Showing papers on "Silt published in 1987"

Dust transport and the question of desert loess formation

Abstract: Although contemporary dust storms are frequent in arid and semi-arid areas, desert loess deposits are poorly developed. Much of the World's loess occurs in mid-latitude areas which experienced glaciation during the Pleistocene. Ocean core evidence indicates that dust transport from sub-tropical deserts increased during cold stages of the Pleistocene, but loess formed only on certain desert margins, for reasons which have not been fully explained. This paper re-examines the mechanisms of dust transport and deposition, and the circumstances leading to the accumulation of thick loess. Typical loess is composed mainly of medium silt grains which are transported in short-term suspension a few metres above the ground. Significant thicknesses of loess form only when dust is trapped within a limited area, often relatively close to the source. Dust particles finer than 20 μm are transported mainly in long-term suspension over a greater height range and may be widely dispersed. The availability of silt and the frequency, magnitude and direction of dust-transporting winds are important factors governing the potential for loess formation, but the existence of a suitable dust trap is particularly important. Traps may be formed by topographic obstacles, areas of moist ground, or vegetated surfaces. Vegetation adjacent to glacial outwash plains and braided meltwater streams trapped dust in mid-latitudes during the Pleistocene. Dust blown during glacial periods from certain deserts, notably in Sinai, Soviet Central Asia and China, accumulated as loess in neighbouring semi-arid regions. On the margins of other deserts loess formation was inhibited partly by the absence of vegetation traps. During most of the Holocene net dust deposition rates in all desert-marginal areas have been too low for significant loess accumulation. This is mainly due to a reduction in silt availability and a tendency towards landscape stability. Reported dust storm frequencies during the past 50 years over-estimate the longer-term Holocene dust flux due to the effects of human activities. Much modern dust owes its origin to erosion of cultivated soils in semi-arid areas and is finer than typical loess.

A micro‐pipette method for soil mechanical analysis

Abstract: Determination of soil texture, particularly the clay (<2 μm) fraction, is an important measurement in most soil investigations. In this study a modified method for mechanical analysis was evaluated that eliminated the need for bulky laboratory equipment and long settling times associated with standard pipette and hydrometer methods. Termed a “micro‐pipette”; method, the modified procedure uses 2 to 4 g soil with 40 mL of dilute dispersant, shaken overnight in 50 mL centrifuge tubes. Clay is determined by sampling 2.5 mL using an adjustable volume pipettor from a depth of 2.5 cm after approximately 2 h of settling, as calculated from Stokes’ law. The dried suspension weight is used to compute clay content after correction for salt content of the dispersant. Sand can be determined by sieving at 50 μm after clay analysis, with silt calculated by difference. Using 12 soils with a range of particle sizes, the proposed method was found to give textural values nearly identical to those found with the st...

Experiments on turbidity currents over an erodible bed

Abstract: Experiments were conducted to determine the behaviour of turbidity currents laden with non-cohesive silt (silica flour) moving down a slope the bed of which is covered with similar silt. Each current was sustained with constant inlet conditions for six to eight minutes. The motion of the head was not studied; measurements were concentrated on the continuous part of the current that was essentially constant in time but developing in space. Only supercritical currents were studied. The currents were free to erode sediment from, and deposit sediment on, the bed. Measurements of vertical profiles of downstream velocity and sediment concentration allowed for the development of approximate similarity relations. These relations can be used to estimate most of the shape factors that occur in the vertically-integrated equations of motion. The values do not deviate grossly from those found by using a "top-hat" function, i.e. a step function, such that velocity or concentration equals its average value in the layer ...

