海南高隆湾海滩生物碎屑分布及其对沉积物粒度特征的影响

在热带砂质海岸,生物碎屑是海滩沉积物的重要组成部分,其分布和变化影响海滩沉积物的粒度特征。采用添加过量盐酸和双氧水的方法对海南高隆湾海滩表层沉积物进行了去生物碎屑前后粒径分布的对比,并结合海滩高程测量分析,结果表明:（1）海滩剖面上沉积物中生物碎屑的含量主要受到海水动力的影响,且生物碎屑颗粒较粗、重量较大,在高潮时由波浪流推上海滩并滞留。（2）海滩沉积物中生物碎屑的分布主要受三方面影响:一是地形及常涌浪向;二是区域中养殖业产生的生物碎屑;三是岸外发育的珊瑚礁坪。（3）海滩沉积物中生物碎屑的含量随季节变化,冬季至春季由海水供给的生物碎屑减少、进一步分解,使得北部海滩生物碎屑含量减少;而珊瑚碎屑的稳定供给和春季较强的波浪动力条件又使研究区南部断面的生物碎屑含量增加。（4）海滩沉积物中粗颗粒所含生物碎屑多于细颗粒中的生物碎屑,生物碎屑粒径主要为-1~2,即粗砂至细砾。沉积物中生物碎屑含量愈大,对粒径参数的影响也愈大。     

Microscopic morphology and elemental composition of size distributed atmospheric particulate matter in Urumqi, China

Microscopic morphology and elemental composition of atmospheric particulate matter (PM) in 13 different size fractions from 0.01 to 10 μm were studied using a Field Emission Scanning Electron Microscope with Energy-Dispersive Spectrometer (FESEM–EDX). The relative mass fractions exhibited a bimodal distribution with a major mode in the fine range (0.18–1 μm) and a minor mode in the coarse range (>1 μm), suggesting that the major pollution of PM is fine particles in this area of Urumqi atmosphere. The PM could be classified as follows: aluminosilicate/silica mineral, Si–Al rich fly ash, Fe oxide particle, Ti dominant particle, sulfate/carbonate crystal, carbonaceous aerosols (including soot, organic carbon, tar ball and irregularly shaped carbon). The soot and organic carbon with anthropogenic sources are dominant types in fine range samples (<1 μm). The natural source minerals and secondary synthesized sulfate/carbonate crystals were accumulated in the coarse range (>1 μm). Elemental composition of various types of particles (0.056–5.6 μm) was also analyzed by EDX. C, S, O, N, Si, Al, Fe, Ca, Na, K, Mg, Cl, F, Hg were detected in most samples. Si, Al and Ca accumulated in coarse fractions, while S and Hg mainly accumulated in fine fractions. Concentrations of 15 metallic elements in size range from 0.1 μm to 5.6 μm were divided into three groups based on their possible sources. (1) The crustal elements (Al, Mg, Fe, Mn and V), mainly present in coarse particles (>1 μm); and (2) the anthropogenic source elements (Ca, Ni, As, Cu, Pb, Cd and Hg). The concentrations of Ca and Ni increased with increasing particle size, while As, Cu, Pb, Cd and Hg showed opposite trends. As, Cu, Pb, Cd and Hg accumulated mainly in fine fraction (<1 μm). (3) The multi sources elements (Cr, Co and Se) possibly come from both natural and anthropogenic sources. High levels of heavy metals, especially Hg in nanosize particles, may pose great risk to human health.     

Carbonate drifts as marine archives of aeolian dust (Santaren Channel,Bahamas)

