Annual regime of bedforms, roughness and flow resistance, Lillooet River, British Columbia, BC

A field study to examine the statistical character of dune morphology and the correlation among dune morphology, discharge, and flow resistance was conducted in a meandering reach of Lillooet River, near Pemberton, British Columbia, Canada. The field season spanned the 1995/1996 hydrologic year with sample day discharge events ranging between 33 and 425 m³ s⁻¹. Surveys of the bed morphology along the thalweg in two dune fields (A and B) were completed using an echo sounder with chart recorder that enabled the measurement of more than 4000 dunes. Dune height ranged between 0.08 and 0.96 m, length between 2.01 and 20.99 m, and steepness between 0.02 and 0.10. Histograms of each dune shape (height, length, steepness) sample most often displayed positive skewness and non-Gaussian distributions (Gamma, Beta, and Weibull); median sample height and length histograms displayed positive skewness; and steepness was nearly Gaussian. Histograms of all dimensionless dunes (i.e., measurement divided by average measurement of sample) were Gaussian and slightly leptokurtic. Neither the height nor length of dunes measured in this investigation were successfully predicted by the empirical models of Allen [Allen, J.R.L., 1984. Developments in Sedimentology. Sedimentary Structures: Their Character and Physical Basis, 2nd edn. Elsevier, New York, vol. 30 (A and B), 1256 pp.], Fredsøe [J. Hydraul. Div., Am. Soc. Civ. Eng. 108(HY8) (1982) 932.], or Yalin [J. Hydraul. Div., Am. Soc. Civ. Eng. 90(HY5) (1964) 105.]. Least-squares regression models for dune–height relations produced here are similar to models published by other field researchers; regression models for dune length only conform to those developed elsewhere if the discharge of the study rivers was similar. The energy gradient in dune field A varied within a smaller range than in field B, enabling dune size to become more fully equilibrated with respect to flow environment. Although the average Froude numbers were much less than critical, dunes appeared to wash out towards a plane bed as discharge increased due to a change from a bedload to suspended-load dominated sediment-transport regime. Flow resistance increased most rapidly during changes in base flow and at the beginning and end of the seasonal flood; resistance tended to be smaller in field A than B, reflecting local differences in energy gradient. Flow resistance increased until a dune steepness of 0.070 was attained and then decreased. The steepness value was considered to be coincident with kolk generation [Dyer, K.R., 1986. Coastal and Estuarine Sediment Dynamics. Wiley, NY, 342 pp.], suggesting that macroturbulent flow structures play an important role in defining the roughness of dunes on a channel bed.     

The decline and recovery of a persistent Texas brown tide algal bloom in the Laguna Madre (Texas, USA)

The Laguna Madre has experienced a persistent bloom ofAureoumbra lagunensis for over eight years. The persistence of this bloom may be due in part to the often hypersaline conditions in Laguna Madre (40–60 psu) that favor the growth ofA. lagunensis. Above-normal rainfall in the fall of 1997 reduced the salinities in Baffin Bay from >40 to<20 psu.A. lagunensis cell densities dropped from>10⁶ cells ml⁻¹ in July 1997 to c. 200 cells ml⁻¹ in January 1998. During this time of low brown tide density, phytoplankton biomass generally remained high and the Laguna Madre experienced successive blooms of diatoms (Rhizosolenia spp.) and cyanobacteria. Hypersaline conditions returned in 1998 and brown tide densities increased to>0.5 × 10⁶ cells ml⁻¹ by summer. The extraordinary persistence of the brown tide and the unusual sequence of intense blooms may be related in part to the reduction of zooplankton populations. Microzooplankton populations declined following the above-normal rain in the fall of 1997; populations did not recover until fall 1998. Copepod populations also declined sharply and remained low in Laguna Madre, but recovered by summer 1998 in Baffin Bay. Dilution experiments indicated that microzooplankton grazing and phytoplankton growth were usually balanced when measured during our cruises. The rapid recovery of theA. lagunensis bloom suggests that this alga may be a more resilient component of the Laguna Madre flora than previously suspected.     