Historical Valley Floor Sedimentation in the Upper Mississippi Valley

Abstract: This research evaluates the response of floodplain sedimentation rates to post-1820 A.D. human activity in the Lead-Zinc district of the Upper Mississippi Valley. Although mine waste has contributed very minor amounts of sediment to historical overbank floodplain sedimentation that commonly is 30–50 cm deep on tributary floodplains to as much as 3–4 m deep on floodplains in the lower reaches of main valleys near the Mississippi River, it is well represented as trace metals. The dating of stratigraphic horizons was accomplished by matching abrupt rises in the concentration of zinc adsorbed on silt and clay particles with the date of opening of a nearby upstream mining and milling operation. Results show that decadal-scale average historical rates of overbank floodplain sedimentation range from about 0.3 cm/yr to 4.0–5.0 cm/yr and greatly exceed the average presettlement post-glacial (Holocene) floodplain vertical accretion rate of 0.02 cm/yr. Overbank sedimentation rates rose rapidly on both tribu...

Decomposability of organic matter in particle size fractions from field soils with straw incorporation

Abstract: Soils from two field experiments on straw disposal were fractionated according to particle size using ultrasonic dispersion and gravity-sedimentation in water. Samples of whole soils, clay. silt and sand-size fractions were held for 49 days at 20°C and the CO2 evolution measured on 14 dates by gas chromatography. Recovery of soil solids. C and N was 99, 98 and 93%, respectively. Most of the soil C and N was in the clay (<2μm). (loamy sand, 50% C and 56% N; sandy loam. 65% C and 68% N), the silt (2–20 μm) having smaller proportions (loamy sand, 41% C and 38% N; sandy loam. 29% C and 27% N). The sand fraction (20–6000 μm) accounted for 4–7% of the organic matter, and 1–2% of the C was water soluble. Straw incorporation generally increased the C and N content of whole soils and size fractions. The decomposition rate constants were higher for the sandy loam than for the loamy sand soil. For both soils, the decomposability of the organic matter decreased in the order: sand > clay ⩾ whole soil > silt. Straw incorporation increased the decomposition rate of whole soil and sand organic matter. whereas the effect of straw on clay and silt respiration was small. Between 58 and 73% of the respiration was from clay, 21–25% from silt and 6–19% from the sand size fraction.

Cohesion Development in Disrupted Soils as Affected by Clay and Organic Matter Content and Temperature

Abstract: Soils were dispersed and separated into sand, silt, and clay fractions that were reconstituted to give mixtures of each soil with 5 to 40% clay. In the range from 0 to 35% clay, higher clay contents resulted in greater stability. Rate of cohesion recovery was over 10 times as fast at 90°C as it was at 23°C, showing that the processes Involved are physical-chemical rather than biological. Maximum rates of cohesion recovery occurred at moderate soil water tensions, probably because some tension is needed to pull the particles into direct contact, but a continuous water phase is also essential to allow diffusion of bonding agents to the contact points. Since diffusion rates in water increase 3e0o*, while rate of cohesion recovery increased 1000% when temperature was raised from 23 to 90°C, other factors, such as higher Mobilities at higher temperatures of compounds contributing hooding ions to the solution. probably play a role In the rate of cohesion recovery. Recovery of cohesion was more rapid in the soil with organic C contents of 0.004 kg/lig than in the soil with 0.012 kg/kg. When the organic matter was removed with H 20 1 from the soil with 0.012 kg C/kg, its rate of cohesion recovery increased. Rate of cohesion recovery of this high organic matter soil was also increased by aging it at 0.1 kg HP/kg soil compared to 0.2 kg/kg. A possible explanation is that organic coatings, tending to prevent direct contact and bonding of adjacent projections of mineral surfaces, are forced away from contact points by extremely strong forces that pull the adjacent minerals together when soil water tensions are high. When the higher organic matter soil had been consolidated by air-drying and rehydrated, its rate of cohesion recovery was just as rapid as that of the soil with low organic matter.

