Variations In Sediment Grain Size Research Articles

Geochemical Characterization of Sediments from the Bibione Coastal Area (Northeast Italy): Details on Bulk Composition and Particle Size Distribution

The aim of this study is to characterize the sediments of the coastal area of Bibione and the Baseleghe Lagoon (Province of Venice, Italy). The characterization includes the assessment of particle size distribution, carbonate content, composition of major oxides, and heavy metal concentrations. The results indicated that the sediments primarily consisted of carbonate but showed significant heterogeneity in both composition and grain size within the different environments of the Bibione coastal area. Carbonate content decreased from the beach to the seabed, which does not appear to be solely influenced by variations in sediment grain size. This finding has potential implications for the Bibione area’s coastal erosion processes and sediment quality. Significant differences in grain size and composition were observed between the mouth and the inner region in the lagoon area. The textural characteristics of the sediments in the inner part of the lagoon make it particularly vulnerable to pollution, with potential environmental and economic consequences. Different pollution indices have indicated the presence of heavy metal contamination in both the coastal and, especially, the lagoon area. The source of these metals appears to be predominantly natural, although there may be some contribution from anthropogenic sources for certain metals. However, the comparison of the metal concentrations in the samples with the limits set by the Italian legislation showed that the sediments were still of good quality.

Mapping depth-integrated microphytobenthic biomass on an estuarine tidal flat using Sentinel satellite data

Microphytobenthos (MPB) are central to benthic tidal flat ecological networks. Large-scale information on total MPB biomass is difficult to obtain from traditional in-situ measurements. Here, we assessed the effectiveness of using surface sediment properties as proxies for predicting the total (depth-integrated) MPB biomass with Sentinel satellite data. First, the best subset regression was applied to the in-situ data to determine the properties that best predict the biomass decay rate with depth (sampling every cm up to 10 cm depth). Then, data from a controlled laboratory experiment were used to analyse the spectral response to different sediment properties (i.e., variations in sediment grain size, organic matter and water content). Subsequently, an algorithm was developed to obtain the spatial distribution of the sediment properties and the depth-integrated total MPB biomass from remote sensing data. Finally, we presented a case study in which the seasonal dynamics of total (depth-integrated) MPB biomass were obtained from Sentinel-2 Multispectral Instrument (MSI) satellite data using the Google Earth Engine (GEE) platform. The results showed that (1) the vertical distribution of MPB biomass on a tidal flat could be predicted by the surface MPB biomass, median grain-size (D50) and water content (W0) of the sediment (top 1 cm); (2) the near infrared (NIR) band absorption depth was a key feature in estimating D50 and W0; and (3) the seasonal variation in the total (depth-integrated) MPB biomass was prominent in Chongming Dongtan, Changjiang Estuary. We concluded that the depth-integrated exponential decay algorithm was useful for estimating total MPB biomass and, combined with mappable sediment-surface data, could help map the total MPB biomass in estuarine tidal flats at a large scale.

Coastal aquifer behaviour related to the textural and mineralogical characteristics of the sands in the eastern coast of the province of Buenos Aires

There are many coastal aquifers in South America on which there is little data, even lacking preliminary hydrological studies. When analyzing the regional hydrogeological behaviour of the coastal plain of the province of Buenos Aires, Argentina, the Pleistocene–Holocene evolution is considered to be especially relevant. This is particularly so in the case of the sand formations, as they constitute aquifer units due to their high permeability and storage capacity. It was the objective of this work to undertake a detailed granulometric and mineralogical study of the formations that occur in this coastal aquifer system in order to determine its parameters and hydrochemical behaviour. Hydrologically speaking, the aquifer unit is composed of the Punta Médanos Formation, the Mar de Ajó Facies and the upper section of the Destacamento Río Salado Facies, while the clayey levels act as an aquitard/aquiclude. The decrease in grain size with depth suggests a decrease in effective porosity in the same direction, with the resulting reduction in flow velocity. This was corroborated by the fact that the hydraulic conductivity values obtained depended on the sediment grain-size variations in each formation. The mineralogical study shows abundant magnetic, opaque (sulphides and oxides) and mafic minerals with Fe and Mg–Mn in their composition. These could be the source of the high concentrations of Fe and Mn in the regional groundwater, which exceed the values that are fit for human consumption; this represents a problem, both for water service providers and for inhabitants with no access to a water supply network.