Sediment data from the Bahamian Santaren carbonate drift reveal the variability of trans‐Atlantic Saharan dust transport back to about 100 ka bp (Marine Isotope Stage 5·3) and demonstrate that carbonate drifts are a valuable pelagic archive of aeolian dust flux. Carbonate drift bodies are common around tropical carbonate platforms; they represent large‐scale accumulations of ocean‐current transported material, which originates from the adjacent shallow‐water carbonate factory as well as from pelagic production, i.e. periplatform ooze. Subordinately, there is a clay‐size to silt‐size non‐carbonate fraction, which typically amounts to less than 10% of the sediment volume and originates from aeolian and fluvial input. Sedimentation rates in the 5·42 m long core GeoHH‐M95‐524 recovered 25 km west of Great Bahama Bank in the Santaren Channel ranges from 1·5 to 24·5 cm ka^?1 with lowest values during the last glacial lowstand and highest values following platform re‐flooding around 8 ka bp . These sedimentation rates imply that carbonate drifts have not only the potential to resolve long‐term environmental changes on orbital timescales, but also millennial to centennial fluctuations during interglacials. The sediment core has been investigated with the aim of characterizing the lithogenic dust fraction. Laboratory analyses included X‐ray fluorescence core scanning, determination of carbonate content and grain‐size analyses (of bulk and terrigenous fraction), as well as visual inspections of the lithogenic residue; the age model is based on oxygen isotopes and radiocarbon ages. Data show that the input of aeolian dust in the periplatform ooze as indicated by Ti/Al and Fe/Al element ratios abruptly increases at 57 ka bp , stays elevated during glacial times, and reaches a Holocene minimum around 6·5 ka bp , contemporary to the African Humid Period. Subsequently, there is a gradual increase in dust flux which almost reaches glacial levels during the last centuries. Grain‐size data show that the majority of dust particles fall into the fine silt range (below 10 μm); however, there is a pronounced coarse dust fraction in the size range up to 63 μm and individual ‘giant’ dust particles are up to 515 μm in size. Total dust flux and the relative amounts of fine and coarse dust are decoupled. The time‐variable composition of the grain‐size spectrum is interpreted to reflect different dust transport mechanisms: fine dust particles are delivered by the trade winds and the geostrophic winds of the Saharan Air Layer, whereas coarse dust particles travel with convective storm systems. This mode of transport ensures continuous re‐suspension of large particles and results in a prolonged transport. In this context, grain‐size data from the terrigenous fraction of carbonate drifts provide a measure for past coarse dust transport, and consequently for the frequency of convective storm systems over the dust source areas and the tropical Atlantic.     

Changes in beach gravel lithology caused by anthropogenic activities along the southern coast of Lake Michigan, USA

The southern coast of Lake Michigan is the most urbanized and most densely populated area in the Great Lakes region. Development of steel mills, harbors, and municipalities in NW Indiana and in NE Illinois in the last century and a half altered the nearshore environment so much that native beach gravel (>8 mm) now exist only in the exhumed paleo-beach remnants from the Nipissing Phase (~4,500 years ago) of Lake Michigan. Native gravel, collected from paleo-beach remnants at Mount Baldy Dune and Beach House Blowout, contain predominantly beach shingle, very platy siltstones (71–78 %), with secondary crystalline pebbles (18 %) in the east, and carbonate pebbles (12 %) in the west. A large amount of anthropogenic fill (steel industry waste, waste from power generating plants, construction debris, railroad, and road fill) has been added since the late 1800s to fill Lake Michigan and expand industrial land. Four areas of major coastal structures—Michigan City Pier and Breakwater, Burns Harbor Pier and Breakwater, Gary Works Pier, and Indiana Harbor Peninsula—altered the natural littoral drift and created four independent sectors on Indiana’s coast—Northeastern, Eastern, Central, and Western—between which no natural transfers of coarse sediments occur. Downdrift from the coastal structures, severe beach erosion has prompted extensive beach nourishment with non-native sandy gravel. Four distinct populations of modern beach gravel now exist along Indiana’s coast of Lake Michigan: (1) native gravel with diluted beach nourishment influence, (2) native gravel with a minor industrial influence, (3) compact gravel of nourished origin, and (4) anthropogenic gravel of industrial origin. Native gravel with diluted nourishment influence is found in the western, downdrift areas of the Northeastern (from Long Beach to Washington Park Beach) and Eastern Sectors (from eastern Indiana Dunes State Park to western Dune Acres) and contains up to 40 % compact carbonate and crystalline pebbles in addition to native beach shingle. Native gravel with minor industrial influence is found in the Central Sector of Indiana’s coast (from central Ogden Dunes to Marquette Beach) and contains predominantly beach shingle, platy clastic lithology, but also up to 30 % of chert and other pebbles released by industry. Compact gravel of nourished origin contains 60–90 % of carbonate and crystalline pebbles, and is found in the eastern, updrift areas of the Northeastern (Michiana Beach and Duneland Beach) and Eastern Sectors (from Crescent Beach to the western Beverly Shores). Anthropogenic gravel of industrial origin contains 70–90 % compact chert and slag and is found in every beach of the Western Sector and in the westernmost beach of the Central Sector. Streams draining into southern Lake Michigan generally contain little coarse sediment except in their channels near the roads and railroads, where angular to subangular anthropogenic pebbles predominate (70–90 %). However, streams have very little influence on gravel lithology along the coast because they seldom discharge anthropogenic gravel into Lake Michigan. Recent changes in gravel lithology along the southern Lake Michigan coast may affect changes in nearshore benthic flora and fauna as well as algal and bacterial blooms during warm summer months.     