Rates and processes of soil development on Quaternary terraces in Cajon Pass, California

Abstract: Field and laboratory analyses of soils on 11 well-dated fluvial terraces spanning the past 0.5 m.y. demonstrate that a threshold governs changes in several morphological and chemical characteristics of increasingly older soils. Correlations with respect to time among iron species, soil morphology, and soil silt and clay demonstrate that the chronosequence at Cajon Pass reflects primarily an evolutionary, largely time-dependent trend and does not reflect differences in external factors such as climate. Most of the soil development on Holocene terraces of the Cajon Pass area is due to physical incorporation of eolian dust and organic material into initially very permeable gravels. This process decreases soil permeability and is conducive for an increase in the magnitude of chemical weathering. Latest Pleistocene and older Pleistocene soils have developed clay and authigenic iron oxide-rich B horizons at the expense of organic-matter-rich A horizons and color B horizons as the extent of chemical weathering has increased. This conversion of the soil from a noncolloidal system to a much more colloidal system takes place over a relatively short period of time (< 4,000 yr) and is herein defined as a type of pedologic threshold. In the Cajon Pass area, the attainment of the threshold and subsequent development of the argillic B horizon of soils on latest Pleistocene terraces occurred during the Holocene; thus, the absence of argillic horizons in soils on Holocene terraces is attributable to simply their younger age rather than to the Pleistocene-to-Holocene climatic change. The threshold is a function of several variables, including influx rate of eolian dust and initial soil permeability; therefore, the time required to attain the threshold will vary in chronose-quences characterized by geomorphic or geographic settings that are different from conditions found in Cajon Pass.

The effect of cultivation on sediment composition and deposition in prairie pothole wetlands

Abstract: Texture, major nutrient content, and deposition rate of sediments were compared for five prairie pothole wetlands surrounded by native grassland and seven otherwise similar wetlands surrounded by row crop and small grain farmland. Specific differences in the nature of the sedimentation cycle of cultivated and noncultivated watersheds were indicated. Flux of total inorganic material into sediments averaged 80 and 43 mg cm−2 yr−1 in cultivated and grassland wetlands, respectively. Cultivated sediments contained significantly higher clay percentages, but lower percentages of silt and sand than grassland sediments. Deposition rates of clay at cultivated sites averaged five times that of grassland locations. Enrichment ratios (the quotient of sediment concentration divided by upland soil concentrations) suggested that sand was selectively retained in equal proportions on uplands in both types of watersheds, that silt was selectively removed (although in different proportions) from uplands in both types of watersheds, and that clay was selectively retained only on grasslands. Total N and organic matter concentrations were significantly higher in both the soils and sediments of grassland watersheds, but there were no differences in total P concentrations with respect to land use. Sediment flux rates for total N and organic matter were similar in the two land use types; however, P was transported at nearly twice the rate to cultivated wetlands. Enrichment ratios indicated that N and P were selectively removed in similar proportions from upland soils in both types of watersheds.

Genesis of marine terrace soils, Barbados, West Indies: Evidence from mineralogy and geochemistry

Abstract: Well-developed, clay-rich soils dominated by interstratified kaolinite-smectite are found on the uplifted coral reef terraces on the island of Barbados The reef limestone is unlikely to have been the soil parent material however, because it is 98 per cent CaCO3 and geomorphic evidence argues against the 20 m of reef solution required to produce the soils by this process The mineralogy of the sand, silt, and clay fractions of the soils, and trace element geochemistry, suggest that aeolian materials carried on the trade winds from Africa, volcanic ash from the island of St Vincent, and quartz from Tertiary bedrock on the island itself are the parent materials for the soils

Aeolian abrasion of quartz particles and the production of silt-size fragments: preliminary results

Abstract: Summary Experimental abrasion of quartz sand in an air stream shows that particles with initially high angularity are progressively rounded by the chipping of edges and corners. Grains closely resemble dune sand grains from deserts with respect to rounding and microscopic surface textures after 48 hours of abrasion. Results show an initially high weight loss from the original grains (primarily by edge chipping), compared with subsequent abrasion. High percentages of coarse and medium silt are produced but there is a gradual increase in the percentage of fine silt as rounding increases and the quantity of fines generated declines. Silica extraction from the fines using water shows concentrations of several hundred ppm. Implications for loess production and silcrete formation are suggested.