A Conversion for Average Grain Size Based on Gamma Ray Well Log: Application in the Second Member of Xujiahe Formation of Anyue Area, Sichuan Basin

The grain sizes of clastic rock sediments serve as important depositional indicators that are significant in sedimentology and petroleum geology studies. Generally, gamma ray, spontaneous-potential and resistivity well logs are utilized to qualitatively characterize variations in sediment grain size and determine the lithology in clastic reservoirs. However, grain size analysis of modern sedimentary samples collected from active rivers and deltas indicates that the percentage of fine depositional component has a logarithmic relationship with the average grain size in delta and river systems. Using the linear relationship to process the lithology interpretation, siltstones or mudstone is likely to be interpreted as sandstone. Therefore, a logarithmic conversion formula was built up between the gamma ray logs and measured grain size for the second member of the Xujiahe Formation of Anyue Area in the Sichuan Basin. Using the formula, the average grain size and lithology of the exploration wells were determined for the interest intervals. Furthermore, the calculated grain size gives a better understanding of the controlling factors of hydrocarbon-bearing reservoirs in the study area.

TiO2–Zr–Th–Y–rare‐earth elements ore deposit in unconsolidated fine‐grained sediments from Tongo Gandima in the Bétaré Oya Gold district, Eastern Cameroon

Tongo Gandima is an area located in the southeastern part of the Bétaré Oya Gold district (Eastern Cameroon). Geochemical investigations on rare metals (TiO2, Th, Y, and rare‐earth elements [REE]) were carried out on sample types (source rocks, unconsolidated fine‐grained sediments, and concentrated sediments) using X‐ray diffraction (XRD), X‐ray fluorescence (XRF), inductively coupled plasma‐mass spectrometry (ICP‐MS), and scanning electron microscope equipped with EDAX spectrometer (SEM‐EDS) methods. In this study, five layers of river terraces were selected (organic matter layer, clayey layer, fine sandy layer, coarse sandy layer, and gravel layer). One of them, the gravel layer, is mineralized. TiO2, Zr, and REE are more enriched in monzonites than in granites. Unconsolidated sediments derive mainly from the erosion of weathered materials developed on monzonites and granites. The mineral assemblage of fine‐grained sediments from the gravel layer is muscovite, kaolinite, quartz, rutile, smectite, and others, and the average concentrations in Th, Zr, and Y are high. Fifty concentrated fractions from the same samples were analysed separately and their mineralogy is smectite, kaolinite, gibbsite, muscovite, quartz, rutile, anatase, zircon, titanomagnetite, and ilmenite. The heavy mineral fractions are opaque minerals, biotite, zircon, rutile, tourmaline, garnet, diopside, zoisite, and kyanite. TiO2, Zr, Th, Y, and REE contents are high. The REE oxides (REO) total grade ranges from 0.0088 to 15.66 wt%, marked by LREO‐abundance, especially in La2O3, Ce2O3, Pr2O3, and Nd2O3. The Tongo Gandima REO deposit was classified as LREO‐dominated type. The chondrite‐normalized REE patterns exhibit strong negative Eu anomalies; their V‐shaped patterns are typical of ore varieties. The abundant TiO2, Zr, Y, Th, and REE might have been influenced by surface processes (peneplanation and chemical weathering) and variations in sediment grain size. SEM‐EDS data reveal that apatite with Ce‐ and La‐dominant, monazite‐(Ce), and Ce‐Mn‐bearing ilmenite are rare metal‐bearing minerals. This study demonstrates that the economic importance of the alluvial‐fluvial placers from Tongo Gandima results from their high Ti, Zr, Th, Y, and REE contents.

Variations in Sediment Grain Size from a Lake in the Tianshan Mountain of Central Asia: Implications for Paleoprecipitation Reconstruction

The Tianshan Mountain is the largest mountain range in Central Asia, and the source area of many river systems. Changes in precipitation result in significant alterations to regional hydrological processes. Lake sediment from the Tian Shan representative of the last 90 years was chosen as the object of this research study. The grain-size data were used in conjunction with instrumental data to provide a method for determining changes in paleoprecipitation. The results showed the three-point moving average curve of the silty fraction content with a size of 16 to 32 μm to be significantly consistent with the curve of total precipitation from April to September since 1950. The total content of clay and fine-silty fraction (0–16 μm) was clearly consistent with the monthly precipitation in July. The total precipitation from April to September showed a significant downward trend from 1930 to 1975, and then an overall increasing trend beginning in 1975, which may have been influenced by the North Atlantic Oscillation. The change in precipitation reconstructed by the grain size of lake sediments was significantly different from the high-resolution gridded datasets (Climatic Research Unit Time-Series version 4.04) because of the lack of data from meteorological stations in China before 1950. The conclusions of this study are significant for evaluating the validity of climatic research unit (CRU) data in arid areas of Western China. In addition, the results of this study serve as a bridge between modern instrumental records and long time-scale paleoclimate research and provide important reference values for future reconstructions of long time-scale paleoclimate.

