Origin and palaeo‐environments of calcareous sediments in the Moshaweng dry valley,southeast Botswana

Quaternary sedimentation in the Moshaweng dry valley of southeastern Botswana is evaluated on the basis of geomorphological evolution and sedimentological analyses. Stratigraphic evidence reveals an upper surface (1095 m) containing abundant sil‐calcrete, an intermediate surface (1085 m) in which sil‐calcrete underlies nodular calcrete and lower (1075 m) surface in which sil‐calcrete and nodular calcrete are interbedded. This subdivision is reflected in the geochemical composition of the sediments which show an overall trend of decreasing SiO₂ content (and increasing CaCO₃ content) with depth from the highest to the lowest surface levels. The calcretes and sil‐calcretes represent modifications of pre‐existing detrital Kalahari Group sand and basal Kalahari pebbles which thinned over a Karoo bedrock high. Modification took place during wet periods when abundant Ca⁺⁺‐rich groundwater flowed along the structurally aligned valley system. With the onset of drier conditions, water table fluctuations led to the precipitation of nodular calcretes in the phreatic layer to a depth of about 20 m. A major geochemical change resulted in the preferential silicification of the nodular calcrete deposits. Conditions for silica mobilization may be related to drying‐induced salinity and in situ geochemical differentiation brought about by pebble dissociation towards the top of the sediment pile. As calcretization and valley formation progressed to lower levels, silica release took place on a diminishing scale. Thermoluminescence dating infers a mid‐Pleistocene age for sil‐calcrete formation suggesting that valley evolution and original calcrete precipitation are much older. Late stage dissolution of CaCO₃ from pre‐existing surface calcretes or sil‐calcretes led to the formation of pedogenic case‐hardened deposits during a time of reduced flow through the Moshaweng system possibly during the upper or late Pleistocene. Copyright © 2002 John Wiley & Sons, Ltd.     

Rock fall dynamics and deposition: an integrated analysis of the 2009 Ahwiyah Point rock fall,Yosemite National Park,USA

We analyzed a combination of airborne and terrestrial LiDAR, high‐resolution photography, seismic, and acoustic data in order to gain insights into the initiation, dynamics, and talus deposition of a complex rock fall. A large (46 700 m³) rock fall originated from near Ahwiyah Point in eastern Yosemite Valley and fell a total of 730 m to the valley floor on 28 March 2009. Analyses of remote sensing, seismic, and acoustic data were integrated to reconstruct the rock fall, which consisted of (1) the triggering of a 25 400 m³ rock block in an area of intersecting and sometimes highly weathered joint planes, (2) the sliding and subsequent ballistic trajectory of the block from a steeply dipping ledge, (3) dislodging of additional rock from the cliff surface from beneath the rock fall source area, (4) a mid‐cliff ledge impact that detached a volume of rock nearly equivalent in volume to the initial block, (5) sliding of the deteriorating rock mass down the remainder of the cliff, and (6) final impact at the base of the cliff that remobilized the existing talus downward and outward and produced an airblast that knocked down hundreds of trees. The depositional geomorphology indicates that the porosity of the fresh talus is significantly lower than that expected for typical blocky talus slopes, likely because the rock debris from this event was pulverized into smaller, more poorly sorted fragments and densified via dynamic compaction when compared to less energetic, fragmental‐type rock falls. These results suggest that accumulation of individual rock‐fall boulders tends to steepen talus slopes, whereas large, energetic rock falls tend to flatten them. Detachment and impact signals were recorded by seismic and acoustic instruments and highlight the potential use of this type of instrumentation for generalized rock fall monitoring, while LiDAR and photography data were able to quantify the cliff geometry, rock fall volume, source and impact locations, and geomorphological changes to the cliff and talus. Published in 2012. This article is a US Government work and is in the public domain in the USA.     

