Theoretical analysis of secondary cratering on Mars and an image-based study on the Cerberus Plains

The issue of crater retention age estimates on planetary surfaces is discussed with an attempt to quantify the effect of overlapping primary and secondary impact crater populations in restricted crater diameter ranges. The approach to this problem is illustrated with a simple model production function where the secondary crater input is artificially enhanced. Extrapolation of such a secondary crater model distribution to a global record results in extraordinarily high crater frequencies that do not exist on Mars, and implies the need of detailed studies of the size-frequency distribution for remote secondary craters, to date poorly known. A key case, the martian crater Zunil and its secondary crater field, illustrate that reasonable predictions for the secondary crater size-frequency distribution at small (<100 m) crater diameters affected the standard model crater retention age for the Cerberus plains less than the statistical uncertainty. These observations show that age determination based on appropriate crater counting statistics is valid in a wide primary crater diameter range.     

Snowmelt runoff and soil moisture dynamics on steep subalpine hillslopes

Snowmelt water supplies streamflow and growing season soil moisture in mountain regions, yet pathways of snowmelt water and their effects on moisture patterns are still largely unknown. This study examined how flow processes during snowmelt runoff affected spatial patterns of soil moisture on two steep sub‐alpine hillslope transects in Rocky Mountain National Park, CO, USA. The transects have northeast‐facing and east‐facing aspects, and both extend from high‐elevation bedrock outcrops down to streams in valley bottoms. Spatial patterns of both snow depth and near‐surface soil moisture were surveyed along these transects in the snowmelt and summer seasons of 2008–2010. To link these patterns to flow processes, soil moisture was measured continuously on both transects and compared with the timing of discharge in nearby streams. Results indicate that both slopes generated shallow lateral subsurface flow during snowmelt through near‐surface soil, colluvium and bedrock fractures. On the northeast‐facing transect, this shallow subsurface flow emerged through mid‐slope seepage zones, in some cases producing saturation overland flow, whereas the east‐facing slope had no seepage zones or overland flow. At the hillslope scale, earlier snowmelt timing on the east‐facing slope led to drier average soil moisture conditions than on the northeast‐facing slope, but within hillslopes, snow patterns had little relation to soil moisture patterns except in areas with persistent snow drifts. Results suggest that lateral flow and exfiltration processes are key controls on soil moisture spatial patterns in this steep sub‐alpine location. Copyright © 2014 John Wiley & Sons, Ltd.     

Sources of uncertainty in estimating stream solute export from headwater catchments at three sites

Uncertainty in the estimation of hydrologic export of solutes has never been fully evaluated at the scale of a small‐watershed ecosystem. We used data from the Gomadansan Experimental Forest, Japan, Hubbard Brook Experimental Forest, USA, and Coweeta Hydrologic Laboratory, USA, to evaluate many sources of uncertainty, including the precision and accuracy of measurements, selection of models, and spatial and temporal variation. Uncertainty in the analysis of stream chemistry samples was generally small but could be large in relative terms for solutes near detection limits, as is common for ammonium and phosphate in forested catchments. Instantaneous flow deviated from the theoretical curve relating height to discharge by up to 10% at Hubbard Brook, but the resulting corrections to the theoretical curve generally amounted to <0.5% of annual flows. Calibrations were limited to low flows; uncertainties at high flows were not evaluated because of the difficulties in performing calibrations during events. However, high flows likely contribute more uncertainty to annual flows because of the greater volume of water that is exported during these events. Uncertainty in catchment area was as much as 5%, based on a comparison of digital elevation maps with ground surveys. Three different interpolation methods are used at the three sites to combine periodic chemistry samples with streamflow to calculate fluxes. The three methods differed by <5% in annual export calculations for calcium, but up to 12% for nitrate exports, when applied to a stream at Hubbard Brook for 1997–2008; nitrate has higher weekly variation at this site. Natural variation was larger than most other sources of uncertainty. Specifically, coefficients of variation across streams or across years, within site, for runoff and weighted annual concentrations of calcium, magnesium, potassium, sodium, sulphate, chloride, and silicate ranged from 5 to 50% and were even higher for nitrate. Uncertainty analysis can be used to guide efforts to improve confidence in estimated stream fluxes and also to optimize design of monitoring programmes. © 2014 The Authors. Hydrological Processes published John Wiley & Sons, Ltd.     