Sediment grain size does matter: implications of spatiotemporal variations in detrital zircon provenance for early Paleozoic peri-Gondwana reconstructions

A comprehensive understanding of the relationship between the Sino-Korean Craton (SKC) and the Gondwana supercontinent is crucial for accurate reconstruction of the tectonic evolution and early Paleozoic paleogeography of East Asia. To explore the link between the SKC and peri-Gondwana, we provide new detrital zircon U–Pb age data from litho- and bio-stratigraphically constrained lower Cambrian to Lower Ordovician sandstone deposits from the eastern margin of the SKC (Taebaeksan Basin). Results indicate that the two distinct age spectra of detrital zircon resulted from provenance change combined with a strong function of sediment grain size within host siliciclastic rocks of the Taebaek Group. The age spectra from coarse-grained sandstones display Paleoproterozoic (1.9 Ga) and Neoarchean (~ 2.5 Ga) peaks, indicating that the sediments were supplied from the basement rocks of the SKC. Conversely, age spectra from fine-grained siliciclastics contain late Mesoproterozoic (~ 1.0 Ga) and Neoproterozoic (~ 0.6 Ga) peaks, with or without a Paleoproterozoic signal, consistent with a Gondwanan sediment origin. These different age populations have been documented in multiple Cambrian and Ordovician sequences of the SKC and are consistently well correlated with sediment grain size. Coarse-grained sediments, sourced primarily from local basement rocks, were deposited in coastal, nearshore, and shelf environments, whereas fine-grained sediments were derived from more distal sources (i.e., the Gondwana mainland and/or the Gyeonggi Marginal Belt) and deposited in inner to outer shelf settings. Therefore, the repeated occurrence of sediment with two distinct provenances in the Cambrian–Ordovician siliciclastics of the SKC is likely a result of provenance shift integrated with changing depositional environments in an epeiric platform. This yielded variations in sediment grain size and source rock provenance, with and/or without a tectonic activity. Consequently, our results indicate that the SKC was adjacent to Gondwana during the early Paleozoic.

Changing sediment sources in the Bay of Bengal: Evidence of summer monsoon intensification and ice-melt over Himalaya during the Late Quaternary

A gravity core retrieved from the southern Bay of Bengal (BoB) was studied to determine the sources of sediments and factors controlling their distributions during the Late Quaternary. The parameters used for this study was sediment grain size, organic carbon, clay mineralogy and environmental magnetic properties. Radiocarbon ages of sediments indicate that the 3.06 m long sediment core covers a period since ~45 cal kyr B.P. The sediments deposited before ~14 cal kyr B.P. were clayey silt with higher magnetic susceptibility (MS) values (av: 26 × 10−8 m3 kg−1) and those deposited after ~14 cal kyr B.P. were sandy silt with lower MS values (avg: 13 × 10−8 m3 kg−1). The latter period (after ~14 cal kyr B.P.) indicated high energy turbidites from the Himalayan Rivers into the BoB due to the strengthening of summer monsoon and ice-melt in the Himalayas. Higher smectite and lower illite contents during the last glacial maximum (LGM) reflected low sediment supply from the Ganges-Brahmaputra River System (GBRS) and higher illite and lower smectite during ~14 cal kyr B.P. indicated increased sediment supply from GBRS during this interglacial period. The magnetic grain size of sediments deposited from ~15 cal kyr B.P. was more fine-grained at the start of the Bølling-Ållerød (B-A) event and coarse-grained during ~13 cal kyr B.P. (Younger Dryas event). Fine-grained magnetic minerals also showed a peak at ~11 cal kyr B.P. along with higher summer solar insolation. The down-core variations in sediment grain size, magnetic properties and clay minerals were interpreted in relation to the significant changes in sources of sediments in the BoB during different climatic events. The changing sediment supply in the BoB was due to the combined influence of ice-melt by increased solar insolation and intensification of summer monsoon over Himalaya. Increase of fine-grained magnetic minerals at ~15 cal kyr B.P. and coarser sediment fraction at ~14 cal kyr B.P. indicated time gap between intensification of summer monsoon and ice-melt over Himalaya, respectively. The trend of the magnetic grain size record seems to be modulated by orbital-scale variations in June insolation, which coincides with the summer monsoon variations also. These observed climatic variations are correlating well with the climate changes of the Northern Hemisphere during the Late Quaternary.

