Merging Massive Star Clusters as Building Blocks of Dwarf Galaxies?

Recent spectroscopic observations of galaxies in the Fornax-Cluster reveal nearly unresolved ‘star-like’ objects with red-shifts appropriate to the Fornax-Cluster. These objects have intrinsic sizes of ≈ 100 pc and absolute B-band magnitudes in the range - 14 < M_B < -11.5 mag and lower limits for the central surface brightness μ_B ≥ 23 mag/arcsec² (Phillipps et al., 2001, Hilker et al., 1999), and so appear to constitute a new population of ultra-compact dwarf galaxies (UCDs). Such compact dwarfs were predicted to form from the amalgamation of stellar super-clusters (= clusters of star clusters; not to confuse with super stellar clusters (SSC)) by P. Kroupa (1998), which are rich aggregates of young massive star clusters (YMCs) that can form in collisions between gas-rich galaxies. Here we present the evolution of super-clusters in a tidal field. The YMCs merge on a few super-cluster crossing times. Super-clusters that are initially as concentrated and massive as Knot S in the interacting Antennae galaxies (Whitmore et al., 1999) evolve to merger objects that are long-lived and show properties comparable to the newly discovered UCDs. This revised version was published online in September 2006 with corrections to the Cover Date.     

Comparison of 14C and 210Pb - 137Cs - 241Am dating methods of a recent bat guano deposit (Lot,SW France)

Bat guano deposits are increasingly used as records of past environmental changes, an approach that requires a precise chronology of the guano layers. This paper presents a comparison between the well-established ¹⁴C dating method and methods based on natural ²¹⁰Pb excesses, (²¹⁰Pb)_ex, and artificial radionuclides ¹³⁷Cs and ²⁴¹Am. The studied example is a bat guano deposit from a cave in SW France (the Brantites III cave), which is currently investigated for paleo-environmental reconstructions using stable isotopes. ¹⁴C data show that the upper part of the guano deposit accumulated during the last 150 years with a marked increase in accumulation rates after around 1960 AD. While the incorporation of atmospheric ¹⁴C in guano is a well-understood process, the origin of ²¹⁰Pb excesses is more complex. Based on consideration of ¹³⁷Cs and (²¹⁰Pb)_ex inventories recorded in undisturbed soils in France, and the measured inventories in the guano deposit, we suggest that most of the ²¹⁰Pb excess is produced by ²²²Rn decay in the cave air and then adsorbed onto the guano. As Radon concentrations in caves can vary significantly on both short and long-term timescales, one needs to be cautious before applying the often-used CRS (constant rate of supply of ²¹⁰Pb excess) model to guano dating. Our (²¹⁰Pb)_ex data are best interpreted by two successive periods of roughly constant, but widely different accumulation rates (0.3 cm/y and 2.6 cm/y before and after 1960, respectively) and (²¹⁰Pb)_ex fluxes. We suggest that these relatively abrupt variations result from a change in cave ventilation leading to a more favourable shelter for bats after 1960. The upper 40 cm of the deposit shows evidence of ²¹⁰Pb mobility, adding a further complexity to the interpretation of (²¹⁰Pb)_ex profiles in guano deposits. However, the existence of well-defined ¹³⁷Cs and ²⁴¹Am peaks allows a precise identification of the year of maximum atmospheric fallouts (∼1963–1964). When the ages provided by artificial radionuclides are combined with the ²¹⁰Pb-derived accumulation rates, an age model can be built, which is in good agreement with the ¹⁴C age model. This example shows that the (²¹⁰Pb)_ex method, when associated with ¹³⁷Cs (²⁴¹Am) data, can be used to date recent guano deposit, although its application is not as straightforward as the ¹⁴C method.     

