Areas Of Bedrock Research Articles

A new aquatic beetle family, Meruidae, from Venezuela (Coleoptera: Adephaga)

Abstract. An unknown beetle collected in the Territorio Federal Amazonas, Venezuela, and designated as belonging to a new family, is described (as Meru phyllisaegen.n., sp.n.) and illustrated by line drawings and scanning electron micrographs. The new family, Meruidae, is diagnosed and assigned to the suborder Adephaga with a discussion of some characters and likely phylogenetic affinities. With a body length of 0.85–0.9 mm, M. phyllisae is the smallest known member of aquatic Adephaga. Based on the observed skeletal and behavioural characters to date, the taxon appears to represent the sister clade to the remaining families of Dytiscoidea, but also has features suggesting a close relationship among Noteridae and/or Haliplidae. A description and photographs of the habitat are included, with a list of other associated water beetle taxa. Some observations on the behaviour of captive beetles are given. The minute ‘comb‐clawed cascade beetles’ are known only from the type locality, El Tobogán, where streams flow among exposed areas of bedrock in the geologically ancient Guyana Shield region.

Effects of ephemeropterans and shrimps on periphyton and sediments in a coastal stream (Atlantic forest, Rio de Janeiro, Brazil)

AbstractWe investigated the importance of shrimps (Atyidae and Palaemonidae) and ephemeropterans (Baetidae) in the removal of periphyton and sediments in a Neotropical stream. The experimental site was open with homogeneous bedrock, shallow depth (8–15 cm), and intermediate water current velocity (0.2–0.3 m/s). We used 2 intensities of electrical current to exclude both shrimps and ephemeropterans (high-intensity treatment) or only shrimps (low-intensity treatment) from fixed areas (180 × 30 cm) of bedrock. When both ephemeropterans and shrimps were excluded in 2 experiments, matter accumulated on the bedrock to 5 and 20× the level in controls; when only shrimps were excluded, no accumulation was observed. Chlorophyll a increased significantly in the high-intensity exclusion, but most of the accumulation was fine organic and inorganic matter. In experiment 1, benthic matter and chlorophyll a decreased by an order of magnitude in areas in which ephemeropterans increased ∼40×. The increase in ephemeropteran...

The frictional resistance characteristics of a bedrock‐influenced river channel

AbstractMuch of the published literature relating to reach‐scale total flow resistance concentrates on alluvial rivers and little mention is made of the more extreme resistance found in bedrock‐influenced river systems. This paper presents the results of a detailed investigation of total flow resistance variation for five channel types on the Sabie River, South Africa (cohesive mixed anastomosed, uncohesive mixed anastomosed, mixed pool–rapid, alluvial braided and alluvial single thread) over a range of discharges. Manning's ‘n’ and Darcy‐Weisbach f resistance coefficients are calculated using Barnes' methodology (H. H. Barnes, US Geological Survey Water Supply Paper 1849, 1967). These data are analysed at the scales of channel type and morphologic unit and the resultant resistance values are compared with the literature. Generally the low‐flow friction values estimated for the bedrock‐influenced channel types are higher than any reported. Flow resistance quantification of the alluvial channel type study sites on the Sabie River produced values slightly higher than earlier studies of similar morphologies. In situations where significant areas of irregular bedrock are exposed (cohesive mixed anastomosed and mixed pool–rapid channel types) the high flow resistance coefficients at low discharges probably reflect energy dissipation due to hydraulic jumps and internal distortions; however, these values fall as discharge increases to values comparable to those recorded for step–pool rivers. It is suggested that the slightly elevated resistance coefficients recorded in the alluvial channel types are the result of the vegetation roughness component, operating when the macro‐channel bar features are inundated. Copyright © 2004 John Wiley & Sons, Ltd.