Sedimentary variations of geomorphic subenvironments at Al-Lith area,central-west coast of Saudi Arabia,Red Sea

Sediment characteristics are the indicators of the intensity and geological history of the processes active in an area. Their association with different geomorphic features also signifies the present day conditions of deposition. In this study, variations in sediment characteristics associated with different geomorphic features, such as the coastal zone, two islands and a lagoon in the Al-Lith area of central-west coast of Saudi Arabia have been analysed. Whereas, the detrital sediments (sand?+?mud) are common (61–87 %) in most of the subunits of the coastal zone; the nondetrital (carbonate rich) sediments are more common (54–95 %) on the two islands as well as the lagoon; indicating distinct sources of sediments in these regions. The variation of sediment texture between sand and sandy silt in most geomorphic units, also shows that they are exposed to high-energy conditions, whereas occurrence of heavy minerals in small proportions (<7 %) indicates limited inputs from land-based igneous and metamorphic rocks. Sediment mean size vs. standard deviation shows that the sediment characteristics of a geomorphic unit (e.g. beach or sand bar) on the coast and on the island are different owing to different processes responsible for their formation.     

Changing depositional environment revealed from sediment components,west of “Swatch of No Ground”, northern Bay of Bengal

The sediment cores of 20 cm length collected from 31 to 83 m range of water depth from the inner shelf of Bay of Bengal west of “Swatch of No Ground” were analyzed for grain size, organic carbon, total nitrogen, calcium carbonate and clay mineralogy. Grain size analysis revealed that the sediment size decreases from west to east within the study area indicating a possible direction of transport towards east parallel to the coast. The grain size also decreases with increasing water depth, i.e. north to south, indicating sediment contribution to the area of study from Hooghly and other local rivers. In the cores studied, illite was the dominant clay mineral which seems to be a product of glacial weathering under arid conditions with its source being the Himalayan region. The organic carbon content was low in the sediments and was attributed to low plankton production or intense oxidation processes. Low calcium carbonate percentage also supports the view of a low productivity in this region. Organic carbon increases gradually from west to east indicating its association with finer sediments and also indicates transport due to currents which drift the organic carbon away before it settles down within the sediments. Higher values relatively closer to the coast in the eastern side indicate additional contribution from Sunderban mangroves. The C/N ratio obtained indicates mixed source of terrestrial and marine for the organic matter in the sediments. Sedimentation rates available for deltaic plains and shelves off Bangladesh when adopted for the present area revealed that a 20 cm long sediment column was deposited in around 10 years. In most of the cores studied, a change in the trend of sediment components and organic carbon was observed at around 10 cm where grain size increases and organic carbon decreases towards the surface inferring that during the last 5 years, possible floods during NE monsoon or due to melting of ice in Himalaya are responsible for releasing additional water and material which brought a change in hydrodynamic conditions. The surface depletion of organic carbon indicates its dilution by the addition of coarser materials.     

