Toward a radiometric ice clock: uranium ages of the Dome C ice core

Ice sheets and deep ice cores have yielded a wealth of paleoclimate information based on continuous dating methods while independent radiometric ages of ice have remained elusive. Here we demonstrate the application of (²³⁴U/²³⁸U) measurements to dating the EPICA Dome C ice core based on the accumulation of ²³⁴U in the ice matrix from recoil during ²³⁸U decay out of dust bound within the ice. Measured (²³⁴U/²³⁸U) activity ratios within the ice generally increase with depth while the surface areas of the dust grains are relatively constant. Using a newly designed device for measuring surface area for small samples, we were able to estimate reliably the recoil efficiency of nuclides from dust to ice. The resulting calculated radiometric ages range between 80 ka and 870 ka. Measured samples in the upper 3100 m fall on the previously published age-depth profile. Samples in the 3200–3255 m section show a marked change from 723–870 ka to 85 ka indicating homogenization of the deep ice prior to resetting of the (²³⁴U/²³⁸U) age in the basal layers. The mechanism for homogenization is likely enhanced lateral ice flow due to high basal melting and geothermal heat flux.     

Sedimentology of the December 26, 2004, Sumatra tsunami deposits in eastern India (Tamil Nadu) and Kenya

The December 26, 2004 Sumatra tsunami caused severe damage at the coasts of the Indian ocean. We report results of a sedimentological study of tsunami run-up parameters and the sediments laid down by the tsunami at the coast of Tamil Nadu, India, and between Malindi and Lamu, Kenya. In India, evidence of three tsunami waves is preserved on the beaches in the form of characteristic debris accumulations. We measured the maximum run-up distance at 580 m and the maximum run-up height at 4.85 m. Flow depth over land was at least 3.5 m. The tsunami deposited an up to 30 cm thick blanket of moderately well to well-sorted coarse and medium sand that overlies older beach deposits or soil with an erosional unconformity. The sand sheet thins inland without a decrease of grain-size. The deposits consist frequently of three layers. The lower one may be cross-bedded with foresets dipping landward and indicating deposition during run-up. The overlying two sand layers are graded or parallel-laminated without indicators of current directions. Thus, it remains undecided whether they formed during run-up or return flow. Thin dark laminae rich in heavy minerals frequently mark the contacts between successive layers. Benthic foraminifera indicate an entrainment of sediment by the tsunami from water depths less than ca. 30 m water depth. On the Indian shelf these depths are present at distances of up to 5 km from the coast. In Kenya only one wave is recorded, which attained a run-up height of 3 m at a run-up distance of ca. 35 m from the tidal water line at the time of the tsunami impact. Only one layer of fine sand was deposited by the tsunami. It consists predominantly of heavy minerals supplied to the sea by a nearby river. The sand layer thins landward with a minor decrease in grain-size. Benthic foraminifera indicate an entrainment of sediment by the tsunami from water depths less than ca. 30 m water depth, reaching down potentially to ca. 80 m. The presence of only one tsunami-related sediment layer in Kenya, but three in India, reflects the impact of only one wave at the coast of Kenya, as opposed to several in India. Grain-size distributions in the Indian and Kenyan deposits are mostly normal to slightly positively skewed and indicate that the detritus was entrained by the tsunami from well sorted pre-tsunami deposits in nearshore, swash zone and beach environments.     

Microthermometry, Raman and LA-ICP-MS of Fluid Inclusions Constrain on Fluids Chemistry and Evolution of Carlin-Type Gold System, Guizhou, China: Implications for Ore Genesis

The Shuiyindong and Yata Carlin-type gold deposits, located in Southwestern Guizhou, China, are hosted by Permianbioclastic limestone in the form of stratabound mineralization and Middle Triassic calcareous clastic rocks as fault-controlled mineralization, respectively. In these deposits, quartz crystals in the veins or veinlets associated with mineralization have contained several populations of fluid inclusions with relatively quite big size and clear paragenetic relationship of entrapment. Petrography, microthermometry, Raman and LA-ICPMS of fluid inclusions analyses are used to characterize fluids chemistry and their evolution of Carlin-type system in Guizhou.     

