On the environmental isotope method for determining the water balance components of some lakes

The paper deals with the isotope balance method applied to lakes, which can be assumed as being under steady-state conditions. It is shown that the long-term averages and non-normalized temperatures can be used in the balance equations. The applicability of the proposed approach was tested by the interpretation or reinterpretation of several known case studies (Chala, Titicaca, Waid, Burdur, Beysehir and Egridir lakes). An isotope study of a small artificial lake (1.2 · 10⁶ m³) is presented and compared with whole-body tracing by K₃⁶⁰ Co(CN)₆. This artificial tracer does not appear useful for long-term tracing of surface waters.The water balance equation for each of the lakes considered was solved with the aid of the isotope balance equation of ¹⁸O. Applying the calculated components of the water balance, the isotope balance equation of deuterium was solved for finding the values of kinetic enrichment for deuterium. These values, which give the fit of the evaporation lines to the experimental data, do not agree with the values known from direct laboratory experiments, thus they have to be treated as apparent values, which may be useful for future field work.     

Lithospheric necking: a dynamic model for rift morphology

Rifting is examined in terms of the growth of a necking instability in a lithosphere consisting of a strong plastic or viscous surface layer of uniform strength overlying a weaker viscous substrate in which strength is either uniform or decreases exponentially with depth. As the lithosphere extends, deformation localizes about a small imposed initial perturbation in the strong layer thickness. For a narrow perturbation, the resulting surface topography consists of a central depression and uplifted flanks; the layer thins beneath the central depression. The width of the rift zone is related to the dominant wavelength of the necking instability, which in turn is controlled by the layer thickness and the mechanical properties of the lithosphere. For an initial thickness perturbation with a width less than the dominant wavelength, deformation concentrates into a zone comparable to the dominant wavelength. If the initial perturbation is wider than the dominant wavelength, then the width of the zone of deformation is controlled by the width of the initial perturbation; deformation concentrates in the region of enhanced thinning and develops periodically at the dominant wavelength. A surface layer with limiting plastic (stress exponent n = ∞) behavior produces a rift-like structure with a width typical of continental rifts for a strong layer thickness consistent with various estimates of the maximum depth of brittle deformation in the continental lithosphere. The width of the rift is essentially independent of the layer/substrate strength ratio. For a power law viscous surface layer (n = 3), the dominant wavelength varies with layer/substrate strength ratio to the one-third power and is always larger than for a plastic surface layer of the same thickness. The great widths of rift zones on Venus may be explained by unstable extension of a strong viscous surface layer.     

Seasonal Neutron-Flux Variations in the Polar Caps of Mars as Revealed by the Russian HEND Instrument Onboard the NASA 2001 Mars Odyssey Spacecraft

We analyze the flux of epithermal neutrons from the Martian surface recorded by the Russian High-Energy Neutron Detector (HEND) from February 19 through December 19, 2002. The HEND was installed onboard the NASA 2001 Mars Odyssey spacecraft and is designed to measure neutron fluxes with energies above 1 eV. Over the period of observations, statistically significant variations in the flux of epithermal (10–100 keV) neutrons were found in the northern and southern polar caps. The largest neutron-flux variations were found at subpolar latitudes, where the relative difference between the summer and winter values can reach severalfold. This correlation becomes weaker with increasing distance from the poles. Thus, the relative change in the neutron flux near the 60° parallel is slightly more than 10%. We assume that the detected variations result from the global circulation of atmospheric carbon dioxide in subpolar Martian regions. To additionally test this assumption, we compared the HEND neutron measurements onboard 2001 Mars Odyssey and the seasonal variations in the CO₂-layer thickness as observed by the Mars Orbital Laser Altimeter (MOLA) onboard Mars Global Surveyor (MGS).     

Diagenetic and other highly mineralized waters in the Polish Carpathians

Highly mineralized waters of different chemical types and origin occur in the flysch formations and their bedrocks in the western part of the Polish Carpathians. The marine sedimentation water of the flysch formations is not preserved, as the most mineralized and the heaviest isotopic values of flysch waters are characterized by δ¹⁸O and δ²H values in the ranges of 5–7‰ and −(20–30)‰, respectively. Their origin is related to the dehydration of clay minerals during burial diagenesis, with molecules of marine water completely removed by molecules of released bound water. They are relatively enriched in Na⁺ in respect to the marine water, supposedly due to the release of Na⁺ during the illitization of smectites and preferable incorporation of other cations from the primary brine into newly formed minerals. In some parts of younger formations, i.e. in the Badenian sediments, brines occur with isotopic composition close to SMOW and Cl⁻ contents greatly exceeding the typical marine value of about 19.6 g/L, supposedly due to ultrafiltration. Most probably, the marine water of the flysch formations was similarly enriched chemically in its initial burial stages. Final Cl⁻ contents in diagenetic waters depend on different Cl⁻ contents in the primary brines and on relationships between diagenetic and further ultrafiltration processes. In some areas, diagenetic waters migrate to the surface along fault zones and mix with young local meteoric waters becoming diluted, with the isotope composition scattering along typical mixing lines. In areas with independent CO₂ flow from great depths, they form chloride CO₂-rich waters. Common CO₂-rich waters are formed in areas without near-surface occurrences of diagenetic waters. They change from the HCO₃–Ca type for modern waters to HCO₃–Mg–Ca, HCO₃–Na–Ca and other types with elevated TDS, Mg²⁺ and/or Na⁻ contents for old waters reaching even those of glacial age. Bedrocks of the flysch are represented by Mesozoic and Paleozoic mudstones, sandstones and carbonates, and in some areas by Badenian sediments. Brines of the Mesozoic and Paleozoic bedrocks are usually significantly enriched in Ca²⁺ and Mg²⁺ in comparison with the Badenian brines. By analogy to the deepest brines in the adjacent Upper Silesian Coal Basin, they are supposed to originate from paleometeoric waters of a hot climate.     