Magnetic tracing of sediment dynamics of mudbanks off southwest coast of India

In this study, we utilized environmental magnetic in combination with sedimentological and hydrodynamic data to investigate the formative processes of mudbanks along southwest coast of India. We document the linkages between enrichment of silt-sized magnetic particles and formation processes of mudbanks along Alappuzha coast. A trend of increasing magnetite concentration and coarsening in magnetic grain size is observed at mudbank stations M2 and M3, while the mud-deficient station (M1) showed an opposite trend. A strong relationship between magnetic and physical grain size for all samples implies that the magnetic particle size and clastic grain size are largely adjunct. Analysis of rock magnetic and grain size data of surficial and suspended sediments from non-mudbank (M1) and two mudbank stations (M2, M3) reflect the differential sediment partitioning and transport regimes which controlled the formation of mudbanks along Alappuzha coast. Two plausible mechanisms responsible for the formation of mudbanks are identified: grain size-selective entrainment is the dominant process during pre-monsoon; weaker hydrodynamics (waves and bottom currents) favors accumulation of silt-sized (fine and coarse) magnetic and non-magnetic fractions resulting in the formation of magnetically low-enriched sediment bed of mixed grain sizes. At the onset of monsoon, wave-induced energetic bottom currents enhance the suspension of entire sediment bedload at stations M2, M3 to form fluid mud. Concurrently, mineral-density-based selective fractionation allows the settling of coarse silt-sized magnetic particles, while the fine magnetic silt-size particles accumulate forming thick fluid mud as a suspension load resulting in the formation of mudbanks. An observed increase in magnetic susceptibility and coarsening in magnetic grain size of surficial sediments at mudbank stations (M2, M3) during monsoon period supports the interpretation. Our findings are summarized in a conceptual model which can be very well applied to investigate sediment dynamics associated with mudbank formation in coastal and shelf sedimentary systems.     

Paleo-Tsunami deposits on the Red Sea beach, Egypt

The characteristics of the recent tsunami deposits threw light on some sources of deposition on the beach of the Red Sea. The studied area is delineated by latitudes 25°6′ N and 25°9′ N and longitudes 34°50′ E and 34°53′ E; it covers an elongate area of about 12 km² along the Red Sea coast, North Marsa Alam City (4 km). The area is bounded by Wadi Asalay to the north and Wadi Sifayn from the south. For evaluation of the area, a lot of information allows us to interpret the conditions prevailing during deposition of the sediments especially at the coast. To achieve the target, five wells were drilled to study core samples, well logging measurements, and 69 vertical electrical sounding stations were carried out. The studied area and adjacent areas were geologically surveyed to note geological features related to Paleo-earthquakes. From geological and geophysical studies, the dominant rock types at the western portions of the studied area are sandstone, sandy clay, clay, clayey sandstone, and gravels; at the middle portion of the studied area, the rocks are hard, but the eastern side of the area, especially at the beach of the Red Sea, several cycles of depositions of coral reefs occurred with intercalations of clastic deposits such as clay, sand, sandstone, conglomerate, gravels, pebbles, and a lot of fossils and shell fragments. The rocks are characterized by heterogeneous properties and ill-sorted. The area includes large numbers of faults due to highly tectonism of the area. The results indicated that the area has lateral variation of sediments. The carbonate rocks at the beach contain clastic fragments, and carbonate blocks are included within clastic rocks. With increasing the distance from the beach to the west, the sediments are less heterogeneous. The beach of the Red Sea was subjected to Paleo-tsunami waves due to highly Paleo-seismic activity inside the Red Sea and left their signature in geological column especially at the beach. The observation of some geological features such as Paleo-liquefaction and landslides indicate that the area subjected to strong earthquakes related to rifting of the Red Sea.     

An analysis of beach profile changes subsequent to the Colombo Harbor Expansion Project,Sri Lanka

Man-made coastal structures directly affect sediment balance and sediment dynamics on the surrounding beaches. The Colombo Harbor Expansion Project has created about 5-km-long breakwater nearly perpendicular to the beach. The present study is focused on quantitatively and qualitatively analyzing the effect of the Colombo Harbor Expansion Project on economically important beaches in and around Colombo city area. In this study, the authors measured monthly variations of beach width, beach profile and the mean grain-size of the sediments at mean sea level for complete annual monsoon cycle. Data were analyzed to establish site-specific erosion vulnerability. Monitoring results show that cumulative beach erosion has increased after the construction of the breakwater (rate = 0.7 m/year from May 2000 to April 2011 and rate = 28.2 m/year from April 2011 to June 2012). In addition, the cumulative and site-specific sand accretion and erosion patterns have a clear relationship with the monsoon seasonality. Beaches were narrower during the stormy southwestern monsoon, whereas beaches were wider during fair weather of northeast monsoon and inter-monsoon periods. In contrast, the constructed breakwater obstructs natural longshore sediment dynamics. For example, a significant amount of sediments from the Kelani-Ganga River were buried in the Colombo Harbor due to alteration of prominent longshore sediments transportation on the western coast of Sri Lanka. Therefore, this study shows enhancement of coastal erosion in the studied southern beaches due to a lack of sediment deposition.     

