Rb-Sr whole-rock dating of late diagenetic to anchimetamorphic,palaeozoic sediments in southern France (Montagne Noire)

Rb-Sr whole-rock and mineral analyses are reported from Early Paleozoic sediments of the southern rim of the French Central Massif (Montagne Noire). The sediments—mainly pelites, silt- and sandstones—can be separated into three different zones (A, B, and C) according to their varying degrees of deformation and transformation, which took place during the Hercynian (= Variscan) orogeny (Carboniferous). For the least deformed, folded but not schistose, fossiliferous Lower Ordovician sediments (zone A), a scatter of the data points resulted, at least partly due to the varying amount of detrital muscovites, which did not equilibrate their Sr isotopes with those of the clay minerals. Fossiliferous Middle Cambrian sediments (zone B), however, are colinear in an isochron plot. The rocks are also folded, and show a beginning schistosity in the pelitic layers. The age of 333±16 m.y. therefore does not correspond to the time of sedimentation, as one would expect from most of the interpretations given in the literature. It is, however, in close agreement with the stratigraphically known time of folding and anchimetamorphism. The same result, within analytical error, of 326±17 m.y. was obtained for Lower Ordovician sediments (zone C), which show a schistosity not only in the pelites but also in some psammitic layers. As for the sediments of zone B, this result indicates open whole-rock systems for Rb and/or Sr during folding and anchimetamorphism. Minerals extracted from psammites of zones B and C gave the following results: albite coincides with the isochrons; detrital muscovites do not coincide and yield ages of about 650 m.y. (Cadomian orogeny) for both zone B and C sediments; detrital chlorites from zone B sediments seem to be only partly reset by anchimetamorphism and therefore plot closer to the whole-rock isochron. From this it must be concluded that an increasing amount of these detrital micas could significantly shift the position of the total-rock points, as might be the case for zone A sediments. From the whole-rock results of zone B and C sediments and the mineralogical and petrological changes during diagenesis and anchimetamorphism we conclude that many of the isochrons obtained from stratigraphically uncontrollable Precambrian sediments should be reinterpreted in favour of metamorphic and not sedimentary ages, especially if the metamorphism reached greenschist facies conditions. Devonian and Cambrian limestones intercalated between the pelites, silstones and sandstones contain Sr of a variable and slightly radiogenic isotopic composition in contrast to the expected ancient sea water Sr. This fact is interpreted again in favour of open whole-rock systems during the postdepositional history of the rock, rather than primary scatter of ancient, radiogenic sea water Sr.     

Noble gas investigations of lunar rocks 10017 and 10071

The noble gases He, Ne, Ar, Kr and Xe and also K and Ba were measured in the Apollo 11 igneous rocks 10017 and 10071, and in an ilmenite and two feldspar concentrates separated from rock 10071. Whole rock K/Ar ages of rocks 10017 and 10071 are (2350 ± 60) × 10⁶ yr and (2880 ± 60) × 10⁶ yr, respectively. The two feldspar concentrates of rock 10071 have distinctly higher ages: (3260 ± 60) × 10⁶ yr and (3350 ± 70) × 10⁶ yr. These ages are still 10 per cent lower than the Rb/Sr age obtained by Papanastassiouet al. (1970) and some Ar⁴⁰ diffusion loss must have occurred even in the relatively coarse-grained feldspar.The relative abundance patterns of spallation Ne, Ar, Kr and Xe are in agreement with the ratios predicted from meteoritic production rates. However, diffusion loss of spallation He³ is evident in the whole rock samples, and even more in the feldspar concentrates. The ilmenite shows little or no diffusion loss. The isotopic composition of spallation Kr and Xe is similar to the one observed in meteorites. Small, systematic differences in the spallation Kr spectra of rocks 10017 and 10071 are due to variations in the irradiation hardness (shielding). The Kr spallation spectra in the mineral concentrates are different from the whole rock spectra and also show individual variations, reflecting the differences in target element composition. The relative abundance of cosmic ray produced Xe¹³¹ differs by nearly 50 per cent in the two rocks. The other Xe isotopes show no variations of similar magnitude. The origin of the Xe¹³¹ yield variability is discussed.Kr⁸¹ was measured in all the samples investigated. The Kr⁸¹/Kr exposure ages of rocks 10017 and 10071 are (480 ± 25) × 10⁶ yr and (350 ± 15) × 10⁶ yr, respectively. Exposure ages derived from spallation Ne²¹, Ar³⁸, Kr⁸³ and Xe¹²⁶ are essentially in agreement with the Kr⁸¹/Kr ages. The age of rock 10071 might be somewhat low because of a possible recent exposure of our sample to solar flare particles.     

Hydrodynamical simulations of viscous overstability in Saturn’s rings

We perform axisymmetric hydrodynamical simulations that describe the nonlinear outcome of the viscous overstability in dense planetary rings. These simulations are particularly relevant for Cassini observations of fine-scale structure in Saturn’s A and B-ring, which take the form of periodic microstructure on the 0.1 km scale, and irregular larger-scale variations on 1-10 km. Nonlinear wavetrains dominate all the simulations, and we associate them with the observed periodic microstructure. The waves can undergo small chaotic fluctuations in their phase and amplitude, and may be punctuated by more formidable ‘wave defects’ distributed on longer scales. It is unclear, however, whether the defects are connected to the irregular larger-scale variations observed by Cassini. The long-term behaviour of the simulations is dominated by the imposed boundary conditions, and more generally by the limitations of the local model we use: the shearing box. When periodic boundary conditions are imposed, the system eventually settles on a uniform travelling wave of a predictable wavelength, while reflecting boundaries, and boundaries with buffer zones, maintain a disordered saturated state. The simulations omit self-gravity, though we examine its influence in future work.     

