Determination of volatile fatty acid turnover rates in organic-rich marine sediments

Volatile fatty acid (VFA) apparent turnover rates in organic-rich marine sediments were determined by measuring whole sediment VFA concentration and the corresponding first-order reaction rate constants. In order to measure VFA concentrations, bulk wet sediment samples were basified, freeze-dried, extracted with methanol, derivatized to form methyl esters of the VFAs, and analyzed by packed-column gas chromatography using hexanoic acid as an internal standard. The detection limits for acetate, propionate, iso-butyrate and butyrate were 1.0, 0.4, 0.2 and 0.2 μmol l^?1_s, respectively, for 600 ml samples. Rate constants for acetate and propionate were determined by anaerobically incubating samples at in-situ temperatures with tracer levels of ¹⁴C-labelled VFAs. Metabolized label was recovered as CO₂, CH₄, cellular material, water-soluble material, and VFA (ether-soluble) fractions. Apparent turnover rates measured during summer and winter in anoxic Cape Lookout Bight, North Carolina (U.S.A.) sediments were in the range of 19–330 μmol l^?1_s h^?1 for acetate and 0.7–7.0 μmol l^?1_s h^?1 for propionate.     

Possible production by lightning of aerosols and trace gases in Titan's atmosphere

Although lightning has not yet been observed in Titan's atmosphere, the presence of condensable vapors and the deposition of a significant amount of solar energy at the surface suggest the possibility of lightning activity. Based on an understanding of the relationship of lightning activity to the amount of convective energy available on Titan, a lightning energy dissipation rate of 4 × 10^?6, W/m² can be expected. This value is much lower than that for Earth or Jupiter, and is a result of both the reduced solar flux at Titan and the absorption of sunlight by the aerosols that lie above the convective layer. For this dissipation rate, the amount of HCN and C₂N₂ produced by lightning should be greater than that by solar UV, but could be less than that produced by electron precipitation and galactic cosmic rays. Equilibrium calculations indicate that large mole fractions of elemental solid phase carbon will also be produced. Using a simplified model of aerosol formation, coagulation, and settling, it is estimated that a lightning-produced aerosol could have a typical optical depth of 10^?2, with values as high as 0.1. The accumulation of soot over geological time might reach a meter or more in depth.     

Two-tier analysis in palaeomagnetism

Summary. The statistical analysis necessary for isolating the between-site dispersion in palaeomagnetic studies is presented, both for the case of a common within-site precision and for variable within-site precision.
Approximate distributions are derived for all the relavant statistics. It is shown that if it is necessary to perform a two-tier analysis the overall true mean direction should be estimated as the mean of individual site mean directions and an equation for the semi-angle of the cone of confidence about this mean direction is derived.
The last section of the paper lays out the process by which a two-tier analysis should be performed together with relevant equations.     

Fish assemblage stability over fifty years in the Lake Pontchartrain estuary; Comparisons among habitats using canonical correspondence analysis