珠江口及琼东北海区元素地球化学特征及物质来源

为了更好的了解南海北部近陆表层沉积物特征及分布,通过对珠江口至海南岛东北海区进行表层沉积物取样测试,共获取了约3 500个样品粒度数据,700余个样品元素地球化学数据。该区域沉积物粒度以粉砂及细砂为主,分选性为较差及差,沉积物常量元素组合以SiO2、Al2O3、Fe2O3、CaO、K2O、MgO及Na2O为主,这7种氧化物占沉积物总量的90％以上（平均值）。对沉积物常量元素百分含量值进行了相关性分析,最终将调查海域沉积物划分为3个不同的区域,分别为珠江口及粤西区域（Ⅰ）、琼州海峡东分布区（Ⅱ）及琼东分布区（Ⅲ）,同时综合分析得出了各区域沉积物运移的路径趋势。     

Environmental texture and geochemistry of the sediments of a subtropical mangrove ecosystem and surrounding areas,Red Sea Coast,Egypt

The textural and geochemical aspects of the sediments of subtropical mangrove ecosystem and surrounding areas have been studied and discussed. Forty sediment samples were collected from different areas of mangrove environment and surrounding areas along the Egyptian Red Sea coast. The sediments of each study areas are characterized by the abundance of sand with minor amounts of mud and gravel. The mean size of the sediments ranged from medium grained to fine grained at the study areas. Cluster analysis showed that the distribution of gravel, sand, and mud fractions is related to bottom facies and type of sediment source. Generally, sand fraction is the main category among the three constituents. Carbonate content recorded minor values in the study areas. The CaCo₃ content of the sediments ranged from 4.7 % at Hamata area to 64.9 % at Erier area. Terrigenous and biogenic components are the factor controlling of the carbonate content of studies sediments. The organic carbon content ranged from 1.10 at Hamata area to 3.1 % at Sharm el-Qibli and Erier areas, which is controlled by particle size of the sediments. Phosphorus content in the different studied localities is related to the sources of phosphorus to the area. Our observations provide evidence that there are no significant differences in environmental texture and geochemistry of the sediments of subtropical mangrove ecosystem and surrounding areas of the Egyptian Red Sea coast.     

Geochemical elements characteristics and sources of the riverbed sediment in the yellow river’s desert channel

The Yellow River flows through an extensive area of aeolian desert and extends from Xiaheyan, Ningxia Province, to Hekouzhen, Inner Mongolia Province, for a total of 1,000 km. In the desert channel of the Yellow River, the major chemical components are SiO₂, Fe₂O₃, Al₂O₃, Ti, CaO, MgO, Ba, P, Mn, Ce, Co in which SiO₂, Al₂O₃, Fe₂O_3, and Ti have significant value as indicators of the sources of the coarse sediment. The spatial distribution of the elements’ weight percentages and the ratios of Si/Ti, Si/Al between sediment sources and riverbed sediment of the Gansu reach and the desert channel of the Yellow River, indicating that the coarse sediments deposited in the desert reach of the Yellow River are mostly controlled by the local sediment sources; and due to the higher mobility of the fine sediments, they are primarily contributed by the local sediment sources and the tributaries that originate from the loess area of the upper reach of Yellow River. The results of R-factor analysis illustrate that the first component score mainly shows negative values in the desert channel, suggesting that the coarse sediments in this reach contain a higher content of Si and a lower content of Mg + Ca + Al + Fe, which further proves that the Ningxia Hedong, Inner Mongolian Wulanbuhe, and Kubuqi deserts are the primary sources of the coarse sediment in the desert channel of the Yellow River.     

Depositional environment in and around Tamiraparani estuary,and off Tuticorin,Tamil Nadu,India: clues from grain size studies