Infiltration capacity of disturbed soils: temporal change and lithologic control

Abstract: The hydrologic character and response of disturbed land is controlled, to a large degree,by soil infiltration characteristics. Reconstructed soils on surface mines (mine soils) of different age (1 to 4 years old) are used to investigate infiltration rates on disturbed landscapes. The data consist of soil/surface properties and runoff volumes fit to the Horton infiltration equation. Infiltration rates on newly reclaimed mine soils are an order of magnitude lower than adjacent, undisturbed soil. Few significant correlations exist between soil/surface properties and infiltration parameters for newly reclaimed soils. However, the correlation between infiltration and mine soil characteristics increases with soil age. Multiple regressions are used to explore relationships between infiltration parameters and soil/surface properties for each soil age. Regression models of 30-min infiltration volume and the steady-state rate consistently include the percent silt and clay, slope, bulk density, and vegetation. Mean infiltration volumes at different mines are equal in the first year following reclamation, but become significantly different with surface age. The magnitude of the increase is controlled by the soil texture, vegetation, slope, and bulk density. Soil characteristics are determined ultimately by the overburden lithology and its effect on mineralogy and grain size during physical redistribution of soil particles andmore » initial weathering.« less

Downstream changes in sand composition, roundness, and gravel size in a short-headed, high-gradient stream, northwestern Italy

Abstract: We assessed changes in grain composition and roundness of the 1-2 and 2-3 size fractions of sand along a 16-km length of the Graveglia River, Liguria, Italy, to its mouth at the Tyrrhenian Sea and then for 1 km along the beach. Rocks exposed in the drainage area include ophiolites, ribbon chert, limestone, shale, sandstone, and Quaternary sands. The erosion of shale and limestone bedrock yields large amounts of sand, but sand grains of these rock types decrease rapidly in abundance downstream, chiefly through dilution rather than abrasion. Carbonate rock fragments that reach the beach survive wave abrasion, but most shale grains do not. Predictably, monocrystalline and polycry talline quartz and chert are the most durable grain types, but serpentine, basalt, and unidentified rock fragments also show significant resistance to both stream and wave abrasion. Polycrystalline quartz, feldspar, basalt, serpentine, sandstone, and shale grains are more abundant in the coarser-sand-size fraction; monocrystalline quartz, carbonate rock fragments, and unidentified rock fragments are more abundant in the finer-sand-size fraction; and chert and plutonic rock fragments have no size preference. A comparison of the outcrop areas of the different rock types with the corresponding percentages of different grain types present in each of three drainage areas indicates that shale, which breaks down readily to silt- and clay-size particles, and chert, which does not readily break down into sand-size fragments, are underrepresented in the sands. Limestone and serpentine, however, are overrepresented in comparison to outcrop area in certain drainage areas. The overrepresentation of limestone reflects, in part, a sampling bias (sand was sampled adjacent to limestone outcrops), but the overrepresentation of serpentine reflects its resistance to weathering (compared to other aluminosilicates) and its durability. Shale, limestone, and serpentine increase in roundness downstream and along the beach to a maximum (limiting?) value of 5 rho units. Medium-grained shale clasts attain high rounding in only a few kilometers of transport. Downstream decrease in gravel size follows an exponential formula that can be expressed in linear form using phi size: size () = 0.1167 distance (km)- 10.29.

Physical properties of some Prince Edward Island soils in relation to their tillage requirement and suitability for direct drilling

Abstract: Successful reduced tillage systems depend on suitable soil types and the maintenance of optimum soil structure. Studies were conducted to assess the tillage requirement of some well-drained Podzolic and Luvisolic soils in Prince Edward Island and their suitability for direct drilling. Relatively high amounts of silt and fine sand indicated that the soils were structurally unstable with a propensity for compaction. Compactability was inversely related (r2 = 0.90) to concentrations of soil organic carbon over a range of 0.2–2.0%. In the Ap horizons, where organic carbon exceeded 1.6%, the level of macroporosity (equivalent pore diam. > 50 μm) was generally greater than 10%. The B horizon and lower level of the Ap horizon, generally, had low porosity and organic matter, although pore continuity was adequate. Macroporosity was closely related to soil bulk density (r2 = 0.81). Penetrometer resistance down the soil profile indicated a relatively shallow rooting depth potential of 25–40 cm. A 3-yr direct drillin...

