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.     

Rates of oxygen exchange between the Al2O8Al28(OH)56(H2O)26(aq) (Al30) molecule and aqueous solution

Rates of steady exchange of oxygens between bulk solution and the largest known aluminum polyoxocation: Al₂O₈Al₂₈(OH)₅₆(H₂O)₂₆¹⁸⁺(aq) (Al₃₀) are reported at pH≈4.7 and 32-40°C. The Al₃₀ molecule is a useful model for geochemists because it is ≈2 nm in length, comparable to the smallest colloidal solids, and it has structural complexity greater than the surfaces of most aluminum (hydr)oxide minerals. The Al₃₀ molecule has 15 distinct hydroxyl sites and eight symmetrically distinct bound waters. Among the hydroxyl bridges are two sets of μ₃-OH, which are not present in any of the other aluminum polyoxocations that have yet been studied by NMR methods. Rates of isotopic equilibration of the μ₂-OH and μ₃-OH hydroxyls and bound water molecules fall within the same range as we have determined for other aluminum solutes, although it is impossible to determine rate laws for exchange at the large number of individual oxygen sites. After injection of ¹⁷O-enriched water, growth of the ¹⁷O-NMR peak near 37 ppm, which is assigned to μ₂-OH and μ₃-OH hydroxyl bridges, indicates that these bridges equilibrate within two weeks at temperatures near 35°C. The peak at +22 ppm in the ¹⁷O-NMR spectra, assigned to bound water molecules (η-OH₂), varies in width with temperature in a similar fashion as for other aluminum solutes, suggesting that most of the η-OH₂ sites exchange with bulk solution at rates that fall within the range observed for other aluminum complexes. Signal from one anomalous group of four η-OH₂ sites is not observed, indicating that these sites exchange at least a factor of ten more rapidly than the other η-OH₂ sites on the Al₃₀.     

Intermittent Turbulence Associated with a Density Current Passage in the Stable Boundary Layer

Using the unprecedented observational capabilities deployed duringthe Cooperative Atmosphere-Surface Exchange Study-99 (CASES-99),we found three distinct turbulence events on the night of 18October 1999, each of which was associated with differentphenomena: a density current, solitary waves, and downwardpropagating waves from a low-level jet. In this study, we focus onthe first event, the density current and its associatedintermittent turbulence. As the cold density current propagatedthrough the CASES-99 site, eddy motions in the upper part of thedensity current led to periodic overturning of the stratifiedflow, local thermal instability and a downward diffusion ofturbulent mixing. Propagation of the density current induced asecondary circulation. The descending motion following the head ofthe density current resulted in strong stratification, a sharpreduction in the turbulence, and a sudden increase in the windspeed. As the wind surge propagated toward the surface, shearinstability generated upward diffusion of turbulent mixing. Wedemonstrate in detail that the height and sequence of the localthermal and shear instabilities associated with the dynamics ofthe density current are responsible for the apparent intermittentturbulence.     

Dynamics of pink shrimp (Farfantepenaeus duorarum) recruitment potential in relation to salinity and temperature in Florida Bay

Progress is reported in relating upstream water management and freshwater flow to Florida Bay to a valuable commercial fishery for pink shrimp (Farfantepenaeus duorarum), which has major nursery grounds in Florida Bay. Changes in freshwater inflow are expected to affect salinity patterns in the bay, so the effect of salinity and temperature on the growth, survival, and subsequent recruitment and harvest of this ecologically and economically important species was examined with laboratory experiments and a simulation model. Experiments were conducted to determine the response of juvenile growth and survival to temperature (15°C to 33°C) and salinity (2‰ to 55‰), and results were used to refine an existing model. Results of these experiments indicated that juvenile pink shrimp have a broad salinity tolerance range at their optimal temperature, but the salinity tolerance range narrows with distance from the optimal temperature range, 20–30°C. Acclimation improved survival at extreme high salinity (55‰), but not at extremely low salinity (i.e., 5‰, 10‰). Growth rate increases with temperature until tolerance is exceeded beyond about 35°C. Growth is optimal in the mid-range of salinity (30‰) and decreases as salinity increases or decreases. Potential recruitment and harvests from regions of Florida bay were simulated based on local observed daily temperature and salinity. The simulations predict that potential harvests might differ among years, seasons, and regions of the bay solely on the basis of observed temperature and salinity. Regional differences in other characteristics, such as seagrass cover and tidal transport, may magnify regional differences in potential harvests. The model predicts higher catch rates in the September–December fishery, originating from the April and July settlement cohorts, than in the January–June fishery, originating from the October and January settlement cohorts. The observed density of juveniles in western Florida Bay during the same years simulated by the model was greater in the fall than the spring, supporting modeling results. The observed catch rate in the fishery, a rough index of abundance, was higher in the January–June fishery than the July–December fishery in most of the biological years from 1989–1990 through 1997–1998, contrary to modeling results and observed juvenile density in western Florida Bay.     

