Deep Surface Layer Research Articles

Effects of seedbed properties on crop emergence: 2. Effects of aggregate size, sowing depth and initial water content under dry weather conditions

Rapid and uniform crop establishment is a prerequisite for efficient crop production and minimal environmental impact. Extensive experiments were carried out in shallow plastic boxes placed in the field directly on the ground for studies of the effects of seedbed properties on emergence of small-grain cereal crops in a cool, temperate climate. This paper presents studies of the seedbed as a protective layer against evaporation and the requirements for good emergence under dry weather conditions. Without rainfall after sowing, nearly complete emergence of barley (Hordeum vulgare L.) was obtained, when rapidly germinating seed was placed on a moderately compacted basal layer containing at least 5% (w/w) plant-available water and covered by a 4 cm deep surface layer consisting of aggregates < 5 mm. This applied even when initial water content in the surface layer was below the wilting point and potential evaporation was high. If the soil at 4 cm depth is drier or if a sufficiently fine seedbed cannot be obtained, then seedbed and sowing depth should be somewhat greater, and if the soil is wetter, then the depth may be smaller. There were only small differences in emergence between surface layers consisting of aggregates < 2 mm, 2–4 mm or 2–5 mm, or dominated by these aggregate fractions, but when the surface layer consisted of coarser aggregates, emergence was usually much lower. Determinations of water losses from the soil during the experimental period showed that the differences in emergence were caused by differences in efficiency of the seedbed to control evaporation. The results indicate that, in an initially wet soil, it is always possible to find a time and method for seedbed preparation and sowing that lead to good emergence of small-grain cereal crops, even with a lengthy period without rainfall after sowing.

Phytoplankton composition and biomass across the southern Indian Ocean

Phytoplankton composition and biomass was investigated across the southern Indian Ocean. Phytoplankton composition was determined from pigment analysis with subsequent calculations of group contributions to total chlorophyll a (Chl a) using CHEMTAX and, in addition, by examination in the microscope. The different plankton communities detected reflected the different water masses along a transect from Cape Town, South Africa, to Broome, Australia. The first station was influenced by the Agulhas Current with a very deep mixed surface layer. Based on pigment analysis this station was dominated by haptophytes, pelagophytes, cyanobacteria, and prasinophytes. Sub-Antarctic waters of the Southern Ocean were encountered at the next station, where new nutrients were intruded to the surface layer and the total Chl a concentration reached high concentrations of 1.7 μg Chl a L −1 with increased proportions of diatoms and dinoflagellates. The third station was also influenced by Southern Ocean waters, but located in a transition area on the boundary to subtropical water. Prochlorophytes appeared in the samples and Chl a was low, i.e., 0.3 μg L −1 in the surface with prevalence of haptophytes, pelagophytes, and cyanobacteria. The next two stations were located in the subtropical gyre with little mixing and general oligotrophic conditions where prochlorophytes, haptophytes and pelagophytes dominated. The last two stations were located in tropical waters influenced by down-welling of the Leeuwin Current and particularly prochlorophytes dominated at these two stations, but also pelagophytes, haptophytes and cyanobacteria were abundant. Haptophytes Type 6 ( sensu Zapata et al., 2004), most likely Emiliania huxleyi, and pelagophytes were the dominating eucaryotes in the southern Indian Ocean. Prochlorophytes dominated in the subtrophic and oligotrophic eastern Indian Ocean where Chl a was low, i.e., 0.043–0.086 μg total Chl a L −1 in the surface, and up to 0.4 μg Chl a L −1 at deep Chl a maximum. From the pigment analyses it was found that the dinoflagellates of unknown trophy enumerated in the microscope at the oligotrophic stations were possibly heterotrophic or mixotrophic. Presence of zeaxanthin containing heterotrophic bacteria may have increased the abundance of cyanobacteria determined by CHEMTAX.

