclone intensity. Increased erosion and changes to the supply of reef-derived sediment may put sand reef cays at risk of dramatic morphological changes. These changes may have negative implications for the existence of reef cay environments, which host vital sea turtle and bird rookery habitats. Beachrock, consolidated carbonate beach sediment in the intertidal zone, forms naturally on many tropical beaches and reduces the erosion rates of these beaches when compared to unconsolidated sand. In spite of the critical role beachrock plays in stabilizing some reef cay shores, the method of beachrock formation is still incompletely understood. In this investigation, beachrock was synthesized using beach sand and beachrock samples from Heron Island (Great Barrier Reef, Australia) in aquarium experiments in which natural beachrock formation conditions were simulated. Beachrock was produced in two aquaria wherein the water chemistry was influenced by microorganisms derived from the natural beachrock ‘inoculum’, whereas no cementation occurred in an aquarium that lacked a microbial inoculum and was controlled only by physicochemical evapoconcentration. The new cements in the synthesized beachrock were analyzed using synchrotron-based X-ray fluorescence microscopy and were identified using a Sr-tracer added to the experimental seawater. The resulting precipitates cement sand grains together and contain abundant microfossils of the microorganisms on whose exopolymer they nucleated, demonstrating the fundamental role microbes play in beachrock formation. These results are of interest because beachrock could be utilized as a natural coastline stabilization strategy on sand reef cays, and in turn, protect the unique habitats reef islands support.

A study of carbonate beach sands from the Yucatan Peninsula, Mexico: a case study

Sedimentological, geochemical, and X-ray diffraction (XRD) analyses in addition to the determination of biogenic fraction components (BFC) were performed in mollusk shells from carbonate beach samples collected on the northern coast of the Yucatan Peninsula, Mexico. The most abundant components of the BFC were bivalves and gastropods shells, ranging from 88.1 to 99.2 % and from 0.1 to 6.0 %, respectively, followed by minor percentages of foraminifera, coral fragments, bryozoa, oolites, spicules, and ostracods. A combination of major element, trace element, and XRD techniques were performed to show how the Mg/Ca and Mg/Sr ratios determine the possibility of a recrystallization process in bivalves and gastropods, which may be attributed to early diagenetic mechanisms caused by groundwater flows, subaerial exposure, high evaporation rates, and neomorphism. The rare earth element analyses gave the average anomalies for the following ratios (Pr/Yb)PAAS and MuQ ratio > 1, (Eu/Eu*)PAAS and MuQ and (Ce/Ce*)PAAS and MuQ < 1, suggesting little incorporation of sea water in the BFC of the shells from the Yucatan beach sands. This is also supported by the flat pattern trends of bivalves and gastropods based on the normalized patterns of light rare earth element compared to heavy rare earth element concentrations. This paper is also a contribution to the methodology of analysis of carbonate sands, i.e., whole bulk composition isolation into their main biogenic components and the accumulation of major, trace, and rare earth elements deposited into their shells, initially under subaquatic marine conditions and currently exposed to subaerial coastal influences based on the allochthonous character of the shells.

Sea-level controls on carbonate beaches and coastal dunes (eolianite): Lessons from Pleistocene Bermuda

© 2015 Geological Society of America. Coastal dune accumulations at locations around the world have formed during sea-level highstands, rising sea levels, falling sea levels, and fluctuating sea levels. Initially, episodes of maximum Pleistocene dune activity in Bermuda, which is the type location for eolianite, were correlated with glacio-eustatic regressions. This model had been abandoned by the 1970s in favor of a rising sea-level model for Bermuda's north shore and autogenic sediment-supply model for the south shore. We report the results of a detailed investigation of the intercalated mid-late Pleistocene beach and coastal dune deposits on the islands of Bermuda, and we test the existing models for these deposits. We contend that the north shore model is invalidated by past misidentification of eoliandeposits as transgressive beach deposits. On the south shore, facies analysis of the marine isotope stage 7 (MIS 7) Belmont Formation reveals that coastal deposition was divided into two phases. S1, which includes foreshore and shoreface progradation, is predominantly marine and coincided with rising relative sea level. S2, which commenced with protosol development on top of the marine section prior to burial by advancing dunes, is nonmarine. We conclude that the two successions S1 and S2 resulted from forcing by glacio-hydro-isostatic relative sea-level change spanning a highstand (rising and then falling), rather than from transgressive or autogenic processes. The finding that, in Bermuda, the construction of protective coastal dunes depends on a falling relative sea level has potential implications for many vulnerable shorelines that face rising relative sea levels associated with climate change.

