Threshold of motion and settling velocities of mollusc shell debris: Influence of faunal composition

Bioclastic particles derived from mollusc shell debris can represent a significant fraction of sandy to gravelly sediments in temperate and cool‐water regions with high carbonate productivity. Their reworking and subsequent transport and deposition by waves and currents is highly dependent on the shape and density of the particles. In this study, the hydrodynamic behaviour of shell debris produced by eight mollusc species is investigated for several grain sizes in terms of settling velocity (measurements in a settling tube) and threshold of motion under unidirectional current (flume experiments using an acoustic profiler). Consistent interspecific differences in settling velocity and critical bed shear stress are found, related to differences in shell density, shell structure imaged by scanning electron microscopy and grain shape. Drag coefficients are proposed for each mollusc species, based on an interpolation of settling velocity data. Depending on the shell species, the critical bed shear stress values obtained for bioclastic particles fall within or slightly below empirical envelopes established for siliciclastic particles, despite very low settling velocity values. The results suggest that settling velocity, often used to describe the entrainment of sediment particles through the equivalent diameter, is not a suitable parameter to predict the initiation of motion of shell debris. The influence of the flat shape of bioclastic particles on the initiation of motion under oscillatory flows and during bedload and saltation transport is yet to be elucidated.     

Entrainment of planktonic foraminifera: effect of bulk density

Depositional hydrodynamics have been studied using settling rate distributions of Norwegian deep sea sediments (between Jan Mayen Island and the Vøring Plateau), together with Shields’ critical shear stress velocities. Planktonic foraminifera are the dominant sand sized component of these sediments. The bulk density of the foraminifera was calculated from their settling velocity, sieve size and shape. Density decreases from 2·39 g cm^?3 at 0·05 mm diameter to 1·37 g cm^?3 at 0·35 mm diameter. These density and size data were used to construct a threshold sediment movement curve. From the similarity in their Shield's critical shear-stress velocities and the observed correlation of foraminifera size with decreasing percentage of fine fraction, it is concluded that the two components, the sand size foraminifera and the quartz and carbonate silt, are transport-equivalent.     

Estimating the settling velocity of bioclastic sediment using common grain‐size analysis techniques

Most techniques for estimating settling velocities of natural particles have been developed for siliciclastic sediments. Therefore, to understand how these techniques apply to bioclastic environments, measured settling velocities of bioclastic sedimentary deposits sampled from a nearshore fringing reef in Western Australia were compared with settling velocities calculated using results from several common grain‐size analysis techniques (sieve, laser diffraction and image analysis) and established models. The effects of sediment density and shape were also examined using a range of density values and three different models of settling velocity. Sediment density was found to have a significant effect on calculated settling velocity, causing a range in normalized root‐mean‐square error of up to 28%, depending upon settling velocity model and grain‐size method. Accounting for particle shape reduced errors in predicted settling velocity by 3% to 6% and removed any velocity‐dependent bias, which is particularly important for the fastest settling fractions. When shape was accounted for and measured density was used, normalized root‐mean‐square errors were 4%, 10% and 18% for laser diffraction, sieve and image analysis, respectively. The results of this study show that established models of settling velocity that account for particle shape can be used to estimate settling velocity of irregularly shaped, sand‐sized bioclastic sediments from sieve, laser diffraction, or image analysis‐derived measures of grain size with a limited amount of error. Collectively, these findings will allow for grain‐size data measured with different methods to be accurately converted to settling velocity for comparison. This will facilitate greater understanding of the hydraulic properties of bioclastic sediment which can help to increase our general knowledge of sediment dynamics in these environments.     

Packing and resistance to compaction of shells

The separated valves of bivalve molluscs, certain gastropods, and many brachiopods closely resemble the regular geometrical form known as the shell. The packing concentration of natural accumulations of such organic shells may be placed within bounds with the aid of packing models making use of ordered arrangements of equal geometrical shells, either conical, cylindrical or spherical. The low packing concentrations, comparable with 0·1–0·2, indicated by these models are confirmed by experiments using the shells of four common British species of bivalve or gastropod mollusc. Packings of these shells have a substantial intrinsic strength, and experimentally appear able to support without failing sedimentary overburdens equivalent to loads comparable with 1000 kg mass/m². The observed and predicted low packing concentrations suggests that natural shell beds can hold relatively very large amounts of pore fluids or mineral cements. The resistance to compaction of the packings means that the high original porosities of natural shell beds have a good chance of being permanently preserved.     