Numerical modelling of ground motion in the Taipei Basin: basin and source effects

SUMMARY The Taipei basin in northern Taiwan is located in a high seismicity region and was affected by several earthquakes in the past (ML= 7.3 on 1909 April 15; ML= 6.8 on 1986 November 15; the Chi-Chi ML= 7.3 earthquake on 1999 September 21 and ML= 6.8 on 2002 March 31). The main characteristic of the Taipei basin is its complex shape with a deep western and shallow eastern part. The uppermost Sungshan formation with its low shear wave velocities (90–200 m s−1) is also a distinct feature of the basin. Based on the large data base of earthquake records obtained from the Taiwan Strong Motion Instrumentation Program network, many studies on ground motion within the Taipei basin exist. However, the influence of the various subsurface structures on the observed ground motions as well as the variability of ground motion with respect to earthquake location is not fully understood. We apply a 3-D finite-difference method to simulate wave propagation up to 1 Hz for a small earthquake close to the basin in order to resolve these open questions. By varying source and structural parameters, we explore the variability of ground motion. Our study includes a subsurface model that is based on recent studies on the basin structure and on the crustal structure of Taiwan. From our simulations we find a good fit between simulated and observed waveforms and peak ground accelerations for the considered small earthquake near the basin. We also explore the influence of fault plane orientation, hypocentre location, deep basin structure and soft soil surface layers of the Sungshan formation by varying the subsurface structure and earthquake position. Our studies reveal that the basin structure produces an amplification factor of about 4 compared to hard rock conditions. Additionally, the soft soil Sungshan formation produce amplification of a factor of 2. This results in a maximum amplification of the basin structure of about 8, which is in good comparison with amplification values larger than 5 found from the analysis of observed earthquakes. These values clearly exceed the amplification values of about 2–3 obtained when applying standard 1-D site effect analysis. Our simulations for different earthquake positions show that ground motion depends strongly on earthquake location and fault orientation. Therefore, the application of average values of spectral amplification obtained from the analysis of recorded data from distant earthquakes with different azimuths and fault planes may significantly underestimate future ground motions of possible earthquakes on known faults close to the Taipei basin. The simulation of a small earthquake near the Taipei basin presented in this study will help to set up adequate simulation parameters for a possible large earthquake close to the Taipei basin. Such a simulation of a scenario earthquake close to the Taipei basin would allow to significantly improve hazard assessment as no observations of strong earthquakes in the vicinity of the basin exist.

Centennial‐scale evolution of Dansgaard‐Oeschger events in the northeast Atlantic Ocean between 39.5 and 56.5 ka B.P.

There is much uncertainty surrounding the mechanisms that forced the abrupt climate fluctuations found in many palaeoclimate records during Marine Isotope Stage (MIS)‐3. One of the processes thought to be involved in these events is the Atlantic Meridional Overturning Circulation (MOC), which exhibited large changes in its dominant mode throughout the last glacial period. Giant piston core MD95‐2006 from the northeast Atlantic Ocean records a suite of palaeoceanographic proxies related to the activity of both surface and deep water masses through a period of MIS‐3 when abrupt climate fluctuations were extremely pronounced. A two‐stage progression of surface water warming during interstadial warm events is proposed, with initial warming related to the northward advection of a thin warm surface layer within the North Atlantic Current, which only extended into deeper surface layers as the interstadial progressed. Benthic foraminifera isotope data also show millennial‐scale oscillations but of a different structure to the abrupt surface water changes. These changes are argued to partly be related to the influence of low‐salinity deepwater brines. The influence of deepwater brines over the site of MD95‐2006 reached a maximum at times of rapid warming of surface waters. This observation supports the suggestion that brine formation may have helped to destabilize the accumulation of warm, saline surface waters at low latitudes, helping to force the MOC into a warm mode of operation. The contribution of deepwater brines relative to other mechanisms proposed to alter the state of the MOC needs to be examined further in future studies.

The relationship between energy distribution and space distribution of charge traps in linear low density polyethylene

Space charge formation and evolution, charge trap distribution, and depolarization process in linear low density polyethylene (LLDPE) samples with semiconducting electrodes subjected to dc electric stress were systematically studied using the techniques of laser induced pressure pulse and thermally stimulated discharge. Space charge distribution and evolution in the samples showed obviously the characteristics of rapid and symmetrical injection of homo-charges from the two electrodes. The interface between the semiconducting electrode and LLDPE showed approximately ohmic contact properties. The charge trap distribution in LLDPE was characterized by shallow bulk traps and deep surface layer traps. Hot-pressing conditions between the semiconducting electrode and LLDPE or contamination of the surface layer of LLDPE by the semiconducting electrode material has a remarkable effect on energy distribution of the surface layer traps, resulting in depth and density decrease of the deeper traps and density increase of the shallower traps in the surface layer. During the whole short-circuit depolarization process, the centers of positive charges and negative charges in the sample shifted to their nearer electrodes, respectively, whereas behaviors different from the short-circuit depolarization were seen in the later stage of the open-circuit depolarization, that is, the positive charges captured in the deep surface layer traps were detrapped and shifted to the rearer electrode.