UK′37 temperature estimates from Eemian marine sediments in the southern coast of Hainan Island, tropical China

This study concerns high-resolution sea surface temperature (SST) changes for the Eemian interglacial period reconstructed from the southern coast of Hainan Island, China. The lower marine unit in the sediment core is composed of monotonous mud (31m thick), dated to be from the Eemian interglacial period or lower part of marine isotope stage 5 (MIS 5), determined by optically stimulated luminescence (OSL). The high sedimentation rate and the richness of organic matter and marine fossils provide good opportunity for the study of palaeo-environments in the northern margin of the South China Sea (SCS). Sediment grain-size, foraminifer, sediment color index and long-chain alkenone were used to reveal the near-shore sedimentary environments and temperature at millennial-scale. Specifically, a high-resolution UK′37-SST record was reconstructed for the Eemian interglacial period. The new evidence in northwestern margin of SCS shows that the maximum value of SST during the Eemian interglacial is approximately 29°C and the lowest is 26.5°C. The trend of decreasing SST from 29°C throughout the marine sequence is consistent with the variations in sediment grain size, reflecting the cooling trend of SST and sea-level lowering tendency during the last interglacial period. This new Eemian SST result agrees with other records in the region of SCS, showing a coherent descending trend, but spatial heterogeneity exists in timing and amplitude. The insolation changes might be the main force for initiating the SST evolution during the penultimate interglacial period.

Late Quaternary loess deposition in the southern Chaiwopu Basin of the northern Chinese Tian Shan foreland and its palaeoclimatic implications

The piedmont aeolian sediment in Central Asia is has been widely used for Quaternary palaeoclimatic reconstructions. Here we present detailed sedimentological and rock magnetic investigations on 188 samples from the ~9.4‐m‐thick Urumqi River section (URS) in the southern Chaiwopu Basin of the northern Chinese Tian Shan foreland. Optically stimulated luminescence (OSL) dating constrains the time‐span of the URS loess sedimentation from c. 28 to c. 6.8 ka. This chronology implies that the onset of the URS loess sedimentation is approximately synchronous with the Shangwangfeng Glaciation (MIS 2) in the headwaters of the Urumqi River. Analyses of sediment grain size and rock magnetic parameters indicate that sediment grain‐size composition controls magnetic properties of the URS loess, i.e. the wind intensity model, distinct from the pedogenic model interpreting the variations in magnetic properties of the loess in the Chinese Loess Plateau (CLP). Overall, the variations in sediment grain size and magnetic properties throughout the URS indicate that the present study area was dominated by a climate of strengthened wind associated with gradually intensified aridity during the latest Pleistocene. In contrast, the intensity of aridity and coldness during the early Holocene gradually became relatively weakened.

Reproduction of estuarine bathymetry by means of a process-based model: Western Scheldt case study, the Netherlands

The current study aims to investigate the processes governing the evolution of major morphological features in alluvial estuaries as well as their morphodynamic interaction with the geometry by means of a 3D morphodynamic, process-based numerical model (Delft3D). The main hypothesis is that channel-shoal patterns evolve through the interaction between tidal movement, the available sediments and the tidal basins' geometry. Once such a system is not disturbed too much by extreme events, stable morphological patterns will emerge that compare well with observed features.Starting from a flat bed and imposing the Western Scheldt geometry, model results show two distinct time-scales, i.e. channel-shoal pattern formation within decades and a slow longitudinal profile development continuing for centuries. The developing channel-shoal patterns reflect major morphological features observed in the Western Scheldt estuary. Using the 1998 bathymetry as a reference, we applied different indicators and techniques to evaluate the model performance over time, namely: visual comparison, hypsometries, Brier Skill Score (BSS), mean basin depth and longitudinal profiles. Sensitivity analysis shows that trends in development are similar for different model parameter settings, like introducing salt-fresh water density differences or variations in sediment grain size and sediment transport formulation. Varying river discharge and including non-erodible layers, dredging and dumping activities and 3D hydrodynamics led to occasional improvements. The geometry is an important governing factor for pattern development. Applying an erodible geometry reveals a timescale for geometric adaptation that is much longer than the pattern development timescale within the basin. Model runs including highly schematized extreme conditions suggest that extreme events play a limited role.The main conclusion of the study is that the interaction between tidal forcing and basin geometry plays a major role in the development of morphological features in an alluvial tidal basin such as the Western Scheldt. A process-based model taking into account these major forcing mechanisms can predict such features with significant skill. The bathymetries generated in this study may form the basis for further studies considering, for example, the impact of sea level rise on estuarine morphodynamic development, or a rapid assessment of estuarine bathymetry in the absence of measured data.