Critical factors for the formation of a nickel–copper deposit in an evolved magma system: lessons from a comparison of the Pants Lake and Voisey's Bay sulfide occurrences in Labrador, Canada

The Voisey's Bay nickel–copper deposit and the Pants Lake sulfide occurrences are the most important mineral systems discovered to date within the Nain Plutonic Suite in northern Labrador. There are many intriguing similarities at both locations. Both are hosted by relatively small troctolite/gabbro bodies that intrude the sulfide-bearing paragneiss of the Churchill Province, and these intrusions contain inclusions of the paragneiss. Similar chemical reactions of the gneissic inclusions with the host magmas are observed at both locations. The reactions resulted in the addition of SiO₂, K₂O, Na₂O and sulfur to the magmas, and are responsible for sulfide-saturation and resultant segregation of immiscible sulfide liquids from the magmas. The initial sulfide liquids in both cases were relatively poor in metals, containing <2.5 wt% Ni and 2 wt% Cu. The sulfides at Pants Lake remained poor in metals because of a lack of subsequent interaction with new, chalcophile-undepleted magma. At Voisey's Bay, the initial sulfides segregated in a dynamic conduit, and were subsequently upgraded in metals to ∼6 wt% Ni and 3 wt% Cu by a new surge of undepleted magma using the same conduit. These sulfides were then concentrated to form large sulfide bodies in the wider parts of the conduit and its entry to an upper chamber in response to a sudden change of liquid velocity in these environments. This study confirms three of the most important factors for the formation of magmatic sulfide deposits in an evolved magma system: (1) contamination of magma with sulfide-bearing country rock to achieve sulfide saturation; (2) a dynamic magmatic system such as a magma conduit to transport large volume of sulfide liquid and to concentrate them in limited localities, and (3) upgrading of metals in the sulfide by new, chalcophile-undepleted magma. Received: 20 February 2000 / Accepted: 14 September 2000     

Uplift, exhumation and precipitation: tectonic and climatic control of Late Cenozoic landscape evolution in the northern Sierras Pampeanas, Argentina

Deciphering the evolution of mountain belts requires information on the temporal history of both topographic growth and erosion. The exhumation rate of a mountain range undergoing shortening is related to the erodability of the uplifting range as well as the efficiency of erosion, which partly depends on the available precipitation. Young, rapidly deposited sediments have low thermal conductivity and are readily eroded, in contrast to underlying resistant basement rocks that have a higher thermal conductivity. Apatite fission‐track thermochronology can quantify cooling; thermal models constrain the relationship between this cooling and exhumation. By utilizing geological relations for a datum, we can examine the evolution of rock uplift, surface uplift and exhumation. In the northern Sierras Pampeanas of Argentina, a young sedimentary basin that overlay resistant crystalline basement prior to rapid exhumation provides an ideal setting to examine the effect of contrasting thermal and erosional regimes. There, tectonically active reverse‐fault‐bounded blocks partly preserve a basement peneplain at elevations in excess of 4500 m. Prior to exhumation, the two study areas were covered by 1000 and 1600 m of recently deposited sediments; this sequence begins with shallow marine deposits immediately overlying the regional erosion surface. Apatite fission‐track data were obtained from vertical transects in the Calchaquíes and Aconquija ranges. At Cumbres Calchaquíes, erosion leading to the development of the peneplain commenced in the Cretaceous, probably as a result of rift‐shoulder uplift. In contrast, Sierra Aconquija cooled rapidly between 5.5 and 4.5 Myr. At the onset of this rapid exhumation, the sediment was quickly removed, causing fast cooling, but relatively slow rates of surface uplift. Syntectonic conglomerates were produced when faulting exposed resistant bedrock; this change in rock erodability led to enhanced surface uplift rates, but decreased exhumation rates. The creation of an orographic barrier after the range had attained sufficient elevation further decreased exhumation rates and increased surface uplift rates. Differences in the magnitude of exhumation at the two transects are related to both differences in the thickness of the sedimentary basin prior to exhumation and differences in the effective precipitation due to an orographic barrier in the foreland and hence differences in the magnitude of headward erosion.     