Effects of crayfish (Paranephrops planifrons: Parastacidae) on in‐stream processes and benthic faunas: A density manipulation experiment

The effects of New Zealand freshwater crayfish or koura (Paranephrops planifrons: Parastacidae) on organic matter processing, sediment accumulation, and benthic invertebrate communities were investigated using four replicate treatments of 0 (control), 4–5 (medium), and 8–11 (high) similar‐sized koura in 0.5 m² artificial stream channels colonised by benthic invertebrates from a pasture stream, Waikato, New Zealand. Wineberry (Aristotelia serrata) leaf packs were placed in each channel and after 7 weeks the leaf matter remaining was significantly lower in both medium and high koura channels than in controls. The amount of sediment (surficial cover by fines and weight of suspendable sediment) was also significantly reduced in high koura density channels. Densities of invertebrates other than crayfish were not significantly different among treatments; however, taxa richness and invertebrate biomass were significantly lower in high koura channels than in controls. Our results suggest that freshwater koura may play a keystone role in structuring benthic invertebrate communities either directly through predation, or indirectly by sediment bioturbation and increasing organic matter processing rates.     

Colony growth of corals transplanted for restoration depends on their site of origin and environmental factors

We determined that growth differences among coral fragments transplanted for restoration were influenced by both source population and environmental factors. In two common garden experiments, storm‐generated fragments of Acropora palmata were transplanted from two source populations in the British Virgin Islands to a restoration site (a ‘common garden’) that lacked A. palmata. In the first experiment, colonies from different sources grew at different rates in the first year after transplanting, suggesting either genetic differences among source populations or enduring acclimation to conditions at the source site. No differences in growth among source populations were detected in the second common garden experiment. To isolate environmental effects on growth, we subdivided fragments from three source populations to create genetically identical pieces that were attached separately at both source and restoration sites. Genetically identical pieces from all source populations grew slightly faster at their source than at the restoration site, implying a subtle home‐site advantage. Overall, our results suggest that matching environmental conditions at source and restoration sites may increase the success of restoration projects.     

Arsenic mobility in weathered gold mine tailings under a low-organic soil cover

Historical gold mining operations in Nova Scotia, Canada, resulted in numerous deposits of publicly accessible, arsenic (As)-rich mine waste that has weathered in situ for 75–150 years, resulting in a wide range of As-bearing secondary minerals. The geochemical heterogeneity of this mine waste creates a challenge for identifying a single remediation approach that will limit As mobility. A 30-cm-thick, low-organic content soil cover was evaluated in a laboratory leaching experiment where, to simulate natural conditions, the equivalent of 2 years of synthetic rainwater was leached through each column and two dry seasons were incorporated into the leaching protocol. Each column was a stratigraphic representation of the four major tailings types found at the historical Montague and Goldenville gold mine districts: hardpan tailings, oxic tailings, wetland tailings, and high Ca tailings. Hardpan tailings released acidic, As-rich waters (max 12 mg/L) under the soil cover but this acidity was buffered by surrounding oxic tailings. Leachate from the oxic tailings was circumneutral, with average As concentrations between 4.4 and 9.7 mg/L throughout the experiment. The presence of carbonates in the high Ca tailings resulted in near-neutral to weakly alkaline leachate pH values and average As concentrations between 2.1 and 6.1 mg/L. Oxidation of sulfides in the wetland tailings led to acidic leachate over time and a decrease in As concentrations to values that were generally less than 1 mg/L. This study shows that the use of a low-organic content soil cover does not create reducing conditions that would destabilize oxidized, As-bearing secondary phases in these tailings. However, oxygen penetration through the cover during dry seasons would continue to release As to tailings pore waters via sulfide oxidation reactions.     

A structural phase-transition in K(Mg1−xCux)F3 perovskite

A complete solid-solution series between cubic (Pm 3 m) KMgF₃ and tetragonal (I4/mcm) KCuF₃ was synthesized at 730–735 °C in an inert atmosphere. X-ray powder-diffraction at room temperature shows that the transition between the cubic and tetragonal perovskite structures in the series K (Mg_1?xCu_x) F₃ occurs at x ～ 0.6. Rietveld structure-refinements were done for selected compositions. In the cubic phase, all parameters are linear with composition up to the transition point. At the transition point, there is a strong discontinuity in the cell volume; this is strongly anisotropic with expansion along the a axes and contraction along the c axis due to a pronounced axial elongation of the (Mg, Cu) F₆ octahedron that increases with increasing Cu content. The phase transition is first-order, with a discontinuity of ≈2% in the symmetry-breaking strain at x_C. It is proposed that the phase transition in K (Mg, Cu) F₃ is due to the onset of the cooperative Jahn-Teller effect. Compositional relationships for lattice vibrations in this solid solution were established using thin-film infrared spectroscopy. A phase transition occurring above 60 mole % KCuF₃ is indicated by the appearance of one of the two modes expected for the tetragonal phase; the weaker mode is not resolved below 80 mole % KCuF₃. Modes common to both structures vary smoothly and continuously across the binary; however, frequencies do not depend linearly on composition, nor is mode-softening discernable. Two-mode behaviour is observed only for the bending motion of the cubic phase, because this peak alone has non-overlapping end-member components.     