Anomalous passive subsidence of deep‐water sedimentary basins: a prearc basin example,southern New Caledonia Trough and Taranaki Basin,New Zealand

Stratigraphic data from petroleum wells and seismic reflection analysis reveal two distinct episodes of subsidence in the southern New Caledonia Trough and deep‐water Taranaki Basin. Tectonic subsidence of ~2.5 km was related to Cretaceous rift faulting and post‐rift thermal subsidence, and ~1.5 km of anomalous passive tectonic subsidence occurred during Cenozoic time. Pure‐shear stretching by factors of up to 2 is estimated for the first phase of subsidence from the exponential decay of post‐rift subsidence. The second subsidence event occured ~40 Ma after rifting ceased, and was not associated with faulting in the upper crust. Eocene subsidence patterns indicate northward tilting of the basin, followed by rapid regional subsidence during the Oligocene and Early Miocene. The resulting basin is 300–500 km wide and over 2000 km long, includes part of Taranaki Basin, and is not easily explained by any classic model of lithosphere deformation or cooling. The spatial scale of the basin, paucity of Cenozoic crustal faulting, and magnitudes of subsidence suggest a regional process that acted from below, probably originating within the upper mantle. This process was likely associated with inception of nearby Australia‐Pacific plate convergence, which ultimately formed the Tonga‐Kermadec subduction zone. Our study demonstrates that shallow‐water environments persisted for longer and their associated sedimentary sequences are hence thicker than would be predicted by any rift basin model that produces such large values of subsidence and an equivalent water depth. We suggest that convective processes within the upper mantle can influence the sedimentary facies distribution and thermal architecture of deep‐water basins, and that not all deep‐water basins are simply the evolved products of the same processes that produce shallow‐water sedimentary basins. This may be particularly true during the inception of subduction zones, and we suggest the term ‘prearc’ basin to describe this tectonic setting.     

Ein Beitrag zur Geochemie und Entstehung der Oberpfälzischen Kaolin-Feldspat-Lagerstätten

The plagioclase of the kaolinised granite of Tirschenreuth is decomposed quantitatively and the biotite almost quantitatively, on the contrary potassium feldspar and mucovite of the parent rock remained unaffected by kaolinisation. The quantity of produced kaolinite is equivalent to the quantity of decomposed plagioclase and biotite. The rare elements Pb, Cu, Cr, Ni, P, and Ti from the kaolinised minerals are adsorbed quantitatively by the kaolin. All other elements are diminished, no element has been added to the kaolin from other sources than from parent rock material. The selective decomposition of plagioclase and biotite in the kaolin-feldspar deposit of Tirschenreuth and the association of rare elements in the kaolin are explainable by weathering processes only. Kaolins from Hirschau-Sehnaittenbach are characterised by extremely high concentrations of the rare elements Ba, Sr, Pb, Cu, and P. The association of rare elements of the kaolin has been originated from the decomposed potassium feldspar of the kaolinised arcoses. The rare elements Pb, Cu, Cr, and P are adsorbed quantitatively by the kaolin. The other elements are diminished, Ba and Sr inclusive and in spite of their high absolute concentrations. The composition of the arcoses of Hirschau-Schnaittenbach prior to kaolinisation is calculated to 56% of quartz and 44% of potassium feldspar by means of the quantities of the elements Pb and Cu analysed in the feldspar and in the kaolin. The kaolin of Hirschau-Schnaittenbach has originated by weathering of the potassium feldspar of the Triassic arcoses. The kaolinisation is of Triassic age because kaolins of the Hirschau-Schnaittenbach type were redeposited near Ehenfeld in the Cenomanian. Kaolin deposits of hydrothermal origin are characterised by a zonal structure and are associated by ores. They are different in geological behaviour and mineral association from kaolin deposits which originated by weathering of feldspathic rocks like the deposits of Tirschenreuth and Hirschau-Schnaittenbach (Upper Palatinate, Bavaria).     

Computer simulation of crystal structures

It is impossible to calculate from first principles the details of crystal structures. However, it is possible to rely on information which is certain, namely on interatomic distances, and calculate the details of a crystal structure based on these. Individual distances between atoms can be predicted accurately if one considers the coordination numbers of anions and cations, the extended electrostatic valence rule, and the effects of shared edges between different coordination polyhedra. The predicted interatomic distances are used as observations in a distance least squares refinement in which the positional parameters and the cell edges are adjusted until the calculated interatomic distances correspond as closely as possible to the predicted distances. The topology of the simulated structure has to be known or assumed. Appropriate weighting can facilitate proper modelling. Applications include: a) solution of pseudosymmetric structures; b) preliminary refinement of trial structures; c) geometric refinement without X-ray intensities; d) comparison of hypothetical structures with observed polymorphs of the same compositions; e) simulation of temperature dependence of structures; f) simulation of pressure dependence; g) calculation of structures isomorphous to known structures; h) calculation of thermal ellipsoids; i) calculation of local environments deviating from the overall symmetry of a structure; j) testing of hypotheses about the behavior of structures at varying conditions.     

Rigorous computation of proper motions and their effects on star positions

New rigorous formulas are given for the computation of the effects of proper motions and radial velocities on star positions, and for the transformation of proper motion components and radial velocities from one epoch to another. These expressions depend explicitly only on the values of the star's coordinates and distance, proper motion components and radial velocity, at the initial epoch.