Which way do you lean? Using slope aspect variations to understand Critical Zone processes and feedbacks

Soil‐mantled pole‐facing hillslopes on Earth tend to be steeper, wetter, and have more vegetation cover compared with adjacent equator‐facing hillslopes. These and other slope aspect controls are often the consequence of feedbacks among hydrologic, ecologic, pedogenic, and geomorphic processes triggered by spatial variations in mean annual insolation. In this paper we review the state of knowledge on slope aspect controls of Critical Zone (CZ) processes using the latitudinal and elevational dependence of topographic asymmetry as a motivating observation. At relatively low latitudes and elevations, pole‐facing hillslopes tend to be steeper. At higher latitudes and elevations this pattern reverses. We reproduce this pattern using an empirical model based on parsimonious functions of latitude, an aridity index, mean‐annual temperature, and slope gradient. Using this empirical model and the literature as guides, we present a conceptual model for the slope‐aspect‐driven CZ feedbacks that generate asymmetry in water‐limited and temperature‐limited end‐member cases. In this conceptual model the dominant factor driving slope aspect differences at relatively low latitudes and elevations is the difference in mean‐annual soil moisture. The dominant factor at higher latitudes and elevations is temperature limitation on vegetation growth. In water‐limited cases, we propose that higher mean‐annual soil moisture on pole‐facing hillslopes drives higher soil production rates, higher water storage potential, more vegetation cover, faster dust deposition, and lower erosional efficiency in a positive feedback. At higher latitudes and elevations, pole‐facing hillslopes tend to have less vegetation cover, greater erosional efficiency, and gentler slopes, thus reversing the pattern of asymmetry found at lower latitudes and elevations. Our conceptual model emphasizes the linkages among short‐ and long‐timescale processes and across CZ sub‐disciplines; it also points to opportunities to further understand how CZ processes interact. We also demonstrate the importance of paleoclimatic conditions and non‐climatic factors in influencing slope aspect variations. Copyright © 2017 John Wiley & Sons, Ltd.     

Using δ15N in Fish Larvae as an Indicator of Watershed Sources of Anthropogenic Nitrogen: Response at Multiple Spatial Scales

There is growing interest in applying ??¹⁵N in biota as an indicator of anthropogenic nutrient inputs to coastal environments because changes in ??¹⁵N correlate to inputs of land-based nutrients. In complex coastal receiving waters, however, land-use effects on biota ??¹⁵N may be masked by local hydrologic processes, especially exchange with coastal waters of different geochemical character. We examined ??¹⁵N differences among larval fish, a novel biotic indicator, in coastal receiving waters at both among and within watershed scales. Our goal was to characterize how hydrologic processes within coastal river mouths and embayments mediate the effect of land-based N sources on larval fish ??¹⁵N. We sampled three Lake Superior river-embayment systems from watersheds that span a large population density gradient. Over all stations, mean fish ??¹⁵N ranged from 2.7??? to 10.8???. Within each system, we found a different pattern in ??¹⁵N across the river?Clake transition zone. Correlations between fish ??¹⁵N and water quality, particularly NH ₄ ⁺ and total nitrogen, were highly significant and corresponded to known differences in sewage waste water inputs. A multivariate model that included both watershed-based population density and NH ₄ ⁺ was found to provide the best fit to the ??¹⁵N data among a series of multi- and univariate candidate models. These results demonstrate that: (1) fish larvae ??¹⁵N responded at within watershed scales, and (2) within coastal receiving waters, fish larvae ??¹⁵N was related to waste water inputs at the watershed scale; however, expression at specific locations within a coastal system was strongly influenced by local hydrologic processes.     

U-Pb ages and tectonomagmatic relationships of early Ordovician low-Ti tholeiites, boninites and related plutonic rocks in central Newfoundland, Canada

The Wild Bight Group (WBG) and South Lake Igneous Complex (SLIC) together comprise one of the Ordovician accreted oceanic terranes of the central mobile belt of the Newfoundland Appalachians. Combined detailed mapping, geochemistry, Sm-Nd isotopic studies and U-Pb geochronology have shown that sheeted dykes and hornblende diorite and tonalite plutons of the SLIC are genetically related to a discrete package of volcanic rocks in the WBG. These igneous rocks are geochemically, isotopically and temporally distinct from volcanic rocks in the rest of the WBG. Plutonic rocks of the SLIC range in age from 486 ± 3 Ma to 489 ± 3 Ma, and a cross-cutting gabbro dyke gives a minimum age of 486 ± 4 Ma for the related volcanic sequence. Volcanic rocks in the rest of the WBG sequence are predominantly younger than 472 ± 3 Ma. The older volcanic sequence of the WBG and the SLIC occur as fault-bounded packages interleaved within the younger WBG sequence. A conformable stratigraphic relationship between the older and younger sequences of the WBG has not been demonstrated. The mafic rocks of the older package include boninites and low-Ti, high-Mg tholeiitic island arc basalts which are interpreted to be genetically related, and normal island arc tholeiites (IAT). The high-Mg mafic rocks are interpreted to have formed in an extensional setting during subduction zone initiation, and the normal IAT are thought to represent stabilisation of the volcanic front. The associated high-Si, low-K rhyolite and tonalite are interpreted to be the products of secondary melting at the base of thickened early arc crust. Sm-Nd isotopic compositions indicate that the characteristic trace element signature of the boninites developed at or near their time of generation and was not a long lived characteristic of the source region. The boninites and low-Ti tholeiites are interpreted to have originated from a similar source, which was metasomatized by different subduction-related components. Apparent decoupling of Sm-Nd geochemical and isotopic compositions suggests that these very depleted rocks may be recording the effect of subduction zone processes not yet fully understood. Received: 31 October 1997 / Accepted: 6 May 1998     

