Clay Grains Research Articles

Ritual Practice in a Domestic Space: Evidence from Melora Hilltop, a Late Iron Age Stone-Walled Settlement in the Waterberg, Limpopo Province, South Africa

This paper reports on preliminary observations and interpretations pertaining to a previously little-researched area and time frame. Artefacts, namely a bored stone, a grinding stone, an iron adze and fragments of what is concluded to be a clay grain bin, found in association during recent excavations on Melora Hilltop in Limpopo Province, are discussed in terms of both their mundane and ritual connotations. The ritual importance of seemingly mundane artefacts is implied. Their association and location argue for a shrine or place of ancestral ritual in a domestic space, possibly belonging to a person of status.

Nitrogen adsorption method for determining the mesopore slit-like size distribution of expansive soils

One of the most important structural parameters involved in the characterization of the void space of soils is the pore size distribution. In this work, a nitrogen adsorption method is presented as an alternative to determine the mesopore contribution of slit-shaped pores to the abovementioned parameter in montmorillonite-containing soils. The mesopore size structural parameter obtained via this sorption method is related to montmorillonite d001 interlayer distances proceeding from either X-ray or electron diffraction analysis, if only by assuming during sorption analysis that mesopores resident inside clay grains adopt slit-shaped geometries. Mineralogical analysis and determination of some important geotechnical properties of these montmorillonite-containing soils complement the preceding sorption and structural studies.

Acoustic and electromagnetic properties of soils saturated with salt water and NAPL

Electrical, seismic, and electromagnetic methods can be used for noninvasive determination of subsurface physical and chemical properties. In particular, we consider the evaluation of water salinity and the detection of surface contaminants. Most of the relevant properties are represented by electric conductivity, P-wave velocity, and dielectric permittivity. Hence, it is important to obtain relationships between these measurable physical quantities and soil composition, saturation, and frequency. Conductivity in the geoelectric frequency range is obtained with Pride's model for a porous rock. (The model considers salinity and permeability.) White's model of patchy saturation is used to calculate the P-wave velocity and attenuation. Four cases are considered: light nonaqueous phase liquid (LNAPL) pockets in water, dense nonaqueous phase liquid (DNAPL) pockets in water, LNAPL pockets in air, and DNAPL pockets in air. The size of the pockets (or pools), with respect to the signal wavelength, is modeled by the theory. The electromagnetic properties in the GPR frequency range are obtained by using the Hanai–Bruggeman equation for two solids (sand and clay grains) and two fluids (LNAPL or DNAPL in water or air). The Hanai–Bruggeman exponent (1/3 for spherical particles) is used as a fitting parameter and evaluated for a sand/clay mixture saturated with water. Pride's model predicts increasing conductivity for increasing salinity and decreasing permeability. The best-fit exponent of the Hanai–Bruggeman equation for a sand/clay mixture saturated with water is 0.61, indicating that the shape of the grains has a significant influence on the electromagnetic properties. At radar frequencies, it is possible to distinguish between a water-saturated medium and a NAPL-saturated medium, but LNAPL- and DNAPL-saturated media have very similar electromagnetic properties. The type of contaminant can be better distinguished from the acoustic properties. P-wave velocity increases with frequency, and has dissimilar behaviour for wet and dry soils.

Significance of mineralogy in soil mechanics

The research of consistency limits according to Atterberg is of key importance in soil mechanics because it determines, in a simple way, results of interactions between solid and liquid phases in soils, and thus provides possibility to classify them in groups with similar mechanic properties. In most cases, the results of these investigations provide a good basis for predicting other properties such as deformability, expansion, hydraulic conductivity and strength of soils. This proves that basis factors influencing the values of consistency limits and other mechanic properties are the same. It is known that the values of consistency limits primarily depend on the type and quantity of clay minerals in soils. The article focuses on experimental evidence of dependence between the quantity of water at the liquid limit for soils and type, portion and specific surface of clay grains. Experiments were performed with monomineral soils, their mixtures and composed samples of clay- and non-clay components. It was established that in parallel orientation of clay particles the quantity of water between grains at liquid limit is dependent on their external specific surface.