Hydraulic characteristics of bioclastic deposits: new possibilities for environmental interpretation using settling velocity fractions

This paper examines the hydraulic behaviour of heterogeneous bioclastic sediments using settling velocity fractions. Bioclastic deposits are divided into fractions by splitting hydraulically sorted samples. Compositional analysis of fractions shows that grains exhibit marked variation in composition, shape, size and density which control the hydraulic behaviour of bioclastic deposits. Despite the heterogeneous grain properties of fractions (analogous to sieve fractions) sedimentation analysis shows that they possess a narrow range of settling velocity values. Flume experiments also show that the settling velocity fractions possess a narrow range of threshold velocity values. A threshold relationship is identified in which the velocity required to entrain mixed settling fractions increases from 20.5 to 65-0cm s ^?1 for settling fractions with mean settling values of 5.5 chi (2-2 cm s ^?1) to 1.4 chi (36.0 cm s ^?1), respectively. Heterogeneous settling fractions, due to intergrain effects, were found to possess larger threshold velocities than individual components showing that the composition of bioclastic deposits controls hydraulic behaviour. Thus, caution must be exercised in using the threshold relationship for homogeneous sediments or deposits of markedly different composition. As settling velocity fractions reflect narrow hydraulic (settling and threshold) properties of sediments the mean settling velocity of fractions is considered a good indicator with which to interpret transport and depositional processes. Comparison of settling and sieve-size distributions shows that size distributions do not reflect the hydraulic behaviour of bioclastic deposits and should not be used to interpret environmental processes. The study indicates that examination of settling velocity fractions (for which hydraulic settling and threshold properties are known) coupled with compositional analysis of these fractions will allow much greater environmental interpretation of deposit-forming and energy processes in reef environments.     

Settling velocities and entrainment thresholds of biogenic sands (shell fragments) under unidirectional flow

Settling velocities and entrainment thresholds of biogenic sedimentary particles, under unidirectional flow conditions, are derived on the basis of settling tower and laboratory flume experiments. Material consisting predominantly of equant blocks (shell fragments of Cerastoderma edule , density, ρ _s=2800 kg m⁻³) or of mica-like flakes and elongate rods ( Mytilus edulis fragments, ρ _s=2720 kg m⁻³) are used in separate series of experiments. Differences in the measured settling and threshold properties are related primarily to particle shape. The selection of a characteristic length scale for non-spherical grains is investigated by comparing two approaches used to define the grain size ( D ) of the sediment samples: grain settling and sieve analysis that are used to derive data for the threshold criteria, in terms of the Shields and Movability diagrams. The empirical curves effectively predict the threshold conditions for any grain shape, provided that grain size is defined in terms of grain settling velocity. However, a functional distinction is made between the characteristic `hydraulic' grain size, defined by grain settling for grain transport applications, and the actual (physical) grain size defined by sieve analysis.     

单向流边界层泥沙起动规律

Shields曲线常用于表示泥沙起动的临界条件,基于边界层理论,对Shields曲线各个流区的线型进行了推证;考虑粘结力的作用,对Shields参数及Shields曲线进行了修正,并给出修正Shields曲线表达式;在此基础之上,从边界层角度重新阐述了Shields曲线。结果表明:Shields曲线在光滑紊流及层流区呈直线分布,在过渡区与阻力系数线型保持一致,在粗糙紊流区呈水平直线分布;修正后Shields曲线与原始Shields曲线在形式上保持一致,修正Shields曲线表达式与实测数据吻合较好,适用于粗、细颗粒泥沙起动条件的计算;Shields曲线事实上代表了Shields参数与沙粒周围绕流流态的关系,同一颗粒处于不同流区起动时,其起动切应力不同。     

Laboratory investigations into the threshold of movement of natural sand-sized sediments under unidirectional, oscillatory and combined flows

