Geomorphic portrait of the Little Rann of Kutch

Little Rann of Kutch is an integral part of the Rann of Kutch, occupying southeastern part of it. It remains a saline-inundated regime, partially seasonal and partially around the year. The study area experiences a very high ambient temperature at one point of time while it gets flooded by both the tidal water ingression and freshwater poured by the seasonal rivers during the monsoon season. And as a result, the area experiences a seasonal reversal of geomorphic processes which in turn is reflected through its distinctive and dynamic landforms. Geologically, this area was a part of oceanic floor and has immerged in the recent past. Geomorphology of the area plays a dominant role in the pattern and cause of saline water intrusion. The surface of Rann is at or slightly above sea level and possesses a monotonously plain character with some outcrop in the form of sandstone capped by basalts, resembling to island in the mid of the Rann. During 3 months of the southwest monsoon, storm tides, aided by the wind, force water from the Arabian Sea over the flat surface of the Ranns. Rainfall is fairly low, so that as the water recede and evaporate, they leave behind a crust of halite and gypsum crystals which grow in the clay and sands. This monotonously saline flat surface with annual inundation, have executed the Rann a mysterious piece of terrain. The characteristics and role of sediments in terms of erosional and depositional characteristics, their plasticity, facilitating capillary rise, their hygroscopic absorption capacity, infiltration capacity, permeability, etc. gives both short- and long-term implication to the area. Apart from the above concern, the soil has a great deal to play in terms of the vegetal and faunal cover. Considering these above characteristics, study area has been regionalized in order to have a better understanding and planning for the study area. The study area being an economic zone, also confronts with several environmental problems.     

Storm‐dominated sedimentation in a protected basin fringed by coral reefs,Nara Inlet,Whitsunday Islands,Great Barrier Reef,Australia

Hydrodynamic data and samples of bed sediment were collected from Nara Inlet, a small incised embayment in the Whitsunday Islands, central Great Barrier Reef. Measured tidal currents in the inlet do not exceed 0.2 ms^–1 even at spring tides. Swell waves dominate much of the inner shelf of the Great Barrier Reef but are absent from in the inlet due to the presence of a fringing reef at the inlet mouth. On the silty sand floor of the inlet, particle size decreases towards the inlet head. Most of the bed is too coarse to be remobilised by fair‐weather wave and tides, and we predict that bedload sediment transport thresholds are only exceeded in the inlet during cyclones. The observed distribution of bed sediments is consistent with landward dispersal of sediment under storm conditions. Over 20 m of (presumably Holocene) sediments occurs in the inlet and the seismic character of the infill is consistent with the observed textural variation of the modern sediments. We infer that sediment accumulation on the floor of the inlet has been storm dominated throughout much of the Holocene.     

Contents volume 40

The Attawapiskat River is one of the major low-gradient rivers which cross the flat wetlands of the Hudson Bay Lowland. It has a nival regime leading to strong, short-duration spring floods during which considerable amount of sediment is carried. For the remainder of the year only organic matter is transported in solution and suspension. The land is isostatically rebounding after the Pleistocene glaciations. The river downcuts through the softer Holocene estuarine basal clays and the Pleistocene tills, to the Paleozoic carbonate bedrock. The river develops an anastomosing pattern in the lower reaches and it acquires an irregular meandering character inland. It does not construct a significantly thick delta as the river-borne sediments are dispersed along the coasts by tides, longshore currents and ice rafting. The river is subjected to frequent ice jams in its lower reaches as warmer southern waters flow toward the still frozen mouth and sea. During the jams, secondary channels become active, floodings develop and sedimentation occurs on the levees. Ice scours and ice-rafted materials affect the river banks greatly.The principal sedimentary environments within the river consist of: (1) erosional shoals covered by boulder and pebble lags over till: (2) sorted coarse to medium-sized sands in junction bars downstream from islands; these sands frequently develop gas bubbles, and they contain characteristic fining-upward sequences from boulder pavements at the base, grading up into rippled sand with abundant organic matter, grading up into laminated silt and sand and organic matter; (3) secondary channels, which develop thin silty and clay drapes on eroded hard substratum and are filled eventually by thick peats; and (4) river banks which show well-developed regressive sequences from Pleistocene tills at the base, overlain by estuarine sparsely fossiliferous clays, capped by thin conglomeratic units with abundant reworked Macoma balthica shells, grading upward into upper tidal flat sand, irregularly laminated sand and silt of marshes, and regular interbedding of sand, silt and organic layers of the levees at the top.     