Millennial‐ to decadal‐scale paleoenvironmental change during the Holocene in the Palmer Deep, Antarctica, as recorded by particle size analysis

A record of Holocene paleoenvironmental variability from the Antarctic Peninsula has been produced using sediment cores from Site 1098 gathered on Ocean Drilling Project Leg 178. The results provide an accurate and continuous record of biotic and sedimentologic responses to Holocene environmental change. Holes 1098B and 1098C were used to produce the first ultrahigh‐resolution record of grain size variability for the past 13,000 years. We utilized samples every 2.5 cm (at 3–5 yr cm−1) which involved the analysis of 1466 samples from two holes at the site. Particle size was measured on uniform suspensions of dispersed particles using a laser diffraction method. The particles are mainly grains of silt (65–85%) and clay (15–35%) size. The analyses revealed that in most intervals, biotic particles in the form of diatom frustules (mostly spores) dominate the silt component, which is inversely reflected by magnetic susceptibility (MS). In intervals of the core that deviate from this relationship it was found that the MS signal correlated with medium to fine or coarse silt that was dominated by terrigenous rather than a biogenic (diatom) component. Spectral analyses of the variability in clay and medium‐fine silt content over the last ∼9000 calendar years reveal cycles of 1800, 400, 200, 100, 70, 60, and 50 years. The analyses revealed that particle size, specifically medium‐fine silt, is an excellent proxy for environmental change in the Palmer Deep region. Such changes may be forced by solar variability (the 200 and 400 year cycles), lunar tidal changes (the 1800 year cycles), and an as yet undetermined multidecadal forcing phenomena that operates in the SE Pacific sector of the Southern Ocean.

Physical and chemical effects of grain aggregates on the Palos Verdes margin, southern California

Large discharges of wastewater and particulate matter from the outfalls of the Los Angeles County Sanitation Districts onto the Palos Verdes shelf since 1937 have produced an effluent-affected sediment deposit characterized by low bulk density, elevated organic matter content, and a high percentage of fine silt and clay particles relative to underlying native sands and sandy silts. Comparison of the results of grain-size analyses using a gentle wet-sieving technique that preserves certain grain aggregates to the results of standard size analyses of disaggregated particles shows that high percentages (up to 50%) of the silt and clay fractions of the effluent-affected mud are incorporated in aggregates having intermediate diameters in the fine-to-medium sand size range (63–500 μm). Scanning electron microscope images of the aggregates show that they are predominantly oval fecal pellets or irregularly shaped fragments of pellets. Deposit-feeding polychaete worms such as Capitella sp . and Mediomastus sp ., abundant in the mud-rich effluent-affected sediment on Palos Verdes shelf, are probably responsible for most of the grain aggregates through fecal pellet production. Particle settling rates and densities, and the concentrations of organic carbon and p, p′-DDE, a metabolite of the hydrophobic pesticide DDT, were determined for seven grain-size fractions in the effluent-affected sediment. Fecal pellet grain densities ranged from about 1.2 to 1.5 g/cc, and their average settling rates were reduced to the equivalent of about one phi size relative to spherical quartz grains of the same diameter. However, repackaging of fine silt and clay grains into the sand-sized fecal pellets causes an effective settling rate increase of up to 3 orders of magnitude for the smallest particles incorporated in the pellets. Moreover, organic carbon and p, p′-DDE exhibit a bimodal distribution with relatively high concentrations in the finest size fraction (0–20 μm), as expected, and a second concentration peak associated with the sand-sized fecal pellets. The repackaging of fine-grained particles along with their adsorbed chemical compounds into relatively fast-settling pellets has important implications for the mobilization and transport of the sediment and the desorption of chemicals from grain surfaces.

Reaction pathways and reaction progress for the smectite‐to‐chlorite transformation: evidence from hydrothermally altered metabasites

AbstractThe transformation from smectite to chlorite has been interpreted as involving either a disequilibrium chlorite/smectite mixed‐layering sequence, or an equilibrated discontinuous sequence involving smectite–corrensite–chlorite. Here, analysis of the smectite to chlorite transition in different geothermal systems leads us to propose that the transformation proceeds via three contrasting reaction pathways involving (i) a continuous mixed‐layer chlorite/smectite series; (ii) a discontinuous smectite–corrensite–chlorite series and (iii) a direct smectite to chlorite transition. Such contrasting pathways are not in accord with an equilibrium mineral reaction series, suggesting that these pathways record kinetically controlled reaction progress. In the geothermal systems reviewed the style of reaction pathway and degree of reaction progress is closely correlated with intensity of recrystallization, and not to differences in thermal gradients or clay grain size. This suggests a kinetic effect linked to variation in fluid/rock ratios and/or a contrast between advective or diffusive fluid transport. The mode of fluid transport provides a means by which the rates of dissolution/nucleation/growth can control the reaction style and the reaction progress of the smectite to chlorite transition. Slow rates of growth are linked to the first reaction pathway involving mixed‐layering, while increasing rates of growth, relative to nucleation, promote the generation of more ordered structures and ultimately lead to the direct smectite to chlorite transition, representative of the third pathway.