Monitoring Moisture Migration in the Vadose Zone with Resistivity

Abstract: Studies of moisture migration in the vadose zone were conducted at four field sites using a reverse Schlumberger resistivity array. Gravimetric moisture measurements on soil samples taken at each of the field sites were made to a maximum depth of 1.83 meters, and these results were correlated with resistivity values. Tensiometer measurements were also taken at two of the four sites. The soils at the sites have combined clay and silt content (less than .0625 mm grain size) which varies from 13% to 84%. Three of the sites are underlain by sand or gravel in the unsaturated zone. The fourth site has fractured but relatively impermeable shales and siltstones under the surface soils. Depths to water table varied from 1.58 m to 13.7 m. Resistivity and gravimetric moisture measurements were carried out prior to the addition of water to the surface, and following the application of water, either by watering of the site (at two sites) or during and after rainfall events (at two sites). Monitoring was carried out for periods of one to three weeks. Results indicate that (1) moisture is retained for long periods of time in clay/silt-rich soils; (2) moisture migration is slow below a moist soil zone and is not readily detected by surface resistivity measurements; and (3) near-surface moisture changes can be defined by surface resistivity.

Spatial distribution of denitrifying activity in a stream draining an agricultural catchment

Abstract: SUMMARY 1. A field study was made of the spatial distribution of denitrification activity in the sediment of the River Dorn, Oxfordshire, England. 2. An assay of denitrifying enzyme activity was used to examine the distribution of denitrification with depth in cores of sediment representative of the types found in the stream. The maximum activity recorded in a predominantly silt sediment core was 5 times greater than that recorded in a sandy gravel core. In both fine sand and silt cores, peaks in denitrifier enzyme activity were shown to correspond to the limit of the nitrate diffusion front. At this depth the redox potential dropped rapidly from + 300 mV to 0 or less. Denitrifying enzyme activity in the stream water was negligible. 3.In situdenitrification activity (I DA) measurements were carried out in an 800 m reach of the Dorn using the acetylene inhibition technique on small sediment cores. Concurrent measurements were also made of stream depth and velocity, nitrate concentration in the interstitial water, and the wet bulk density, loss on ignition, mineraliz- able carbon and total nitrogen contents of the sediment. Mineralizable carbon was the variable which showed the best correlation with I DA. Highest IDAs were associated with accumulations of fine-grained sediment at meander bends. Mean IDAs measured under flood conditions were significantly higher (P<0.05) than those measured under baseflow. It was estimated that denitrification reduced the nitrate load in the River Dorn by 15% under summer baseflow conditions

Response of excised gill tissue from the New Zealand scallop Pecten novaezelandiae to suspended silt

Abstract: The role of turbidity in scallop mortality was investigated for two size classes of Pecten novaezelandiae (70 ± 2 mm and 20 ± 1 mm shell height), by using the “crawl” velocity of excised pieces of gill tissue as an indicator of the response to suspended silt. The crawl velocity of 3 × 5 mm pieces of gill tissue was measured in six different silt concentrations (0.025, 0.05, 0.1, 0.2, 0.3. 0.5% silt, by dry weight) and two size classes (< 10 μm and 10–30 um) of silt. Results of the study indicate that juvenile (20 mm shell height) scallops may be less tolerant of suspended silt than larger (70 mm shell height) scallops and that both size classes may be less tolerant of fine (< 10 μm) silt/clay sized particles than coarse (10–30 μm) particles. These conclusions are supported in part by data from the mortality of whole 20 mm scallops exposed to two size classes (< 10 μm and 10–30 μm) and three concentrations (0.05, 0.3, 0.6%, by dry weight) of silt. Increasing concentrations and decreasing particle ...

