A reappraisal of the Rb-Sr systematics of early Archaean gneisses from Hebron,Labrador

A re-investigation of the Rb-Sr isotope systematics of early Archaean granodiorite gneisses from Hebron, northern Labrador, confirms the existence of early Archaean crust in that area, but has failed to corroborate both the high degree of coherence and the high initial⁸⁷Sr/⁸⁶Sr reported by Barton [1]. Instead, marked geological scatter is observed in the data for both slabbed gneisses and large bulk samples, a scatter unequivocally due to the development of secondary whole-rock isochrons at ～ 1800 Ma.Regression analysis of the new data gives very large uncertainties in the age and initial⁸⁷Sr/⁸⁶Sr because of this secondary disturbance, viz. 3645_?470⁺⁸⁷⁵ Ma and 0.702_+0.005^?0.009. We interpret the previous indications of high initial⁸⁷Sr/⁸⁶Sr at 3600 Ma as due to chance.The Hebron data cannot be distinguished from similar populations of geologically-disturbed Rb-Sr results from the Uivak I gneisses to the north [5] and the Amîtsoq grey gneisses [6]. This supports a field-based contention that the so-called “Hebron gneisses” are both lithologic and stratigraphic equivalents of the Uivak I gneisses.     

Satellite-based rainfall data reveal a recent drying trend in central equatorial Africa

West-central Uganda, a biodiversity hotspot on the eastern edge of central equatorial Africa (CEA), is a region coping with balancing food security needs of a rapidly growing human population dependent on subsistence agriculture with the conservation of critically endangered species. Documenting and understanding rainfall trends is thus of critical importance in west-central Uganda, but sparse information exists on rainfall trends in CEA during the past several decades. The recently created African Rainfall Climatology version 2 (ARC2) dataset has been shown to perform satisfactorily at identifying rainfall days and estimating seasonal rainfall totals in west-central Uganda. Therefore, we use ARC2 data to assess rainfall trends in west-central Uganda and other parts of equatorial Africa from 1983–2012. The core variables examined were three-month rainfall variables for west-central Uganda, and annual rainfall variables and seasonal rainfall totals for a transect that extended from northwestern Democratic Republic of the Congo to southern Somalia. Significant decreases in rainfall in west-central Uganda occurred for multiple three-month periods centered on boreal summer, and rainfall associated with the two growing seasons decreased by 20 % from 1983–2012. The drying trend in west-central Uganda extended westward into the Congo rainforest. Rainfall in CEA was significantly correlated with the Atlantic Multidecadal Oscillation (AMO) at the annual scale and during boreal summer and autumn. Two other possible causes of the decreasing rainfall in CEA besides North Atlantic Ocean sea-surface temperatures (e.g., AMO), are the warming of the Indian Ocean and increasing concentrations of carbonaceous aerosols over tropical Africa from biomass burning.     

Passive interference localization within the GNSS environmental monitoring system (GEMS): TDOA aspects

Due to their low power levels, global positioning system (GPS) signals are very susceptible to interference from intentional and unintentional sources. With ever increasing reliance on global navigation satellite systems (GNSS) for everyday operation of safety–critical infrastructure, the detection, localization and elimination of interference to GNSS is of paramount importance. The GNSS environmental monitoring system (GEMS) II provides the capability to detect and localize interferers in real time in a given area. It consists of a number of spatially distributed sensor nodes connected to a central processing unit. Interference is localized using hybrid direction-of-arrival (DOA) and time-difference-of-arrival (TDOA) techniques. We describe the GEMS II environment and provide an in-depth analysis and evaluation of the TDOA aspects of the system. During evaluation, signals generated from Spirent GPS signal generators as well as data from actual field-test trials are used to provide extensive performance analysis and comparison, with a view to final system integration.     

Submarine channel initiation,filling and maintenance from sea‐floor geomorphology and morphodynamic modelling of cyclic steps

Advances in acoustic imaging of submarine canyons and channels have provided accurate renderings of sea‐floor geomorphology. Still, a fundamental understanding of channel inception, evolution, sediment transport and the nature of the currents traversing these channels remains elusive. Herein, Autonomous Underwater Vehicle technology developed by the Monterey Bay Aquarium Research Institute provides high‐resolution perspectives of the geomorphology and shallow stratigraphy of the San Mateo canyon‐channel system, which is located on a tectonically active slope offshore of southern California. The channel comprises a series of crescent‐shaped bedforms in its thalweg. Numerical modelling is combined with interpretations of sea‐floor and shallow subsurface stratigraphic imagery to demonstrate that these bedforms are likely to be cyclic steps. Submarine cyclic steps compose a morphodynamic feature characterized by a cyclic series of long‐wave, upstream‐migrating bedforms. The bedforms are cyclic steps if each bedform in the series is bounded by a hydraulic jump in an overriding turbidity current, which is Froude‐supercritical over the lee side of the bedform and Froude‐subcritical over the stoss side. Numerical modelling and seismic‐reflection imagery support an interpretation of weakly asymmetrical to near‐symmetrical aggradation of predominantly fine‐grained net‐depositional cyclic steps. The dominant mode of San Mateo channel maintenance during the Holocene is interpreted to be thalweg reworking into aggrading cyclic steps by dilute turbidity currents. Numerical modelling also suggests that an incipient, proto‐San Mateo channel comprises a series of relatively coarse‐grained net‐erosional cyclic steps, which nucleated out of sea‐floor perturbations across the tectonically active lower slope. Thus, the interaction between turbidity‐current processes and sea‐floor perturbations appears to be fundamentally important to channel initiation, particularly in high‐gradient systems. Offshore of southern California, and in analogous deep‐water basins, channel inception, filling and maintenance are hypothesized to be strongly linked to the development of morphodynamic instability manifested as cyclic steps.     