New chronological constraints on the Plio-Pleistocene uplift of the Guizhou Plateau,SE margin of the Tibetan Plateau

The Guizhou Plateau represents a geomorphic transition between the Tibetan Plateau and the Yangtze River Plain. It likely formed in response to the propagation of surface uplift in southeastern Tibet during India-Eurasia continental collision. However, the uplift history of the region is unclear largely due to a lack of datable material. The bedrock geology is dominated by carbonate rocks, which contains numerous multi-level caves in the main river valleys that are linked to the river incision history. Cosmogenic ²⁶Al and ¹⁰Be burial dating of sediments in caves and river terraces from the northwestern and southern plateau reveals the fluvial chronology and provides the first direct determination of long-term river incision rates. The caves and terraces on the Liuchong River in NW Guizhou yield burial ages of between 0.41 ± 0.12 Ma and 2.85 ± 0.21 Ma, indicating an average incision rate of 57 ± 3 m/Ma. Four level caves at Libo in southern Guizhou yield burial ages of between 0.56 ± 0.16 Ma and 3.54 (+0.25/-0.22) Ma, indicating slightly slower incision rate (47 ± 5 m/Ma). These new results imply that the high elevation of the Guizhou Plateau had developed before the Late Pliocene, and that surface uplift during the Late Cenozoic was largely uniform across the region.     

The role of inherited Pb in controlling the quality of speleothem U-Pb ages

Over two decades of technical and application-based advances to the speleothem U-Pb chronometer have cemented this terrestrial archive at the forefront of landscape reconstruction, palaeoclimatology, and palaeoanthropology. The ability to access speleothem palaeoclimate records beyond the 650 ka limit of the U-Th system has opened many avenues to such ‘deep-time’ considerations. Yet still this chronometer remains a challenging analytical exercise, more-so as the technique becomes routinely applied to carbonates with less-than-ideal U/Pb ratios. In this contribution, we review the vadose speleothem U-Pb system, as revealed by 68 previously published isochrons produced from 474 solution-mode analyses across three separate geographical regions. We develop a new statistical parameter of ‘average distance’ to quantify the dispersion or ‘spread’ along the previously published isochrons and compare this with available U and Pb elemental concentrations. Our findings highlight the importance of regional geology and karst morphology in controlling the speleothem's overall U/Pb ratio. Furthermore, we show that variability in the amount of inherited Pb across the sampling layer (average variability of 63% relative to sample average) – not uranium (23% variability) or by extension radiogenic Pb – is a main factor controlling the resulting isochron's quality. This is demonstrated using Z-score distributions of the U and Pb concentrations and isochron average distance values. By making this distinction we hope to initiate further research into Pb-specific transportation vectors through the karst, in addition to physical/chemical processes that fractionate inherited Pb at the speleothem precipitation site. We go on to extend the use of Tera-Wasserburg common Pb anchors to the ‘classical’ U-Pb isochrons as an example of the utility gained by considering the role of inherited Pb within speleothems. Finally we argue that an improved theoretical understanding of inherited Pb distributions within speleothems would greatly benefit solution mode sampling strategies, especially for poor-to-moderate U/Pb speleothems.     

神农架石笋记录的近200年东亚季风变化及其ENSO响应

通过湖北省神农架永兴洞石笋(编号:YX92)230Th年龄和纹层计数年代学研究,建立了1788AD-1956AD时段年际分辨率石笋氧同位素序列。该序列δ18O值变化范围为-9.70‰~-7.87‰,最大振幅约达1.83‰,呈现显著的年代际尺度振荡旋回特征。功率谱分析结果揭示存在显著的60~70年准周期信号。该记录与同区域和尚洞石笋δ18O曲线具有很好的重现性,说明其可能受控于共同的气候因子,指示了东亚夏季风大尺度环流特征。将石笋重建的东亚夏季风指标与全印度夏季降水量指示的印度夏季风指数对比,结果显示两者在年代际旋回特征上具有很好的相似性,但是呈反相位关系。进一步与ENSO指数对比,发现东亚夏季风年代际尺度变化与ENSO存在密切联系,表现为东亚夏季风增强,对应于ENSO暖期,反之亦然。因此,东亚季风与印度季风在年代际尺度上的反相位关系可能源于两者对ENSO机制的不同响应。     

Condensation water in heritage touristic caves: Isotopic and hydrochemical data and a new approach for its quantification through image analysis

Condensation water is a major factor in the conservation of heritage caves. It can cause dissolution of the rock substrate (and the pigments of rock art drawn on it) or covering thereof with mineral components, depending on the chemical saturation degree of the condensation water. In show caves, visitors act as a source of CO₂ and thus modify the microclimate, favouring negative processes that affect the conservation of the caves. In spite of their interest, studies of the chemical composition of this type of water are scarce and not very detailed. In this work we present research on the condensation water in the Nerja Cave, one of the main heritage and tourist caves in Europe. The joint analysis of isotopic, hydrochemical, mineralogical and microbiological data and the use of image analysis have allowed us to advance in the knowledge of this risk factor for the conservation of heritage caves, and to demonstrate the usefulness of image analysis to quantify the scope of the possible corrosion condensation process that the condensation water could be producing on the bedrock, speleothem and rock art. To our knowledge, this application of image analysis (relative to the condensation water in caves) is the first one of this type that has been documented.     