The sediments of the estuaries and offshore Tuticorin along the southern coast of Tamil Nadu were studied for their textural variation. Ninety sediment samples were collected from three sectors (river, estuary, offshore) on the basis of prevailing energy conditions and oceanographic parameters. Frequency curves show unimodal to bimodal in nature. Offshore samples also fall in bimodal with medium and fine sand where sediment is additionally deposited by the Tamiraparani River. The mean values ranging from 1.69 to 2.01 φ with medium to fine sand. The fine sand in the riverine region indicates the depositional nature of the sediments. In pre-monsoon, the standard deviation ranges from 0.29 to 0.81 φ, which falls in the very well sorted to moderately well sorted. Estuary ranges from 0.63 to 1.30 φ, with moderately sorted, whereas in the marine sediments ranges from 0.4 to 1.01 φ, with moderately to moderately well- sorted and some patches of poorly sorted is observed. In both monsoons, the grain size variations are mostly influenced in river and offshore samples compared to the estuaries. The CM pattern indicates the deposition of sediments in graded suspension. In estuary, the sediments are medium sand with moderately to poorly sorted, fine skewed which are indicative of denudation processes taking place there. In marine, the sediments are medium-to-fine, moderately-to-well sorted, and fine skewed to very fine skewed sediments, probably as a result of the influence of palaeo-sediments deposited by rivers from inland as well as by waves and currents from offshore. Further, the marine samples also highlight depositional processes.     

Nearshore cool-water carbonate sedimentation and provenance of Holocene calcareous strandline dunes,southeastern Australia

The southeastern coastal plain of South Australia contains a spectacular and world-renowned suite of Quaternary calcareous eolianites. This study is focused on the provenance of components in the Holocene, actively forming sector, of these carbonate eolian deposits. Research was carried out along seven transects across a lateral distance of 120 km from ～30 m water depth offshore across the beach and into the dunes. Offshore sediments were acquired via grab sampling and SCUBA. Results indicate that dunes of the southern Lacepede and Bonney coasts are composed of siliciclastic particles (mainly quartz), relict allochems, Cenozoic and limestone pieces, but dominated by Holocene invertebrate and calcareous algal biofragments. The most numerous grains are from molluscs > benthic foraminifera ≥ coralline algae, > echinoids and > bryozoans. Most of these particles originate in carbonate factories such as macroalgal forests, rocky reefs, seagrass meadows and low-relief sea-floor rockgrounds. Incorporation of Holocene carbonate skeletons into coastal dunes, however, depends on a combination of: (1) the addition of infauna from intertidal and nearshore environments; (2) the physical characteristics of different allochems and their ability to withstand bioerosion, fragmentation and abrasion; (3) the character of the wave and swell climate; and (4) the nature of eolian transport. Most eolian dune sediment is derived from nearshore and intertidal carbonate factories. This is well illustrated by the abundance of robust infaunal bivalves that inhabit the nearshore sands and virtual absence of bryozoans that are common as sediment particles in offshore water depths >15 m. Importantly, the calcareous eolianites in this cool-water, open-platform carbonate setting are not simply an allochthonous reflection of the offshore marine shelf factories, but more a product of autothonous shallow nearshore–intertidal skeletal production and modification. These findings explain the preponderance of mollusc fragments and lack of bryozoans in similar older Pleistocene calcareous eolianites up to ca 1 million years old across ～2000 km of southern Australia with implications for the older rock record.     

Grain-size parameters of insoluble residues in mixed terrigenous-skeletal carbonate sediments and sedimentary rocks: some New Zealand examples

Grain-size parameters derived from the mechanical analysis of acid-insoluble residues from mixed terrigenous-shell beach sediments from Auckland, New Zealand, are more effective for interpreting the depositional processes, and for characterizing the beach environment on textural scatter plots, than are parameters based on the analysis of the total beach samples. This results mainly from the wide variations in size of hydraulically equivalent skeletal carbonate grains in the sediment, especially in the coarser size grades, but may also be accentuated by the local origin and susceptibility to alteration of the shell material. Compared with the total sediment grain-size distribution, the insoluble residues from the beach deposits are characteristically finer-grained, better sorted, and more consistently coarse-skewed and leptokurtic. The grain-size distribution characteristics of insoluble residues from several stratigraphic units in a sequence of Oligocene mixed terrigenous-skeletal shelf sediments in the South Auckland region distinguish depositional mechanisms and environmental energy levels for each unit that are consistent with interpretations made on the basis of carbonate petrography and palaeontology. With regard to the bulk content of terrigenous mud in the insoluble residues, the grain-size distribution of the fraction coarser than 4φ is alone diagnostic of the energy-time trends in these sediments. The grain-size parameters of the acid-insoluble residues in modern and ancient mixed terrigenous-skeletal carbonate sediments may provide more reliable criteria for distinguishing and characterizing the depositional environment of these deposits than do the parameters obtained from the size distribution of the total grain population.     