Pb isotopic variability in the modern-Pleistocene Indus River system measured by ion microprobe in detrital K-feldspar grains

The western Himalaya, Karakoram and Tibet are known to be heterogeneous with regard to Pb isotope compositions in K-feldspars, which allows this system to be used as a sediment provenance tool. We used secondary ion mass spectrometry to measure the isotopic character of silt and sand-sized grains from the modern Sutlej and Chenab Rivers, together with Thar Desert sands, in order to constrain their origin. The rivers show a clear Himalayan provenance, contrasting with grains from the Indus Suture Zone, but with overlap to known Karakoram compositions. The desert dunes commonly show ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁶Pb/²⁰⁴Pb values that are much higher than those seen in the rivers, most consistent with erosion from Nanga Parbat. This implies at least some origin from the trunk Indus, probably reworked by summer monsoon winds from the SW, a hypothesis supported by bulk Nd and U-Pb zircon dating. Further data collected from Holocene and Pleistocene sands shows that filled and abandoned channels on the western edge of the Thar Desert were sourced from Himalayan rivers before and at 6-8 ka, but that after that time the proportion of high isotopic ratio grains rose, indicating increased contribution from the Thar Desert dunes prior to ∼4.5 ka when flow ceased entirely. This may be linked to climatic drying, northward expansion of the Thar Desert, or changes in drainage style including regional capture, channel abandonment, or active local Thar tributaries. Our data further show a Himalayan river channel east of the present Indus, close to the delta, in the Nara River valley during the middle Holocene. While this cannot be distinguished from the Indus it is not heavily contaminated by reworking from the desert. The Pb system shows some use as a provenance tool, but is not effective at demonstrating whether these Nara sediments represent a Ghaggar-Hakra stream independent from the Indus. Our study highlights an important role for eolian reworking of floodplain sediments in arid rivers such as the Indus.     

Pesticides associated with suspended sediemnts entering San Francisco Bay following the first major storm of water year 1996

Estuaries receive large quantities of suspended sediments following the first major storm of the water year. The first-flush events transport the majority of suspended sediments in any given year, and because of their relative freshness in the hydrologic system, these sediments may carry a significant amount of the sediment-associated pesticide load transported into estuaries. To characterize sediment-associated pesticides during a first-flush event, water and suspended sediment samples were collected at the head of the San Francisco Bay during the peak in suspended sediment concentration that followed the first major storm of the 1996 hydrologic year. Samples were analyzed for a variety of parameters as well as 19 pesticides and degradation products that span a wide range of hydrophobicity. Tidal mixing at the head of the estuary mixed relatively fresh suspended sediment transported down the rivers with suspended sediments in estuary waters. Segregation of the samples into groups with similar degrees of mixing between river and estuary water revealed that transport of suspended sediments from the Sacramento-San Joaquin drainage basin strongly influenced the concentration and distribution of sediment-associated pesticides entering the San Francisco Bay. The less-mixed suspended sediment contained a different distribution of pesticides than the sediments exposed to greater mixing. Temporal trends were evident in pesticide content after samples were segregated according to mixing history. These results indicate sampling strategies that collect at a low frequency or do not compare samples with similar mixing histories will not elucidate basin processes. Despite the considerable influence of mixing, a large number of pesticides were found associated with the suspended sediments. Few pesticides were found in the concurrent water samples and in concentrations much lower than predicted from equilibrium partitioning between the aqueous and sedimentary phases. The observed sediment-associated pesticide concentrations may reflect disequilibria between sedimentary and aqueous phases resulting from long equilibration times at locations where pesticides were applied, and relatively short transit times over which re-equilibration may occur.     