Low resistivity oil(gas)-bearing reservoir conductive model

Shaly sands reservoir is one of the most distributive types of the oil(gas)-bearing reservoirs discovered in China, and low resistivity oil(gas)-bearing reservoirs are mostly shaly sands reservoirs. Therefore, shaly sands reservoir conductive model is the key to evaluate low resistivity oil(gas)-bearing reservoirs using logging information. Some defects were found when we studied the clay distribution type conductive model, dual-water conductive model, conductive rock matrix model, etc. Some models could not distinguish the conductive path and nature of microporosity water and clay water and some models did not consider the clay distribution type and the mount of clay volume. So, we utilize the merits,overcome the defects of the above models, and put forward a new shaly sands conductive model—dual water clay matrix conductive model (DWCMCM) in which dual water is the free water and the microporosity water in shaly sands and the clay matrix(wet clay) is the clay grain containing water. DWCMCM is presented here, the advantages of which can tell the nature and conductive path from different water (microporosity water and freewater), in consideration of the clay distribution type and the mount of clay volume in shaly sands. So, the results of logging interpretation in the oil(gas)-bearing reservoirs in the north of Tarim Basin area, China with DWCMCM are better than those interpreted by the above models.

Pulsed field gradient proton NMR study of the self-diffusion of H2O in montmorillonite gel: Effects of temperature and water fraction

Self-diffusion coefficients of water molecules (H 2 O) in Na-montmorillonite gel were measured as a function of water fraction (54.8 to 100 wt%) and temperature (30.4 to 60.0 °C) using proton nuclear magnetic resonance (NMR). Spin-echo pulse sequences with magnetic field gradient pulses for the diffusion measurement were applied to the montmorillonite gel at the Larmor frequency of 20 MHz. The self-diffusion coefficient, D , of H 2 O in the clay gel is expressed phenomenologically by ln( D / D 0 ) = 1.77{exp[0.0798( w − 100)] − 1}, where D 0 is the water diffusivity in bulk water and w is the water fraction of the gel (wt%). The data for w > 84.7 wt% can be explained by a theoretical diffusion model for the randomly distributed clay grains. The activation energy of the water diffusivity in the montmorillonite gel was nearly equal to that in bulk water, so the normalized diffusivity, D/D 0 , obeys the temperature-independent master curve. The transition from the free diffusion to the restricted diffusion was not observed for gradient pulse intervals ranging from 5 to 120 ms. This indicates that the average pore size of the gel is much smaller than a few tens of micrometers, so the random walk trajectory of water molecules in the gel is geometrically restricted by the packing of clay mineral grains. Water diffusivity higher than that of the present NMR study was found by computer simulations and neutron scattering experiments in which effects of bound water were considered but those of the tortuosity of the grain packing were neglected. Thus, the predominant factor controlling the diffusivity in the NMR experiments is not the bound water near the clay surface but the geometrical tortuosity of the packing of clay mineral grains.

Paleopedology and Paleohydrology of a Volcaniclastic Paleosol Interval: Implications for Early Pleistocene Stratigraphy and Paleoclimate Record, Olduvai Gorge, Tanzania

ABSTRACT A cumulative red paleosol interval developed on volcaniclastic parent material under semiarid conditions in Olduvai Gorge, Tanzania. It contains a complex history of pedogenesis that was affected by: (1) episodic pyroclastic and debris-fan processes and (2) episodic expansions and contractions of an adjacent alkaline lake and the associated fluctuations in the water table. The paleosol interval is 130-320 cm thick, represents 25 ka of the 50 kyr duration of lowermost Bed II, and is defined by early Pleistocene ( 1.75 Ma) Tuffs IF and IIA. The paleosol interval records a paleocatena related to both landscape and drainage--the slope position on a pyroclastic fan relative to an alkaline lake, the proximity to freshwater wetlands, and the position of water table. Biogenic paleosol structures include grass and sedge root traces, zeolite rhizocretions, and soil fauna (termite and ant) traces. Abundant pedogenic features sensitive to soil moisture conditions, including redoximorphic mottles in the paleosol matrix, Fe oxide glaebules, grain and pore coatings, illuviated clay grain and pore coatings, and vadose siliciclastic and zeolite crystal silt, record episodic water-table fluctuations. The geochemistry of whole-rock samples distinguishes two parent materials (early low Ti/Zr, weathered volcaniclastic sediment; and late high Ti/Zr, tuffaceous sediment), which represent two distinct pedogenic phases. The Lower Paleosol developed at both sites, whereas the Upper Paleosol developed only at the upslope site. Mass-balance calculations indicate greater weathering, higher Eh and pH, and greater zeolite precipitation at the upslope site than at the downslope site. These relationships are compatible with the upslope site having had a lower overall water table and better-drained conditions than the downslope site, which had a higher water table and poorly drained conditions. The Lower Paleosol provides evidence of a fluctuating water table consistent with a wetter climate followed by a prolonged arid period. The Upper Paleosol began to form after a return to wetter conditions and ended under arid conditions. The position of the Olduvai Subchron, C2n (1.942-1.785 Ma) in Bed I, directly beneath the paleosol interval, is used to make a tentative correlation at 1.75 Ma with global climate (dust) records (wet/dry cycles).