The old alluvium of Singapore and the extinct drainage system to the South China Sea

Abstract: In both sand quarry faces and boreholes the Old Alluvium of Singapore is a matrix supported pebbly sand with minor beds of better sorted sands and clays. The beds can be grouped into four textural classes (pebbles, coarse sand with fine pebbles, medium to coarse sand, clay, and silt), each with characteristic sedimentary structures. The deposit appears to be the proximal facies of an ancient braided river alluvium of possible Pleistocene age. This widespread but scattered alluvium is found both on land and offshore on the Sunda Shelf in Southeast Asia. It is believed to have been deposited during low sea levels. In Singapore, the mineralogy of the grains forming the Old Alluvium suggests a mixed provenance of granitic and low-grade metamorphic origin. The volume and freshness of the deposited material indicate an environment of considerable relief, seasonal rainfall, and extremely active erosional processes, conditions considerably different from the current ones. The Old Alluvium was probably deposited by seasonal rivers which experienced periodic large floods. Such regional conditions could have prevailed over much of Southeast Asia at the time of deposition of the Old Alluvium.

Chemical weathering of fine debris from a series of Holocene moraines: Storbreen, Jotunheimen, Southern Norway

Abstract: Chemical weathering trends were detected within the silt/clay fractions of material taken from the 1750, 1852, and 1928 moraines, of Storbreen, Jotunheimen Mountains, southern Norway. In addition t...

Virgin Hardwood Forest Soils of the Southern Appalachian Mountains: II. Weathering, Mineralogy, and Chemical Properties1

