Anatomy of a granophyre intrusion

Various structural and textural features of a drusy granophyric ring-dyke intrusion can be explained in terms of the behaviour of magmatic water. Initial crystallization of water-rich magma close to ternary eutectic composition resulted in textures grading from granophyric to microgranitic. Eventual entrapment of water-rich pockets resulted in crystallization from a silicate liquid and a hydrothermal liquid simultaneously, probably at magmatic overpressure, to form isolated bodies of pegmatite containing giant euhedral quartz crystals. Later aplite, rhyolite breccia, and tuffisite veins reflect the escape of magma and magmatic gas through the congealed parts of the intrusion, at first permissively, but later with increasingly explosive effects.     

New X.R.F. data retrieval techniques and their application to U.S.G.S. standard rocks

Accurate and consistent X-ray fluorescence analysis of major and trace elements using a single pressed pellet for each sample has been achieved with a wide variety of rock compositions. Counting techniques for use with data reduction and correction programs in Standard Fortran IV are described; the latter depend throughout on data developed from a wide range of standard rock compositions. A comparison of ten major and fifteen trace element analyses for U.S.G.S. standard rocks with averages taken from the literature testifies to the precision of these techniques.     

Geomorphology,acoustic backscatter,and processes in Santa Monica Bay from multibeam mapping

Santa Monica Bay was mapped in 1996 using a high-resolution multibeam system, providing the first substantial update of the submarine geomorphology since the initial compilation by Shepard and Emery [(1941) Geol. Soc. Amer. Spec. Paper 31]. The multibeam mapping generated not only high-resolution bathymetry, but also coregistered, calibrated acoustic backscatter at 95 kHz. The geomorphology has been subdivided into six provinces; shelf, marginal plateau, submarine canyon, basin slope, apron, and basin. The dimensions, gradients, and backscatter characteristics of each province is described and related to a combination of tectonics, climate, sea level, and sediment supply. Fluctuations of eustatic sea level have had a profound effect on the area; by periodically eroding the surface of Santa Monica plateau, extending the mouth of the Los Angeles River to various locations along the shelf break, and by connecting submarine canyons to rivers. A wetter glacial climate undoubtedly generated more sediment to the rivers that then transported the increased sediment load to the low-stand coastline and canyon heads. The trends of Santa Monica Canyon and several bathymetric highs suggest a complex tectonic stress field that has controlled the various segments. There is no geomorphic evidence to suggest Redondo Canyon is fault controlled. The San Pedro fault can be extended more than 30 km to the northwest by the alignment of a series of bathymetric highs and abrupt changes in direction of channel thalwegs.     

The dust distribution within the inner coma of comet P/Halley 1982i: encounter by Giotto's impact detectors

Analysis of the data from Giotto's Dust Impact Detection System experiment (DIDSY) is presented. These data represent measurement of the size of dust grains incident on the Giotto dust shield along its trajectory through the coma of comet P/Halley on 1986 March 13/14. First detection occurred at some 287000 km distance from the nucleus on the inbound leg; the majority of the DIDSY subsystems remained operational after closest approach (604 km) yielding the last detection at about 202000 km from the nucleus. In order to improve the data coverage (and especially for the smallest grains, to approximately 10(-19) kg particle mass), data from the PIA instrument has been combined with DIDSY data. Flux profiles are presented for the various mass channels showing, to a first approximation, a 1/R2 flux dependence, where R is the distance of the detection point from the cometary nucleus, although significant differences are noted. Deviations from this dependence are observed, particularly close to the nucleus. From the flux profiles, mass and geometrical area distributions for the dust grains are derived for the trajectory through the coma. Groundbased CCD imaging of the dust continuum in the inner coma at the time of encounter is also used to derive the area of grains intercepted by Giotto. The results are consistent with the area functions derived by Giotto data and the low albedo of the grains deduced from infrared emission. For the close encounter period (-5 min to +5 min), the cumulative mass distribution function has been investigated, initially in 20 second periods; there is strong evidence from the data for a steepening of the index of the mass distribution for masses greater than 10(-13) kg during passage through dust jets which is not within the error limits of statistical uncertainty. The fluences for dust grains along the entire trajectory is calculated; it is found that extrapolation of the spectrum determined at intermediate masses (cumulative mass index alpha = 0.85) is not able to account for the spacecraft deceleration as observed by the Giotto Radio Science Experiment and by ESOC tracking operations. Data at large masses (>10(-8) kg) recently analysed from the DIDSY data set show clear evidence of a decrease in the mass distribution index at these masses within the coma, and it is shown that such a value of the mass index can provide sufficient mass for consistency with the observed deceleration. The total particulate mass output from the nucleus of comet P/Halley at the time of encounter would be dependent on the maximum mass emitted if this change in slope observed in the coma were also applicable to the emission from the nucleus; this matter is discussed in the text. The flux time profiles have been converted through a simple approach to modeling of the particle trajectories to yield an indication of nucleus surface activity. There is indication of an enhancement in flux at t approximately -29 s corresponding to crossing of the dawn terminator, but the flux detected prior to crossing of the dawn terminator is shown to be higher than predicted by simple modelling. Further enhancements corresponding to jet activity are detected around +190 s and +270 s.     