Studies on vertical total-beta-activity profiles of fission products in the accumulation area of the Stubacher Sonnblickkees (Hohe Tauern,Salzburg, Austria)

Summary In the years of 1965 and 1966 continuous vertical sets of samples were bored from pits comprising the uppermost perannual snowcover in the accumulation area of the Stubacher Sonnblickkees (Hohe Tauern, Salzburg, Austria). The total-beta-activity of the samples was measured in order to study the changes of the vertical distribution of fission products by summerly melting processes. An enrichment of fission products was found in the layer under the late, summer horizon. However, the weak activity of the layer underneath accumulated during the winter period is hardly influenced by meltwater percolation from the superimposed layer with high activity.An 11.75 m deep firn pit covers, the annual layers of net accumulation from 1960/61 to 1965/66. The most remarkable activity variation of the entire total-beta-activity profile of this pit is a maximum at the coinciding late summer horizons of 1962 and 63. This activity maximum proves that mass budget years with heavy fission product debris and high melting conditions cause a strong enrichment and high retention of the remaining layer for fission products. It can be assumed that this characteristic feature of the vertical activity distribution will be also found on neighboring glaciers in the region of the Mittlere Hohe Tauern. Because its relatively long time stability it is especially interesting for future dating purposes.Moreover, it was found that the period from 1961/62 to 1963/64 shows a high fission product activity in contrast to the low radioactive period from 1964/65 to 1965/66. This behavior of the firn activity is in agreement with the one of the air and the precipitation during these years. The characteristic changes of the total-beta-activity of the firn at the late summer horizon of 1964 can be also used for dating purposes.

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Zusammenfassung In den Jahren 1965 und 1966 wurden im Akkumulationsgebiet des Stubacher Sonnblickkees, (Hohe Tauern, Salzburg) aus Schneeschächten kontinuierliche Probenserien entnommen, welche die Akkumulation des laufenden hydrologischen Jahres erfassen. Die Proben wurden auf Gesamt-Beta-Aktivität untersucht, um die Veränderung der Vertikalverteilung der Spaltprodukte durch sommerliche Abschmelzvorgänge zu studieren. Als Ergebnis zeigt sich eine Spaltproduktanreicherung der Schicht unter dem Spätsommerhorizont, wobei jedoch die geringe Aktivität der darunterliegenden Winterschicht durch Einsickern von Schmelzwasser aus der darüberliegenden stark mit Spaltprodukten angereicherten Schicht kaum beeinflußt wird.Ein 11.75 m tiefer Firnschacht umfaßt die Jahresrücklagen von 1961/60 bis 1965/66. Als hervorstechendstes Merkmal der Vertikalverteilung der Gesamt-Beta-Aktivität ergibt sich ein Maximum an den zusammenfallenden Spätsommerhorizonten 1962 und 1963. Dieses Aktivitätsmaximum zeigt, daß in Haushaltsjahren mit starkem Fallout und starker Abschmelzung eine beachtliche Anreicherung sowie ein gutes Retentionsvermögen der verbleibenden Schicht für Spaltprodukte auftreten. Dieses charakteristische Aktivitätsmerkmal kann auch auf benachbarten Gletschern im Gebiet der Mittleren Hohen Tauern erwartet werden. Zufolge der guten Stabilität dieses Merkmals ist es für künftige Datierungszwecke besonders interessant.Weiters weist die Periode von 1961/62 bis 1963/64 hohe Spaltproduktaktivitäten im Gegensatz zur Periode 1964/65 bis 1965/66 auf. Dieses Ergebnis stimmt auch mit dem Verlauf der Luft-und Niederschlagsaktivität in den entsprechenden Jahren überein. Die charakteristische Abnahme der Gesamt-Beta-Aktivität des Firns über dem Spätsommerhorizont 1964 kann ebenfalls für Datierungszwecke verwendet werden.