Surface nitriding of Ti–6Al–4V alloy with a high power CO 2 laser

Surface nitriding of a Ti–6Al–4V alloy by laser melting in a flow of nitrogen gas has been investigated, with the aim of increasing surface hardness and hence improving related properties such as wear and erosion resistance. The effect of the scanning speed, nitrogen dilution, and nitrogen flow rate on microstructure, microhardness, and cracking of the nitrided layers was studied. Optical, scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction (XRD) were used to reveal the microstructure and to identify the phases formed. It is shown that smooth, deep, and crack-free nitride layers of a surface hardness ranging between 500 and 800 HV can be obtained by controlling the processing parameters. Cracks are present in the sample processed at slow scanning speed and high laser power. Dilution of the nitrogen gas with argon gas leads to a crack-free nitride layer at the expense of a reduction in surface hardness. Slow scanning speeds lead to the formation of a deep and hard surface layer, and increasing the nitrogen flow rate results in a rough surface with a slight increase in hardness.

Grain structure and homogeneity of pulsed laser treated surfaces on Al-aerospace alloys and FSWs

The effect of pulsed excimer laser surface treatment, on typical Al-aerospace alloys and friction stir weld substrates, has been studied. A thin <10 μm deep surface layer can be achieved, where the majority of the re-solidified material does not exhibit phase separation, and has a higher level of homogeneity relative to the substrate material. However, due to the extremely short melt times, the layer is not as homogeneous as has previously been claimed and melted constituent particles leave solute rich trails throughout the layer. The treated layer also contains a series of second phase bands that are the result of a transient region, caused by the growth front from each pulse initially accelerating through speeds below ultimate stability, combined with the effects of composition variations within the liquid from the solute trails. The layer was further found to develop a grain structure that was very sensitive to that of the substrate and the presence of Al 3Zr β′ dispersoids within the treated alloy, which can alter the grain structure from coarse epitaxial, to very fine columnar.

On the validity of thermal wind balance in alongshelf currents off the New Jersey coast

The validity of thermal wind balance (TWB) as an approximation for the observed vertical shear in coastal currents (depth 10–30 m) is analyzed based on the ADCP and CTD transects conducted off the New Jersey coast in early August of 1996. At that time, the study area was forced simultaneously by a brief upwelling-favorable wind event and the arrival of a buoyant plume from the Hudson Estuary. As a result, both downwelling- and upwelling-favorable alongshelf currents resided within the same area, the former inshore and the latter offshore. The very intense pycnocline (buoyancy frequency ∼0.06 s −1) was shaped as a dome and thus the TWB shear of both signs was observed within the same across-shelf transects and under the same wind conditions. In average, the geostrophic (TWB) shear overestimated the observed by a factor of 2. The maximum of the ageostrophic shear (which is the observed minus TWB shear) was within the sloping pycnocline and not near the bottom (where one would expect to find a strong frictional shear). For both upwelling and downwelling-favorable currents, the geostrophic shear had a sharp maximum in the pycnocline (localized within a few meters), while the ageostrophic shear tended to smooth the actual velocity profile, counteracting the TWB shear in the pycnocline and adding to it (changing sign) below. The result was a more linear velocity distribution with depth compared to the TWB prediction. The significant reduction of the TWB shear in the pycnocline described in this study was likely related to the turbulence production by the internal wave breaking. The wind-induced stresses were trapped in the shallow, ∼5 m deep surface layer, and were inhibited below by a strong pycnocline. Nonfrictional mechanisms (inertia or momentum advection) did not contribute significantly to the ageostrophic shear.

Surface segregation in FeSi alloys

The surface segregation of a Fe 94Si 6 and Fe 76Si 24 alloys was studied using high resolution photoemission spectroscopy with synchrotron radiation at 150–166 eV photon beam energy. During Ar + ion sputtering and following heat treatment, a SiO 2 layer and segregation of Si atoms in three clearly resolved phases occurred. This indicates formation of silicides, mainly Fe 3Si and cubic FeSi superstructures. The photoemission measurements were complemented by 57Fe Mössbauer spectroscopy in different modes which gave information about deeper surface layers (approximately 10–30 000 nm) of the samples. The results derived from the Mössbauer spectra support the conclusions concerning phase composition of the surface deduced from photoemission spectroscopy.