Quantitative bedrock geology of east and Southeast Asia (Brunei, Cambodia, eastern and southeastern China, East Timor, Indonesia, Japan, Laos, Malaysia, Myanmar, North Korea, Papua New Guinea, Philippines, far‐eastern Russia, Singapore, South Korea, Taiwan, Thailand, Vietnam)

We quantitatively analyze the area‐age distribution of sedimentary, igneous and metamorphic bedrock based on data from the most recent digital geologic maps of East and Southeast Asia (Coordinating Committee for Coastal and Offshore Geosciences Programmes in East and Southeast Asia (CCOP) and the Geologic Survey of Japan, 1997; 1:2,000,000), published as Digital Geoscience Map G‐2 by the Geological Survey of Japan. Sedimentary rocks, volcanic rocks, plutonic rocks, ultramafic rocks and metamorphic rocks cover 73.3%, 8.5%, 8.8%, 0.9%, and 8.6% of the surface area, respectively. The average ages of major lithologic units, weighted according to bedrock area, are as follows: sedimentary rocks (average stratigraphic age of 123 Myr/median age of 26 Myr), volcanic rocks (84 Myr/20 Myr), intrusive rocks (278 Myr/195 Myr), ultramafic rocks (unknown) and metamorphic rocks (1465 Myr/1118 Myr). The variability in lithologic composition and age structure of individual countries reflects the complex tectonic makeup of this region that ranges from Precambrian cratons (e.g., northeast China and North Korea) to Mesozoic‐Cenozoic active margins (e.g., Japan, the Philippines, Indonesia and New Guinea). The spatial resolution of the data varies from 44 km2 per polygon (Japan) to 1659 km2 per polygon (Taiwan) and is, on average (490 km2/polygon), similar to our previous analyses of the United States of America and Canada. The temporal and spatial resolution is sufficiently high to perform age‐area analyses of individual river basins larger than ∼10,000 km2 and to quantitatively evaluate the relationship between bedrock geology and river chemistry. As many rivers draining tropical, mountainous islands of East and Southeast Asia have a disproportionate effect on the dissolved and particulate load delivered to the world oceans, bedrock geology in such river drainage basins disproportionately affect ocean chemistry.

Mobility of palladium in the surface environment: data from a regional lake sediment survey in northwestern Ontario

This paper describes part of a project to examine geochemical methods in exploration for Pd deposits in northwestern Ontario. Palladium is one of the platinum group elements (PGE) that is mobile in dilute surface waters as hydroxy complexes, a feature that can facilitate its use in exploration. Earlier studies around the Lac des Iles Pd mine have shown that Pd is widely dispersed in solution even from sulphide-poor mineralization ( Hattori & Cameron 2004 ). The region that includes Lac des Iles has been covered by a lake sediment survey by the Ontario Geological Survey ( Dyer & Russell 2002 ). These lake sediment data provide a further means of better understanding the geochemistry of Pd in the surface environment and its application to exploration for PGE deposits. Factor analysis of the lake sediment data reveals three associations: Factor 1, an association of Cr, Co, Ni, Pt and Pd; Factor 2, an association of Pd, S and As; and Factor 3, an association of Cu, Ag and Au. High scores for Factor 1 extend for 10 km down-ice from the mineralized mafic rocks of Archean age at Lac des Iles and reflect the dispersion of clastic material by ice sheets, supplemented by hydromorphic dispersion. High scores for Factor 2 are much more widespread and are spatially associated with Quaternary glaciofluvial deposits, notably eskers. A plot of Pd alone in lake sediments throughout the area largely reflects this association with eskers. The elemental assemblage of Factor 2 originates from the leaching of fragments of Nipigon diabase (Pd) and Sibley sedimentary rocks (S, As), both of Proterozoic age, contained within the eskers. The high permeability of the eskers permits soluble elements to be leached. All three elements, Pd, S and As, form neutral and anionic complexes and are not adsorbed by negatively-charged iron oxyhydroxide colloids and coatings, which restrict the dispersion of elements that dissolve as cations, such as Cu, Ni and Co. Factor 3, Cu, Ag and Au, occurs in areas of Proterozoic bedrock.

Guidelines to prevention of slope failure related disasters in granitic bedrock areas of Malaysia