Mesodensity organo–clay associations in a near-shore sediment

Abstract The well-known correlation between mineral grain size and organic matter concentrations in sediments has recently been parameterized in terms of organic matter per unit surface area. Normalized thus, organic matter concentrations in many ocean margin sediments are on the order of 1 mg organic carbon m −2 (Mayer, L.M., 1994a. Relationships between mineral surfaces and organic carbon concentrations in soils and sediments. Chem. Geol. 114, 347–363; Mayer, L.M., 1994b. Surface area control of organic carbon accumulation in continental shelf sediments. Geochim. Cosmochim. Acta 58, 1271–1284.). If this organic matter is physically associated with fine clay grains, the resultant organo–clay aggregates should have a density of ca. 2.2 g cm −3 , but if associated with larger grains the densities should be much closer to those of pure minerals. We found organo–mineral aggregates of density 2.0–2.3 g cm −3 (“mesodensity”) in a shelf sediment to contain the majority of the sedimentary organic matter and surface area. Their mineralogy was composed almost exclusively of fine clays. The correlation of organic matter concentration with sediment surface area may thus be derivative of associations with the fine clay minerals that largely control sediment surface area. Pore size distributions of these aggregates indicate that most surface area is contained within small mesopores of

Synsedimentary Faulting in a Mesozoic Deltaic Sequence, Svalbard, Arctic Norway--Fault Geometries, Faulting Mechanisms, and Sealing Properties

Distributary channel sandstones of the Lower Cretaceous Helvetiafjellet Formation and underlying prodelta shales and thin-bedded sandstones of the Upper Jurassic Janusfjellet Formation exposed along the east coast of Spitsbergen are cut by syndepositional planar and listric faults forming collapse scars with depths of 35-90 m and widths up to 1.5 km. The fault zones mostly have a dip of 40-60°, a width of up to 2.3 m, and contain fault-parallel 3-5-m-long overlapping sandstone sheets with widths of 5-40 cm, and up to 2-m-thick fault-parallel sandy mudstones. The intrafault sandstones show fault-parallel banding resulting from differences in detrital clay content and grain size. The banding has been enhanced by selective, late diagenetic quartz cementation of the clay-poor bands. Thin clay laminae, now developed into fault-parallel stylolites, occur along the margins of the intrafault sandstones. The clay laminae do not emerge from clay layers in the fault blocks and are not clay smears. The laminae probably formed during faulting when fluidization within the fault zones allowed clay particles to move laterally and accumulate along the margins of the fault zones. There is no enhanced cementation or cataclastic deformation within the fault zones. The ability of the fault zones to act as capillary seals or barriers to fluid flow is therefore mostly determined by the clay laminae rimming the intrafault sandstones.

Making diagenesis obey thermodynamics and kinetics: the case of quartz cementation in sandstones from offshore mid-Norway

Calculation of the quantity and distribution of quartz cement as a function of time and temperature/depth in quartzose sandstones is performed using a coupled dissolution/diffusional–transport/precipitation model. This model is based on the assumptions that the source of the silica cement is quartz surfaces adjoining mica and/or clay grains at stylolite interfaces within the sandstones, and the quantity of silica transport into and out of the sandstone by advecting fluids is negligible. Integration of the coupled mass transfer/transport equations over geologically relevant time frames is performed using the quasi-stationary state approximation. Results of calculations performed using quartz dissolution rate constants and aqueous diffusion coefficients generated from laboratory data, are in close agreement with both the overall porosity and the distribution of quartz cement in the Middle Jurassic Garn Formation only after optimizing the product of the effective surface area and quartz precipitation rate constants with the field data. When quartz precipitation rate constants are fixed to equal corresponding dissolution rate constants, the effective surface area required to match field data depends on the choice of laboratory generated quartz rate constant algorithm and ranges from 0.008 cm −1 to 0.34 cm −1. In either case, these reactive surface areas are ∼2 to 4 orders of magnitude lower than that computed using geometric models.

