Variations in air density,satellite drag coefficient and atmospheric rotation rate from analysis of the orbit of 1966-92D

Variations in air density, the satellite drag coefficient, and the atmospheric rotation rate at 60°S lat and 120–130 km height during the period September 1968–June 1969 have been determined from analysis of the high-eccentricity orbit of the 4th Molyniya 1 upper-stage rocket body, 1966-92D. The results show good correlation between density increases and strong geomagnetic activity, although solar flares of equal geomagnetic index value do not consistently produce density changes of equal magnitude. A 30 per cent semi-annual variation was observed, but there was no indication of the 50 per cent lower thermosphere seasonal-latitudinal variation that was predicted from the CIRA 1972 atmosphere. The satellite drag coefficient was observed to begin decreasing with height at an altitude where the molecular mean free path, λ, was twice the satellite's length. The coefficient decreased to a value approaching 1.0 as the satellite's perigee height fell below the altitude where λ was one-half the length. A mean atmospheric rotation rate of 1.1 ± 0.1 Earth rot/day was obtained for the last 20 days of decay. However, variations were observed with west-to-east wind speeds of ?100 m/sec measured for a local time of 13 hr.     

Equifinality of formal (DREAM) and informal (GLUE) Bayesian approaches in hydrologic modeling?

In recent years, a strong debate has emerged in the hydrologic literature regarding what constitutes an appropriate framework for uncertainty estimation. Particularly, there is strong disagreement whether an uncertainty framework should have its roots within a proper statistical (Bayesian) context, or whether such a framework should be based on a different philosophy and implement informal measures and weaker inference to summarize parameter and predictive distributions. In this paper, we compare a formal Bayesian approach using Markov Chain Monte Carlo (MCMC) with generalized likelihood uncertainty estimation (GLUE) for assessing uncertainty in conceptual watershed modeling. Our formal Bayesian approach is implemented using the recently developed differential evolution adaptive metropolis (DREAM) MCMC scheme with a likelihood function that explicitly considers model structural, input and parameter uncertainty. Our results demonstrate that DREAM and GLUE can generate very similar estimates of total streamflow uncertainty. This suggests that formal and informal Bayesian approaches have more common ground than the hydrologic literature and ongoing debate might suggest. The main advantage of formal approaches is, however, that they attempt to disentangle the effect of forcing, parameter and model structural error on total predictive uncertainty. This is key to improving hydrologic theory and to better understand and predict the flow of water through catchments.     

One‐ and two‐dimensional modelling of overland flow in semiarid shrubland,Jornada basin,New Mexico

Two distributed parameter models, a one‐dimensional (1D) model and a two‐dimensional (2D) model, are developed to simulate overland flow in two small semiarid shrubland watersheds in the Jornada basin, southern New Mexico. The models are event‐based and represent each watershed by an array of 1‐m² cells, in which the cell size is approximately equal to the average area of the shrubs. Each model uses only six parameters, for which values are obtained from field surveys and rainfall simulation experiments. In the 1D model, flow volumes through a fixed network are computed by a simple finite‐difference solution to the 1D kinematic wave equation. In the 2D model, flow directions and volumes are computed by a second‐order predictor–corrector finite‐difference solution to the 2D kinematic wave equation, in which flow routing is implicit and may vary in response to flow conditions. The models are compared in terms of the runoff hydrograph and the spatial distribution of runoff. The simulation results suggest that both the 1D and the 2D models have much to offer as tools for the large‐scale study of overland flow. Because it is based on a fixed flow network, the 1D model is better suited to the study of runoff due to individual rainfall events, whereas the 2D model may, with further development, be used to study both runoff and erosion during multiple rainfall events in which the dynamic nature of the terrain becomes an important consideration. Copyright © 2006 John Wiley & Sons, Ltd.     

