Parametric analysis of modeled ion escape from Mars

We develop a parametric fit to the results of a detailed magnetohydrodynamic (MHD) study of the response of ion escape rates (O⁺, and ) to strongly varied solar forcing factors, as a way to efficiently extend the MHD results to different conditions. We then use this to develop a second, evolutionary model of solar forced ion escape. We treat the escape fluxes of ion species at Mars as proportional to the product of power laws of four factors - that of the EUV flux R_euv, the solar wind particle density R_ρ, its velocity (squared) R_v², and the interplanetary magnetic field pressure R_B², where forcing factors are expressed in units of the current epoch-averaged values. Our parametric model is: , where ?(i) is the escape flux of ion i. We base our study on the results of just six provided MHD model runs employing large forcing factor variations, and thus construct a successful, first-order parametric model of the MHD program. We perform a five-dimensional least squares fit of this power law model to the MHD results to derive the flux normalizations and the indices of the solar forcing factors. For O⁺, we obtain the values, 1.73 × 10²⁴ s⁻¹, 0.782, 0.251, 0.382, and 0.214, for ?₀, α, β, γ, and δ, respectively. For , the corresponding values are 1.68 × 10²⁴ s⁻¹, −0.393, 0.798, 0.967, and 0.533. For , they are 8.66 × 10²² s⁻¹, −0.427, 1.083, 1.214, and 0.690. The fit reproduces the MHD results to an average error of about 5%, suggesting that the power laws are broadly representative of the MHD model results. Our analysis of the MHD model shows that by itself an increase in R_EUV enhances O⁺ loss, but suppresses the escape of and , whereas increases in solar wind (i.e., in , and R_B², with R_euv constant) favors the escape of heavier ions more than light ions. The ratios of escaping ions detectable at Mars today can be predicted by this parametric fit as a function of the solar forcing factors. We also use the parametric model to compute escape rates over martian history. This second parametric model expresses ion escape functions of one variable (per ion), ?(i) = ?₀(i)(t/t₀)^−ξ(i). The ξ(i) are linear combinations of the epoch-averaged ion escape sensitivities, which are seen to increase with ion mass. We integrate the and oxygen ion escape rates over time, and find that in the last 3.85 Gyr, Mars would have lost about mbars of , and of water (from O⁺ and ) from ion escape.     

Influence of shear rate,organic matter content,pH and salinity on mud flocculation

The purpose of this paper is to establish a relation between a few measurable quantities (the so-called ζ potential, organic matter content, and shear rate) and the flocculation behavior of mud. The results obtained with small-scale flocculation experiments (mixing jar) are compared to results of large-scale experiments (settling column). The mud used for all experiments has been collected in October 2007 in the lower Western Schelde, near Antwerp, Belgium. From this study, it was found that the mean floc size and the Kolmogorov microscale vary in a similar way with the shear rate for suspensions with different pH and salt concentrations. The size of flocs at a given shear rate depends on the properties of the suspension, which affect the electrokinetic properties of the sediment; these can be described by means of the ζ potential. The main findings of this paper are: (1) In saline suspensions at pH = 8, the mean floc size increases when the salt concentration and the ζ potential increase. (2) For a given ζ potential, the mean floc size at low pH is larger than observed at pH = 8 for any added salt. (3) The mean floc size increases with increasing organic matter content. (4) Mud with no organic matter at pH = 8 and no added salt flocculates very little. The response of mud suspensions to variations in salinity and pH is similar to that of kaolinite. This suggests that a general trend can be established for different and complex types of clays and mud. This systematic study can therefore be used for further development of flocculation models.     

Drifter observations of the Gulf of Maine Coastal Current

Two-hundred and twenty seven satellite-tracked drifters were deployed in the Gulf of Maine (GoM) from 1988 to 2007, primarily during spring and summer. The archive of tracks includes over 100,000 km logged thus far. Statistics such as transit times, mean velocities, response to wind events, and preferred pathways are compiled for various areas of the coastal GoM. We compare Lagrangian flow with Eulerian estimates from nearby moorings and evaluate drifter trajectories using Ekman theory and 3-D ocean circulation models.     