Impact of river damming on sediment texture and trace metals distribution along the watershed and the coastal zone of Nestos River (NE Greece)

This study examines the sediment particle size distribution and the trace metal concentrations from a dammed-river watershed (Nestos River) to its deltaic zone in NE Greece. The study area is relatively unpolluted. The distribution of trace metals (Cu, Cr, Cd, Ni, Pb, Hg) in sediments throughout the catchment area showed selective “trapping” of certain elements behind the two artificial dams (Thissavros and Platanovrisi dams) in the watershed and a sudden reduction downstream (83% for Cd, 81% for Cr, 94% for Cu, 90% for Ni, 86% for Hg and 33% for Pb). Marked sediment particle separation is observed at the upstream dam (Thissavros), where coarse material including sand is trapped (coarse fraction 12.9–49.3%). Fine-grained material (<63 μm) is trapped behind the Platanovrisi dam (68.1%), and the reservoir showed elevated metal concentrations, especially for Cu and Cd (16.3 and 0.5 μg/g, respectively). Lead exhibited a homogenous distribution throughout the watershed (20.1–32.3 μg/g). All other trace metals (Cu, Cr, Cd, Ni and Hg) decline sharply downstream of the dam complex. In the delta system, nearshore sediments consist of shallow deposits in the vicinity of river mouth and are enriched in Cr (4.4–53.0 μg/g) and Ni (2.6–44.3 μg/g), while the further offshore and slightly deeper (20–40 m) sediments illustrate elevated Hg (0–0.07 μg/g), Cd (0.09–0.18 μg/g), Cu (11.5–18.3 μg/g) and Ni (38–54.5 μg/g).     

Sedimentological characteristics of the surficial sediments of the Kuwaiti marine environment, northern Arabian Gulf

The textural characteristics, carbonate content and the coarse fraction components of the Recent bottom sediments of the marine environment off Kuwait are described and the faunal-sediment associations discussed. The sediments were subdivided into seven textural classes, namely sand, silty sand, muddy sand, sandy silt, sandy mud, silt and mud. Most of the study area is covered with muddy sediments whereas sandy deposits are restricted to the rocky bottoms near the southern flat of Kuwait Bay, the southern coast of Kuwait and around the islands and bathymetric highs. The textural classes, carbonate contents and faunal types of the coarse fraction were used to construct a biolithofacies map of the marine bottom sediments off Kuwait, in which nine facies are identified. The sedimentological characteristics of the Recent marine bottom sediments off Kuwait reflect the interaction between autochthonous calcareous fragments mostly of biogenic origin, lime rock fragments derived from beachrocks and submerged reef flats, and allochthonous terrigenous detritus transported to the area mainly by dust storms. The Kuwaiti offshore area is generally a low energy depositional environment with little sediment transport.     

Application of Plutonium Isotopes to the Sediment Geochronology of Coarse-Grained Sediments from Englebright Lake,California (USA)

The determination of sediment accumulation rates in environments with temporal variations in texture is challenging using traditional radioisotope methods, largely due to low activities associated with coarse sediments. This study used Englebright Lake, an impoundment in northern California, as a model system to examine the application of plutonium isotopes in lacustrine environments where the interlayering of coarse and fine sediments complicates the geochronology. Inductively coupled plasma mass spectrometry was used to quantify plutonium isotopes and low limits of detection allowed for the measurement of plutonium in sand, clay, and silt fractions. Although measurable levels of plutonium were found in sand fractions, over 75 % of the total plutonium activity was found in fine-grain-size fractions (<63 μm). Correlations between cesium-137 and plutonium activities in fine-grained sediments (r = 0.81–0.98, p < 0.005) suggest that plutonium isotopes may be substituted for cesium isotopes in coarse-grained sediments where cesium is typically below detectable levels. Sediment accumulation rates calculated from grain-size normalized plutonium activity profiles ranged from 6 to 145 cm year^?1 in Englebright Lake and identified a sediment depocenter at the delta front upstream of Englebright Dam. Progradation of the delta front reflected changes in sediment supply from the watershed in response to flood events, whereas average annual accumulation responded to human impacts. This study extends the application of plutonium isotopes for sediment geochronology to aquatic environments dominated by coarse sediments and provides new information that contributes to a better understanding of the processes influencing sediment deposition in Englebright Lake.     