The threshold of movement of sediment obtained from sandbanks within the Bristol Channel (UK) is investigated under unidirectional, oscillatory and combined flows. The experiments were undertaken in a recirculating, unidirectional laboratory flume containing an oscillating plate to simulate wave action, with movement along the same axis as the unidirectional flows. The sand samples consisted of cohesionless quartz grains with median grain sizes between 0·315 and 0·513 mm. The experiments were performed under flow velocities (measured at 2 cm above the bed) ranging between 0 and 24 cm s^–1 and oscillatory currents (wave periods of 5, 12 and 15 s) ranging from 0 to 28 cm s^–1. The critical conditions for the initiation of sediment movement were assessed, by visual observation, using the Yalin criterion. The results show that, under unidirectional flow, there is a slight overestimation of the threshold of naturally graded sediments derived on the basis of empirically derived threshold curves for artificially prepared sediments under similar flow conditions. In the case of oscillatory flows, the threshold for the natural sands is found to be higher than that predicted by previously derived empirical curves. Under combined flows, wave period is shown to control threshold conditions, with the unidirectional and oscillatory flow components combining in a linear fashion for long-period (12 s and 15 s) waves. In contrast, in the presence of short-period (5 s) waves, the unidirectional and oscillatory components of the flow appear to 'decouple'. For high orbital velocities, in both cases, the effect of the wave period on threshold diminishes.     

Threshold of sediment motion under unidirectional currents

Carefully selected data for the threshold of sediment movement under unidirectional flow conditions have been utilized to re-examine the various empirical curves that are commonly employed to predict this threshold. After a review of the existing data, we employed only that data obtained from open channel flumes with parallel sidewalls where flows were uniform and steady over flattened beds of unigranular, rounded sediments. Without these restrictions, an unmanageable amount of scatter is introduced. This selected data is used to develop a modified Shields-type threshold diagram that extends the limits of the original diagram by three orders of magnitude in the grain-Reynolds number. The equally general but more easily employed Yalin diagram for sediment threshold is also examined. Although the Shields and Yalin diagrams are general in that they apply to a wide range of different liquids, in both cases somewhat different curves are obtained for threshold under air than for the liquids. The often used empirical curves of the friction velocity u_*, the velocity 100 cm above the bed u₁₀₀, the bottom stress θ_t, and Shields’ relative stress θ_t, all versus the grain diameter D, are limited in their ranges of application to certain combinations of grain density, fluid density, fluid viscosity and gravity. These conditions must be selected before the curves are generated from either the more general Shields or Yalin curves. For example, on the basis of the data selected for use in this paper, empirical threshold relationships for quartz density material in water are where the velocity u₁₀₀ measured 100 cm above the sediment bed is given in cm/sec and the grain diameter D is in cm. The limitations on any of the threshold relationships are severe. These limitations should be properly understood so that the empirical curves and relationships are not improperly employed.     

A first look at oxygen isotope records from modern and Holocene-aged gastropod (Stenomelania) shells from Lake Kutubu,Papua New Guinea

The oxygen isotopic composition of Stenomelania gastropod shells was investigated to reconstruct Holocene palaeoclimate change at Lake Kutubu in the southern highlands of Papua New Guinea. Oxygen isotope (δ¹⁸O) values recorded in aquatic gastropod shells change according to ambient water δ¹⁸O values and temperature. The gastropod shells appear to form in oxygen isotopic equilibrium with the surrounding water and record a shift in average shell oxygen isotopic composition through time, probably as a result of warmer/wetter conditions at ca. 600–900 and 5900–6200 cal a bp. Shorter term fluctuations in oxygen isotope values were also identified and may relate to changes in the intensity or source of rainfall. Further δ¹⁸O analyses of gastropod shells or other carbonate proxies found in the Lake Kutubu sediments are warranted. © 2020 John Wiley & Sons, Ltd.     

The use of settling velocity in defining the initiation of motion of heavy mineral grains, under unidirectional flow

The results of critical threshold experiments on four commonly occurring heavy minerals are described. The data are presented, in conjunction with comparable quartz data, using the non-dimensional Shields’ Curve and the Movability Number (U*/w_s). The results indicate that critical shear stress for material of high density is overestimated by use of Shields’ Curve, under smooth boundary conditions. Grain settling velocity is found to be a good indicator for the critical shear stress for grains of a wide density range. A physical explanation for the results is proposed.     