River tributaries as sediment sinks: Processes operating where the Tapajós and Xingu rivers meet the Amazon tidal river

The Amazon River is the largest fluvial source of fresh water and sediment to the global ocean and has the longest tidally influenced reach in the world. Two major rivers, the Tapajós and Xingu, enter the Amazon along its tidal reach. However, unlike most fluvial confluences, these are not one‐way conduits through which water and sediment flow downstream towards the sea. The drowned‐river valleys (rias) at the confluences of the Tapajós and Xingu with the Amazon River experience water‐level fluctuations associated not only with the seasonal rise and fall of the river network, but also with semidiurnal tides that propagate as far as 800 km up the Amazon River. Superimposed seasonal and tidal forcing, distinct sediment and temperature signatures of Amazon and tributary waters, and antecedent geomorphology combine to create mainstem–tributary confluences that act as sediment traps rather than sources of sediment. Hydrodynamic measurements are combined with data from sediment cores to determine the distribution of tributary‐derived and Amazon‐derived sediment within the ria basins, characterize the sediment‐transport mechanisms within the confluence areas and estimate rates of sediment accumulation within both rias. The Tapajós and Xingu ria basins trap the majority of the sediment carried by the tributaries themselves in addition to ca 20 Mt year^?1 of sediment sourced from the Amazon River. These findings have implications for the interpretation of stratigraphy associated with incised‐valley systems, such as those that dominated the transfer of sediment to the oceans during lowstands in sea level.     

Mineralogy,chemistry, and grain size composition of recent sediments in the northern Tyrrhenian Sea: Contribution to the study of sediment transport and distribution

As a contribution to an environmental study on Tuscany's marine shelf, textural, mineralogical, and chemical characteristics of recent sediments in the northern Tyrrhenian Sea between Elba Island and Livorno have been analyzed. The data provide clear information on sediment sources and movement patterns. The main sediment source appears to be the Cecina River; very subordinate is the contribution of the Fine River. As to sediment movement patterns, the surveyed basin appears to be divided into two subbasins: (1) a greater, southern subbasin, bounded by Tuscany mainland, Elba Island and Elba Ridge, and Capraia Isle, and Vada Shoals; and (2) a smaller, northern subbasin, enclosed between Vada Shoals and Meloria Shoals (off Livorno port).In the former the coarse-grade Cecina River sediments are transported southward along most of the mainland coast by littoral drift and coastal current; a northward flowing countercurrent seems to be active only in the southernmost part. Silts and clays are distributed in concentric belts around basin's central part, being transported mainly by slow circular water movements. In the marginal western zone, very little terrigenous influx is coupled with a high biologic productivity, and biogenic debris sedimentation prevails there.In the northern subbasin the sediment contribution from Fine River is more appreciable. Here the environmental energies are very low and the coarse sediments are restricted to a few small pocket beaches where they are deposited by local creeks. Most of the subbasin area is occupied by sapropelitic muds, again distributed by slow small-scale gyres.     