Mountain-block recharge, present and past, in the eastern Espa?ola Basin, New Mexico, USA

Noble gas recharge temperatures (NGTs) and radiocarbon ages were determined for 43 groundwater samples collected in the eastern Espa?ola Basin, New Mexico (USA), to identify mountain-block recharge in waters <10 thousand years (ka) old and to evaluate possible changes in mountain-block recharge over the past ??35?ka. For Holocene samples from the southeastern area, NGTs are dominantly 2?C4° cooler than the measured water-table temperature near the mountain front. Computed minimum mountain-block recharge fractions are dominantly 0.2?C0.5, consistent with previous large mountain-block recharge estimates. NGTs do not display the distinct low during the last glacial maximum observed in other paleorecharge studies; samples recharged 15?C25?ka ago are on average only 1.3° cooler than Holocene samples. Instead, samples with the coldest NGTs were recharged 25?C35?ka ago. A proposed explanation is that higher precipitation rates during the last glacial maximum resulted in a lower mean recharge elevation for the basin, essentially buffering the effect of the lower mean annual air temperature and producing NGTs similar to the Holocene. In the period preceding the last glacial maximum, precipitation rates more like today??s resulted in Holocene-like mountain-block recharge fractions, producing a mean NGT ??5° cooler than the Holocene, as expected.     

Impacts of cyclic hypoxia on reproductive and gene expression patterns in the grass shrimp: field versus laboratory comparison

Identification of organisms able to act as biological indicators of hypoxia exposure is important given the increasing frequency of hypoxic events worldwide. Grass shrimp, Palaemonetes pugio, are ubiquitous estuarine residents in the southeastern US, where they commonly experience cyclic hypoxia. We hypothesized that reproductive and gene expression patterns of grass shrimp in cyclic hypoxic field sites relative to normoxic sites would be similar to previous results from cyclic hypoxia laboratory experiments showing differential up- and down-regulation of hypoxia-responsive genes. There were few differences in gene expression of indigenous shrimp collected during summer from two Gulf of Mexico estuarine systems [East Bay, FL (EB) and Weeks Bay, AL (WB)], although down-regulation of the gene coding for vitellogenin-1 corresponded with a decreased relative fecundity and fewer ovigerous females at cyclic hypoxic field sites, suggesting an overall impact on reproduction. The gene expression profiles of grass shrimp caged for 7 days in field sites differed by estuary, but few hypoxia-responsive genes identified in laboratory studies were differentially expressed in field shrimp. Overall, genes coding for protein synthesis, protein degradation, carbohydrate and lipid metabolism and electron transport were mostly up-regulated in EB caged shrimp but generally down-regulated in WB caged shrimp and laboratory shrimp. Thus, caged grass shrimp from different bay systems exhibited profoundly different gene expression profiles. Such profiles may serve as sensitive bioindicators of differences in water quality, habitat quality or food resources among estuaries but are not effective as indicators of hypoxia exposure.     