Modern carbonate and terrigenous clastic sediments on a cool water, high energy, mid-latitude shelf: Lacepede, southern Australia

The wide Lacepede Shelf and narrow Bonney Shelf are contiguous parts of the south-eastern passive continental margin of Australia. The shelves are open, generally deeper than 40 m, covered by waters cooler than 18°C and swept by oceanic swells that move sediments to depths of 140 m. The Lacepede Shelf is proximal to the ‘delta’of the River Murray and the Coorong Lagoon. Shelf and upper slope sediments are a variable mixture of Holocene and late Pleistocene quartzose terrigenous clastic and bryozoa-dominated carbonate particles. Bryozoa grow in abundance to depths of 250 m and are conspicuous to depths of 350 m. They can be grouped into four depth-related assemblages. Coralline algae, the only calcareous phototrophs, are important sediment producers to depths of 70 m. Active benthic carbonate sediment production occurs to depths of 350 m, but carbonate sediment accumulation is reduced on the open shelf by continuous high energy conditions. The shelf is separated into five zones. The strandline is typified by accretionary sequences of steep shoreface, beach and dune carbonate/siliciclastic sediments. Similar shoreline facies of relict bivalve/limestone cobble ridges are stranded on the open shelf. The shallow shelf, c.40–70 m deep, is a wide, extremely flat plain with only subtle local relief. It is a mosaic of grainy, quartzose, palimpsest facies which reflect the complex interaction of modern bioclastic sediment production (dominated by bryozoa and molluscs), numerous highstands of sea level over the last 80 000 years, modern mixing of sediments from relatively recent highstands and local introduction of quartz-rich sediments during lowstands. The middle shelf, c.70–140 m deep, is a gentle incline with subtle relief where Holocene carbonates veneer seaward-dipping bedrock clinoforms and local lowstand beach complexes. Carbonates are mostly modern, uniform, clean, coarse grained sands dominated by a diverse suite of robust to delicate bryozoa particles produced primarily in situ but swept into subaqueous dunes. The deep shelf edge, c. 140–250 m deep, is a site of diverse and active bryozoa growth. Resulting accumulations are characteristically muddy and distinguished by large numbers of delicate, branching bryozoa. The upper slope, between 250 and 350 m depth, contains the deepest platform-related sediments, which are very muddy and contain a low diversity suite of delicate, branching cyclostome bryozoa. This study provides fundamental environmental information critical for the interpretation of Cenozoic cool water carbonates and the region is a good model for older mixed carbonate-terrigenous clastic successions which were deposited on unrimmed shelves.     

Textural characteristics of sediments along the southern Red Sea coastal areas,Saudi Arabia

A lack of understanding exists of the origin and textural characteristics of Saudi Arabian Red Sea coastal sediments. This paper concerns the southern coastline of Jizan on the Saudi Red Sea. It is some 160 km long characterised by either narrow rocky headlands with intermittent pocket beaches or wide low-lying beaches dissected by wadis. Granulometric testing of samples from 135 locations showed that beach sand size was mainly very fine to medium grained (M _z = 3.93 Ø), sorting ranged from 1.65 to 0.41 and skewness values from ?051 to 0.39, being mainly negative; dune sands were medium to fine grained (M _z = 1.13 Ø; average sorting 2.8), while skewness variations within dune samples indicated symmetrical to fine skewed values (б _Ι = 0.55 to 0.89). Most foreshore samples were derived from wadis. Wadi mud levels can be high, e.g. Baysh (84%), and wadi Samrah (90%) with mean grain size ranging from very fine to medium sand (M _z = 3.9 Ø), sorting being well to poor (0.45 to 1.52) due to sediment influxes. Sabkha had a wide range of sand/mud and significantly higher carbonate percentages than other environments. Sediment source differences and littoral reworking contributed to grain size variation. The carbonate content varied between 1.5 and 31.5% due to hinterland contributions, and spatial analysis showed increasing quantities of carbonate minerals towards the south. On the wider geographical front, findings from Jizan are similar to those of the Northern United Arab Emirates (UAE), including sabkhas, being composed of sand, skeletal carbonate, fine fluvial material and wind-blown silt and clay components of wadi origin. Further work on the northeastern Red Sea edge can hopefully confirm these findings.