Parasitic barnacles (Cirripedia: Rhizocephala) from New Zealand off‐shore waters

Nine genera and species of rhizocephalans were recorded from the off‐shore waters around New Zealand. Mitochondrial and nuclear gene sequences were used to examine base differences between the European and New Zealand species of Parthenopea. Serial sections to study the internal structures of the reproductive organs were made for the genera Thylacoplethus and Thompsonia. Two species, Parthenopea australis n. sp. and Thylacoplethus novaezealandiae n. sp. are new to science and described in detail. Parthenopea australis n. sp. is the first rhizocephalan species recorded from the vicinity of active cold seeps. Three rhizocephalans could not be identified as they were parasitised by hyperparasitic cryptoniscine isopods. The decapodhost species comprised the taxa Paguridae, Lithodidae, Galatheidae, Chirostylidae, and Callianassidae.     

The challenge of dating early pleistocene fossil teeth by the combined uranium series–electron spin resonance method: the Venta Micena palaeontological site (Orce,Spain)

The palaeontological site of Venta Micena (Orce, Andalusia, Spain) lies in the eastern sector of the Guadix–Baza basin, one of the best documented areas in Europe for Plio‐Pleistocene biostratigraphy. The combination of biochronological and palaeomagnetic results, combined with the radiometric data obtained for Atapuerca Sima del Elefante, indicated that the Venta Micena stratum was formed between the Jaramillo and Olduvai palaeomagnetic events, most likely between 1.22 and 1.77 Ma. Five fossil teeth from two outcrops (sites A and B) were selected to assess the potential of combined uranium series–electron spin resonance (US‐ESR) dating of Early Pleistocene sites. Although the US‐ESR results of the first outcrop showed a large scatter between the three teeth, the mean age of 1.37 ± 0.24 Ma can be considered a reasonable age estimate for Venta Micena. The mean ESR age of 0.62 ± 0.03 Ma obtained for site B seems to be a severe underestimation when compared with the independent age control. This underestimation is attributed to a relative recent U‐mobilization event that led to some U‐leaching. The results show that any ESR age calculations of old samples are extremely sensitive to variations in the measured ²³⁰Th/²³⁴U ratios in dental tissues. Although the results demonstrate that ESR can in principle be applied to Early Pleistocene sites, they also reveal the complexity of dating such old teeth. It is necessary to continue research in several directions, such as study of the behaviour of ESR signals in old teeth and understanding recent U‐mobilization processes, to improve the reliability of the combined US‐ESR dating method applied to Early Pleistocene times, a period for which the number of available numerical dating techniques is very limited. Copyright © 2011 John Wiley & Sons, Ltd.     

Urban green commons: Insights on urban common property systems

The aim of this paper is to shed new light on urban common property systems. We deal with urban commons in relation to urban green-space management, referring to them as urban green commons. Applying a property-rights analytic perspective, we synthesize information on urban green commons from three case-study regions in Sweden, Germany, and South Africa, and elaborate on their role for biodiversity conservation in urban settings, with a focus on business sites. Cases cover both formally established types of urban green commons and bottom-up emerged community-managed habitats. As our review demonstrates, the right to actively manage urban green space is a key characteristic of urban green commons whether ownership to land is in the private, public, the club realm domain, or constitutes a hybrid of these. We discuss the important linkages among urban common property systems, social–ecological learning, and management of ecosystem services and biodiversity. Several benefits can be associated with urban green commons, such as a reduction of costs for ecosystem management and as designs for reconnecting city-inhabitants to the biosphere. The emergence of urban green commons appears closely linked to dealing with societal crises and for reorganizing cities; hence, they play a key role in transforming cities toward more socially and ecologically benign environments. While a range of political questions circumscribe the feasibility of urban green commons, we discuss their usefulness in management of different types of urban habitats, their political justification and limitation, their potential for improved biodiversity conservation, and conditions for their emergence. We conclude by postulating some general policy advice.     

Viscous overstability and eccentricity evolution in three-dimensional gaseous discs

We investigate the growth or decay rate of the fundamental mode of even symmetry in a viscous accretion disc. This mode occurs in eccentric discs and is known to be potentially overstable. We determine the vertical structure of the disc and its modes, treating radiative energy transport in the diffusion approximation. In the limit of very long radial wavelength, an analytical criterion for viscous overstability is obtained, which involves the effective shear and bulk viscosity, the adiabatic exponent, and the opacity law of the disc. This differs from the prediction of a two-dimensional model. On shorter wavelengths (a few times the disc thickness), the criterion for overstability is more difficult to satisfy because of the different vertical structure of the mode. In a low-viscosity disc a third regime of intermediate wavelengths appears, in which the overstability is suppressed as the horizontal velocity perturbations develop significant vertical shear. We suggest that this effect determines the damping rate of eccentricity in protoplanetary discs, for which the long-wavelength analysis is inapplicable and overstability is unlikely to occur on any scale. In thinner accretion discs and in decretion discs around Be stars overstability may occur only on the longest wavelengths, leading to the preferential excitation of global eccentric modes.