A variety of slope failures have occurred in the granitic bedrock areas of Malaysia; the more important of which are slump and debris flows that have sometimes led to considerable economic loss and loss of life. These failures have occurred at cut and fill slopes, as well as at natural ground slopes, having a varied vegetation cover ranging from primary and secondary forest to agricultural crops and grass. The failures have mainly involved weathered materials from morphological Zones I and II of the weathering profiles (or rock mass weathering grades 3 to 6) over granitic bedrock. Several factors are responsible for the failures, though the main cause is saturation and loss of negative pore water pressures within slope materials as the failures have mostly occurred during, or following, short periods «3 he) of intense rainfall (when total rainfall >70 mm), or longer periods (>1 day) of continuous rainfall. In order to prevent slope failure related disasters, it is necessary to evaluate the various factors that give rise to the failures. The regional and local topographic settings of any site or area proposed for development needs to be first evaluated in order to allow recognition of the earth materials present and the earthworks that may be necessary. Evaluation of the local topographic setting is particularly important as the location of buildings and other structures needs to be considered with reference to the surrounding terrain. Surface and subsurface drainage patterns at the proposed site and surrounding area should then be evaluated as they influence variations in moisture contents and pore water pressures within slope materials. Stream channels and valleys also directly control the movement of debris flows in hilly to mountainous terrain. The rainfall at the proposed site and surrounding area also needs to be monitored in order to allow recognition of significant rainfall intensities and/or durations that increase the likelihood of slope failures. The vegetation cover at the proposed site and surrounding area also needs to be monitored as changes in this cover can also give rise to variations in moisture contents and pore water pressures within slope materials. The stability of cut and fill slopes associated with earthworks at the proposed site or area should finally be evaluated. Consideration and evaluation of all these factors will serve as guidelines that can prevent slope failure related disasters in the granitic bedrock areas of Malaysia.

Quantitative bedrock geology of Alaska and Canada

We quantitatively analyze the area‐age distribution of sedimentary, igneous, and metamorphic bedrock on the basis of data from the most recent geologic maps of Alaska [Beikman, 1980] (1:2,500,000) and Canada [Douglas, 1969] (updated; 1:5,000,000), made available in digital form by the U.S. Geologic Survey and the Geologic Survey of Canada (National Resources Canada), respectively. Sediments cover 72.9% and 52.4% of the surface of Alaska and Canada. Volcanic rocks comprise 11.6% and 6% of the surface area in Alaska and Canada, respectively, whereas intrusive rocks cover 6.6% and 24% of the surface, respectively. Ultramafic rocks account for 0.20% and 0.08% of the bedrock area, whereas metamorphic rocks cover 3.4% and 16.1%. The average ages of major lithologic units, weighted according to bedrock area, are significantly younger in Alaska than in Canada: marine sediments (stratigraphic age of 206 Myr in Alaska versus 599 Myr in Canada), continental sediments (stratigraphic age of 81 Myr versus 150 Myr), volcanic rocks (126 Myr versus 1377 Myr), intrusive rocks (114 Myr versus 2109 Myr), ultramafic rocks (261 Myr versus 874 Myr), and metamorphic rocks (474 Myr versus 2545 Myr). This age difference reflects the contrast between the young active margin influence on the geology of Alaska and the old cratonic nature of much of the bedrock exposed in Canada. Glacial erosion, and the resulting exposure of Precambrian bedrock, primarily in Canada, contributes to this age contrast. The average temporal and spatial resolution of the digital data is sufficiently high to perform age‐area analyses on individual river basins larger than ∼20,000 km2 and to evaluate the relationship between bedrock geology and river chemistry.

Environmental correlates of meso-scale plant species richness in the province of Härjedalen, Sweden

We investigated the species richness of vascular plants at the scale of 5 3 5 km in the Swedish province of Harjedalen, and the relationship between richness and environment. Environmental variables included geographical, altitudinal, topographical, bedrock, soil type, and land-cover descriptors. The species richness was subdivided into groups of species with similar life-form, i.e. trees, dwarf shrubs, hydro- and helophytes, vascular cryptogams, forbs, graminoids, and mountain plants. The data were split at random into two equal subsets. Explanatory models were built by multiple linear regression on the first subset, and the models were validated on the second subset. The total species richness of vascular plants could be explained by sandy and clayey soil, the heterogeneity in bedrock types and the area of acid volcanic bedrock. The model could explain about 46% of the variation in species richness. The richness of trees and hydrophytes tended to decrease with altitude, whereas this was not the case for mountain plants. The latter group occurred frequently at low elevation, but then predominantly along streams and rivers. Clayey soils, sandy soils, and basic volcanic bedrock were the variables most frequently included in the regression models.