Episodic reductions in bottom-water currents since the last ice age

Although North Atlantic deep-water formation was greatly reduced during the last glacial maximum, bottom-water currents were as vigorous as at present. However, they were weakened during periods of North Atlantic surface freshening. A strong correlation can be seen between bottom-water-current strength and Greenland air-temperature records, thus confirming a close connection between ocean circulation and abrupt climate change. Past changes in the freshwater balance of the surface North Atlantic Ocean are thought to have influenced the rate of deep-water formation, and consequently climate1,2. Although water-mass proxies are generally consistent with an impact of freshwater input on meridional overturning circulation3, there has been little dynamic evidence to support this linkage. Here we present a 25,000 year record of variations in sediment grain size from south of Iceland, which indicates vigorous bottom-water currents during both the last glacial maximum and the Holocene period. Together with reconstructions of North Atlantic water-mass distribution, vigorous bottom currents suggest a shorter residence time of northern-source waters during the last glacial maximum, relative to the Holocene period. The most significant reductions in flow strength occur during periods that have been associated with freshening of the surface North Atlantic. The short-term deglacial oscillations in bottom current strength are closely coupled to changes in Greenland air temperature, with a minimum during the Younger Dryas cold reversal and a maximum at the time of rapid warming at the onset of the Holocene. Our results support a strong connection between ocean circulation and rapid climate change.

Black carbon contents and distribution in sediments from the southeastern Brazilian coast (Guanabara Bay)

The concentration of black carbon (BC), as well as the relative contribution of BC to total organic carbon, was determined for the first time in sediments from Guanabara Bay. BC was quantified by a thermal oxidation method (CTO-375) and total organic carbon by dry oxidation. Sedimentary organic carbon showed a large range in concentrations (0.82 to 10.3%), but it was possible to define distinct regions of the Bay accordingly to variations in sediment grain size, location and magnitude of allochthonous and autochthonous sources of organic matter and hydrodynamic features. Low concentrations of BC were observed (0.03 to 0.31%, with a mean of 0.19 ± 0.07%, n = 25) when considering the large spectrum of possible sources on land and at sea. However, in some stations levels are sufficiently high as to influence the fate of pollutants of concern, such as PAH and dioxins. The space distribution of BC concentrations in Guanabara Bay cannot be fully understood at this stage since further information on contributions from various sources and on natural dispersion processes are required.

Spatial and temporal variations in sediment grain size in tidal wetlands, Yangtze Delta: On the role of physical and biotic controls

To examine the spatial and temporal variability of sediment grain size in exposed tidal wetlands with ample sediment supply, we sampled sediments and measured hydrodynamics, accretion/erosion rates, and vegetation characteristics in the Yangtze Delta. Sediment grain size exhibited a landward/upward decreasing trend. This trend is mainly attributed to attenuation of hydrodynamics. A 630-day series of daily surface sediment sampling at a fixed site on an unvegetated intertidal flat revealed significant seasonal and storm-cyclic changes in grain size. This temporal variability was related to alternating accretion/erosion events, with erosion associated with coarser grain size. Such temporal dynamics were not present in vegetation, where sediment remained fine grained throughout the year. In the marsh, vegetation cover enables the trapping of fine-grained sediments in the following ways: (a) adherence of suspended sediments onto plants; (b) deposition of suspended sediments stimulated by attenuation of hydrodynamics through plant obstruction; and (c) prevention of resuspension of fine-grained deposits due to the protection of the plant canopy. The influence of vegetation on sediment grain size was clearly seen when comparing sediment trapped by different vegetation types and seasonal patterns of trapped sediment on different vegetation canopy densities. The relatively high plant biomass of the recently introduced Spartina alterniflora enhanced the trapping effect, whereas plant degradation due to buffalo grazing reduced the trapping effect. We conclude that for exposed tidal wetlands with ample sediment supply such as the Yangtze Delta, the spatial and temporal variability of sediment grain size is governed predominantly by physical controls on the unvegetated flat and predominantly by biophysical interaction of hydrodynamics and vegetation in the salt marsh, rather than by sediment supply.