Geomorphic and stratigraphic evidence for an unusual tsunami or storm a few centuries ago at Anegada, British Virgin Islands

Waters from the Atlantic Ocean washed southward across parts of Anegada, east-northeast of Puerto Rico, during a singular event a few centuries ago. The overwash, after crossing a fringing coral reef and 1.5?km of shallow subtidal flats, cut dozens of breaches through sandy beach ridges, deposited a sheet of sand and shell capped with lime mud, and created inland fields of cobbles and boulders. Most of the breaches extend tens to hundreds of meters perpendicular to a 2-km stretch of Anegada??s windward shore. Remnants of the breached ridges stand 3?m above modern sea level, and ridges seaward of the breaches rise 2.2?C3.0?m high. The overwash probably exceeded those heights when cutting the breaches by overtopping and incision of the beach ridges. Much of the sand-and-shell sheet contains pink bioclastic sand that resembles, in grain size and composition, the sand of the breached ridges. This sand extends as much as 1.5?km to the south of the breached ridges. It tapers southward from a maximum thickness of 40?cm, decreases in estimated mean grain size from medium sand to very fine sand, and contains mud laminae in the south. The sand-and-shell sheet also contains mollusks??cerithid gastropods and the bivalve Anomalocardia??and angular limestone granules and pebbles. The mollusk shells and the lime-mud cap were probably derived from a marine pond that occupied much of Anegada??s interior at the time of overwash. The boulders and cobbles, nearly all composed of limestone, form fields that extend many tens of meters generally southward from limestone outcrops as much as 0.8?km from the nearest shore. Soon after the inferred overwash, the marine pond was replaced by hypersaline ponds that produce microbial mats and evaporite crusts. This environmental change, which has yet to be reversed, required restriction of a former inlet or inlets, the location of which was probably on the island??s south (lee) side. The inferred overwash may have caused restriction directly by washing sand into former inlets, or indirectly by reducing the tidal prism or supplying sand to post-overwash currents and waves. The overwash happened after A.D. 1650 if coeval with radiocarbon-dated leaves in the mud cap, and it probably happened before human settlement in the last decades of the 1700s. A prior overwash event is implied by an inland set of breaches. Hypothetically, the overwash in 1650?C1800 resulted from the Antilles tsunami of 1690, the transatlantic Lisbon tsunami of 1755, a local tsunami not previously documented, or a storm whose effects exceeded those of Hurricane Donna, which was probably at category 3 as its eye passed 15?km to Anegada??s south in 1960.     

Isotopic fractionation of the major elements of molten basalt by chemical and thermal diffusion

Samples produced in piston cylinder experiments were used to document the thermal isotopic fractionation of all the major elements of basalt except for aluminum and the fractionation of iron isotopes by chemical diffusion between a natural basalt and rhyolite. The thermal isotopic fractionations are summarized in terms of a parameter Ω_i defined as the fractionation in per mil per 100 °C per atomic mass units difference between the isotopes. For molten basalt we report Ω_Ca = 1.6, Ω_Fe = 1.1, Ω_Si = 0.6, Ω_O = 1.5. In an earlier paper we reported Ω_Mg = 3.6. These fractionations represent a steady state balance between thermal diffusion and chemical diffusion with the mass dependence of the thermal diffusion coefficient being significantly larger than the mass dependence of the chemical diffusion coefficients for isotopes of the same element. The iron isotopic measurements of the basalt-rhyolite diffusion couple showed significant fractionation that are parameterized in terms of a parameter β_Fe = 0.03 when the ratio of the diffusion coefficients D₅₄ and D₅₆ of ⁵⁴Fe and ⁵⁶Fe is expressed in terms of the atomic mass as D₅₄/D₅₆ = (56/54)^β_Fe. This value of β_Fe is smaller than what we had measured earlier for lithium, magnesium and calcium (i.e., β_Li = 0.215, β_Ca = 0.05, β_Mg = 0.05) but still significant when one takes into account the high precision with which iron isotopic compositions can be measured (i.e., ±0.03‰) and that iron isotope fractionations at magmatic temperatures from other causes are extremely small. In a closing section we discuss technological and geological applications of isotopic fractionations driven by either or both chemical and thermal gradients.     