Soft sediment as a constraint on the spread of the zebra mussel in western Lake Erie: Processes and impacts

The zebra mussel (Dreissena), inadvertently introduced to the Great Lakes in 1986, has since expanded to cover most of the shallow-water, hard substrates in Lakes Erie and Ontario. Colony densities exceed 300,000 per m2 in some bedrock areas of western Lake Erie. The objective of this study was to investigate the spread of zebra mussel onto soft sediment areas of the western basin of Lake Erie and to identify natural controls on the large-scale colonization of such sediments. Combined side scan sonar, underwater video imagery, and direct diver observation showed three modes of viable zebra mussel colonies in soft substrates: 1) attachment to zebra mussel shell debris deposited in linear troughs (stripes); 2) attachment to shells built up over hard substrates intermittently covered by soft sediments (footballs); and 3) as isolated clumps (druses) attached to dropstones, unionid clams, or their shells. Their spatial distribution suggests that zebra mussel expansion onto soft sediments is supplied primarily by nearby hard substrate areas. Zebra mussel populations living on soft sediment have the normal size distribution as those found on hard surfaces, often with a bimodal or trimodal character, representing cohorts of different ages. At the sites studied, there was a large proportion of dead shells, suggesting colonization over an extended period, as well as a relatively high mortality rate due to burial by periodic catastrophic sedimentation after storms. This vulnerability and the need to be near to source areas make it unlikely that the zebra mussel (D. polymorpha) will continue to expand into areas of soft sediment remote from hard substrate areas. The impact of the zebra mussel colonies on important textural properties of the substrate was not dramatic, but median grain diameter was significantly finer, organic carbon significantly higher, and sediment consistency more clayey below zebra mussel mats. Metal content was generally higher in the samples below zebra mussel mats, but the differences were statistically significant only in the case of iron and maganese. However, the sediment concentrations of metals at all sites were much greater than those at the remote Mid-basin site that was barren of zebra mussels, suggesting an overall metal enrichment near zebra mussel colonies.

Primary Surface Rupture Associated with the Mw 7.1 16 October 1999 Hector Mine Earthquake, San Bernardino County, California

The M w 7.1 Hector Mine earthquake occurred within the Mojave Desert portion of the eastern California shear zone and was accompanied by 48 km of dextral surface rupture. Complex northward rupture began on two branches of the Lavic Lake fault in the northern Bullion Mountains and also propagated southward onto the Bullion fault. Lesser amounts of rupture occurred across two right steps to the south. Surface rupture was mapped using postearthquake, 1:10,000-scale aerial photography. Field mapping provided additional detail and more than 400 fault-rupture observations; of these, approximately 300 measurements were used to characterize the slip distribution. En echelon surface rupture predominated in areas of thick alluvium, whereas in the bedrock areas, rupture was more continuous and focused within a narrower zone. Measured dextral offsets were relatively symmetrical about the epicentral region, with a maximum displacement of 5.25 ± 0.85 m.[*][1] Vertical slip was a secondary component and was variable, with minor west-side-down displacements predominating in the Bullion Mountains. Field and aerial photographic evidence indicates that most of the faults that ruptured in 1999 had had prior late-Quaternary displacement, although only limited sections of the rupture show evidence for prior Holocene displacement. [1]: #fn-1

Physical modelling of bedrock brecciation by ice segregation in permafrost

AbstractThe lower half of a large block of moist chalk was maintained at subzero temperatures while the upper half was cyclically frozen and thawed, simulating 19 seasonal temperature cycles in an active layer above permafrost. During the experiment, the rock surface heaved vertically by at least 34.7 mm. Sixty‐one percent of the heave occurred during freezing periods, and is attributed primarily to ice segregation accompanying upward freezing from the permafrost table during the early stages of simulated winters. Thirty‐nine percent of the heave occurred during thawing periods, and is attributed to ice segregation in the frozen rock beneath the thaw front during the mid to late stages of simulated summers. By the end of the experiment, the middle horizon of the chalk, representing the upper part of the simulated permafrost and the basal part of the active layer, had become strongly brecciated and rich in segregated ice. The style of brecciation has similarities with that in perennially‐frozen limestone, sandstone and shale in Svalbard and Canada, and chalk frozen during Quaternary cold stages in France and England. These similarities suggest that ice segregation during perennial and seasonal freezing is an important process of weathering and coarse‐sediment supply in areas of frost‐susceptible bedrock. Copyright © 2001 John Wiley & Sons, Ltd.

