Stereochemistry of amino acids in surface samples of a marine sediment

In two surface samples of marine sediment, the percentages of d-alanine and d-aspartic acid are significantly higher than the other d-amino acids and are similar to the range found in soils. The percentage of d-glutamic acid is also higher than the other amino acids but less than d-alanine and d-aspartic acid. These d-amino acids may come mainly from bacteria.     

Hillslope hydrographs by the finite element method

Simulation models may be used to explore the implications of making specific assumptions about the nature of a real world system, and then to make predictions of the behaviour of that system under a set of naturally occurring conditions. It is important that understanding generated by the former should be gained before predictive use of the system model. This paper describes and uses a finite-element model of transient, partially saturated water flow within a hillslope soil mantle overlying an impermeable bedrock, to make an investigation into the effects of parameter variations and initial conditions on the hillslope hydrograph. The results clearly demonstrate that the response of the hillslope system to rainfall is highly non-linear and that the initial conditions, particularly in the unsaturated zone, are of paramount importance in governing the timing and magnitude of the hydrograph peak. Hillslope convergence appears as the dominant topographic parameter but the non-linearity of the response and the complex interdependence between the soil and topographic parameters restrict the possibility of further definite conclusions about the relative sensitivity of the simulated hillslope hydrograph to changes in these parameters.     

Subsidence analysis on the Sardinian margin and the central Tyrrhenian Basin: Thermal modelling and heat flow control; deep structure implications

The subsidence of the Tyrrhenian Basin is analysed based on numerous MCS reflection data and related seismic velocities, and calibrated using subsidence curves versus time at ODP Leg 107 sites 654 and 652 on the Sardinian margin. Two main phases of rifting are clearly defined for the margin-basin evolution: the first, beginning at 7 Ma, split the Sardinian-Calabrian margins, the second, beginning at 5 Ma formed the Central oceanic subbasins.Thermal models, based on thinned crust geometry from reflection-refraction data, are used which consider both vertical and horizontal conduction over time along a NW-SE transect across the entire Tyrrhenian Basin from Sardinia to Calabria. In general, there is a close agrrement between the predicted and the observed surface heat flow and the rate of subsidence over time. However, within the oceanic domain an initial subsidence must be chosen about 1000 m greater than observed on typical spreading centers, i.e., at about 3500 m as postulated in numerous back-arc basins.Thermal models, constrained by subsidence analysis and gravity observations, imply a large difference for the Moho depth beneath the Sardinian margin as determined after refraction seismic experiments. The upper mantle, hot but with high density, would be situated much higher than is postulated by seismic data and probably includes 7.3–7.4 km/s velocity layers previously interpreted as deep crustal layers.     

Stable-isotope exchange fronts, Damköhler numbers, and fluid to rock ratios

The advection of extraneous fluid into permeable solid rock along a one-dimensional path is a fundamental scenario of geochemistry. Model solutions are presented for advection with “instantaneous” as well as with kinetically restricted equilibration. An initial step input of a stable-isotope δ-value leads to the propagation of a “geochemical” or isotope front. For pure advection and instantaneous isotope exchange between fluid and solid, fronts are sharp and their positions and velocities determined by the carrier porosity Ψ (e.g., oxygen porosity) of the aquifer. For limited exchange rates the dimensionless “Damköhler number” N_D = (κ/q)L, where κ is an exchange rate constant, q the interstitial fluid velocity, and L the total theoretical infiltration distance, determines the isotope front shape, i.e. the degree of degradation of the original sharp fronts. The effects of temperature (Δ) variations and, for the first time, of variations in initial rock composition, are shown and a model calculation for a one-dimensional system with two isotopic elements of distinct Ψ (O and Sr) is given. Dispersion in the fluid is considered, and for an idealised geothermal system the combined effects of (1) dispersion in the fluid and (2) limited exchange rates, characterised by their respective Péclet and Damköhler numbers, are also calculated. Fluid/rock ratios need to be treated differently in one-dimensional and zero-dimensional models and in general it is preferable in flow models to replace them with information on porosity, infiltration distance and Damköhler numbers.     

Geologic influences on fluvial hydrology and bedload transport in small mountainous watersheds,Northern New Mexico,U.S.A.

Discharge characteristics in six adjacent mountainous watersheds in northern New Mexico, U.S.A., vary substantially between basins underlain by different lithologies. Relatively resistant gneisses and granites underlie two basins (drainage areas: 43 and 94 km²) that have high unit discharge (0·010 to 0·14 m³s^?1 km^?2), high bankfull discharge, and sustained high discharge. Less resistant sandstones and shales underlie four basins (drainage areas: 96 to 215 km²) that have relatively low unit discharge (0·001 to 0·005 m³s^?1 km^?2), relatively low bankfull discharge, and peak discharges that are not sustained as long as those in the crystalline terrane. Analysis of snowmelt-runoff water budgets suggests that three factors control hydrologic conditions in the basins. First, area-elevation distributions appear to control the timing and amounts of water input. These distributions probably reflect the erosional resistance of the different lithologies. Second, lithology appears to control runoff production in areas having minor amounts of storage. Third, glacial deposits in headwater regions control discharge duration and timing via storage and return flow releases. The amount of return flow released by glacial deposits, however, is probably controlled by the permeability of underlying bedrock. Therefore it appears that the duration, timing, and magnitude of discharge events in the study area are controlled both directly and indirectly by lithology. Stream power and shear stress estimates derived from bankfull discharge and bed-material size data suggest that higher bedload transport rates and larger bedload particle sizes exist in streams draining crystalline rocks than in streams draining sedimentary terrane. It appears that source-area lithology, by controlling discharge production, also influences stream power, bedload transport capabilities, and therefore total amounts of bedload transport.     

Dominance of Tilapia mossambica,an introduced fish species,in three Puerto Rican estuaries

We compared species presence, abundance, and size characteristics of fish in three brackish, coastal marshes at Humacao, Roosevelt Roads, and Boqueron, Puerto Rico, in February and March 1988. The three marsh ecosystems were similar with respect to the presence of large expanses of open water bordered by emergent vegetation, creeks, and mangroves, and all had some recreational use. We sampled fish using gill nets. Tilapia (Oreochromis) mossambica were the most abundant fish, accounting for 55–79% of the samples at all three marshes. Overall, tilapia were both the largest (North Lagoon) and the smallest (Frontera Creek) at Humacao. Tilapia were most common in open lagoons rather than creeks or bays (except for Mandri Creek), and their distribution seemed unrelated to salinity. Tarpon (Megalops atlantica) were more abundant at low salinities, whereas other fish were more abundant at higher salinities.     

Air motion and precipitation growth in frontal systems

In recent years there has been much progress in describing and understanding the detailed dynamical and microphysical structure of frontal precipitation systems. In this article we consider phenomena on a hierarchy of scales: the broad synoptic-scale flows in mid-latitude depressions, the mesoscale patterns of precipitation, the nature and origin of convection at the surface and aloft, and the microphysical properties of the resulting precipitation. The influence of orography is also briefly discussed.