Holocene storage of siliciclastic sediment around islands on the middle shelf of the Great Barrier Reef Platform, north-east Australia

Thick sequences of sediment surround the Whitsunday Islands on the middle shelf of the Great Barrier Reef (GBR) Platform. Much of this sediment is siliciclastic material deposited since the sea‐level highstand at around 6·5 ka. This raises a mass balance dilemma because modern terrigenous discharge to the GBR Platform is restricted to the inner shelf. Shallow seismic profiles and sediment samples were collected over 450 km² around the Whitsunday Islands to quantify the mass of siliciclastic sediment for a dynamic model of the shelf. The sea floor and pre‐Holocene surfaces were mapped using 4584 stations along the seismic profiles and a graphical computer program. The total volume of sediment between these two surfaces is 3·67 ± 0·45 × 10⁹ m³. This volume is composed of buried reefs (0·13 ± 0·01 × 10⁹ m³), medium‐ (0·70 ± 0·30 × 10⁹ m³) and fine‐grained shoals (2·84 ± 0·35 × 10⁹ m³). The volume estimates combined with measurements of carbonate concentration, density and porosity indicate that 1850 ± 380 Mt of Holocene siliciclastic sediment surround the Whitsunday Islands in medium‐ (510 ± 225 Mt) and fine‐grained shoals (1340 ± 155 Mt). The total mass of siliciclastic material is 1·7–2·6 times that stored in Cleveland Bay, a similar sized repository on the inner shelf. A simple numerical model has been constructed to explain this large quantity of Holocene siliciclastic sediment. The model results in the appropriate siliciclastic mass next to the Whitsunday Islands by integrating regional shelf processes over time. Unlike the present day, rivers discharged sediment to the middle shelf during the early Holocene. This material was subsequently focused by northward transport into the vicinity of the islands, a geomorphologically complex region that serves as a sediment trap. Although direct riverine inputs to the middle shelf have stopped during the present sea‐level highstand, previously deposited siliciclastic sediment is continually being winnowed from the middle shelf and redeposited next to the Whitsunday Islands. The transport and distribution of siliciclastic sediment on the GBR Platform is thus influenced significantly by storage around islands on the middle shelf.     

Settling velocity and drag coefficient of platy shell fragments

Settling velocity of bioclastic particles in coastal and shallow marine environments is essential for interpreting depositional facies and processes. There is, however, a paucity of accurate formulae for predicting the settling velocities and drag coefficients of platy biogenic particles in particular. This study provides experimental settling data based on 320 platy shell fragments from a sediment core recovered in Li'an Lagoon, south-eastern Hainan Island, China. The results indicate that the settling velocities of platy shell fragments are strongly correlated with nominal diameters and Corey shape factors (ranging from 0.02 to 0.20 in this study). On this basis, a practical equation of acceptable accuracy was established for platy particles, relating dimensionless settling velocities to dimensionless diameters and Corey shape factors. Similarly, another empirical formula for quickly calculating the equivalent diameter of platy shell fragments in practice was proposed as well. Regarding the strong dependence of the drag coefficients using equivalent spherical diameters to Corey shape factors, the drag coefficient based on the diameter of the equivalent maximum projected area remains almost constant and is hence physically well-suited for the definition of grain drag coefficients. The settling data of this study has extended the lower Corey shape factors limit of bioclastic particles, and the equations presented here can be used for quantitative interpretations of sedimentary records, modelling of depositional processes and investigations of other platy particles.     

Threshold studies of gravel size particles under the co-linear combined action of waves and currents

Gravel size sediment beds are tested under the combined influence of simulated wave action and co-linear currents in a laboratory flume. Critical current speed, at threshold, increases with increasing size. Superimposed wave energy causes a small reduction in the unidirectional current energy. For low values of wave-induced near bed current velocities, the resistance to erosion increases when the wave period decreases from 10 to 6 s. Finally, combined critical shear stresses are found to be lower than those predicted using the Shields curve, as modified for oscillatory flow. Grain protrusion is suggested as a mechanism to explain this divergence.     