Temporal and Spatial Dynamics of Estuarine Shoreline Change in the Albemarle-Pamlico Estuarine System,North Carolina,USA

Many shoreline studies rely on historical change rates determined from aerial imagery decades to over 50 years apart to predict shoreline position and determine setback distances for coastal structures. These studies may not illustrate the coastal impacts of short-duration but potentially high-impact storm events. In this study, shoreline change rates (SCRs) are quantified at five different sites ranging from marsh to sediment bank shorelines around the Albemarle-Pamlico estuarine system (APES) for a series of historical (decadal to 50-year) and short-term (bimonthly) time periods as well as for individual storm events. Long-term (historical) SCRs of approximately ?0.5 ± 0.07 m year^?1 are observed, consistent with previous work along estuarine shorelines in North Carolina. Short-term SCRs are highly variable, both spatially and temporally, and ranged from 15.8 ± 7.5 to ?19.3 ± 11.5 m year^?1 at one of the study sites. The influence of wave climate on the spatial and temporal variability of short-term erosion rates is investigated using meteorological observations and coupled hydrodynamic (Delft3D) and wave (SWAN) models. The models are applied to simulate hourly variability in the surface waves and water levels. The results indicate that in the fetch-limited APES, wind direction strongly influences the wave climate at the study sites. The wave height also has an influence on short-term SCRs as determined from the wave simulations for individual meteorological events, but no statistical correlation is found for wave height and SCRs over the long term. Despite the significantly higher rates of shoreline erosion over short time periods and from individual events like hurricanes, the cumulative impact over long time periods is low. Therefore, while the short-term response of these shorelines to episodic forcing should be taken into account in management plans, the long-term trends commonly used in ocean shoreline management can also be used to determine erosion setbacks on estuarine shorelines.     

Costs and benefits of tropical cyclones,severe thunderstorms and bushfires in Australia

The historical frequency, distribution and impact of tropical cyclones, severe thunderstorms and bushfires in Australia are discussed. Although the climatological record of frequency and distribution is incomplete for some hazards, this information is more reliable than that available on the impacts of the hazards. Insurance payout costs form the best quantitative measure of negative impacts, but such figures represent only a fraction of the true costs of damage from severe weather. For tropical cyclones the insurance payout since 1967 has been $1715 million, for severe thunderstorms $1808 million and for bushfires $488 million. Tropical cyclones and storms each result in the loss of 4 to 6 lives each year, while bushfires have an average annual death toll of about 10. Although significant benefits arising from severe weather events can also be identified, quantitative estimates of their alue are not available.     

Dolomite dissolution: An alternative diagenetic pathway for the formation of palygorskite clay

Palygorskite is a fibrous, magnesium‐bearing clay mineral commonly associated with Late Mesozoic and Early Cenozoic dolomites. The presence of palygorskite is thought to be indicative of warm, alkaline fluids rich in Si, Al and Mg. Palygorskite has been interpreted to form in peritidal diagenetic environments, either as a replacement of detrital smectite clay during a dissolution–precipitation reaction or solid‐state transformation, or as a direct precipitate from solution. Despite a lack of evidence, most diagenetic studies involving these two minerals posit that dolomite and palygorskite form concurrently. Here, petrological evidence is presented from the Umm er Radhuma Formation (Palaeocene–Eocene) in the subsurface of central Qatar for an alternative pathway for palygorskite formation. The Umm er Radhuma is comprised of dolomitized subtidal to peritidal carbonate cycles that are commonly capped by centimetre‐scale beds rich in palygorskite. Thin section, scanning electron microscopy and elemental analyses demonstrate that palygorskite fibres formed on both the outermost surfaces of dissolved euhedral dolomite crystals and within partially to completely dissolved dolomite crystal cores. These observations suggest that dolomite and palygorskite formed sequentially, and support a model by which the release of Mg²⁺ ions and the buffering of solution pH during dolomite dissolution promote the formation of palygorskite. This new diagenetic model explains the co‐occurrence of palygorskite and dolomite in the rock record, and provides valuable insight into the specific diagenetic conditions under which these minerals may form.     

Sub-Micrometre Resolution FIB-SEM-based ToF-SIMS Used to Map Geochemical Zoning in Four Zircon Reference Materials

Zircon geochemistry can vary over micrometre scales; therefore, natural reference materials need to be well characterised before being used to calculate trace element mass fractions in unmeasured samples. Moreover, reference material homogeneity needs to be ensured with the accelerating rate of geoanalytical developments to map mineral chemistry at increasingly finer scales. Here, we investigate trace element zoning in four widely used zircon reference materials: 91500, Mud Tank, Temora and Plešovice, as well as zircon crystals from the Mount Dromedary/Gulaga Igneous Complex, Australia. Sub-micrometre resolution focused ion beam scanning electron microscope (FIB-SEM) based time-of-flight secondary ion mass spectrometry (ToF-SIMS) and 5 μm resolution LA-ICP-MS mapping show that trace elements are zoned in all reference materials, though 91500 exhibited the least zonation. We demonstrate that FIB-SEM-based ToF-SIMS can rapidly resolve variations in trace elements (e.g., U, Th, Sc, Y, Gd, Dy, Yb and Li) at sensitivities down to the μg g^-1 level with a spatial resolution of 195 nm for areas 100 × 85 μm to 959 × 828 μm. Zircon 91500 is recommended for future quantitative analyses provided that (1) the spatial distribution of elements is imaged before analysis of unknown samples and (2) it is used in conjunction with a doped glass as the primary reference material.