A study of debris flow block on the model of percolation

When a debris flow joins a main river, it may block the river. The collapse of the block is often a sudden change that can be simplified into a phase transition. In this paper we study the problem on the model of percolation which is a second order phase transition. Rather than general random percolation model, we introduce a self-similar percolation model and develop it from two dimensions to three dimensions. Under this model, we conclude that a block made of grains (for example stone, sandy grains, clay grains, silt grains, etc.) with more different sizes has a better stability. Moreover, with the help of efflux theory, we explain why the joint on the other side of the block with the main river is easier to break than other places of the block.

Acúmulo de macronutrientes em plantas de crisântemo sob cultivo hidropônico em argila expandida para flor-de-corte

Para avaliar a eficiência do cultivo hidropônico no estado nutricional de plantas de crisântemo em relação ao cultivo convencional, realizou-se um experimento no outono/inverno e outro na primavera/verão, utilizando-se argila expandida, nas classes granulométricas 4-10, 4-13, 10-13, 13-20 mm de diâmetro, saturada duas ou três vezes ao dia por solução nutritiva recirculante com relação N-P-K:1-0,3-2.5, mais o cultivo no sistema convencional. Os plantios ocorreram em casa de vegetação da Universidade Federal de Viçosa, no ano de 1994. As plantas cultivadas em argila expandida no outono/inverno tiveram produção de material seco e conteúdos de N-P-K significativamente superiores aos das cultivadas no sistema convencional, e foram favorecidas quando o cultivo ocorreu nas três classes menores de argila expandida e com uma saturação pela solução nutritiva duas vezes ao dia. As plantas cultivadas em argila expandida no período primavera/verão tiveram uma produção de material seco e conteúdo de N, P, K e Ca semelhantes às cultivadas no sistema convencional. Essas características não foram afetadas pela freqüência de saturação com solução nutritiva e granulometria da argila expandida. Fatores como época do cultivo, temperatura e umidade, podem ter limitado a absorção dos nutrientes pelas plantas cultivadas em argila expandida, nesse período. Em todos os tratamentos, as plantas apresentaram concentrações adequadas de macronutrientes nas folhas superiores.

ESR isochron dating of the Nojima Fault gouge, southwest Japan, using ICP-MS: an approach to fluid flow events in the fault zone

Abstract The absolute age of the earliest fault movement during the Quaternary period of the Nojima Fault, which moved in the 1995 Southern Hyogo Prefecture Earthquake (SHPE) in Japan, has been determined from clay grains in the fault gouge by election spin resonance (ESR) isochron dating using inductively coupled plasma mass spectrometry (ICP-MS). According to ICP-MS analysis, the concentrations of minor elements tend to increase with decreasing clay grain size, and they may adhere to the surface of clay grains, especially montmorillonite, in the fault gouge. The adherence of minor elements is possibly attributed to electrical charges caused on the surface of montmorillonite by substituting Al for some of the Si in the tetrahedral sites of the Si 2 O 5 sheets. 238 U, 232 Th and 208 Pb also increase with decreasing grain size, whereas the 238 U/ 232 Th ratio is almost unchanged within the measurement error. This indicates that a closed system in the 238 U- and 232 Th- series had pertained in the fault gouge collected since the earliest fault movement. On the other hand, the 208 Pb/ 232 Th ratio decreases with the grain size, and the 208 Pb/ 232 Th ratio from a grain of <1 µm is about 14% of that from <500 µm. 232 Th decays to radioactively stable 208 Pb and produces 220 Rn in the decay process, but 220 Rn which is an inactive and non-ionized gas cannot be absorbed electrically on the surface of montmorillonite. Therefore, most of the 220 Rn gases produced on the surface of clay grains may be released from the fault fracture zone. A radon anomaly in ground water observed before the 1995 SHPE may be attributed to radon loss by huge clay mineralization in the Earth’s crust. The ESR isochron age of the fault clay gouge of the Nojima Fault is estimated as 0.84 ± 0.12 Ma to 1.23 ± 0.1 Ma from each clay grain by considering the calibration of radon loss, water content and defect creation efficiency by α-rays. This age is consistent with a date of about 0.53 −3 Ma that was estimated as the beginning of the Nojima Fault from recent drilling core data.