Deep-water sediment wave fields, bottom current sand channels and gravity flow channel-lobe systems: Gulf of Cadiz, NE Atlantic

Abstract A study of the seafloor of the Gulf of Cadiz west of the Strait of Gibraltar, using an integrated geophysical and sedimentological data set, gives new insights into sediment deposition from downslope thermohaline bottom currents. In this area, the Mediterranean Outflow (MO) begins to mix with North Atlantic waters and separates into alongslope geostrophic and downslope ageostrophic components. Changes in bedform morphology across the study area indicate a decrease in the peak velocity of the MO from >1 m s^?1 to <0·5 m s^?1. The associated sediment waves form a continuum from sand waves to muddy sand waves to mud waves. A series of downslope‐oriented channels, formed by the MO, are found where the MO starts to descend the continental slope at a water depth of ≈700 m. These channels are up to 40 km long, have gradients of <0·5°, a fairly constant width of ≈2 km and a depth of ≈75 m. Sand waves move down the channels that have mud wave‐covered levees similar to those seen in turbidite channel–levee systems, although the channel size and levee thickness do not decrease downslope as in typical turbidite channel systems. The channels terminate abruptly where the MO lifts off the seafloor. Gravity flow channels with lobes on the basin floor exist downslope from several of the bottom current channels. Each gravity flow system has a narrow, slightly sinuous channel, up to 20 m deep, feeding a depositional lobe up to 7 km long. Cores from the lobes recovered up to 8·5 m of massive, well‐sorted, fine sand, with occasional mud clasts. This work provides an insight into the complex facies patterns associated with strong bottom currents and highlights key differences between bottom current and gravity flow channel–levee systems. The distribution of sand within these systems is of particular interest, with applications in understanding the architecture of hydrocarbon reservoirs formed in continental slope settings.     

Experimental determination of oxygen isotope fractionations between CO2 vapor and soda-melilite melt

We report results of experiments constraining oxygen isotope fractionations between CO₂ vapor and Na-rich melilitic melt at 1 bar and 1250 and 1400°C. The fractionation factor constrained by bracketed experiments, 1000^.lnα_{CO2-Na melilitic melt}, is 2.65±0.25 ‰ (±2σ; n=92) at 1250°C and 2.16±0.16 ‰ (2σ; n=16) at 1400°C. These values are independent of Na content over the range investigated (7.5 to 13.0 wt. % Na₂O). We combine these data with the known reduced partition function ratio of CO₂ to obtain an equation describing the reduced partition function ratio of Na-rich melilite melt as a function of temperature. We also fit previously measured CO₂-melt or -glass fractionations to obtain temperature-dependent reduced partition function ratios for all experimentally studied melts and glasses (including silica, rhyolite, albite, anorthite, Na-rich melilite, and basalt). The systematics of these data suggest that reduced partition function ratios of silicate melts can be approximated either by using the Garlick index (a measure of the polymerization of the melt) or by describing melts as mixtures of normative minerals or equivalent melt compositions. These systematics suggest oxygen isotope fractionation between basalt and olivine at 1300°C of approximately 0.4 to 0.5‰, consistent with most (but not all) basalt glass-olivine fractionations measured in terrestrial and lunar basalts.     