The record of tidal cycles in mixed silici–bioclastic deposits: examples from small Plio–Pleistocene peripheral basins of the microtidal Central Mediterranean Sea

The Pliocene–Pleistocene peripheral marine basins of the Mediterranean Sea in southern Italy, from Basilicata and western Calabria to northern and eastern Sicily, represent tectonically formed coastal embayments and narrow straits. Here, units of cross‐stratified, mixed silici–bioclastic sand, 25 to 80 m thick, record strong tidal currents. The Central Mediterranean Sea has had a microtidal range of ca 35 cm, and the local amplification of the tidal wave is attributed to tides enhanced in some of the bays and to the out‐of‐phase reversal of the tidal prism in narrow straits linking the Tyrrhenian and Ionian basins. The siliciclastic sediment was generated by local bedrock erosion, whereas the bioclastic sediment was derived from the contemporaneous, foramol‐type cool‐water carbonate factories. The cross‐strata sets represent small to medium‐sized (10 to 60 cm thick) two‐dimensional dunes with mainly unidirectional foreset dip directions. These tidalites differ from the classical tidal rhythmites deposited in mud‐bearing siliciclastic environments. Firstly, the foreset strata lack mud drapes and, instead, show segregation of siliciclastic and bioclastic sand into alternating strata. Secondly, the thickness variation of the successive silici–bioclastic strata couplets, measured over accretion intervals of 2 to 3 m and analysed statistically, reveal only the shortest‐term, diurnal and semi‐diurnal tidal cycles. Thirdly, the record of diurnal and semi‐diurnal tidal cycles is included within the pattern of neap‐spring cycles. Differences between these sediments and classical tidal rhythmites are attributed to the specific palaeogeographic setting of a microtidal sea, with the tidal currents locally enhanced in peripheral basins. It is suggested that this particular facies of mud‐free, silici–bioclastic arenite rhythmites in the stratigraphic record might indicate a specific type of depositional sub‐tidal environment of straits and embayments and the shortest‐term tidal cycles.     

渤海古近系混合沉积发育特征与沉积模式

陆源碎屑与湖相碳酸盐混合沉积在渤海海域古近系普遍发育,已经成为重要的油气勘探目标.基于90余口已钻井,结合地震、测井以及相关分析化验资料,以渤海海域古近系钻遇的混合沉积为研究对象,开展了沉积特征与成因背景分析,建立了混合沉积发育模式.研究表明:渤海古近系混合沉积发育近岸混积扇、近岸混积滩坝、远岸混积滩坝3种沉积相、7种沉积亚相和14种沉积微相.近岸混积扇以发育混积沟道为典型特征,岩性为含生屑的砂砾岩或中粗砂岩,为陡坡扇三角洲环境短源水道废弃后与生物碎屑间歇性间互沉积;近岸混积滩坝,为近源搬运沉积的陆源碎屑与盆内碎屑之间的混合沉积,典型沉积特征为发育高陆源碎屑含量的各种粒屑滩或坝;远岸混积滩坝为典型的以富含生物碎屑为主的混合沉积,陆源碎屑含量低,一般为盆内低隆或孤立潜山之上原地生长生物与少量供给的陆源碎屑发生的混合沉积.混合沉积发育模式和规模受古气候与古水体环境、古构造与水动力、陆源碎屑物质供给、古地貌条件等共同控制,其中古地貌特征起主导作用.混合沉积发育模式的建立,对混积型储层预测和油气勘探具有重要意义.      