Tidal river sediments in the Washington,D.C. area. I. Distribution and sources of trace metals

Thirty-three bottom sediments were collected from the Potomac and Anacostia rivers, Tidal Basin, and Washington Ship Channel in June 1991 to define the extent of trace metal contamination and to elucidate source areas of sediment contaminants. In addition, twenty-three sediment samples were collected directly in front of and within major storm and combined sewers that discharge directly to these areas. Trace metals (e.g., Cu, Cr, Cd, Hg, Pb, and Zn) exhibited a wide range in values throughout the study area. Sediment concentrations of Pb ranged from 32.0 μg Pb g^?1 to 3,630 μg Pb g^?1, Cd from 0.24 μg Cd g^?1 to 4.1 μg Cd g^?1, and Hg from 0.13 μg Hg g^?1 to 9.2 μg Hg g^?1, with generally higher concentrations in either outfall or sewer sediments compared to river bottom-sediments. In the Anacostia River, concentration differences among sewer, outfall, and river sediments, along with downriver spatial trends in trace metals suggest that numerous storm and combined swers are major sources of trace metals. Similar results were observed in both the Tidal Basin and Washington Ship Channel. Cadminum and Pb concentrations are higher in specific sewers and outfalls, whereas the distribution of other metals suggests a more diffuse source to the rivers and basins of the area. Cadmium and Pb also exhibited the greatest enrichment throughout the study area, with peak values located in the Anacostia River, near the Washington Navy Yard. Enrichment factors decrease in the order: Cd>Pb>Zn>Hg>Cu>Cr. Between 70% and 96% of sediment-bound Pb and Cd was released from a N₂-purged IN HCl leach. On average, ≤40% of total sedimentary Cu was liberated, possibly due to the partial attack of organic components of the sediment. Sediments of the tidal freshwater portion of the Potomac estuary reflect a moderate to highly components area with substantial enrichments of sedimentary Pb, Cd, and Zn. The sediment phase that contains these metals indicates the potential mobility of the sediment-bound metals if they are reworked during either storm events or dredging.     

Sedimentary conditions and deposits of Akimiski Strait,James Bay,Canada

Akimiski Strait is a wide (17–20 km), shallow, emergent (0.70 cm per century) waterway in James Bay. It is localized in a saddle of a Paleozoic reef track, which has been enhanced and molded by Pleistocene glaciers. Drumlinoid ridges form the till cores of shoals and islets of the strait. The boundary conditions of the strait change throughout the year, as it is covered by ice for six months. During spring break-up the strait remains clogged with ice at its northern approach for several weeks, and acts as a large tidal inlet. It is during this period that most of the fluvial sediments are carried to sea. Other sediments are obtained by erosion of the Pleistocene tills and Holocene subtidal clays and silts exposed in nearshore areas. Resuspension of nearshore material is achieved through the action of wind-driven, short choppy waves and ice scour. Tides are the most important process for the redistribution of sediments along the coast, both flooding onshore and flooding and ebbing into and out from the strait generating locally powerful (2 m s^?1) reversing currents. Ice rafting and ice pushing are important processes in this frigid environment, particularly in upwind sides of shaols, and at/or near river mouths.Different intertidal sedimentary sequences develop as functions of sediment supply and exposure of the environments to ice, currents and waves. The eastern shores and the southern shoals of the strait develop pebble lags over till, covered by thin (5–20 cm) drapes of silty sand trapped and protected from erosion by algae. In these shores and in emerging small islands significant sedimentation (1–1.5 m thick) occurs in the marshes where the suspended load of tidal waters is trapped by vegetation. The western shores of the strait receive considerable amounts of sediment from large rivers and are affected by strong tidal longshore currents. Thick (3–4 m) and narrow tidal flats and marshes develop on the maincoast. The shoals of the northern part of the strait have characteristic sediments. Those near the western shore have thin (up to 80 cm) tidal silty sand deposits, locally heavily burrowed by Macoma balthica. Those strung across the northern approach to the strait have well-developed, thin, coarse sand dune fields, indicating a prevalent ebb flow out of the strait.     