Sedimentation rates and trace metal input history in intertidal sediments from San Simón Bay (Ría de Vigo, NW Spain) derived from 210Pb and 137Cs chronology

210Pb and 137Cs dating methods, accompanied by a high-resolution geochemical study, were applied to intertidal sediments containing both fine and coarse-grained particles and also, anthropogenic Pb in order to establish sedimentation rates and historical trends in heavy metal input. Sedimentation rates were established according to the “Constant Flux:Constant Sedimentation” (CF:CS) 210Pb dating model, which was preferred rather than the “Constant Rate of Supply-Minimum Variance” (CRS-MV) model. Variations in sediment grain-size were accounted for by application of several normalization procedures. Al was selected for grain-size correction. Corrected sedimentation rates obtained from 210Pb dating were corroborated through a second independent radionuclide tracer and by comparison of peak trace metal inputs into the environment with peaks in the sediment. The Chernobyl maximum was identified in the normalized 137Cs activity profile. Sedimentation rates of 6.2 ± 1.2 mm y −1 and 3.1 ± 0.5 mm y −1 for the upper (two muddy layers) and 6.0 ± 2.0 mm y −1 for the lower (sandy) intertidal mud flat deposits of San Simón Bay were determined. According to the established chronology, human-induced environmental changes were detected in total metal concentrations (Cu, Pb, Zn) and in sediment grain-size and composition.

Historical Trace Metal Accumulation in the Sediments of an Urbanized Region of the Lake Champlain Watershed, Burlington, Vermont

This study documents the history ofpollution inputs in the Burlington region of LakeChamplain, Vermont using measurements of anthropogenicmetals (Cu, Zn, Cr, Pb, Cd, and Ag) in four age-datedsediment cores. Sediments record a history ofcontamination in a region and can be used to assessthe changing threat to biota over time and to evaluatethe effectiveness of discharge regulations onanthropogenic inputs.Grain size, magnetic susceptibility, radiometricdating and pollen stratigraphy were combined withtrace metal data to provide an assessment of thehistory of contamination over the last 350 yr inthe Burlington region of Lake Champlain. Magneticsusceptibility was initially used to identify land-usehistory for each site because it is a proxy indicatorof soil erosion. Historical trends in metal inputs inthe Burlington region from the seventeenth through thetwentieth centuries are reflected in downcorevariations in metal concentrations and accumulationrates. Metal concentrations increase above backgroundvalues in the early to mid nineteenth century. Themetal input rate to the sediments increases around1920 and maximum concentrations and accumulation ratesare observed in the late 1960s. Decreases inconcentration and accumulation rate between 1970 andthe present are observed for most metals. Theobserved trends are primarily a function of variationsin anthropogenic inputs and not variations in sedimentgrain size. Grain size data were used to removetexture variations from the metal profiles and resultsshow trends in the anthropogenic metal signals remain. Radiometric dating and pollen stratigraphy providewell-constrained dates for the sediments therebyallowing the metal profiles to be interpreted in termsof land-use history.

Morphology and Paleoenvironmental Distribution of Paleozoic Spirophyton and Zoophycos: Implications for the Zoophycos Ichnofacies

Spirophyton producers inhabiting ephemeral ponds on estuarine floodplains marginal to the (Devonian) Catskill sea covering present-day southern New York were subjected to salinity fluctuations and desiccation, whereas Zoophycos producers living farther offshore were buried by storm deposits. Producers of Zoophycos living in interdistributary bays on Pennsylvanian lower delta plains (Tennessee) contended with fluctuations in salinity and oxygen conditions and variations in sediment grain size and in rate of sedimentation. In each case the producer was an r-selected opportunistic species, as shown by the distribution of the Spirophyton or Zoophycos and the paucity of co-occurring trace fossils. Morphologic analysis indicates that the Paleozoic producers of these Zoophycos and Spirophyton: (1) transported sediment downward from overlying layers or the sediment surface, and (2) were capable of a variety of behavioral patterns, only some of which resulted in complex 3-dimensional structures.