The use of Roentgen stereophotogrammetry to study micromotion of orthopaedic implants

Roentgen stereophotogrammetry is the most accurate Roentgen technique for three-dimensional assessment of micromotion of orthopaedic implants. The reported accuracy of Roentgen Stereophotogrammetric Analysis (RSA) ranges between 0.05 and 0.5 mm for translations and between 0.15° and 1.15° for rotations. Because of the high accuracy of RSA, small patient groups are in general sufficient to study the effect on prosthetic fixation due to changes in implant design, addition of coatings, or new bone cements. By assessing micromotion of a prosthesis in a short-term (i.e. 2 years) clinical RSA study, a prediction can be made on the chance of long-term (i.e. 10 years) loosening of the prosthesis. Therefore, RSA is an important measurement tool to screen new developments in prosthetic design, and to prevent large groups of patients from being exposed to potentially inferior designs.In this article, the basics of the RSA technique are explained, and the importance of clinical RSA studies is illustrated with two examples of clinical RSA studies which RSA delivered very valuable information. Thereafter, two recent developments in RSA that have been implemented at Leiden University Medical Center are presented: digital automated measurements in RSA radiographs and model-based RSA.     

Chemical contamination, toxicity, and benthic community indices in sediments of the lower Miami River and adjoining portions of Biscayne Bay, Florida

A large-scale survey of sediment quality in Biscayne Bay, Florida, was conducted in 1995–1996 to characterize the relative degree, geographic patterns, and spatial extent of degraded sediment quality. Chemical analyses and multiple toxicity tests were performed on 226 surficial sediment samples collected over an area of 484 km² in greater Biscayne Bay, including saltwater reaches of several tributaries. Benthic samples were collected and analyzed at one-third of the locations. One or more chemical concentrations exceeded effects range median (ERM) values in 35 samples, representing an area of 5.4 km² (1.1% of the survey area). Highly toxic conditions in amphipod survival tests occurred in 24 of the samples, representing 62 km² (13% of the area). Highly significant results were more frequently observed in three sub-lethal tests: sea urchin fertilization (affecting 47% of the area), sea urchin embryological development (84% of the area), and microbial bioluminescence (51% of the area). The highest levels of chemical contamination (range in mean ERM quotients of 0.2 to 2.0, average 0.76) were observed in samples from the lower Miami River. The high degree of contamination in the river contrasted sharply with conditions in the bay, where chemical concentrations generally were much lower (range in mean ERM quotients of 0.005 to 0.21, average 0.04). Amphipod survival tests showed a very high degree of correspondence with a gradient in chemical contamination in the river and adjoining reaches of the bay. Correlation analyses, scatter plots, and principal component analyses indicated that both amphipod survival in the laboratory tests and the abundance and diversity of the benthos decreased sharply with increasing concentrations of mixtures of organic compounds and trace metals in the sediments. The triad of analyses provided a strong weight of evidence of pollution-induced degradation of sediment quality in the riverine locations.     

Three-dimensional seismic modelling of crustal structure in the TESZ region based on POLONAISE''97 data

This paper reports the results of 3-D tomographic modelling of crustal structure in the Trans European Suture Zone region (TESZ) of Poland, eastern Germany and Lithuania. The data are the product of a large-scale seismic experiment POLONAISE'97, which was carried out in 1997. This experiment was designed to provide some 3-D coverage. The TESZ forms the boundary between the Precambrian crustal terranes of the East European Craton (EEC) and the younger Phanerozoic terranes to the southwest. The 3-D results generally confirm the earth models derived by earlier 2-D analyses, but also add some important details as well as a 3-D perspective on the structure. The velocity model obtained shows substantial horizontal variations of crustal structure across the study area. Seismic modelling shows low (<6.1 km/s) velocities suggesting the presence of sedimentary rocks down to a depth of about 20 km in the Polish basin. The shape of the basin in the vicinity of the profile P4 shows significant asymmetry. Three-dimensional modelling also allowed tracing of horizontal irregularities of the basin shape as well as variations of the Moho depth not only along profiles, but also between them. The slice between P2 and P4 profiles shows about 10-km variations of the Moho over a 100-km interval. The crustal thickness varies from about 30 km in SW, beneath the Palaeozoic platform, to about 42 km beneath East European Craton in NE. High seismic velocities of about 6.6 km/s were found in the depth range 2–10 km, which coincides with K trzyn anorthosite massif. The results of this 3-D seismic modelling of the POLONAISE'97 data will ultimately be supplemented by inversion of seismic data from previous experiments.