Sodium and strontium in mollusc shells: preservation,palaeosalinity and palaeotemperature of the Middle Pleistocene of eastern England

This paper revisits the utility of sodium (Na) content in aragonite and calcite mollusc shells as an indicator of palaeosalinity. The data come mainly from a related suite of Middle Pleistocene marine and freshwater fossils that have been subject to broadly similar diagenetic histories. Environmental salinity is re-affirmed as the primary factor in determining the sodium content of modern and ancient mollusc shells: values <2000 ppm Na are generally indicative of non-marine environments while values >2000 Na ppm are typically from marine shells. There is a positive relationship between Na (salinity) and Sr which is a helpful discriminator of palaeosalinity in the fossil data set. The Na and Sr data give confidence that the fossil shells have not suffered pervasive diagenetic alteration and that the marine fossils lived in fully marine conditions. Oxygen isotope values in the best-preserved, fully marine fossil shells, suggest Middle Pleistocene ‘eastern England’ seawater temperatures were broadly similar to those of the modern North Sea.     

Physical and biological controls on the formation of carbonate and siliciclastic bedforms on the north-east Brazilian shelf

The continental shelf of the State of Rio Grande do Norte, Brazil, is an open shelf area located 5°S and 35°W. It is influenced by strong oceanic and wind-driven currents, fair weather, 1·5-m-high waves and a mesotidal regime. This work focuses on the character and the controls on the development of suites of carbonate and siliciclastic bedforms, based on Landsat TM image analysis and extensive ground-truth (diving) investigations. Large-scale bedforms consist of: (i) bioclastic (mainly coralline algae and Halimeda) sand ribbons (5–10 km long, 50–600 m wide) parallel to the shoreline; and (ii) very large transverse siliciclastic dunes (3·4 km long on average, 840 m spacing and 3–8 m high), with troughs that grade rapidly into carbonate sands and gravels. Wave ripples are superposed on all large-scale bedforms, and indicate an onshore shelf sediment transport normal to the main sediment transport direction. The occurrence of these large-scale bedforms is primarily determined by the north-westerly flowing residual oceanic and tidal currents, resulting mainly in coast-parallel transport. Models of shelf bedform formation predict sand ribbons to occur in higher energy settings rather than in large dunes. However, in the study area, sand ribbons occur in an area of coarse, low-density and easily transportable bioclastic sands and gravels compared with the very large transverse dunes in an offshore area that is composed of denser medium-grained siliciclastic sands. It suggests that the availability of different sediment types is likely to exert an influence on the nature of the bedforms generated. The offshore sand supply is time limited and originates from sea floor erosion of sandstones of former sea-level lowstands. The trough areas of both sand ribbons and very large transverse dunes comprise coarse calcareous algal gravels that support benthic communities of variable maturity. Diverse mature communities result in sediment stabilization through branching algal growth and binding that is thought to modify the morphology of dunes and sand ribbons. The occurrence and the nature of the bedforms is controlled by their hydrodynamic setting, by grain composition that reflects the geological history of the area and by the carbonate-producing benthic marine communities that inhabit the trough areas.     

The isotopic composition of oxygen and carbon in the subfossil mollusc shells of the Baltic Sea as an indicator of palaeosalinity

The carbon (δ¹³ C) and oxygen (δ¹⁸O) isotopic composistion in mollusc shells in mainly determined by the isotopic composition of water and dissolved bicarbonate. The δ¹⁸O values of water show a good correlation with the salinity of the Baltic. This correlation served as a basis for reconstructing palaeosalinity and for stratifying the marine sediments according to the δ¹⁸O values of the carbonate skeletons of subfossil shells. The δ¹³C values in shells are mainly determined by the isotopic composition of land-originating bicarbonate, especially in the carbonate skeleton of Lymnaea balthica , which inhabits the immediate coastal zone. According to the δ¹⁸O data, salinity in the investigated area (the coastal area of W and NW Estonia) was highest (about 9–11%) during the Littorina stage. The Limnae a stage had, in general, a salinity similar to the contemporary one, but during some phases possibly exceeding it by 2–3%.     

Particulate calcium carbonate in New England shelf waters: result of shell degradation and resuspension*

Aragonite and calcite needles, 30–500 μm long, were found to be prominent visual components throughout the water column in some areas of the New England (northeastern U.S.A.) continental shelf during winter months. Further investigation showed these‘needles’to be laths derived from the degradation of mollusc shells which were resuspended from bottom sediments during winter storms. Such degradation and subsequent transport/dissolution of carbonate particles may help explain why the terrigenous 'starved’shelf sediments off New England contain such small amounts of calcium carbonate.