Loop Current Ring Shedding: The Formation of Cyclones and the Effect of Topography

Abstract The formation of cyclones in the vicinity of the Loop Current ring during the separation stage is analyzed in the frame of a high-resolution ECMWF daily wind-forced Miami Isopycnic Coordinate Ocean Model (MICOM) simulation. Mesoscale cyclones, observed in sea surface height maps in the vicinity of the Loop Current in a necking-down position, are found to contribute to the separation of the ring from the Loop Current as they grow between the Loop Current and the ring in the MICOM simulation. To understand the origin of the cyclones, the instability of the Loop Current idealized as an isolated vortex is studied. After noticing the cyclonic vorticity belt around the Loop Current, and based on the vertical distribution of potential vorticity in a Loop Current ring in the MICOM simulation, the linear stability of a shielded vortex is studied in the quasigeostrophic formalism. To simulate the effects of the planetary vorticity gradient and topography on the Loop Current, the nonlinear states of the idealized Loop Current are analyzed using the adiabatic MICOM code. Results from the analysis of the MICOM simulation show that cyclones are the products of a vortex rim instability. The modal analysis of the Loop Current instability reveals that mode 4 is the fastest-growing mode and that baroclinic (barotropic) instability is intensified in the deep (surface) layers. These results are confirmed by the analytical study of the idealized Loop Current. The nonlinear state shows that a Loop Current–like vortex is indeed a pentapole on an f plane. On the β plane, the northern cyclone is separated from the anticyclone by the β effect and both drift westward. When the topography of the Gulf of Mexico is taken into account—namely, the Campeche Bank, the southward slope north of the Loop Current, and the Florida shelf east of the Loop Current—several effects are observed: 1) the northern corner of the Campeche Bank erodes the Loop Current ring and its cyclones and interacts with the vortex’s most unstable mode, 2) the northern southward slope scatters the northern cyclone while the anticyclone remains coherent and propagates to the west, and 3) realistic westward propagation speeds are obtained in the presence of the northern Campeche shelf, which acts as a mirror effect on the Loop Current ring, as opposed to the Florida shelf, which tends to block the ring.

Sea-breeze forced diurnal surface currents in the Thermaikos Gulf, North-west Aegean

Energetic rotary diurnal surface currents of up to 40 cm/s are observed in the Thermaikos Gulf, North-west Aegean. They are principally confined above the pycnocline in the upper 3 m of the water column. Beneath this depth they are much weaker, less regular and oscillate in anti-phase with those at the surface. Averaged over 10-days, the current cycles are seen to vary sinusoidally with the northward component leading the eastward by 0.2 of a cycle (0.4 π), corresponding to clockwise rotating current ellipses. The mean amplitude ratio between the northward and eastward component amplitudes ( u/ v) is 0.83. A simple analytical model, representing the dynamical balance of a homogeneous surface layer, predicts diurnal wind forcing, i.e. a sea-breeze, should drive diurnal currents whose amplitude will be particularly energetic close to 30° latitude, where the diurnal and inertial periods are similar. Assuming that a 3 m deep surface layer is subject to linear frictional drag, the model is used to hindcast the phase of a northward diurnal wind required to force the observed current components. A linear friction coefficient (of 0.6 ω) is derived from the phase difference between the eastward and northward current components. Using this friction coefficient the phase of a diurnal northward wind is hindcast from the absolute phase of the current components. This hindcast wind phase of 1.70 rad is consistent with the observed northward wind of 1.66 rad with a phase difference of 0.04 rad (<10 min), which provides confirmatory evidence of the role of the sea-breeze wind stress cycle in forcing these motions. The model is finally used to predict the variation in the amplitude, ellipticity and rotation of these currents with latitude.