Growth conditions and 3-D seismic delineation of carboniferous barrier reefs in the southwestern Tarim Basin

The Tarim Basin was a craton block during the Carboniferous period. Except for its open southwestern (SWW) deepening basin margin, the central part of the basin was surrounded by northern, eastern, and southern uplifts. In most parts of the basin, there are three layers of carbonate rocks called Biocalcarenite member, standard limestone member and Xiaohaizi Formation. The present-day Maigeti slope, located in the southwestern area of the basin, was ever a paleogeographic carbonate platform margin to clinoform. Boreholes in the area reveal over ten meters of high-energy biospararenite of beach deposition in Xiaohaizi Formation. This feature is quite different from the low-energy micritic limestone of equivalent horizon in the middle and northern basins. Such typical carbonate beach and platform margin backgrounds indicate an environment conducive for reef growth. According to an interpretation of 3-D seismic data on a 600 km2 Xinbaza survey, a 25 km-long barrier reef orientated in an east-westward direction is delineated with a hummocky geometry, a 1–2 km wide south-dip reef front, and a 3–6 km wide north-dip backreef. The reef buttress and outlines are also shown on an isopach map of the Xiaohaizi Formation as well as horizon slices of coherency, amplitude, and frequency attributes. The reef extends beyond the range of the rectangular 3-D seismic survey. The thickness of the Xiaohaizi Formation is over 60 m at the central reef, and 12 m larger than the outside of the reef. The barrier reef consists of segments separated by narrow tidal inlets. The neighborhood of the reef is mainly beach deposition of bioclasticgrainstones. Due to Late Permier and Late Tertial tectonic movements, the barrier reef was tilted to dip in a SWW direction. Its updip part was broken and misplaced by a set of reverse faults. Nevertheless, potential stratigraphic pools of a large amount of hydrocarbon reserves related to the reef are expected to be found in the future based on the substantial reef scale and the recent borehole indications beside it.

Australia's temperate carbonate coast: sources, depositional environments and implications

Abstract The 30 000 km-long Australian open coast is located between tropical and temperate latitudes (9°–42°S), with humid north and east coasts, and generally semi-arid–arid south and west coasts. The 9000 km-long temperate arid coast is dominated by carbonate shelf and inshore sediments that have supplied contemporary carbonate beach and dune systems, as well as very extensive Pleistocene calcarenite barrier systems that dominate much of the inner shelf, coast and near interior, and which, in places, have contributed to a blanketing of the downwind interior with calcareous loess deposits. Much of the contemporary coastal landscape and, to varying degrees, the adjacent interior is therefore dominated by a legacy of carbonate marine sediments. This paper reviews the reasons for the dominance of carbonate along this section of coast. It begins by examining the sources, followed by the transport and depositional mechanisms, the nature of the onshore carbonate deposits, including their diagenesis, and their longer-term impact on the coast.

Wind transport of foraminiferal tests into subaerial dunes: an example from western Ireland