Statistical testing of a new testate amoeba‐based transfer function for water‐table depth reconstruction on ombrotrophic peatlands in north‐eastern Canada and Maine,United States

Proxy reconstructions of climatic parameters developed using transfer functions are central to the testing of many palaeoclimatic hypotheses on Holocene timescales. However, recent work shows that the mathematical models underpinning many existing transfer functions are susceptible to spatial autocorrelation, clustered training set design and the uneven sampling of environmental gradients. This may result in over‐optimistic performance statistics or, in extreme cases, a lack of predictive power. A new testate amoeba‐based transfer function is presented that fully incorporates the new recommended statistical tests to address these issues. Leave‐one‐out cross‐validation, the most commonly applied method in recent studies to assess model performance, produced over‐optimistic performance statistics for all models tested. However, the preferred model, developed using weighted averaging with tolerance downweighting, retained a predictive capacity equivalent to other published models even when less optimistic performance statistics were chosen. Application of the new statistical tests in the development of transfer functions provides a more thorough assessment of performance and greater confidence in reconstructions based on them. Only when the wider research community have sufficient confidence in transfer function‐based proxy reconstructions will they be commonly used in data comparison and palaeoclimate modelling studies of broader scientific relevance. Copyright © 2012 John Wiley & Sons, Ltd.     

Precipitation stable isotope variability and subcloud evaporation processes in a semi‐arid region

The stable isotopic (²H/¹H and ¹⁸O/¹⁶O) composition of precipitation has been used for a variety of hydrological and paleoclimate studies, a starting point for which is the behaviour of stable isotopes in modern precipitation. To this end, daily precipitation samples were collected over a 7‐year period (2008–2014) at a semi‐arid site located at the Macquarie Marshes, New South Wales (Australia). The samples were analysed for stable isotope composition, and factors affecting the isotopic variability were investigated. The best correlation between δ ¹⁸O of precipitation was with local surface relative humidity. The reduced major axis precipitation weighted local meteoric water line was δ ²H = 7.20 δ ¹⁸O + 9.1. The lower slope and intercept (when compared with the Global Meteoric Water Line) are typical for a warm dry climate, where subcloud evaporation of raindrops is experienced. A previously published model to estimate the degree of subcloud evaporation and the subsequent isotopic modification of raindrops was enhanced to include the vertical temperature and humidity profile. The modelled results for raindrops of 1.0 mm radius showed that on average, the measured D‐excess (=δ ²H ? 8 δ ¹⁸O) was 19.8‰ lower than that at the base of the cloud, and 18% of the moisture was evaporated before ground level (smaller effects were modelled for larger raindrops). After estimating the isotopic signature at the base of the cloud, a number of data points still plotted below the global meteoric water line, suggesting that some of the moisture was sourced from previously evaporated water. Back trajectory analysis estimated that 38% of the moisture was sourced over land. Precipitation samples for which a larger proportion of the moisture was sourced over land were ¹⁸O and ²H‐enriched in comparison to samples for which the majority of the moisture was sourced over the ocean. The most common weather systems resulting in precipitation were inland trough systems; however, only East Coast Lows contributed to a significant difference in the isotopic values. Copyright © 2016 Australian Nuclear Science and Technology Organisation. Hydrological Processes. © 2016 John Wiley & Sons, Ltd.     

Tropical Pacific – mid-latitude teleconnections in medieval times

Terrestrial and marine late Holocene proxy records from the western and central US suggest that climate between approximately 500 and 1350 a.d. was marked by generally arid conditions with episodes of severe centennial-scale drought, elevated incidence of wild fire, cool sea surface temperatures (SSTs) along the California coast, and dune mobilization in the western plains. This Medieval Climate Anomaly (MCA) was followed by wetter conditions and warming coastal SSTs during the transition into the “Little Ice Age” (LIA). Proxy records from the tropical Pacific Ocean show contemporaneous changes indicating cool central and eastern tropical Pacific SSTs during the MCA, with warmer than modern temperatures in the western equatorial Pacific. This pattern of mid-latitude and tropical climate conditions is consistent with the hypothesis that the dry MCA in the western US resulted (at least in part) from tropically forced changes in winter NH circulation patterns like those associated with modern La Niña episodes. We examine this hypothesis, and present other analyses showing that the imprint of MCA climate change appears in proxy records from widely distributed regions around the planet, and in many cases is consistent with a cool medieval tropical Pacific. One example, explored with numerical model results, is the suggestion of increased westerlies and warmer winter temperatures over northern Europe during medieval times. An analog technique for the combined use of proxy records and model results, Proxy Surrogate Reconstruction (PSR), is introduced.