Flocculation settling characteristics of mud: sand mixtures

When natural muds become mixed with sandy sediments in estuaries, it has a direct effect on the flocculation process and resultant sediment transport regime. Much research has been completed on the erosion and consolidation of mud/sand mixtures, but very little is known quantitatively about how mixed sediments interact whilst in suspension, particularly in terms of flocculation. This paper presents the settling velocity findings from a recent laboratory study which examined the flocculation dynamics for three different mud/sand mixtures at different concentrations (0.2–5 g.l^?1) and turbulent shear stresses (0.06–0.9 Pa) in a mini-annular flume. The low intrusive video-based Laboratory Spectral Flocculation Characteristics instrument was used to determine floc/aggregate properties (e.g., size, settling velocity, density and mass) for each population. Settling data was assessed in terms of macrofloc (>160 μm) and microfloc (<160 μm) settling parameters: Ws_macro and Ws_micro, respectively. For pure muds, the macroflocs are regarded as the most dominant contributors to the total depositional flux. The parameterised settling data indicates that by adding more sand to a mud/sand mixture, the fall velocity of the macrofloc fraction slows and the settling velocity of microflocs quickens. Generally, a mainly sandy suspension comprising 25% mud and 75% sand (25M:75S), will produce resultant Ws_macro which are slower than Ws_micro. The quickest Ws_micro appears to consistently occur at a higher level of turbulent shear stress (τ?～?0.6 Pa) than both the macrofloc and microfloc fractions from suspensions of pure natural muds. Flocculation within a more cohesively dominant muddy-sand suspension (i.e., 75M:25S) produced macroflocs which fell at similar speeds (±10%) to pure mud suspensions at both low (200 mg l^?1) and intermediate (1 g?l^?1) concentrations at all shear stress increments. Also, low sand content suspensions produced Ws_macro values that were faster than the Ws_micro rates. In summary, the experimental results of the macrofloc and microfloc settling velocities have demonstrated that flocculation is an extremely important factor with regards to the depositional behaviour of mud/sand mixtures, and these factors must be considered when modelling mixed sediment transport in the estuarine or marine environment.     

Applications of a total dissolved gas pressure probe in ground water studies

Measurements of dissolved gases have numerous applications in ground water hydrology, and it is now possible to measure the total dissolved gas pressure in situ using a probe. Dissolved gas pressure is measured by submerging a headspace volume with a gas-permeable membrane, allowing dissolved gases in the water to equilibrate with gases in the headspace, then measuring the pressure in the headspace with a pressure transducer. Total dissolved gas pressure (TGP) probes have many potential uses in ground water studies employing dissolved gases, including: (1) determining approximate excess air levels, which may provide information about the time and location of recharge; (2) screening wells for air contamination, which can compromise the accuracy of dissolved gas tracer techniques: (3) detecting a trapped gas phase, which can significantly reduce hydraulic conductivity and impede the transport of dissolved solutes and gases; (4) enabling the use of gas-filled passive diffusion samplers for determining accurate dissolved gas concentrations; and (5) determining relative concentrations of CH4 and CO2 when they are known to be highly abundant. Although TGP probes designed for surface water have been available for several years, TGP probes suitable for ground water applications have only recently become available. Herein we present what are, to our knowledge, the first reported ground water dissolved gas data collected using a TGP probe. We also explain the basic operating principles of these probes and discuss the potential applications listed.     

The effect of fluorine on liquidus phase relationships in the system Qz-Ab-Or with excess water at 1 kb

Liquidus phase relationships have been determined experimentally for the system Qz-Ab-Or with excess water and 1, 2, and 4 wt.% added fluorine at 1 kb pressure. With increasing fluorine content the position of the quartz-alkali feldspar field boundary moves away from the quartz apex. The position of the minimum melting composition and the minimum liquidus temperature change progressively from Qz₃₇Ab₃₄Or₂₉ and 730° C for the fluorine free system (Tuttle and Bowen 1958) to Qz₁₅Ab₅₈Or₂₇ and 630° C for the system with 4 wt.% added fluorine. Exploratory experiments have been carried out below the liquidus, and have indicated that for certain bulk compositions an assemblage consisting of two alkali feldspars, quartz, melt and vapour can exist at temperatures as low as 550° C at 1 kb.The experimental results suggest that there may be an interaction between fluorine and aluminosilicate complexes present within the melt, to produce aluminofluoride (AlF ₆ ^3– ) complex anions (Manning et al. 1980). The observed changes in liquidus phase relationships with increasing fluorine content indicate that the compositions of certain fluorine-rich granitic rocks are consistent with an origin by crystallisation of residual melts enriched in fluorine by magmatic differentiation. Such residual melts may exist at relatively low temperatures, and may form part of a continuum between granite magmatism and associated hydrothermal activity. Because of the observed preference of fluorine for aluminosilicate phases at the magmatic stage, the presence of fluorine alone is not considered to play a direct part in the generation of residual mineralising hydrothermal fluids.