Short‐ and long‐term sediment toxicity test methods with the amphipod Grandidierella japonica

AbstractMethods are described for conducting flow‐through sediment toxicity tests with the marine amphipod Grandidierella japonica. Short‐term (10‐d) exposures were conducted at 15°C in 1‐liter beakers containing a 2‐cm layer of sediment and 700 ml overlying seawater. Long‐term (28‐d) tests were conducted in 1‐liter beakers at 19°C with the weekly addition of food to the test chambers. Both methods were used to measure the toxicity of sediments from five locations in southern California. These sites included highly contaminated areas adjacent to large municipal wastewater outfalls and within industrialized harbors.Both test methods were sensitive to levels of contamination found in the field, but produced different patterns of effects. Short‐term mortality was greatest in amphipods exposed to the harbor sediments, while long‐term exposure produced the greatest reductions in survival and growth at the Los Angeles County Outfall site. Amphipod survival was unaffected by variations in sediment grain size, while this characteristic appeared to have an important effect on growth.

Regional geochemistry of metal-contaminated surficial sediments and seagrasses in upper Spencer Gulf, South Australia

Surficial sediments, shallow sediment cores and seagrass samples were collected over an area of about 600 km 2 of upper Spencer Gulf, South Australia, adjacent to a major lead-zinc smelter. Regional variation in heavy metal contents was studied in relation to different pathways of contamination from the smelter complex, and geochemical association with natural constituents of the sediments. About 300 km 2 were contaminated to some extent. Areas of appreciable contamination (10 × background) were greatest for Cd and least for As, with Pb and Zn intermediate. Regional trends of heavy metals in the sediment reflected inputs of pollutants from both liquid effluents and atmospheric fallout, modified by tidal currents. Total accessions to Germein Bay were estimated to be at least 25 000 tonnes each of Pb and Zn and 500 tonnes of Cd and As. Arsenic differed both in its distribution with sediment depth and in its geochemical associations when compared to Pb, Zn and Cd. Correlation of Cd with the fine fraction (< 20 μm) permitted a normalization of data which accounted for marked regional variations in sediment grain size and provided a better basis of assessing the regional impact of pollution. Metal concentrations in Posidonia leaves were highly correlated with total and EDTA extractable metal concentrations in the sediments, and may provide a more sensitive indicator of metal dispersion. Background levels of metals in Posidonia were not found in our study area.

Cyclicity and the facies model concept in fluvial deposits

ABSTRACT Since the fining-upward cycle model for meandering-river deposits was published in 1963, about a dozen facies models have been defined for rivers of varying sediment grain size, channel multiplicity and sinuosity. However, this suite of models is inadequate for the interpretation of many fluvial deposits because it does not incorporate the effects of a variety of outside-the-basin (allocyclic) sedimentary controls. The most important of these controls is tectonics, because differential movement between and source area, and changes in the rate of movement, can cause major changes in fluvial sedimentary environments. These changes give rise to fining- or coarsening-upward cyclic sequences which are superimposed on the cycles generated by within-basin (autocylic) mechanisms. Variations in climate can cause variations in sediment grain size and in depositional style as a result of alterations in weathering modes and discharge characteristics. These latter variations have been studied in hot arid, warm temperate and fluvioglacial regimes, but are poorly known in other climatic zones. As a result, many of our preconceptions about fluvial geomorphology and sedimentation may be inappropriate for other climatic conditions such as hot, humid environments. Channel morphology varies between four basic styles: braided, meandering, anastomosed and straight, but the sedimentological characteristics of intermediate morphological types have yet to be quantified. Many pre-Carboniferous rivers appear to have lacked clearly defined channels with cutbanks. The probable cause was low bank cohesion caused by an absence of land vegetation, a condition that cannot be modelled accurately from observations of modern rivers. The absence of vegetation would have favoured a flash-flood depositional process, which tends to generate sheet-like deposits rather than lenticular, channel-bound units. Sedimentological studies in modern rivers are beset by the problem of preservability, in that a river's long-term aggradation behaviour is commonly unclear. The problem is compounded by the fact that many studies have been carried out on rivers that are actively degrading. Cyclic sequences in fluvial sediments can be produced by a variety of mechanisms, and the interpretation of a given deposit solely in terms of a published facies model may be misleading. For most fluvial sequences it is necessary to adopt a multivariate basin analysis approach.