An IR-reflectivity and X-ray diffraction study of high energy He-ion implantation-induced damage in 4H–SiC

Due to recent progress in silicon carbide (SiC) technology, understanding the behaviour of defects in SiC is a crucial challenge for the industrial development of SiC-based devices. High energy helium ion implantation at high fluence in SiC creates a few microns deep damaged layers with nanocavities which are expected to play an important role in gettering of unwanted impurities. Post-implantation evolution of the ion implantation-induced damage is then controlled by thermal annealing. Results of an infrared reflectivity (IRR) and X-ray diffraction (XRD) study of ion implantation-induced damage in crystalline (0 0 0 1) n-type 4H–SiC implanted at room temperature with 1.6 MeV He + ions at the fluence of 10 17 cm −2 show the formation of two layers above the unperturbed crystal: a 3.4 μm deep defective strained surface layer on a 0.4 μm thick strongly perturbed interface layer. Thermal annealing at 1500 °C for 30 min under high vacuum was shown to induce structural recovery of the surface layer.

Applications in food quality and environmental contamination

Cadmium is one of the more mobile heavy metals in the soil ‐ plant system. Its accumulation in plants is especially important from the point of view of its entering the nutrient chain. It is equally toxic for plant, animal and human organisms. Cadmium is easily taken up by plants and is among the biologically most accumulative heavy metals. We carried out pot experiments with different types of soils to examine how the different soil properties influenced cadmium up‐take by plants. The Soil Conservation Information and Monitoring System (TIM) is part of the Environment Protection Information System, the starting point of an agricultural environment management program launched in Hungary. The TIM has 1200 measuring points. In accordance with the examination data, we picked some points where the pH value and the Cd content of the soils may justify the testing of Cd uptake by plants. It was near these points, at five test sites that we collected soil samples from a 0–20 cm deep surface layer for our experiments. We chose lettuce (Lactuca sativa L.) as test plant, as in our earlier experiments we had found that it was this plant that best accumulated Cd in its leaves thus we thought it might prove a good marker/indicator on soils with lower Cd contents. We tried to establish correlations between the total and available Cd content of soil, its pH value, clay content, organic matter content, hydrolytic acidity, cation adsorption capacity, and the Cd concentration and total uptake by lettuce. According to our experiment findings, clay, silt and organic matter content of the soils had no effect on Cd uptake. In lettuce leaves the Cd concentration was 1.4–16 times as much as the total Cd content of the soil. There was a negative linear correlation existing between the pH value of the soil and the Cd uptake by lettuce (r =–0.409, p<0.05). We also, found a close correlation between the Cd concentration of the lettuce leaves, its Cd uptake and the hydrolytic acidity of soils. Consequently the Cd uptake increases with increasing hydrolytic acidity.

On the sea surface temperature high in the Lakshadweep Sea before the onset of the southwest monsoon

The north Indian Ocean becomes the warmest area of the world oceans prior to the onset of southwest monsoon in June. During this period a zonal band of high sea surface temperature (SST), the “thermal equator” (TE), moves over this region concurrently with the Intertropical Convergence Zone (ITCZ). Using a weekly SST data set, we show that another SST high develops off southwest India in the Lakshadweep Sea in March, well before the TE moves in to the area, and that it continues to retain its identity until the onset of monsoon. The SST high has its genesis about 6 months earlier in the Bay of Bengal. The collapse of the southwest monsoon in October and the onset of the northeast monsoon trigger down welling coastal Kelvin waves that propagate along the periphery of the Bay of Bengal, forcing an equatorward East India Coastal Current, which brings low‐salinity water from the bay to the southeastern Arabian Sea during the northeast monsoon (November‐January). As the Kelvin waves propagate poleward along the west coast of India after turning around Sri Lanka, they radiate downwelling Rossby waves that produce a “high” in sea level off southwest India. The downwelling and the surface layer of low‐salinity water provide a breeding ground for the formation of a SST high in January. By March, with the increase in solar insolation due to the northward march of the Sun and the deep stable surface layer, the high reaches a mature phase clearly evident in the Lakshadweep Sea. By May, when the thermal equator and ITCZ move over the region, the high can be seen embedded in the TE. We speculate that at this time the high helps in producing conditions that are conducive for genesis of the monsoon onset vortex.