Abstract: Eight undisturbed soils derived from feldspathic meta-sediments in the Joyce Kilmer Memorial Forest were studied. Gibbsite was common throughout all soils, and increased in abundance in the clay fraction with depth. The clay content in most soils decreased with depth, with intergrade 2:1 type minerals occurring as the dominant phyllosilicates in surface horizons. Kaolinite was low in all soils, but was more abundant in south-facing soils. Gibbsite abundance was not aspect dependent. The silt fractions contained appreciable quantities of weathered 2:1 type minerals similar in type to the clays. The coarse sand fractions were almost entirely quartz, while the fine sands contained quartz, weathered 2:1 minerals, and feldspars. The CEC of the mineral surfaces in these soils was extremely low. Soil CEC was almost entirely derived from organic matter, and was typically 200 cm precipitation) temperate environment and resemble some tropical soils. Erosion could severely damage the natural productivity of these soils. The addition of oxidic mineralogy classes to certain Inceptisol families is recommended. Additional Index Words: Hapludults, Dystrochrepts, Haplumbrepts, meta-sediments, colluvium, residuum. Daniels, W.L., L.W. Zelazny, and CJ. Everett. 1987. Virgin hardwood forest soils of the southern Appalachian Mountains: II. Weathering, mineralogy, and chemical properties. Soil Sci. Soc. Am. J. 51:730-738. O UNDERSTANDING OF SQIL CHEMISTRY and its relationship to mineralogy and soil weathering processes over time has resulted primarily from the study of agricultural soil systems. Even the majority of eastern forest soils studied in recent years supported agricultural production at one time or another. Many steeply sloping Appalachian forest soils were never cultivated, but most have been logged one or more times, frequently followed by fires and severe erosion (Clarkson, 1964). The Joyce Kilmer Memorial Forest in western North Carolina, examined in this study, provides an excellent opportunity to study virgin hardwood forest soils under their original vegetative cover. Two other studies conducted in this region have reported stronger weathering on south-facing slopes than north-facing slopes. Franzmeier et al. (1969) studied the effects of topographic position on steeply sloping soils in the Cumberland Plateau of eastern Kentucky and Tennessee and found argillic horizons on south-facing slopes and weakly weathered cambic horizons on opposing northerly slopes. Soils on northerly slopes had 2°C lower mean annual soil temperature, were considerably wetter year round, and therefore contained more organic matter. Soils in lower 1 Contribution from Dep. of Agronomy, Virginia Polytechnic Inst. and State Univ., Blacksburg, VA 24061. Received 21 Oct. 1985. 2 Instructor and Professor of Agronomy, Virginia Polytechnic Inst. and State Univ., Blacksburg, VA 24061, and Research Soil Scientist, Westvaco Corp., Box 1950, Summerville, SC 29484, respectively. slope positions were higher in base saturation, presumably due to downslope water seepage. Losche et al. (1970) studied the effects of aspect on soils weathered from granitic biotite gneiss in southwestern North Carolina, not far from our research area, and found that kaolinite and hydroxy-interlayered 2:1-type minerals were dominant in soils on north-facing slopes while gibbsite was dominant on the warmer southfacing slopes. The soils were almost devoid of exchangeable Ca and Mg below their A horizons due to the extreme leaching environment. They also reported that the finer soil fractions were more highly weathered than the coarser fractions, and that the soils in lower slope positions were more highly weathered. Intergrade vermiculite-chlorite is commonly reported (Weed and Nelson, 1962; Carlisle and Zelazny, 1974; Karathanasis et al., 1983.) as being the dominant stable surface soil 2:1-type mineral resulting from muscovite weathering in humid regions, while biotite commonly weathers to a kaolinite pseudomorph of the original mica crystal (Harris et al., 1985). A regularly interstratified biotite/vermiculte weathering intermediary (hydrobiotite) is reviewed in detail by Sawnhey (1977), and a similar product results from partial muscovite weathering (Norrish, 1973). Reports of regularly interstratified minerals in southeastern soils are rare, presumably due to the highly weathered stage of the majority of soils studied. Deep feldspar weathering in rock saprolites with recrystallization as kaolinite (O'Brien and Buol, 1984) and at the soil/ saprolite boundary with recrystalization as gibbsite and halloysite (Calvert et al., 1980) have been reported in North Carolina Piedmont soils over granitic gneiss. In the latter study gibbsite was not present within the solum and was considered to be reprecipitated at'depth and then resilicated to halloysite. The overall goal of this study was to determine parent material/soil weathering sequences and to relate the chemical and mineralogical properties of undisturbed soils within the Joyce Kilmer Memorial Forest to variations in aspect, parent materials, and vegetation. While the data and relationships reported here are certainly specific to the region and parent materials studied, we hope that they will serve as benchmarks for other studies. METHODS AND MATERIALS Geology and Site Description Eight soils were sampled from intensive study plots within the Joyce Kilmer Memorial Forest in Graham County, North Carolina. Considerable detail on the watershed, landforms, study site locations, and overall soil distributions is given by Daniels et al. (1987). The classification of each soil is given in Table 1. The underlying bedrock within the watershed consists primarily of massive metasandstones, metasiltstones, graywackes, and phyllites of the Great Smoky Group (King et al., 1968). Schists are occasionally encountered. The geology of the watershed has not been mapped in detail. Regional

Effects of different dispersion and fine fraction determination methods on the results of routine particle size analysis

Abstract: Particle-size analysis (PSA) methods to be used in routine situations need to be rapid, require no prior information about the sample and give precise results over a wide range of soil textures. Effects of three physical dispersion and two fine-fraction determination methods on PSA results were investigated over a wide range of soil textures to find the most appropriate technique for routine PSA. Interactions between physical dispersion and fine-fraction determination methods were also investigated. The reciprocating shaker produced significantly lower silt and fine sand and higher coarse sand (and clay, although not significant) values than either drink mixer or end-over-end shaker dispersions. This result was interpreted as the reciprocating shaker giving the most effective dispersion, with aggregated clay being dispersed to primary particles while coarse sand was not fragmented to fine sand or silt. The end-over-end shaker did not reliably disperse a heavy clay soil, and so cannot be recommended for routine use where similar soils may be encountered. When considered over all soils and dispersion methods, hydrometer clay and clay + silt values were higher than pipette values. These results were due to the effective depth of the hydrometer being greater, on average, than the depth of the pipette. However, there were interactions between dispersion and fine-fraction determination methods for the clay and clay +silt classes. Hydrometer values were greater than pipette values with drink mixer and end-over-end shaker dispersion, but were similar with reciprocating shaker dispersion. For the clay fraction, inferior dispersion given by the drink mixer and end-over-end shaker resulted in a significant mass of particles between the sampling depths of the pipette and hydrometer, causing the higher hydrometer values. For the clay +silt fraction, both drink mixer and end-over-end dispersion methods fragmented sands to a size which was recorded by the hydrometer but not the pipette. These interactions highlighted the requirement for effective dispersion where clay and silt are determined by the hydrometer, and may explain some of the conflicting observations of the precision of the hydrometer compared with the pipette. Reciprocating shaker physical dispersion combined with the hydrometer fine-fraction determination method was found the most appropriate PSA method combination for use in a routine situation.