Temporal and spatial evolution of surface sediments characteristics in the Dagu River estuary and their dynamic response mechanism

Based on the 39 surface sediment samples collected in the flood season and the dry season in 2012 respectively and the measured hydrological data in October 2012, the sediment grain size characteristics has been analyzed and the response mechanism of surface sediments to estuarine hydrodynamics was revealed by calculating the range of waves and tidal currents. The results show that: (1) The grain size of the surface sediment samples decreased gradually from land to sea in the flood season. The fine sediment was redistributed under marine hydrodynamics in the dry season and the sediments showed coarser tendency ingeneral; (2) tidal current stirring sediment was very obvious in Dagu River estuary area, and wave stirring sediments mainly occurred in the tidal flat area and estuary sand bar area; (3) in the flood season, surface sediment sat the estuary were transported towards south and southeast. In the dry season, surface sediments were transported towards southwest at the north area of Jiaozhou Bay Bridge, and sediments were transported towards northeast area at the south of Jiaozhou Bay Bridge.     

Depositional controls on Palaeoproterozoic iron formation accumulation, Gogebic Range, Lake Superior region, USA

Shallowing‐upward, decametre‐scale, Palaeoproterozoic iron formation cycles in northern Wisconsin record the combined effects of tectonism and changing oceanographic conditions on a storm‐dominated shelf. Cycles consist of a lower unit of laminated, Fe‐ and Si‐rich chemical mudstone that is transitional into an upper unit dominated by trough cross‐stratified chert grainstone. Grainstone lenses become progressively thicker upwards in cycles with the largest at cycle tops, where they are sharply overlain by a unit of slumped chemical mudstone. The cycles developed through progradation when offshore‐directed storm currents transported chert sand intraclasts that were formed in nearshore settings into middle and distal shelf environments. Abrupt subsidence events, probably resulting from normal faulting associated with extensional tectonism, repeatedly terminated chert grainstone accumulation and may also have generated the slumped units at cycle boundaries. The episodic storm currents are also interpreted to have transported biologically oxygenated waters from the shallow‐water, inner shelf into otherwise anoxic bottom waters of the strongly stratified distal shelf. The consequence of such transport and mixing was rapid deposition of chemical mud, mainly as precipitated Fe‐oxide. In many cases, the resultant decrease in Fe²⁺ in the water column, together with pelagic inorganic precipitation of SiO₂ and rainout of terrigenous clays, resulted in submillimetre‐ to millimetre‐thick, chemically graded laminae. The concomitant decreasing Fe²⁺/Mn²⁺ ratio also led to increasing Mn‐compound precipitation and enrichment in the upper portions of some chemically graded layers.     

U–U–Th disequilibria and timescale of sedimentary transfers in rivers: Clues from the Gangetic plain rivers

This study uses ²³⁸U–²³⁴U–²³⁰Th disequilibria in river sediments in order to constrain the transfer times of sediments in alluvial plains of rivers from Himalaya and the Gangetic plain. From the observed distributions we infer sediment transfer times of about 100 ka in the Gangetic plain for rivers taking their source in the Himalayan chain, and longer transfer of about 160–250 ka for foothill-fed rivers. This difference is probably related to the difference in the sediment transport dynamics of these two types of rivers.     

南海北部陆架海域表层沉积物黏土矿物分布特征及物源分析

应用X射线衍射(XRD)对南海北部陆架海域225个站位表层沉积物黏土组分进行分析,结果表明,研究区黏土矿物总体以伊利石和绿泥石为主,高岭石和蒙脱石质量分数少,绿泥石、高岭石与蒙脱石质量分数呈明显的负相关关系。根据南海北部陆架海域表层沉积物中黏土矿物空间分布特征,结合邻近河流的黏土矿物组分以及洋流搬运作用,雷州半岛东部海域伊利石主要来源于广东沿海河流和珠江,绿泥石来自台湾岛,蒙脱石主要由吕宋河流提供,高岭石则由广东沿海河流和海南岛入海河流提供;雷州半岛西部海域伊利石来源于珠江,绿泥石和高岭石由红河提供,蒙脱石可能受广西入海河流携带的沉积物影响。     