Ecological interaction and sediment transport on an intertidal mudflat I. Evidence for a biologically mediated sediment-water interface

Abstract Using biological, chemical and physical data, evidence of biologically mediated cohesive sediment dynamics is presented for an intertidal mudflat in the Humber Estuary, UK. The data suggest that in excess of 98% of sediment fluxes to the sediment bed on the mudflat are subject to resuspension, forming a bedload that is important in generating the shear stress required to erode the sediment bed. Benthic micro-algae photosynthesize and are grazed by macro-heterotrophs within this 300–3000 μm deep dynamic surface layer. The data suggest that the algal growth within this layer is limited by nitrogen fluxes, resulting in ‘excess’ carbon fixation by photosynthesis. A significant positive relationship is apparent between ‘excess’ carbon fixation rates and the sediment critical erosion stress. This indicates that nitrogen limitation and sediment stability were directly proportional to each other on the mudflat. Grazing of the algae by Macoma balthica (a burrowing bivalve) profoundly affects the carbon and nitrogen cycles in the dynamic surface layer, and hence also ‘excess’ carbon fixation. We propose that a highly interdependent community of benthic algae, bacteria and macro-heterotrophs, acts to regulate sediment dynamics via small-scale nitrogen cycling and the algal exudation of ‘excess’ carbon as sediment-binding polymers.

Frontogenesis, subduction, and cross‐front exchange at upper ocean fronts

The subduction of water at upper ocean fronts driven by internal instabilities is investigated. A simple analytic model which assumes conservation of potential density and potential vorticity is used to characterize the expected structure of subducted parcels as a function of the surface layer depth and the length scale of the parcel. Parcels subducted from a deep surface layer will be characterized by anomalously low potential vorticity and anticyclonic circulation, while shallow surface layers result in subducted parcels with high potential vorticity and cyclonic circulation. A nonlinear isopycnal primitive equation model is used to demonstrate how baroclinic instability and the resulting frontogenesis force the subduction of parcels below and across the front. Frontogenesis generates vertical velocities of 𝒪(30 m d−1) and ageostrophic cross front flows of 3 to 5 cm s−1. The subduction is achieved by the deep ageostrophic flow which carries water from the surface layer below and across the front. The subducted parcels quickly develop a circulation through balanced adjustment, anticyclonic for deep surface layers and cyclonic for shallow surface layers. The characteristics of parcels subducted in winter conditions closely resemble a class of eddies called submesoscale coherent vortices. The horizontal and vertical scales of the vortices are determined by the frontogenetic mechanism and the depth of the surface layer. The total permanent subduction rate is estimated to be 20 m yr−1 for frontal subduction driven by frontogenesis, although local (and temporary) subduction can be much larger. The subducted anticyclonic vortices are often coupled to upper ocean cyclonic vortices, thus forming a baroclinic dipole that propagates at approximately 3.5 cm s−1 and is capable of transporting the anomalous water properties far from the region of formation. Similarities with observations in the North Atlantic Subtropical Convergence Zone and near the Gulf Stream are discussed.

Depth-resolved investigation of the element and compound inventory of aerosol particles from outdoor air

Plasma-based SNMS has been employed to perform a depth-resolved elemental analysis of aerosol particles collected in a suburban area near Karlsruhe. Two classes of particles could be distinguished. Both of them exhibited a significant depth structure. Submicron particles have been found with a 15 nm deep ammonium sulphate surface layer on carbon rich cores. The other class consisted of coarse particles of several μm size which even showed a double layer system enclosing geogenic cores. For depth-scaling a new internal calibration procedure has been tested and found to be suitable for the complex environmental material. Dynamic SIMS measurements have been used to investigate especially the carbon compounds of the particles by evaluation of small cluster ion signal patterns.

Spatially confined nickel disilicide formation at 400 °C on ion implantation preamorphized silicon

The localized formation of continuous silicide layers via solid state reaction of nickel atoms with ion implantation preamorphized silicon is described. Ni films 12 nm thick were evaporated on 65 nm deep amorphized surface layers. The silicidation reaction was induced by two stage thermal annealing at 360 and 400 °C, well below the solid phase epitaxial regrowth temperature for amorphous Si and the normal NiSi2 formation temperature. Rutherford backscattering with channeling, XTEM, and four-point-probe measurements were used to determine the structure, interfacial morphology, composition, and resistivity of the silicide films. After the lower temperature annealing stage a continuous layer of NiSi2 is formed with an underlying residual amorphous region located above the crystalline substrate. During a second annealing the residual amorphous areas recrystallize epitaxially with respect to the substrate via lateral silicide growth. The process leads to formation of a continuous 35 nm thick nickel disilicide layer completely confined within the original amorphous region. Silicide resistivity was 44 μΩ cm. The process may be viewed as a low-temperature process enhancement to self-aligned silicide (SALICIDE) technology since silicide growth proceeds only on preamorphized areas of the silicon substrate.