Water quality analysis of groundwater in crystalline basement rocks,Northern Ghana

Hydrochemical data are presented for groundwater samples, collected from fractured aquifers in parts of northern Ghana. The data was collected to assess the groundwater suitability for domestic and agricultural use. Results of the study reveal that the pH of the groundwater in the area is slightly acidic to slightly alkaline. The electrical conductivity values, total dissolved solids (TDS) values and calcium, magnesium and sodium concentrations in the groundwater are generally below the limit set by the WHO for potable water supply. On the basis of activity diagrams, groundwater from the fractured aquifers appears to be stable within the montmorillonite field, suggesting weathering of silicate minerals. An inverse distance weighting interpolator with a power of 2 was applied to the data points to produce prediction maps for nitrate and fluoride. The distribution maps show the presence of high nitrate concentrations (50–194 mg/l) in some of the boreholes in the western part of the study area indicating anthropogenic impact on the groundwater. Elevated fluoride level (1.5–4 mg/l), higher than the WHO allowable fluoride concentration of 1.5, is recorded in the groundwater underlying the northeastern part of the study area, more specifically Bongo and its surrounding communities of the Upper East region. Results of this study suggest that groundwater from the fractured aquifers in the area exhibit low sodicity–low salinity (S1–C1), low sodicity–medium salinity (S1–C2) characteristics [United States Salinity Laboratory (USSL) classification scheme]. All data points from this study plot within the ‘Excellent to good’ category on a Wilcox diagram. Groundwater in this area thus appears to provide irrigation water of excellent quality. The hydrochemical results indicate that, although nitrate and fluoride concentrations in some boreholes are high, the groundwater in the study area, based on the parameters analyzed, is chemically potable and suitable for domestic and agricultural purposes.     

Shallow radar (SHARAD) sounding observations of the Medusae Fossae Formation, Mars

The SHARAD (shallow radar) sounding radar on the Mars Reconnaissance Orbiter detects subsurface reflections in the eastern and western parts of the Medusae Fossae Formation (MFF). The radar waves penetrate up to 580 m of the MFF and detect clear subsurface interfaces in two locations: west MFF between 150 and 155° E and east MFF between 209 and 213° E. Analysis of SHARAD radargrams suggests that the real part of the permittivity is ∼3.0, which falls within the range of permittivity values inferred from MARSIS data for thicker parts of the MFF. The SHARAD data cannot uniquely determine the composition of the MFF material, but the low permittivity implies that the upper few hundred meters of the MFF material has a high porosity. One possibility is that the MFF is comprised of low-density welded or interlocked pyroclastic deposits that are capable of sustaining the steep-sided yardangs and ridges seen in imagery. The SHARAD surface echo power across the MFF is low relative to typical martian plains, and completely disappears in parts of the east MFF that correspond to the radar-dark Stealth region. These areas are extremely rough at centimeter to meter scales, and the lack of echo power is most likely due to a combination of surface roughness and a low near-surface permittivity that reduces the echo strength from any locally flat regions. There is also no radar evidence for internal layering in any of the SHARAD data for the MFF, despite the fact that tens-of-meters scale layering is apparent in infrared and visible wavelength images of nearby areas. These interfaces may not be detected in SHARAD data if their permittivity contrasts are low, or if the layers are discontinuous. The lack of closely spaced internal radar reflectors suggests that the MFF is not an equatorial analog to the current martian polar deposits, which show clear evidence of multiple internal layers in SHARAD data.     

Subsurface structure of Planum Boreum from Mars Reconnaissance Orbiter Shallow Radar soundings