Abstract: A Geological Exploration Model for Offstructure Gas Accumulations in Geopressured Lower Miocene Sandstones, Offshore Texas

Abstract: A Geological Exploration Model for Offstructure Gas Accumulations in Geopressured Lower Miocene Sandstones, Offshore Texas

96/03356 - Trace emissions from coal combustion: Measurement and control

Scanning electron microscopy with wavelength-dispersive spectrometry has been used to measure trace metals in coal and ash. Hg, As. Ni, and Se have been detected in individual pyrite grains in Illinois #6 coal at levels up to 2680 ppm, 410 ppm, 320 ppm, and 880 ppm, respectively. These elements were present in fewer than half the grains analyzed. Cr has been detected at up to 950 ppm in half of clay mineral grains analyzed in Illinois #6 coal. The same trace metals were detected in pyrite and clay grains from Pittsburgh #8 coal. Ash samples show a similarly heterogeneous distribution of trace metals. Hg has been detected at up to 700 ppm in 24% of aluminosilicate. particles analyzed in ash from Absaloka coal, a subbituminous Montana fuel. These data confrm that coal cleaning processes which remove pyrite are likely to be suitable for trace metal emissions control. In addition, back-end control devices which target specific types of ash particles may be helpful for control of air toxics emissions.

Kinetic Modeling of Quartz Cementation and Porosity Loss in Deeply Buried Sandstone Reservoirs

A mathematically simple kinetic model simulates quartz cementation and the resulting porosity loss in quartzose sandstones as a function of temperature history. Dissolved silica is considered to be sourced from quartz dissolution at stylolites or individual quartz grain contacts containing clay or mica, and diffuses short distances to sites of precipitation on clean quartz surfaces. The modeled sandstone volume is located between stylolites, and no quartz dissolution or grain interpenetration takes place within this volume. After quartz cementation starts, compactional porosity loss is typically minor, and porosity loss within the modeled sandstone volume is therefore considered to be equal to the volume of precipitated quartz cement. The quartz cementation process is mod led as a precipitation rate-controlled reaction where quartz precipitation rate per unit time and surface area can be expressed by an empirically determined logarithmic function of temperature. When the sandstone's temperature history is known, precipitation rate per unit time and surface area can be expressed as a function of time, and the amount of quartz cement precipitated within a certain time interval can be calculated by multiplying the precipitation rate function with the surface area available for quartz precipitation and integrating with respect to time. Because quartz surface area will change as quartz cement precipitation proceeds, the calculations are performed for short time steps, and quartz surface area is adjusted after each time step. The total amount of quartz cement p ecipitated during a sandstone's burial history and the corresponding porosity loss are found by taking the sum of the increments of quartz cement precipitated during each time step. The effect of variation in parameters such as grain size, detrital quartz content, abundance of clay or other grain coatings, prequartz cementation porosities, and temperature history is easily simulated with the presented algorithm. This flexibility is illustrated by presenting calculated histories of quartz cementation and porosity loss for sandstones with a range of grain sizes, framework grain compositions, degree of clay coat development, prequartz cementation porosities, and temperature histories.

Transport and Burial as a Cause of Whitefish (Coregonus sp.) Egg Mortality in a Eutrophic Lake

Predictions based on the Shields diagram and confirmed by experiments conducted in eutrophic Lake Sempach imply that bottom currents associated with winter storm events are responsible for the simultaneous transport of coregonid eggs and fine silt and clay (grain size [Formula: see text]) from the spawning grounds into deeper lake regions. The critical shear stress required to initiate egg transport is estimated to lie in the range 0.02–0.04 N∙m−2, corresponding to mean current speeds of 10–15 cm∙s−1 at a reference height of 0.5 m above the sediment surface. On settling out, egg burial is likely. This will increase egg mortality not only by physically hindering oxygen transport to the egg, but also, in POC-rich eutrophic lake sediment, by relocating the egg in a zone of steep oxygen gradients and low mean oxygen concentrations. Microelectrode measurements and computation of the thickness of the oxygen diffusive boundary layer over the sediment reveal that even eggs that escape interment are likely to be subjected to ambient oxygen concentrations insufficient for development to hatching. It is suggested that transport and burial may in general be important factors determining coregonid egg mortality in eutrophic lakes.