EFFECTS OF INTENSIVE VEGETABLE PRODUCTION ON CHEMICAL PROPERTIES AND NUTRIENT MINERALIZATION OF A BRITISH COLUMBIA HUMISOL

A study was conducted to determine the effects of long-term vegetable production on some organic matter (OM) chemical properties in an organic soil located near Cloverdale, British Columbia. Producers in this area are concerned about yield reduction and some feel this is due to changes in the soil quality. Paired soil samples were collected from two transects: one in a 40-yr-old vegetable field, and the other in an adjacent fence row. The transects were positioned in order to collect samples from a relatively deep organic deposit and from a shallow deposit where a ridge of mineral subsoil had been exposed. Initial analysis showed that cultivation had changed some chemical properties only in the plow layer. The exception was pH which was higher throughout the sample depth of the cultivated transect. Organic matter loss from the cultivated field was estimated 28% by weight, based on ash accumulation. In the cultivated deep organic section (west of the ridge) OM and C contents were reduced by 7.1 and 5.6% respectively, whereas on the cultivated ridge OM, C, N and S contents and the amount of C, N and S mineralized were reduced by 43.3, 41.6, 43.4, 25.9, 42.8, 60.8 and 52.5%, respectively. On an ash-free basis there were no changes in the deep organic section. In the ridge, the OM sulphur content had increased and the amount of N and S mineralized were decreased compared with the uncultivated transect. In the cultivated surface layer, increased S content in the OM was caused by incorporation of a S-rich subsurface layer during tillage operations. Increases in P content in the cultivated transect were attributed to P fertilization. Within the cultivated transect, C, N and S contents in the exposed mineral ridge were 48, 48, and 44%, respectively, lower than in the deep organic surface layer on the west side of the ridge. The C:N ratios were unchanged at about 16. The chemical stability of the OM was attributed to its well-decomposed nature. At this particular site, the degradation due to excessive cultivation was manifested mainly in OM loss, rather than in changes in OM quality. The main concern of producers is to reduce or prevent further loss of soil OM. Key words: Humisol, organic soil, cultivation, mineralizable C, N and S, incubation

The response of the equatorial Pacific Ocean to a westerly wind burst in May 1986

Western Pacific westerly wind bursts of 1‐ to 3‐week duration are potentially important in triggering and sustaining El Niño‐Southern Oscillation events. One such burst of 10‐day duration and maximum speeds of greater than 10 m s−1 occurred in May 1986 west of the date line. The response to this westerly wind burst is documented from equatorial current meter moorings, thermistor chain moorings, and sea level and hydrographic data. At 0°, 165°E in the western Pacific the thermocline was depressed by 25 m, sea surface temperature dropped by 0.3°–0.4°C, and sea level rose by 10–15 cm a few days after the maximum in westerly wind speed. Likewise, the South Equatorial Current rapidly accelerated eastward and attained speeds in excess of 100 cm s−1. Vertical shear in an approximately 100 m deep surface layer reversed within a few days of the winds, consistent with a simple model of equatorial mixed layer dynamics in which vertical eddy viscosities are inferred to be O(100 cm2 s−1). A sharp Kelvin wavelike pulse in sea level propagated out of the directly forced region into the central and eastern Pacific. The pulse took 45 days to travel from Tarawa (1°N, 173°E) to La Libertad (2°S, 81°W) on the South American coast, at an average phase speed of about 300 cm s−1. This is of the same order of magnitude as, but significantly higher than, the phase speed of a first baroclinic mode Kelvin wave and is probably the result of Doppler shifting by the Equatorial Undercurrent. A rise in sea surface temperature of about 1°C in 2 days occurred at 0°N, 110°W with the passage of the pulse. However, coincidental meridional advection of a sharp sea surface temperature front, rather than zonal advection of downwelling associated with the pulse, appears to be responsible for this warming. The relevance of this wind‐forced pulse to the subsequent evolution of the 1986–1987 El Niño‐Southern Oscillation event is discussed in the light of these observations.