The Origin of Clay Minerals in Soils of King George Island, South Shetland Islands, West Antarctica, and Its Implications for the Clay-Mineral Compositions of Marine Sediments

ABSTRACT Clay minerals in the soils of King George Island, South Shetland Islands, West Antarctica, were investigated to evaluate their influence upon the composition of marine sediments derived from them. Paleohydrothermal activity has developed phyllic-argillic alteration zones in restricted areas of the volcanic bedrock. Illite and kaolinite show a highly localized distribution in the soils of the phyllic-argillic zones, whereas chlorite dominates in the soils of unaltered bedrock. The localized distribution indicates that illite and kaolinite are residues from altered bedrock, whereas chlorite originates from unaltered bedrock. Smectite in the soils is rather uniformly distributed with no relationship to the phyllic-argillic zones. It is mostly the alteration product of eolian pumice shards, which are found scattered throughout the soils on a microscopic scale. The eolian provenance of smectite associated with pumice shards is supported by a uniform distribution of smectite, coexistence of fresh and altered pumice shards, and rather short exposure after deglaciation. Smectites likely have been formed in a remote source around volcanic craters and reworked by repeated eruptions. Formation of clay minerals by chemical weathering as suggested in previous studies appears to be insignificant on King George Island, even though the climate is warmer and more humid than in other Antarctic regions. Smectite and kaolinite in seafloor sediments around the South Shetland Islands do not indicate chemical weathering in the source area. Instead, their occurrence suggests that volcanism, igneous intrusion, and related hydrothermal alterations were active in the South Shetland Islands during the Cenozoic.

The influence of contemporary flow regime on the geomorphology of the Sabie River, South Africa

The existence of a channel-forming, ‘dominant’ or ‘bankfull’ discharge has been applied, with some success, to a variety of alluvial river systems in temperate areas. This paper presents the results of an investigation into the relationship between flow magnitude and frequency, and the geomorphological units of the Sabie River in the Mpumalanga Province, South Africa. The river is perennial, however, it exhibits an extreme seasonal flow regime. The river has been subject to incision (100,000–10,000 years ago), which exposed bedrock areas as topographic highs within a wide macro-channel. Zones of deposition have created alluvial sections resulting in a morphologically diverse river system. Stage–discharge relationships have been observed and constructed for 23 monitoring sites on the Sabie River. These have been used in conjunction with a 62-year calibrated simulated daily discharge record, to generate inundation frequencies for the morphological units present at each site. This includes results using annual maximum series data and time spans for activation, using the continuous daily flow data. The results reveal that the overall form of the cross-section is not related to a single channel-forming discharge, instead, a complex relationship exists whereby the section is influenced by the entire flow regime. The results do however suggest a poorly defined division of morphological units within the incised macro-channel, namely, those influenced by perennial, seasonal and infrequent flows. This relationship is best demonstrated by the morphological units associated with the perennial, active channels in alluvial sections, which correlate with the low flow regime (1–2-year return period on the annual maximum series), and larger macro-channel deposits which are related to rarer higher flows. Climatic wet and dry cycles, human influences and the physical effect of bedrock in the macro-channel further complicate the inundation pattern, resulting in disequilibrium conditions between channel form and contemporary flow regime on the Sabie River. Macro-channel sedimentary deposits, in particular, show no consistent relationship with a particular segment of the flow regime, being inundated by flows of between a 10- and 50-year return period.

Sources of Major Chemical Constituents in Surface Water and Groundwater of Southern Sweden

The chemistry of groundwater has been altered to a large degree through the influence of human activities. Addition of acids through precipitation and dry deposition, nitrogen from agriculture and sewage systems, and chloride from deicing are quantitatively the most important sources. In this paper the chemical characteristics of groundwater are discussed for a crystalline bedrock area located above the highest marine shoreline in southern Sweden. The aim of the study is to establish the background levels for the main chemical constituents in these waters. Focusing on sulphate and chloride, contributions from deposition are evaluated and a comparison with surface water chemistry is performed. Approximately 50% of the base cations in the lakes with extremely low alkalinity originate directly from deposition, while the corresponding figure for lakes with higher, though still low alkalinity, is approximately 39%, and for shallow wells the figure is 36%. There is evidence that the lakes receive water from a shallower depth than the shallow wells, but both groups receive the base cations mostly from the exchange store, or from weathering where the alkalinity produced is consumed by acid inputs. Deposition levels of sulphate and chloride are better approximated by lake water than by groundwater.