Abstract. INTRODUCTIONThe empty tests of dead foraminifera behave as sedimentary particles and are subject to transport, although their different shapes and effectively low density means that their hydraulic equivalence is greater than that of spherical quartz grains (see Haake, 1962). Their estimated traction velocities range from c. 4 cm s−1 to c. 13 cm s−1 (Snyder et al., 1999). The presence of calcareous foraminiferal tests in a fossil sedimentary deposit would normally be taken as an indication of deposition in a marine environment. However, it has long been known that wind can transport tests from a carbonate beach into adjacent carbonate dunes as in Dogs Bay, Connemara, Eire (Murray, 1973) and Abu Dhabi, Persian Gulf (Murray, 1970). The purpose of this Notebook is to provide some details of the Dogs Bay occurrence and to comment on how such deposits might be recognized in the rock record.In western Ireland the coastal geology consists of hard rocks, yet the beaches are commonly composed primarily of calcareous bioclastic sands (Guilcher &amp;amp; King, 1961; Keary, 1967). Dogs Bay (Lat. 53° 24′ N Long. 9° 58′ W) lies on the west-facing side of a tombola which is approximately 200 m wide and 400 m long. The tombola is made up of sand dunes that are mainly vegetated except along the margins adjacent to the beaches. Dogs Bay is exposed to Atlantic storms and the surface layer of the beach is reworked on each tidal cycle. Both the beach and the dunes are composed of . . .

Morphodynamics of carbonate beaches in the Yucatán Peninsula

The concept of beach morphodynamics was used to classify the swash zone of carbonate sandy beaches in the central part of the state of Yucatán (Mexico). Several topobathymetric profiles were analyzed to determine the slope of the swash zone, shape parameter A of Dean’s model, and shoreline advance and recession. Grain size and wave data were also analyzed to determine the dimensionless fall velocity (Ω). The slope of the swash zone was in general greater during the tropical storm season than during the northerly wind (Nortes) season. Mean profile values according to shape parameter A ranged from –0.186 to –0.074. The Ω values indicated intermediate beaches with a reflective tendency. Shoreline change relative to beach width helped to identify and rank security levels of beach segments.

A sustained +21 m sea-level highstand during MIS 11 (400 ka): direct fossil and sedimentary evidence from Bermuda

A small, protected karstic feature exposed in a limestone quarry in Bermuda preserved abundant sedimentary and biogenic materials documenting a transgressive phase, still-stand, and regressive phase of a sea-level in excess of 21.3 m above present during Marine Isotope Stage (MIS) 11 (400 ka) as determined by U/Th dating and amino acid racemization. Cobbles and marine sediments deposited during the high-energy transgressive phase exhibit rim cements indicating a subsequent phreatic environment. This was succeeded stratigraphically by a still-stand deposition of fine calcareous lagoonal sediments containing bioclasts of red algae and benthic and planktonic foraminifera that was intensely burrowed by marine invertebrates, probably upogebiid shrimp, that could not be produced under any condition other than sustained marine submergence. Overlying this were pure carbonate beach sands of a low-energy regressive phase containing abundant remains of terrestrial and marine vertebrates and invertebrates. The considerable diversity of this fauna along with taphonomic evidence from seabird remains indicates deposition by high run-up waves over a minimum duration of months, if not years. The maximum duration has yet to be determined but probably did not exceed one or two thousand years. The most abundant snails in this fauna are two species indicative of brackish water and high-tide line showing that a Ghyben-Herzberg lens must have existed at > + 20 m. The nature of these sediments and fossil accumulation is incompatible with tsunami deposition and, given the absence of evidence for tectonic uplift of the Bermuda pedestal or platform, provide proof that sea-level during MIS 11 exceeded +20 m, a fact that has widespread ramifications for geologists, biogeographers, and human demographics along the world's coastlines.

リーフカレントによる事故状況と海浜の安全利用

A number of tourists would frequently visit coastal areas characterized by coral reef and carbonate beach in tropical and subtropical islands such as in Okinawa, Kagoshima and even in Tokyo, Japan. In Okinawa Prefecture for instance, nearly 6 million people visited as of last year and is projected to increase annually where 60% of them probably enjoy marine activities. However, about one hundred people have been involved in drowning accidents in Okinawa Prefecture per year due to lack of knowledge on nearshore current and topography, and swimming capability. Therefore, it is necessary to compile a set of data on drowning accidents in order to analyze the causes of such incidents, and to conduct a field study on the characteristics of current and topography in a coral reef area. In addition, guidelines for safe utilization of carbonate beach and coral reef, i. e. for swimming, snorkeling, scuba diving and wind surfing, should be proposed for public information and education in order to ensure safety of beach users. So far, it has been established that many drowning accidents are probably caused by a reef current (seaward current in a coral reef) which happens during ebb tide.