We assessed fish assemblage stability over the last half century in Lake Pontchartrain, an environmentally degraded oligohaline estuary in southeastern Louisiana. Because assemblage instability over time has been consistently associated with severe habitat degradation, we attempted to determine whether fish assemblages in demersal, nearshore, and pelagic habitats exhibited change that was unrelated to natural fluctuations in environmental variables (e.g., assemblage changes between wet and dry periods). Collection data from three gear types (trawl, beach seine, and gill nets) and monthly environmental data (salinity, temperature, and Secchi depth) were compared for four collecting periods: 1954 (dry period), 1978 (wet period), 1996–1998 (wet period), and 1998–2000 (dry period). Canonical correspondence analysis (CCA) revealed that although the three environmental variables were significantly associated with the distribution and abundance patterns of fish assemblages in all habitats (with the exception of Secchi depth for pelagic samples), most fish assemblage change occurred among sampling periods (i.e., along a temporal gradient unrelated to changing environmental variables). Assemblage instability was the most pronounced for fishes collected by trawls from demersal habitats. A marked lack of cyclicity in the trawl data CCA diagram indicated a shift away from a baseline demersal assemblage of 50 yr ago. Centroid positions for the five most collected species indicated that three benthic fishes, Atlantic croaker (Micropogonias undulatus), spot (Leiostomus xanthurus), and hardhead catfish (Arius felis), were more dominant in past demersal assemblages (1954 and 1978). A different situation was shown for planktivorous species collected by trawls with bay anchovy (Anchoa mitchilli) becoming more dominant in recent assemblage and Gulf menhaden (Brevoortia patromus) remaining equally represented in assemblages over time. Changes in fish assemblages from nearshore (beach seine) and pelagic (gill net) habitats were more closely related to environmental fluctuations, though the CCA for beach seine data also indicated a decrease in the dominance ofM. undulatus and an increase in the proportion ofA. mitchilli over time. The reduced assemblage role of benthic fishes and the marked assemblage change indicated by trawl data suggest that over the last half century demersal habitats in Lake Pontchartrain have been impacted more by multiple anthropogenic stressors than nearshore or pelagic habitats.     

Containment criterion for contaminant isolation by cutoff walls

A rigorous solution is developed from first principles to guide the preliminary design of cutoff walls installed to contain the migration of contaminants from source zones. The full analytic solution is used to develop a criterion for determining the configuration and hydraulics of optimal wall designs. The solution is used to demonstrate the interaction between the properties of the wall, the Darcy flux, and the concentration of contaminants at the outside face of the well. For a particular wall design, the containment criterion can be used to estimate the long-term concentration that will develop at the outside face of the wall. Alternatively, for a given concentration on the outside face of the cutoff wall, the containment criterion can be used to estimate the Darcy flux required to balance the outward diffusion of contaminants. The results of numerical simulations are presented to evaluate the analytic approach. The numerical results confirm that for a wall with known transport properties, a specified Darcy flux is associated with a unique outside contaminant concentration.     

Light-toned layered deposits in Juventae Chasma, Mars

We examine hypotheses for the formation of light-toned layered deposits in Juventae Chasma using a combination of data from Mars Global Surveyor's Mars Orbiter Camera (MOC), Mars Orbiter Laser Altimeter (MOLA), and Thermal Emission Spectrometer (TES), as well as Mars Odyssey's Thermal Emission Imaging System (THEMIS). We divide Juventae Chasma into geomorphic units of (i) chasm wall rock, (ii) heavily cratered hummocky terrain, (iii) a mobile and largely crater-free sand sheet on the chasm floor, (iv) light-toned layered outcrop (LLO) material, and (v) chaotic terrain. Using surface temperatures derived from THEMIS infrared data and slopes from MOLA, we derive maps of thermal inertia, which are consistent with the geomorphic units that we identify. LLO thermal inertias range from ∼400 to 850 J m⁻² K⁻¹ s^−1/2. Light-toned layered outcrops are distributed over a remarkably wide elevation range () from the chasm floor to the adjacent plateau surface. Geomorphic features, the absence of small craters, and high thermal inertia show that the LLOs are composed of sedimentary rock that is eroding relatively rapidly in the present epoch. We also present evidence for exhumation of LLO material from the west wall of the chasm, within chaotic and hummocky terrains, and within a small depression in the adjacent plateau. The data imply that at least some of the LLO material was deposited long before the adjacent Hesperian plateau basalts, and that Juventae Chasma underwent, and may still be undergoing, enlargement along its west wall due to wall rock collapse, chaotic terrain evolution, and exposure and removal of LLO material. The new data allow us to reassess possible origins of the LLOs. Gypsum, one of the minerals reported elsewhere as found in Juventae Chasma LLO material, forms only at low temperatures () and thus excludes a volcanic origin. Instead, the data are consistent with either multiple occurrences of lacustrine or airfall deposition over an extended period of time prior to emplacement of Hesperian lava flows on the plateau above the chasm.