Soil chronosequences in subalpine superhumid Cropp Basin, western Southern Alps, New Zealand

Soil chronosequences are described from Cropp River, western Southern Alps, New Zealand. In this area of high rainfall and bedrock of quartzofeldspathic schist, soils are developed in alluvium on gentle slopes and colluvium on steeper slopes, and form under indigenous grassland or scrubland, respectively. Soil ages ranging from 70 to c. 10 000 yr were determined from dendrochronology, radiocarbon dating, and stratigraphic relationships. Soil morphological and chemical changes with time reveal a similar soil development sequence on both gentle and steep slopes. Soil development is rapid and progressively forms Fluvial or Orthic Raw Soils (<200 yr), Fluvial or Orthic Recent Soils, Acid Brown Soils and Orthic Podzols (500–1500 yr), and Perch‐gley Podzols (7000–10 000 yr). Strong leaching is indicated by rapid acidification of surface horizons in Recent Soils. Morphological and chemical evidence shows that older soils (>1000 yr) are podzolised and gleyed. Eluvial/illuvial coefficients calculated from total element analyses, using Zr as an internal standard, indicate large relative losses of all elements from A and E horizons, with the greatest losses in the older soils. The formation of low‐permeability silt loam E horizons, as a result of strong weathering of alluvium and colluvium, causes the development of perch‐gley conditions in soils on very steep slopes and in relatively young soils that do not have iron pans. As a result of the very high rainfall, soil development is more rapid than described from anywhere else in Westland.

Distribution of glacial geomorphic features on the Antarctic continental shelf and correlation with substrate: implications for ice behavior

AbstractSurveys were conducted seaward of all the major drainage outlets of the Antarctic ice sheet from the Pennell Coast, north Victoria Land, to Marguerite Bay, Antarctic Peninsula. The results show that the ice sheet extended onto the outer shelf. Glacial troughs occur offshore of all major glacial outlets. Where the substrate is crystalline bedrock, ice flow tended to follow the structural grain of the bedrock, deposited little sediment and eroded the underlying bedrock. Where ice flowed over relatively soft, more easily eroded, sedimentary strata, the direction of ice flow was more directly offshore, and depositional features characterize the sea-floor. In these areas the signature of the grounded ice consists of till deposits and large-scale geomorphic features. Drumlins occur within the region of contact between crystalline and sedimentary substrates. The different geological substrates are interpreted to have exerted a fundamental control on the behavior of past ice sheets. The troughs in the areas of bedrock composed of sedimentary substrate are interpreted to have been occupied by relatively fast-flowing ice, ice streams, and the troughs in the areas of crystalline substrate are interpreted to have been occupied by slower-moving ice. The area between these two zones was characterized by ice acceleration and is marked by drumlins.

Bathymetry and Seafloor Mapping Via One Dimensional Inversion lAnd Conductivity Depth Imaging of AEM

This study examines the application of airborne electromagnetic (AEM) methodologies to bathymetry in shallow seawater and to map seafloor conductivity. Conductivity versus depth sections have been generated from a recent helicopter-borne DIGHEMV survey (operating vertical coaxial and horizontal coplanar transmitter-receiver coil geometries) of lower Port Jackson, Sydney Harbour. The sea depth ranges from about 1 to 30 m. Acoustic bathymetric soundings and marine seismic survey data provide the true seawater layer thickness and estimates of depth to bedrock respectively over most of the EM survey region. This complementary data can be used to evaluate the accuracy of airborne electromagnetic bathymetry. The efficacy of 1D conductivity inversion and rapid conductivity-depth imaging was investigated for shallow seawater overlaying marine sand sediments and sandstone. The inversion constructs layered conductivities which satisfy the AEM data to an accuracy consistent with the observational uncertainties. Inverted frequencies ranged from 328 to 55300 Hz. Resolution of the sea depth gave good agreement with known bathymetry (within about 10% or better) when inversion was unconstrained. Approximate conductivity-depth images obtained using program “EM Flow” gave similar agreement. Both inversion methods clearly identify the location and burial depth of higher resistivity regions associated with shallow marine sandstone bedrock. In addition to measuring water depths to about 30 m, this study has shown that the AEM DIGHEM technique provides a capability for remote sensing of seabed properties and offers the potential to detect areas of shallow bedrock and differentiate between consolidated and unconsolidated sediment in areas of seawater deeper than 25 m.