The Interpretation and Environmental Significance of a Buried Middle Pleistocene Soil Near Ipswich Airport, Suffolk, England

Abstract: A buried Middle Pleistocene soil at Ipswich Airport, Suffolk, England, was studied by using macromorphological, textural, mineralogical, chemical and micromorphological techniques. This soil, developed on a low-level terrace surface in the Kesgrave Sands and Gravels, and buried beneath solifluction deposits and the Barham Sands and Gravels, is a composite of the Valley Farm and Barham Soils which have been recognized over wide areas of East Anglia. Clay illuviation, gleying, rubification (haematite formation) and periglacial disruption were the major pedogenic processes active during its formation; mineral weathering and temperate pedoturbation appear to have played only a minor role. After deposition of the Kesgrave Sands and Gravels and establishment of a stable land surface, clay was translocated from the eluvial horizons into the lower illuvial horizons. Initially, this process consisted solely of fine clay but as the environment deteriorated, coarser and more poorly sorted clay was translocated. Biotic, shrink—swell or frost turbation processes led to localized disruption of some limpid (fine) clay coatings before, or simultaneous with, commencement of this phase of coarser clay illuviation. However, most fragmentation of coatings occurred later when the environment had deteriorated to one characterized by seasonally frozen ground. At this stage, silt grains were translocated and small-scale contraction cracks or microscale cryogenic features (silty clay cappings and duplex textural lamellae features) formed. Further deterioration of climate led to formation of large-scale contraction cracks and soil (or incipient ice) wedges, truncation of the soil and deposition of two solifluction deposits. The older sediment contains components of the eroded eluvial horizons, whereas the other solifluction deposit and the overlying (glacifluvial) Barham Sands and Gravels contain minerals derived from the Anglian ice sheet. The soil at Ipswich Airport is developed in the Waldringfield Member of the Kesgrave Formation, which is assumed to be of Beestonian age. As the overlying sediments were apparently deposited during the Anglian Stage, it appears that the soil probably formed during the Cromerian and early parts of the Anglian. Such a chronology would not be in dispute with the proposed environmental reconstruction derived largely from pedological evidence, which suggests a simple environmental deterioration from a temperate optimum to that of periglacial conditions. However, much depends on the significance of the first disruption phase. If the fragmentation of limpid clay coatings represents a sharp climatic oscillation, the environmental reconstruction and stratigraphic implications of this soil may be more complex.

Oxygen Isotopes and the Origin of Quartz: PERSPECTIVE

Abstract: Published data indicate that in igneous rocks the 18O of quartz averages about +9; in metamorphic rocks, +13 to +14; in sandstones, +11; in shales, +19; quartz overgrowths in sandstones, +20; and cherts, +28. Either a very large proportion of the silt and clay-size quartz in shales (about 90% of the total quartz in shales) is secondary, or the isotopic data are incomplete. It is concluded that the latter interpret tion is correct because published isotopic analyses of metamorphic rocks consider only the coarser-grained rocks. Phyllites and slates, whose quartz is nearly all of silt and clay size and has more positive 18O values than quartz from schists and gneisses, probably supplies the bulk of quartz to mudrocks.