We map the subsurface structure of Planum Boreum using sounding data from the Shallow Radar (SHARAD) instrument onboard the Mars Reconnaissance Orbiter. Radar coverage throughout the 1,000,000-km² area reveals widespread reflections from basal and internal interfaces of the north polar layered deposits (NPLD). A dome-shaped zone of diffuse reflectivity up to 12 μs (∼1-km thick) underlies two-thirds of the NPLD, predominantly in the main lobe but also extending into the Gemina Lingula lobe across Chasma Boreale. We equate this zone with a basal unit identified in image data as Amazonian sand-rich layered deposits [Byrne, S., Murray, B.C., 2002. J. Geophys. Res. 107, 5044, 12 pp. doi:10.1029/2001JE001615; Fishbaugh, K.E., Head, J.W., 2005. Icarus 174, 444-474; Tanaka, K.L., Rodriguez, J.A.P., Skinner, J.A., Bourke, M.C., Fortezzo, C.M., Herkenhoff, K.E., Kolb, E.J., Okubo, C.H., 2008. Icarus 196, 318-358]. Elsewhere, the NPLD base is remarkably flat-lying and co-planar with the exposed surface of the surrounding Vastitas Borealis materials. Within the NPLD, we delineate and map four units based on the radar-layer packets of Phillips et al. [Phillips, R.J., and 26 colleagues, 2008. Science 320, 1182-1185] that extend throughout the deposits and a fifth unit confined to eastern Gemina Lingula. We estimate the volume of each internal unit and of the entire NPLD stack (821,000 km³), exclusive of the basal unit. Correlation of these units to models of insolation cycles and polar deposition [Laskar, J., Levrard, B., Mustard, J.F., 2002. Nature 419, 375-377; Levrard, B., Forget, F., Montmessin, F., Laskar, J., 2007. J. Geophys. Res. 112, E06012, 18 pp. doi:10.1029/2006JE002772] is consistent with the 4.2-Ma age of the oldest preserved NPLD obtained by Levrard et al. [Levrard, B., Forget, F., Montmessin, F., Laskar, J., 2007. J. Geophys. Res. 112, E06012, 18 pp. doi:10.1029/2006JE002772]. We suggest a dominant layering mechanism of dust-content variation during accumulation rather than one of lag production during periods of sublimation.     

Diffusive fractionation of volatiles and their isotopes during bubble growth in magmas

Bubbles grow in decompressing magmas by simple expansion and by diffusive supply of volatiles to the bubble/melt interface. The latter phenomenon is of significant geochemical interest because diffusion can fractionate elements and isotopes (or isotopologues) of dissolved components. This raises the possibility that the character of volatile components in bubbles may not reflect that of volatiles dissolved in the host melt over the lifetime of a bubble—even in the absence of equilibrium vapor/melt isotopic fractionation. Recent experiments have confirmed the existence of an isotope mass effect on diffusion of the volatile element Cl in silicate melt [Fortin et al. (Isotopic fractionation of chlorine during chemical diffusion in a dacitic melt and its implications for isotope behavior during bubble growth (abstract), 2016 Fall AGU Meeting, 2016)], so there is a clear need to understand the efficacy of diffusive fractionation during bubble growth. In this study, numerical models of diffusion and mass redistribution during bubble growth were implemented for both “passive” volatiles—those whose concentrations are generally well below saturation levels—and “active” volatiles such as CO₂ and H₂O, whose elevated concentrations and limited solubilities are the cause of bubble nucleation and growth. Both diffusive and convective bubble-growth scenarios were explored. The magnitude of the isotope mass effect on passive volatiles partitioned into bubbles growing at a constant rate R in a static system depends upon R/D _L, K _d and D _H/D _L (K _d = bubble/melt partition coefficient; D _H/D _L = diffusivity ratio of the heavy and light isotopes). During convective bubble growth, the presence of a discrete (physical) melt boundary layer against the growing bubble (of width x _BL) simplifies outcomes because it leads to the quick onset of steady-state fractionation during growth, the magnitude of which depends mainly upon R?x _BL/D _L and D _H/D _L (bubble/melt fractionation is maximized at R?x _BL/D _L ≈0.1). Constant R is unrealistic for most real systems, so other scenarios were explored by including the solubility and EOS of an “active” volatile (e.g., CO₂) in the numerical simulations. For plausible decompression paths, R increases exponentially with time—leading, potentially, to larger isotopic fractionation of species partitioned into the growing bubble. For volatile species whose isotope mass effects on diffusion have been measured (Cl, Li), predicted isotope fractionation in the exsolved vapor can be as large as ?4‰ for Cl and ?25‰ for Li.