Cathodoluminescence microanalysis of natural hydrated amorphous SiO2; opal

 Natural amorphous hydrated silicon dioxide (opal) has been investigated for the first time using Cathodoluminescence microanalysis in a Scanning Electron Microscope. Defect centers have been identified and imaged with high sensitivity and high spatial resolution. Intrinsic defects identified include the non bridging oxygen hole center (NBOHC) and the oxygen deficient centers (ODC). Impurities are strongly correlated with the grain structure of the precious opal and a higher than average concentration of impurity defects are observed at grain boundaries and cracks. Impurity associated defect centers include the NBOHC with −OH precursor, and the charge compensated substitutional Al, Ti, and Fe centers. Cathodoluminescence microanalysis provides important information about the defect structure and long term stability of precious opal. Received September 4, 1995 / Revised, accepted May 30, 1996     

Cooling rate variation in natural volcanic glasses from Tenerife,Canary Islands

 Silicate melts form glasses in a variety of geological environments. The relaxation (equilibration) of the frozen glass structure provides a means of investigating the quench rates of natural glasses, and this cooling history provides an important constraint for models of melt dynamics. Phonolite glasses from the central volcanic edifice of Tenerife, Canary Islands indicate a range of five orders of magnitude cooling rate, determined by modeling the relaxation of the structure-dependent property, enthalpy (H) across the glass transition. The relaxation of enthalpy is determined by heat capacity (c _p = ΔH/ΔT) measurement of natural glass samples by differential scanning calorimetry (DSC). Upon heating, the heat capacity curve in the vicinity of the glass transition has a geometry characteristic of the previous cooling rate. A series of thermal treatments applied to each individual sample results in a set of sample-specific parameters which are used to model the heat capacity curve of the naturally cooled glass. The cooling rate is then derived. The equivalence of shear and enthalpic relaxation enables the relaxation of enthalpy for these volcanic samples to be described by a general term for the evolution of fictive temperature. Quench rates for thirty-one glasses are calculated to be within the range 10°C s^–1 to 7°C per day. The cooling rates quoted are linear approximations across the glass transition. Within different volcanic facies cooling rates depend on several factors. The most rapidly cooled glasses occur where samples lose heat by radiation from the surface. Our analyses indicate that in certain environments, a natural annealing process results in slow quench rates. This is interpreted as either a slow initial cooling process or the reheating of a glass to an annealing temperature within the glass transition interval. The latter results in relaxation to a lower temperature structure. Controls on these processes include the initial temperature and dissipation of thermal energy from the volcanic body. Our results are consistent with an influence of volatiles on quench rates in volcanic bombs where glass adjacent to vesicular layers is relatively rapidly quenched. We interpret this as a rapid quench rate frozen into the glass resulting from a change in viscosity due to volatile degassing. In lava flows, the conduction of heat from the hot flow interior controls the cooling process and diminishes the effect of volatile exsolution. Relaxation geospeedometry can be applied to glass samples from a variety of geological environments where cooling rates cannot be measured directly. Such measurements provide a means of determining cooling rates for a variety of volcanic processes, an independent calibration for existing temperature and time data and a means for testing cooling-rate-dependent models. Received: 9 January 1996 / Accepted: 13 May 1996     

The X-ray signature of solar coronal mass ejections

The coronal response to six solar X-ray flares has been investigated. At a time coincident with the projected onset of the white-light coronal mass ejection associated with each flare, there is a small, discrete soft X-ray enhancement. These enhancements (precursors) precede by typically 20 m the impulsive phase of the solar flare which is dominant by the time the coronal mass ejection has reached an altitude above 0.5 R . We identify motions of hot X-ray emitting plasma, during the precursors, which may well be a signature of the mass ejection onsets. Further investigations have also revealed a second class of X-ray coronal transient, during the main phase of the flare. These appear to be associated with magnetic reconnection above post-flare loop systems.NCAR is sponsored by the National Science Foundation.     

Hydrogen depletion and the evolution of cataclysmic variables to low mass X-ray binaries

Certain cataclysmic variables may evolve into low mass X-ray binaries if the white dwarfs can steadily accrete sufficient mass to exceed the Chandrasekhar limit. We present spectra of a recurrent nova and a low mass X-ray binary which are very similar to each other, and are also unusual for the strengths of the observed He II emission. We suggest that this similarity is not coincidental, but is evidence for an evolutionary link between the two classes of objects. A hydrogen depletion in the accreting gas is implied from the emission line fluxes, and may be an important parameter in determining whether accreted gas remains bound to the white dwarf, enabling eventual core collapse to occur.Paper presented at the IAU Colloquium No. 93 on Cataclysmic Variables. Recent Multi-Frequency Observations and Theoretical Developments, held at Dr. Remeis-Sternwarte Bamberg, F. R. G., 16–19 June, 1986.