Sedimentology of the Ekwan Shoal,Akimiski Strait,James Bay,Canada

One of the steepest depositional coasts of western James Bay is found along the west shores of Akimiski Strait, north of the mouth of the Ekwan River. This shore receives considerable amounts of sediment during the spring break-up of the rivers. The sediments are stored on the steep narrow tidal flats and marshes, and in thinner (up to 80 cm) drapes on till-cored shoals that parallel and protect the coast. The low areas between the shoals and the mainland are swept and reworked by relatively powerful (2 m s^?1) reversing currents due to flooding and ebbing of tides into the strait.A series of distinct environments and sedimentary facies develop on this western coast and its antecedent longshore shoal. The outer part of the shoal is characterized by tidal bedding, Macoma balthica burrows and considerable ice scour. The inner part of the shoal has winnowed sand, the greatest abundance of Macoma, and well-developed flaser bedding. The longshore tidal channel separating the shoal from the mainland has coarse sand lags in the shallower parts and silty sand in deeper protected areas. The steep tidal flats develop laminated silty sands locally saturated and slumping toward the channel. The high saturation of the sediments inhibits colonization of the flats by Macoma. The narrow marshes have characteristic vegetation zonation, with Puccinellia phryganodes colonizing the lower marsh. The sedimentary sequence of the marsh displays irregular, bioturbated laminated sequences of silt, silty sand and organic matter.     

First record of 1.2 Ga quartz dioritic magmatism in the Archaean Yilgarn Craton,Western Australia,and its significance

Farming of southern bluefin tuna in South Australia currently contributes to more than 30% of the value of the aquaculture production in Australia. This study investigated the natural sedimentary setting of the area designated for this important industry in coastal waters off Port Lincoln, and explored the links between the natural distribution of sediments and potential environmental effects and risks to the industry. Sediments were mostly composed of poorly sorted silts and fine sands, predominantly skeletal remains of carbonate-secreting organisms. The contribution of plankton to the organic matter remaining in the sediments was calculated to be in excess of 80% using concentration-dependent stable-isotope mixing models. An erosional area was identified south of Rabbit Island where sediments contained up to 50% siliciclastic material, grainsize distributions were better sorted and coarser, and organic carbon and total nitrogen contents were very low. In contrast, deeper waters north of Cape Donington were identified as a depocentre for fine sediments, which contained organic matter levels twice those elsewhere in the region despite the extremely high carbonate contents (>80%). The heavier stable isotopic signature of nitrogen suggested that this organic matter comprised a greater fraction of weathered components, probably advected to the area by suspended and bedload transport. This local variability of sediment characteristics in the farming zone suggests that the benthic assimilative capacity of farmed sites will depend on their location. Wastes from pens located south of Rabbit Island in particular are likely to be quickly winnowed out by wave and tidal action. These pens are also less likely to be affected by resuspension of fine sediments that might be associated with unusually severe storms.     

Geochemical investigations on fluvial sediments contaminated by tin-mine tailings,Cornwall, England

Tin-mine tailings containing high concentrations of Sn, Cu, Zn, Fe, Mn, As, and W are discharged into the Red River of cornwall, England and are then transported into St. Ives Bay under normal flow conditions. Most of the tin-bearing particles in the fluvial sediments are smaller than 170 μm, but tin-bearing composite grains or mineral grains with tin interspersed in the crystal lattices also occur in coarser size fractions. Tin distribution in the sediments is controlledby: (1) the distance from the source of the tailings, and (2) the concentration processes operating on the river bed. Suspended sediment and sediment transported by saltation filtered from river water samples also showed high concentrations of metals although, in contrast to the bottom sediments, they vary within a narrow range. Distributions of Cu, Zn, Fe, As, and Pb in the filtered sediments probably are related to the physical and chemical behavior of their sulphide minerals during fluvial transportation. A regional stream-sediment geochemical reconnaissance survey for tin did not show the highest concentration in the Red River; this indicated that in other rivers and streams tin reconcentration by selective removal of light minerals had taken place in the bottom sediments after mining operations had ceased. These rivers and streams also can transport large quantitiies of land-derived sediment including tin-mine tailings discharged into them when mines were operating. The minimum distance of tin transported by the Red River is at least 10 km; however, most of the tin was derived from mine tailings and is considered to be unnatural.     

