Galactic rotation parameters from data on open star clusters

Currently available data on the field of velocities V _r , V _l , V _b for open star clusters are used to perform a kinematic analysis of various samples that differ by heliocentric distance, age, and membership in individual structures (the Orion, Carina-Sagittarius, and Perseus arms). Based on 375 clusters located within 5 kpc of the Sun with ages up to 1 Gyr, we have determined the Galactic rotation parameters ω ₀ = ?26.0 ± 0.3 km s^?1 kpc^?1, ω′₀ = 4.18 ± 0.17 km s^?1 kpc^?2, ω″₀ = ?0.45 ± 0.06 km s^?1 kpc^?3, the system contraction parameter K = ?2.4 ± 0.1 km s^?1 kpc^?1, and the parameters of the kinematic center R ₀ = 7.4 ± 0.3 kpc and l ₀ = 0° ± 1°. The Galactocentric distance R ₀ in the model used has been found to depend significantly on the sample age. Thus, for example, it is 9.5 ± 0.7 and 5.6 ± 0.3 kpc for the samples of young (≤50 Myr) and old (>50 Myr) clusters, respectively. Our study of the kinematics of young open star clusters in various spiral arms has shown that the kinematic parameters are similar to the parameters obtained from the entire sample for the Carina-Sagittarius and Perseus arms and differ significantly from them for the Orion arm. The contraction effect is shown to be typical of star clusters with various ages. It is most pronounced for clusters with a mean age of ≈100 Myr, with the contraction velocity being Kr = ?4.3 ± 1.0 km s^?1.     

Spatial and Temporal Dynamics of Sources and Water Regime of the Ugol’naya-Dionisiya River (Anadyr Lowland,Chukotka)

Water Resources - For the first time, by analyzing the spatial variations and diurnal dynamics of the water regime of the small river of the Anadyr Lowland, the main sources were studied and the...     

Investigating the complex interface where bedrock transforms to regolith

The interface where bedrock transforms to regolith is not planar but rather has a roughness that varies with the scale of observation. The complexity of this surface is manifested in both element-depth and fragment size-depth distributions and may sometimes be related to the longitudinal profiles of watershed streams. The fractal nature of the bedrock-regolith interface means that the interface has a “thickness” which is >20 m in two ridgetop examples from Pennsylvania and Puerto Rico. Such weathering thicknesses, modeled as a function of one-dimensional fluid flow, are affected by the balance between rates of weathering and erosion. One-dimensional models are consistent with weathering advance rates that vary with equilibrium solubility and porefluid velocities (and not reaction kinetics). However, fluid flow is not strictly downward and one-dimensional. Permeability of regolith changes as particle size and bulk density changes with depth. Thus, both downward and lateral flow occurs especially at reaction fronts where reactions change permeability. The rate of weathering advance is, therefore, affected by the 3-dimensional distribution of reaction zones that affect permeability across the watershed. Quantitative models of such phenomena over a range of spatial and temporal scales are needed.     

Influence of sieving time on the efficiency and accuracy of grain‐size analysis of beach and dune sands

Grain size is a fundamental property of sediments and is commonly used to describe sedimentary facies and classify sedimentary environments. Among the various conventional techniques utilized to determine grain‐size frequency distributions, sieving is the most widely applied procedure. The accuracy of such analyses is, among other factors, strongly dependent on the sieving time. However, despite a substantial amount of research in this field, optimal sieving times for different types of sediments have, to date, not been established. In this article, the influence of sieving time on grain‐size analyses of medium‐grained microtidal and mesotidal beach and dune sands has been determined. To assess the precision of important textural parameters, such as median grain size, sorting, skewness and kurtosis, an error analysis was carried out for different sieving times (2, 5, 10, 15 and 20 minutes). After calibrating the analytical and sampling methodologies, significant deviations were registered when sieving time was less than 10 minutes. However, such deviations were very small and grain‐size distributions remained almost identical for sieving times of 10 minutes and longer, relative errors being as low as 0% in some cases.     

Khatyrka,a new CV3 find from the Koryak Mountains,Eastern Russia

A new meteorite find, named Khatyrka, was recovered from eastern Siberia as a result of a search for naturally occurring quasicrystals. The meteorite occurs as clastic grains within postglacial clay‐rich layers along the banks of a small stream in the Koryak Mountains, Chukotka Autonomous Okrug of far eastern Russia. Some of the grains are clearly chondritic and contain Type IA porphyritic olivine chondrules enclosed in matrices that have the characteristic platy olivine texture, matrix olivine composition, and mineralogy (olivine, pentlandite, nickel‐rich iron‐nickel metal, nepheline, and calcic pyroxene [diopside‐hedenbergite solid solution]) of oxidized‐subgroup CV3 chondrites. A few grains are fine‐grained spinel‐rich calcium‐aluminum‐rich inclusions with mineral oxygen isotopic compositions again typical of such objects in CV3 chondrites. The chondritic and CAI grains contain small fragments of metallic copper‐aluminum‐iron alloys that include the quasicrystalline phase icosahedrite. One grain is an achondritic intergrowth of Cu‐Al metal alloys and forsteritic olivine ± diopsidic pyroxene, both of which have meteoritic (CV3‐like) oxygen isotopic compositions. Finally, some grains consist almost entirely of metallic alloys of aluminum + copper ± iron. The Cu‐Al‐Fe metal alloys and the alloy‐bearing achondrite clast are interpreted to be an accretionary component of what otherwise is a fairly normal CV3 (oxidized) chondrite. This association of CV3 chondritic grains with metallic copper‐aluminum alloys makes Khatyrka a unique meteorite, perhaps best described as a complex CV3 (ox) breccia.     