Origin of double‐tower raft cones in hypogenic caves

In the present paper, we describe the genetic mechanism that causes the precipitation of raft cones in caves. These speleothems usually form in a hydrothermal and epiphreatic environment where dripwater, dripping repeatedly over the same spot, sinks calcite rafts that were floating on the water surface of a cave pool. In particular, the paper describes a new variety of raft cones that were recently discovered in the Paradise Chamber of the Sima de la Higuera Cave (Murcia, south‐eastern Spain) based on their morphological and morphometric characteristics. These speleothems, dubbed ‘double‐tower cones’, have a notch in the middle and look like two cones, one superimposed over the other. The genetic mechanism that gave rise to the double‐tower cones must include an intermediate stage of rapid calcite raft precipitation, caused by a drop in the water table and by changes in cave ventilation leading to greater carbon dioxide (CO₂) degassing and evaporation over the surface of the thermal lake where these speleothems formed. Calcite rafts were deposited in Paradise Chamber, completely covering many of the cones. Later, conditions for slower calcite raft precipitation were restored and some of the cones continued to grow at the same points. When the water table finally fell below the level of Paradise Chamber, the tower cones became exposed, as the incongruent deposits of calcite rafts were dissolved and mobilized to lower cave levels. Copyright © 2013 John Wiley & Sons, Ltd.     

Hourglass magnetic field from a survey of current density profiles

Modeling the magnetic field in prestellar cores can serve as a useful tool for studying the initial conditions of star formation. The analytic hourglass model of Ewertowski and Basu (2013) provides a means to fit observed polarimetry measurements and extract useful information. The original model does not specify any radial distribution of the electric current density. Here, we perform a survey of possible centrally-peaked radial distributions of the current density, and numerically derive the full hourglass patterns. Since the vertical distribution is also specified in the original model, we can study the effect of different ratios of vertical to radial scale length on the overall hourglass pattern. Different values of this ratio may correspond to different formation scenarios for prestellar cores. We demonstrate the flexibility of our model and how it can be applied to a variety of magnetic field patterns.     

Bayesian age-depth modelling of Late Quaternary deposits from Wet and Blanche Caves,Naracoorte, South Australia: A framework for comparative faunal analyses

Bayesian age-depth models were constructed for two Late Quaternary aged fossil-bearing sedimentary sequences from caves in south eastern South Australia. The deposits in Wet and Blanche Caves contain dense assemblages of vertebrate fossils, largely the result of owl pellet accumulation. While individually calibrated radiocarbon determinations from the fossil sequences have provided a chronology for their accumulation, there was limited capacity available with such data to (a) temporally constrain assemblages associated with different depositional units and layers within the two sites, (b) interpret the chronological relationships among successive units and layers and (c) correlate sedimentary units and layers of similar age between the two deposits. Here, Bayesian age-depth models were constructed in OxCal for the Wet and Blanche Cave sequences, incorporating the available radiocarbon data and stratigraphic information collected during their excavation. Despite the low precision of the age-depth models for Wet and Blanche Caves which results in part from there being only single radiocarbon determinations available for a number of units and layers, the models enabled the relationships within and between the two sites to be established. Of particular utility for future faunal analyses is quantification of the temporal relationship between strata from the two sites, where groups of individual layers from Blanche Cave were found to be temporally equivalent with the longer-duration units in Wet Cave. We suggest that the use of Phase modelling, as performed here, is useful for cave deposits that have complex depositional histories and even in such instances where, as is common for palaeontological sites, few radiocarbon data are collected relative to the time-spans of tens of millennia that are often represented by them.     

地质雷达在公路山体地质灾害调查中的应用

对临沧市南撒至班考二级公路山体地质灾害勘察中,地质雷达取得了较好的地质效果,确定地下岩溶发育区的空间展布规律、岩溶破碎带的形态及延伸,为进一步认识山体地质灾害及其勘察治理提供了地球物理依据。