On the reliability of stable carbon isotopes for Neoproterozoic chemostratigraphic correlation

The reliability of δ¹³C trends in Neoproterozoic carbonate-dominated successions for regional and global chemostratigraphic correlation is discussed. In the light of recent findings of a predominantly non-marine rare earth element and yttrium signature in most Neoproterozoic carbonates and a comparatively short oceanic residence time of carbon, trends towards enrichment in ¹³C seen in many of these carbonates are considered to reflect facies variations rather than temporal signals of ocean chemistry. Positive δ¹³C_Carb excursions are explained by elevated bioproductivity and/or increased evaporation in shallow marine, near-coastal, temporarily restricted depositional environments. Examples are provided that illustrate that C isotope trends can be highly ambiguous temporal markers and are in the absence of other chemostratigraphic data, such as Sr isotope ratios, and radiometric age control of only limited use for stratigraphic correlation. The overall enrichment in ¹³C recorded by most Neoproterozoic carbonates, except for those in close stratigraphic proximity to glacial deposits, is suggested to reflect a dominance of microbially mediated carbonate formation in the Neoproterozoic. This might explain why C isotope chemostratigraphy in Neoproterozoic successions is less reliable than in Phanerozoic successions in which carbonates are, with only few exceptions, biogenic products of shelly fossils.     

Rumuruti chondrites: Noble gases,exposure ages,pairing, and parent body history

Abstract— In this paper, we present concentration and isotopic composition of the light noble gases He, Ne, and Ar as well as of ⁸⁴Kr, ¹³²Xe, and ¹²⁹Xe in bulk samples of 33 Rumuruti (R) chondrites. Together with previously published data of six R chondrites, exposure ages are calculated and compared with those of ordinary chondrites. A number of pairings, especially between those from Northwest Africa (NWA), are suggested, so that only 23 individual falls are represented by the 39 R chondrites discussed here. Eleven of these meteorites, or almost 50%, contain solar gases and are thus regolithic breccias. This percentage is higher than that of ordinary chondrites, howardites, or aubrites. This may imply that the parent body of R chondrites has a relatively thick regolith. Concentrations of heavy noble gases, especially of Kr, are affected by the terrestrial atmospheric component, which resides in weathering products. Compared to ordinary chondrites, ¹²⁹Xe/¹³²Xe ratios of R chondrites are high.     

The case and fate of HD 75767 – neutron star or supernova?

We report the discovery of the nearby  ( d = 24 pc)  HD 75767 as an eight billion year old quadruple system consisting of a distant M dwarf pair, HD 75767 C–D, in orbit around the known short-period   P = 10.25 d  single-lined binary HD 75767 A–B, the primary of which is a solar-like G star. On the reasonable assumption of synchronous orbital rotation as well as rotational and orbital coplanarity for the inner pair, we get   M _B= 0.96 M_⊙  for the unseen HD 75767 B, that is, the case of a massive white dwarf. Upon future evolution, mass transfer towards HD 75767 B will render the   M _A= 0.96 M_⊙  G-type primary, now a turnoff star, to become a helium white dwarf of   M _A∼ 0.33 M_⊙  . Depending on the mass accretion rate, accretion efficiency and composition of the massive white dwarf, this in turn may result in a collapse of HD 75767 B with the formation of a millisecond pulsar, i.e. the creation of a low-mass binary pulsar (LMBP), or, instead, a Type Ia supernova explosion and the complete disruption of HD 75767 B. Irrespective of which scenario applies, we point to the importance of the distant M dwarfs as the likely agents for the formation of the inner, short-period HD 75767 A–B pair, and hence a path that particularly avoids preceding phases of common envelope evolution.     

Fault imaging in sparsely sampled 3D seismic data using common‐reflection‐surface processing and attribute analysis – a study in the Upper Rhine Graben

Cost reduction in seismic reconnaissance is an issue in geothermal exploration and can principally be achieved by sparse acquisition. To address the adherent decrease in signal/noise ratio, the common‐reflection‐surface method has been proposed. We reduced the data density of an existing 3D dataset and evaluated the results of common‐reflection‐surface processing using seismic attributes. The application of the common‐reflection‐surface method leads in all cases to an improvement of the signal/noise ratio. The most distinct improvement can be seen in the low fold regions. The improvement depends strongly on the midpoint aperture, and there is a tradeoff between reflector continuity and horizontal resolution. If small scale targets are to be imaged, a small aperture size is necessary, which may be far below the Fresnel zone for a specific reflector. The substantial reduction of the data density leads in our case to an irrecoverable information loss.     

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Seasonal oxygen depletion in Chesapeake Bay

The spring freshet increases density stratification in Chesapeake Bay and minimizes oxygen transfer from the surface to the deep layer so that waters below 10 m depth experiece oxygen depletion which may lead to anoxia during June to September. Respiration in the water of the deep layer is the major factor contributing to oxygen depletion. Benthic respiration seems secondary. Organic matter from the previous year which has settled into the deep layer during winter provides most of the oxygen demand but some new production in the surface layer may sink and thus supplement the organic matter accumulated in the deep layer.