Outflow of Pacific water from the Chukchi Sea to the Arctic Ocean

Pacific water exits the Chukchi Sea shelf through Barrow Canyon in the east and Herald Canyon in the west, forming an eastward-directed shelfbreak boundary current that flows into the Beaufort Sea. Here we summarize the transformation that the Pacific water undergoes in the two canyons, and describe the characteristics and variability of the resulting shelfbreak jet, using recently collected summertime hydrographic data and a year-long mooting data set. In both canyons the northward-flowing Pacific winter water switches from the western to the eastern flank of the canyon, interacting with the northward-flowing summer water. In Barrow canyon the vorticity structure of the current is altered, while in Herald canyon a new water mass mode is created. In both instances hydraulic effects are believed to be partly responsible for the observed changes. The shelfl）reak jet that forms from the canyon outflows has distinct seasonal configurations, from a bottom-intensified flow carrying cold, dense Pacific water in spring, to a surface-intensified current advecting warm, buoyant water in summer. The current also varies significantly on short timescales, from less than a day to a week. In fall and winter much of this mesoscale variability is driven by storm events, whose easterly winds reverse the current and cause upwelling. Different types of eddies are spawned from the current, which are characterized here using hydrographic and satellite data.     

Beyond the boundaries: social place in a protected space

Tourism is increasingly promoted worldwide by public and private agencies as a means of achieving sustainable community development. Rare are the economic windfalls that come without social costs, however, and marketing rural communities as tourist 'products' inevitably forces upon their residents a social transfiguration. Yet, concerns about local places have largely been marginalized in the regionalization and commodification of tourist spaces. As a result, important questions remain unanswered. In response, this study uses the cognitive images of visitors and residents of the twin Alaska 'ghost' towns of Kennicott and McCarthy to describe the evolving landscape of tourism in a protected wilderness community. Specifically, it poses two questions. First, how might disparate perceptions of the tourism landscape serve to define the boundaries of 'place' experience for those who live in or visit Kennicott-McCarthy? Second, can this geographic perspective encourage a proactive planning process more cognizant of the effects on – and responsive to the concerns of – the destination community? A preliminary analysis of the results is presented with a focus on tourism as both agent and process in the structure, identity, and meaning of local places, embedded within larger regions and economies.     

Grid transformation for incorporating the Arctic in a global ocean model

A grid transformation is described which isolates the Arctic and North Atlantic, rotates the spherical grid to pass an equator up the Atlantic through the north pole and remaps the Coriolis parameter. Boundary condition information is exchanged along the equatorial Atlantic so that the Arctic-Atlantic model is dynamically coupled to a model of the rest of the global ocean (which remains on the geographic spherical grid). The transform produces a more regular grid over the Arctic and eliminates the need for filtering or special treatment at the pole. The transform has been implemented in the GFDL Modular Ocean Model. After testing with idealized geometry, a 300 y global integration is compared to an integration using the geographic spherical grid and Fourier filtering. Results are similar, with differences in the Arctic and western North Atlantic regions leading to smaller air-sea heat flux near the Gulf Stream separation latitude for the transform case. Use of the transform also leads to a reduction in computation time.     

Comparison of microbial indicators and seasonal temperatures as means for evaluating the vulnerability of water resources from karst and siliciclastic springs

Hydrogeology Journal - Spring water is a critical resource in many parts of the world, however, there are few effective and efficient means of evaluating the vulnerability and sustainability of...