Dissolved Al in sediments and waters of the East China Sea: Implications for authigenic mineral formation

Numerous previous studies indicate that several different authigenic aluminosilicates form in the oceans. In this study we show, using dissolved Al distributions in sediments and waters from the nearshore regions of the East China Sea, that the process of aluminosilicate formation probably begins rapidly upon contact of detrital clays with seawater. Statistical analyses of dissolved Al-Si-H⁺ relations in surface sediments indicate that the minerals forming in East China Sea sediments low in dissolved Fe are dioctahedral chlorites with an average composition EX_0.91Mg_0.77Al_5.0Si_2.7O₁₀(OH)₈ (where EX = exchangeable + 1 cation). This composition is also consistent with dissolved Al and Si measurements as a function of salinity in turbid overlying waters. Results suggest a dissolution—reprecipitation mechanism for clay mineral reconstitution. This mechanism can help to explain why different authigenic clays are found in different areas of the oceans. In the East China Sea the total amount of authigenic clays present must constitute a very minor fraction of the bottom sediments. Thus, the formation of these minerals has a relatively small impact upon dissolved Si distributions. Clay mineral reconstitution in nearshore regions may provide a mechanism for buffering sediments and overlying waters with respect to pH, as the composition of minerals formed should be a direct function of the H⁺ activity in the surrounding environment.     

The importance of changing oceanography in controlling late Quaternary carbonate sedimentation on a high-energy, tropical, oceanic ramp: north-western Australia

The North West Shelf is an ocean‐facing carbonate ramp that lies in a warm‐water setting adjacent to an arid hinterland of moderate to low relief. The sea floor is strongly affected by cyclonic storms, long‐period swells and large internal tides, resulting in preferentially accumulating coarse‐grained sediments. Circulation is dominated by the south‐flowing, low‐salinity Leeuwin Current, upwelling associated with the Indian Ocean Gyre, seaward‐flowing saline bottom waters generated by seasonal evaporation, and flashy fluvial discharge. Sediments are palimpsest, a variable mixture of relict, stranded and Holocene grains. Relict intraclasts, both skeletal and lithic, interpreted as having formed during sea‐level highstands of Marine Isotope Stages (MIS) 3 and 4, are now localized to the mid‐ramp. The most conspicuous stranded particles are ooids and peloids, which ¹⁴C dating shows formed at 15·4–12·7 Ka, in somewhat saline waters during initial stages of post‐Last Glacial Maximum (LGM) sea‐level rise. It appears that initiation of Leeuwin Current flow with its relatively less saline, but oceanic waters arrested ooid formation such that subsequent benthic Holocene sediment is principally biofragmental, with sedimentation localized to the inner ramp and a ridge of planktic foraminifera offshore. Inner‐ramp deposits are a mixture of heterozoan and photozoan elements. Depositional facies reflect episodic environmental perturbation by riverine‐derived sediments and nutrients, resulting in a mixed habitat of oligotrophic (coral reefs and large benthic foraminifera) and mesotrophic (macroalgae and bryozoans) indicators. Holocene mid‐ramp sediment is heterozoan in character, but sparse, most probably because of the periodic seaward flow of saline bottom waters generated by coastal evaporation. Holocene outer‐ramp sediment is mainly pelagic, veneering shallow‐water sediments of Marine Isotope Stage 2, including LGM deposits. Phosphate accumulations at ≈ 200 m water depth suggest periodic upwelling or Fe‐redox pumping, whereas enhanced near‐surface productivity, probably associated with the interaction between the Leeuwin Current and Indian Ocean surface water, results in a linear ridge of pelagic sediment at ≈ 140 m water depth. This ramp depositional system in an arid climate has important applications for the geological record: inner‐ramp sediments can contain important heterozoan elements, mid‐ramp sediments with bedforms created by internal tides can form in water depths exceeding 50 m, saline outflow can arrest or dramatically slow mid‐ramp sedimentation mimicking maximum flooding intervals, and outer‐ramp planktic productivity can generate locally important fine‐grained carbonate sediment bodies. Changing oceanography during sea‐level rise can profoundly affect sediment composition, sedimentation rate and packaging.     