Characteristics of Intercanopy Bare Patches in Juniperus Woodlands of the Southern Edwards Plateau, Texas

Intercanopy bare patches are small, relatively circular communities with an exposed central bedrock, shallow soil, low plant cover, and dominated by non-vascular plants, annual grass- es, and forbs. Although these patches are relatively common in the Edwards Plateau of central Texas, their characteristics have never been described. Twelve circular, intercanopy bare patches in the Hill Country State Natural Area in Bandera and Medina counties, were examined using the line-point procedure. Mean cover (?SD) of plant, litter, rock, and bare soil was 23 - 3%, 23 ? 5%, 44 ? 9%, and 10 ? 4%, respectively. Approximately half of the total plant relative cover consisted of forbs with 25% grasses and 25% non-vascular plants. Fifty-seven species of flowering plants from 30 families were identified, including 27 annuals and 30 perennials. The most com- mon families were the Poaceae (9 species), Asteraceae (5 species), Euphorbiaceae (5 species) and Labiatae (4 species). Flowering plant species encountered in 10 to 12 of the intercanopy bare patches sampled were Chaetopappa bellidifolia, Evax prolifera, Croton monanthogynus, Spermolepis inter- mis, Centaurium texense, Galium virgatum, Sida abutifolia, and Schizachyrium scoparium. Nostoc commune, a common soil blue-green algae, was found in every intercanopy bare patch. Non-vascular plant cover was highest near the edge of the central bedrock and decreased with distance. Total plant and litter cover increased with distance from the central bedrock. Total annual cover was five times higher than total perennial cover and both increased with distance from the central bedrock. Mean number of species found per intercanopy patch was 31 ? 4 with a mean Simpson's diversity of 9.2 ?- 2.8. Soil depth increased with distance from the central bedrock, whereas rock cover decreased. Total plant, annual and perennial cover, and litter cover were positively related to soil depth; rock cover was negatively related to soil depth. Nostoc commune relative cover decreased with distance, but Chaetopappa bellidifolia relative cover increased with distance from the patch center, and then decreased near the outer edge of the patch. Heliotropium tenellum and Bothrichloa ischaemum relative cover increased with distance from the center of the patch, with no indication of a decrease near the outer edge. Nostoc commune relative cover was inversely related to soil depth. Relative cover of C. bellidifolia, H. tenellum and B. ischaemum was positively related to soil depth. Almost all soil chemical characteristics were the same from near the central bedrock area to the outer edge of the patch. Soils were slightly basic, high in calcium, low in nitrogen and organic matter. Shallow soil depth and intermittent water availability are probably the main factors that determine composition, including low cover and the high proportion of annuals, in these com- munities.

Major element chemistry of sediments from the Darling-Barwon river and its tributaries: implications for sediment and phosphorus sources

In 1991, one of Australia's longest rivers, the Darling–Barwon, experienced one of the world's largest recorded algal blooms. Nearly 1000 km of river were affected. At the time of the bloom phosphorus originating from anthropogenic sources was believed to be the principal cause. In this study we have used major element chemistry to examine the sources of sediment and sediment-associated phosphorus delivered to the Darling–Barwon River. We show that the sediments are derived primarily from the sedimentary and granitic bedrock areas of the catchment, not the intensively farmed basalt areas. We also show that the sediment currently in transport in the Darling–Barwon does not originate from contemporary upland erosion, but is probably derived from lowland areas of the catchment that contain more weathered material. Phosphorus concentrations in the sediments are consistent with those in natural soils of the region, and evidence from two sediment cores indicates that phosphorus concentrations have not changed significantly in the last 200 years. The implications of these findings for algal growth control strategies are discussed. Copyright © 2000 John Wiley & Sons, Ltd.

Mapping hydrothermally altered rocks by analyzing hyperspectral image (AVIRIS) data of forested areas in the Southeastern United States

Analysis of Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) data of the Carolina slate belt, a region in the Southeastern United States, has shown that certain types of hydrothermally altered rocks can be delineated based on the spectral reflectance characteristics of forest vegetation growing on them. These altered rocks are typically highly silicified and commonly associated with gold, silver, and pyrophyllite ores. Field evaluation of the resulting forest classification identified several previously unmapped areas of altered rocks, and no areas that were mapped erroneously. Distinction of the altered rocks from unaltered siliceous volcanic bedrock areas, also silica-rich, is especially significant as chestnut oak forests are characteristic of both. We conclude that the dominant chestnut oak canopy and the lack of other species that are usually present here on the unaltered rocks, result in lower apparent canopy density at the altered sites, which is expressed in the AVIRIS data by lower O–H absorption intensity. These differences in species composition and canopy architecture may be caused by one or several factors, including soil depth and physical properties, direction and steepness of slope, moisture availability, and the amount of nutrients present and their availability to the trees.