Origin of the structure and planform of small impact craters in fractured targets: Endurance Crater at Meridiani Planum, Mars

We present observations and models that together explain many hallmarks of the structure and growth of small impact craters forming in targets with aligned fractures. Endurance Crater at Meridiani Planum on Mars (diameter ≈ 150 m) formed in horizontally-layered aeolian sandstones with a prominent set of wide, orthogonal joints. A structural model of Endurance Crater is assembled and used to estimate the transient crater planform. The model is based on observations from the Mars Exploration Rover Opportunity: (a) bedding plane orientations and layer thicknesses measured from stereo image pairs; (b) a digital elevation model of the whole crater at 0.3 m resolution; and (c) color image panoramas of the upper crater walls. This model implies that the crater’s current shape was mostly determined by highly asymmetric excavation rather than long-term wind-mediated erosion. We show that modal azimuths of conjugate fractures in the surrounding rocks are aligned with the square component of the present-day crater planform, suggesting excavation was carried farther in the direction of fracture alignments. This was previously observed at Barringer Crater in Arizona and we show the same relationship also holds for Tswaing Crater in South Africa. We present models of crater growth in which excavation creates a “stellate” transient cavity that is concave-cuspate in planform. These models reproduce the “lenticular-crescentic” layering pattern in the walls of some polygonal impact craters such as Endurance and Barringer Craters, and suggest a common origin for tear faults and some crater rays. We also demonstrate a method for detailed error analysis of stereogrammetric measurements of bedding plane orientations.     

Mars high resolution gravity fields from MRO, Mars seasonal gravity, and other dynamical parameters

With 2 years of tracking data collection from the MRO spacecraft, there is noticeable improvement in the high frequency portion of the spherical harmonic Mars gravity field. The new JPL Mars gravity fields, MRO110B and MRO110B2, show resolution near degree 90. Additional years of MGS and Mars Odyssey tracking data result in improvement for the seasonal gravity changes which compares well to global circulation models and Odyssey neutron data and Mars rotation and precession (). Once atmospheric dust is accounted for in the spacecraft solar pressure model, solutions for Mars solar tide are consistent between data sets and show slightly larger values (k₂ = 0.164 ± 0.009, after correction for atmospheric tide) compared to previous results, further constraining core models. An additional 4 years of Mars range data improves the Mars ephemeris, determines 21 asteroid masses and bounds solar mass loss (dGM_Sun/dt < 1.6 × 10⁻¹³ GM_Sun year⁻¹).     

Interpretation of tracer tests performed in fractured rock of the Lange Bramke basin, Germany

 Two multitracer tests performed in one of the major cross-fault zones of the Lange Bramke basin (Harz Mountains, Germany) confirm the dominant role of the fault zone in groundwater flow and solute transport. Tracers having different coefficients of molecular diffusion (deuterium, bromide, uranine, and eosine) yielded breakthrough curves that can only be explained by a model that couples the advective–dispersive transport in the fractures with the molecular diffusion exchange in the matrix. For the scale of the tests (maximum distance of 225 m), an approximation was used in which the influence of adjacent fractures is neglected. That model yielded nearly the same rock and transport parameters for each tracer, which means that the single-fracture approximation is acceptable and that matrix diffusion plays an important role. The hydraulic conductivity of the fault zone obtained from the tracer tests is about 1.5×10^–2 m/s, whereas the regional hydraulic conductivity of the fractured rock mass is about 3×10^–7 m/s, as estimated from the tritium age and the matrix porosity of about 2%. These values show that the hydraulic conductivity along the fault is several orders of magnitude larger than that of the remaining fractured part of the aquifer, which confirms the dominant role of the fault zones as collectors of water and conductors of fast flow. Received, April 1997 Revised, January 1998, August 1998 Accepted, August 1998     

The morphology of Mercury’s Caloris basin as seen in MESSENGER stereo topographic models

A digital terrain model (1000-m effective spatial resolution) of the Caloris basin, the largest well-characterized impact basin on Mercury, was produced from 208 stereo images obtained by the MESSENGER narrow-angle camera. The basin rim is far from uniform and is characterized by rugged terrain or knobby plains, often disrupted by craters and radial troughs. In some sectors, the rim is represented by a single marked elevation step, where height levels drop from the surroundings toward the basin interior by approximately 2 km. Two concentric rings, with radii of 690 km and 850 km, can be discerned in the topography. Several pre-Caloris basins and craters can be identified from the terrain model, suggesting that rugged pre-impact topography may have contributed to the varying characteristics of the Caloris rim. The basin interior is relatively smooth and shallow, comparable to typical lunar mascon mare basins, supporting the idea that Caloris was partially filled with lava after formation. The model displays long-wavelength undulations in topography across the basin interior, but these undulations cannot readily be related to pre-impact topography, volcanic construction, or post-volcanic uplift. Because errors in the long-wavelength topography of the model cannot be excluded, confirmation of these undulations must await data from MESSENGER’s orbital mission phase.