http://www.sciencedirect.com/science/article/pii/S1674987115000584

The spatial distribution patterns of surficial sediment samples from different sedimentary domains(shallow to deep-sea regions) of the eastern Arabian Sea were studied using sediment proxies viz.environmental magnetism,geochemistry,particle size and clay mineralogy.Higher concentrations of magnetic minerals(high x_(lf)) were recorded in the deep-water sediments when compared with the shallow water sediments.The magnetic mineralogy of one of the shallow water samples is influenced by the presence of bacterial magnetite as evidenced from the x_(ARM)/x_(lf)~(vs).x_(arm_/x_(fd)biplot.However,the other samples are catchment-derived.The high correlation documented for x_(lf).anhysteretic remanent magnetisation(x_(arm)) and isothermal remanent magnetisation(IRM) with Al indicates that the deep-sea surficial sediments are influenced by terrigenous fluxes which have been probably derived from the southern Indian rivers,the Sindhu(the Indus) and the Narmada-Tapti rivers.A lower Mn concentration is recorded in the upper slope sediments from the oxygen minimum zone(OMZ) but a higher Mn/Al ratio is documented in the lower slope and deep-sea sediments.Clay minerals such as illite(24-48.5%),chlorite(14.1-34.9%),smectite(10.6-28.7%) and kaolinite(11.9-27.5%) dominate the sediments of shallow and deep-sea regions and may have been derived from different sources and transported by fluvial and aeolian agents.Organic carbon(OC) data indicate a low concentration in the shallow/shelf region(well oxygenated water conditions) and deeper basins(increased bottom-water oxygen concentration and low sedimentation rate).High OC concentrations were documented in the OMZ(very low bottom-water oxygen concentration with high sedimentation rate).The calcium carbonate concentration of the surface sediments from the continental shelf and slope regions( 1800 m) up to the Chagos-Laccadive Ridge show higher concentrations(average = 58%) when compared to deep basin sediments(average = 44%).Our study demonstrates that particle size as well as magnetic grain size,magnetic minerals and elemental variations are good indicators to distinguish terrigenous from biogenic sediments and to identify sediment provenance.     

Tidal signature recorded in burrow fill

The arrangement of sediment couplets preserved in Thalassinoides shafts suggests that tides regulated the passive filling of these trace fossils and, thus, represent tubular tidalites. The thickness variation in individual layers and couplets implies a mixed diurnal, semi‐diurnal tidal signature where packages of either thick‐layered or thin‐layered couplets alternate. Calcarenitic sediment accumulated when tidal current velocity was too high to allow deposition of mud, whereas a marly mud layer is interpreted to have formed during more tranquil times of a tidal cycle (in particular, low‐tide slack water). The tidal record within the burrows covers a few weeks and the corresponding spring–neap cycles. The fill of the Thalassinoides shafts is the only known record to decipher the tidal signature from otherwise totally bioturbated sediments. These deposits accumulated in a lower‐shoreface to upper‐offshore setting during the late Miocene on a shallow shelf extending from the Atlantic Ocean to the west into northern Patagonia. The fill of all investigated burrows started around spring tide and, thus, the behaviour of the burrow producers – probably crustaceans – is speculated to have been affected by tides or the high water level because all studied burrows became abandoned around the same period of a tidal cycle.