Comments on “Confirmation of beach accretion by grain-size trend analysis: Camposoto beach, Cádiz, SW Spain” by E. Poizot et al. (2013) Geo-Marine Letters 33(4)

This Discussion provides comments on the application of grain-size trend analysis to Camposoto beach (SW Spain) reported by Poizot et al. (2013) in Geo-Marine Letters 33(4):263–272. Some of their results are updated or complemented by existing data from other studies carried out on Camposoto and other nearby beaches. For example, a detailed breakdown of beach nourishment volumes and costs is presented, and the influence of a horizontal reef flat on the tilting of the beach profile around the mean or the low water level is highlighted. Moreover, data from the displacement of dyed samples are used to evaluate the relationship between sediment transport speed (v_a) and current speed (V), the corresponding ratio being consistent with the range of values reported by several other authors. Finally, some minor, although still significant, differences are detected in some granulometric parameters as well as in the profile shape. Determining the reason for these discrepancies could enhance our current knowledge about the factors controlling short-term beach profile responses.     

Seasonal variations in methane concentrations and diffusive fluxes in the Curonian and Vistula lagoons,Baltic Sea

Expected seasonal variations in methane concentrations and diffusive fluxes from surficial sediments into near-bottom waters were investigated in autumn 2012 and winter 2013 in the Curonian and Vistula lagoons of the Baltic Sea, expanding on earlier findings for summer 2011. Methane concentrations in bottom sediments (upper ca. 2 cm) generally ranged from ca. 1 to 1,000 μmol/dm³, and in near-bottom waters from ca. 0 to 1 μmol/l. Highest concentrations were found in the Curonian Lagoon, plausibly explained by the influence of freshwater conditions and finer-grained, organic-rich sediments. Vistula Lagoon methane concentrations and fluxes are dampened by periodic saline water inflow from the open sea, intensifying sulphate reduction. Calculated diffusive methane fluxes from the upper sediment layer (usually 0–5 cm, i.e. excluding any fluffy layer) into near-bottom waters were highest—2.48 mmol/(m² day)—in clayey silts of the Curonian Lagoon in autumn (September) 2012, contrasting strongly with the minimum value of 0.002 mmol/(m² day) observed there in February 2013 under ice-covered conditions. Seasonal and even weekly variations in methane dynamics can be largely explained by two main drivers, i.e. wind and temperature, operating at various spatiotemporal scales via, for example, wind wave-induced resuspension of bottom sediments, and involving regional weather patterns including autumnal low-pressure zones over the Gulf of Gdansk.     

Architecture,structural and tectonic significance of the Seagap fault (offshore Tanzania) in the framework of the East African Rift

The Southeastern portion of the East African Rift System reactivates Mesozoic transform faults marking the separation of Madagascar from Africa in the Western Indian Ocean. Earlier studies noted the reactivation of the Davie Fracture Zone in oceanic lithosphere as a seismically active extensional fault, and new 3D seismic reflection data and exploration wells provide unprecedented detail on the kinematics of the sub-parallel Seagap fault zone in continental/transitional crust landward of the ocean-continent transition. We reconstruct the evolution of the seismically active Seagap fault zone, a 400-km-long crustal structure affecting the Tanzania margin, from the late Eocene to the present day. The Seagap fault zone is represented by large-scale localized structures affecting the seafloor and displaying growth geometries across most of the Miocene sediments. The continuous tectonic activity evident by our seismic mapping, as well as 2D deep seismic data from literature, suggests that from the Middle-Late Jurassic until 125 Ma, the Seagap fault acted as a regional structure parallel to, and coeval with, the dextral Davie Fracture Zone. The Seagap fault then remained active after the cessation of both seafloor spreading in the Somali basin and strike-slip activity on the Davie Fracture Zone, till nowaday. Its architecture is structurally expressed through the sequence of releasing and restraining bends dating back at least to the early Neogene. Seismic sections and horizon maps indicate that those restraining bends are generated by strike-slip reactivation of Cretaceous structures till the Miocene. Finally based on the interpretation of edge-enhanced reflection seismic surfaces and seafloor data, we shows that, by the late Neogene, the Seagap fault zone switched to normal fault behaviour. We discuss the Seagap fault's geological and kinematic significance through time and its current role within the microplate system in the framework of the East African rift, as well as implications for the evolution and re-activation of structures along sheared margins. The newly integrated datasets reveal the polyphase deformation of this margin, highlighting its complex evolution and the implications for depositional fairways and structural trap and seal changes through time, as well as potential hazards.     

Where fast weathering creates thin regolith and slow weathering creates thick regolith

Weathering disaggregates rock into regolith – the fractured or granular earth material that sustains life on the continental land surface. Here, we investigate what controls the depth of regolith formed on ridges of two rock compositions with similar initial porosities in Virginia (USA). A priori, we predicted that the regolith on diabase would be thicker than on granite because the dominant mineral (feldspar) in the diabase weathers faster than its granitic counterpart. However, weathering advanced 20× deeper into the granite than the diabase. The 20 × ‐thicker regolith is attributed mainly to connected micron‐sized pores, microfractures formed around oxidizing biotite at 20 m depth, and the lower iron (Fe) content in the felsic rock. Such porosity allows pervasive advection and deep oxidation in the granite. These observations may explain why regolith worldwide is thicker on felsic compared to mafic rock under similar conditions. To understand regolith formation will require better understanding of such deep oxidation reactions and how they impact fluid flow during weathering. Copyright © 2012 John Wiley & Sons, Ltd.