Pollen- and oxygen-isotope analyses of late- and postglacial sediments from the Schwemm raised bog near Walchsee in Tirol, Austria

Oxygen-isotope and pollen analyses were carried out on late- and postglacial (Late Weichselian and Holo-cene) sediment samples of the raised bog. The ¹⁸O/¹⁶O-ratio results provide the first unequivocal proof for the Eastern Alps of the climatic deterioration of the Younger Dryas period. High NAP-values in the pollen diagram and a strong negative deviation in the oxygen-isotope curve characterize this period. The earlier Gerzensee fluctuation is also indicated by small changes in the oxygen-isotope curve. During the postglacial the Frosnitz climatic deterioration (6,000–6,500 B.P.) is evidenced as a fluctuation in the lake-level. The oxygen-isotope values indicate that the air temperature conditions remained unchanged at that time.     

RAYS AND WAVEFRONT CHARTS IN GRADIENT MEDIA1

Wavefront charts in anisotropic gradient media are a useful tool in ray geometric constructions, particular in shear-wave exploration. They can be constructed by: (i) a family of wavefronts that contains a vertical plane as member - it is convenient to choose constant time increments; (ii) tracing one ray that makes everywhere the angle with the normal to the wavefront that is required by the anisotropy of the medium; (iii) scaling this ray to obtain a set of rays with different ray parameters; (iv) shifting these rays (with wavefront elements attached) so that they pass through a common source point; (v) interpolating the wavefronts between the elements. The construction is particularly simple in linear-gradient media, since here all members of the family of wavefronts are planes. Since the ray makes everywhere the angle prescribed by the anisotropy with the normal of the (plane) wavefronts, the ray has the shape of the slowness curve rotated by ?π/2. For isotropic media the slowness curve is a circle, and thus rays are circular arcs. The circles themselves intersect in the source point and in a second point above the surface of the earth. This provides a simple proof that wavefronts emanating from a point source in an isotropic linear-gradient medium are spheres: inversion of the set of circular rays with the source as centre maps the pencil of circular rays into a pencil of straight lines passing through a point. A pencil of concentric spheres around this point is perpendicular to the pencil of straight lines. On inverting back the pencil of spheres is mapped into another pencil of spheres that is perpendicular to the circular rays.     

Lunar Pyroxene-Phyric Basalts: Crystallization Under Supercooled Conditions

Pyroxene-phyric basalts constitute a distinct petrologic groupin samples from lunar maria at both Apollo 12 and Apollo 15sites. Textures of pyroxene-phyric basalts from both sites aresimilar, but bulk compositions and mineralogy are somewhat different.Pyroxene-phyric basalts are characterized by large pyroxenephenocrysts with cores of magnesian pigeonite and mantles ofaugite grading to ferroaugite, usually set in a distinctly finergroundmass of iron-rich pyroxene, plagioclase, ilmenite, andother minor minerals. Olivine is scarce or absent modally andnormatively. Controversy has arisen over whether the porphyritictexture is a result of (1) a two-stage cooling history, e.g.phenocrysts formed at depth, and groundmass formed on extrusion;or (2) single stage, rapid cooling under supercooled conditions(cooling history here refers to cooling conditions imposed byexternal factors, and is not to be equated with ‘crystallizationhistory’). A study of six rocks belonging to this groupfrom Apollo 15 rake samples is reported here. A considerablerange of textures is present in these rocks, and they may beranked in order of decreasing late-stage cooling rate (15125,15666, 15682, 15118, 15684, 15116) on the basis of groundmasscrystal size. The same ranking is obtained from delta-beta (measuredin X-ray precession photos) of pigeonite and augite exsolvedfrom once homogeneous crystals, or of epitaxially overgrownaugite and pigeonite. The size of the phenocrysts in these rocks tends to be positivelycorrelated with the coarseness of the groundmass. Furthermorethe same correlation is evidently present in almost all otherApollo 12 and Apollo 15 pyroxene-phyric rocks. This constitutesa strong argument in favor of a single-stage cooling historyfor all pyroxene-phyric rocks, because the correlation wouldbe fortuitous for a two-stage cooling history. On the basisof this and many other arguments advanced previously and inthis paper, it is concluded that all the pyroxene-phyric rocksoriginated in a single-stage cooling process. A crystallization model for pyroxene-phyric rocks accounts forthe bimodal distribution of crystal size by two episodes ofsupercooling and nucleation during the continuous cooling process,the first of pyroxene, which nucleates homogeneously, the secondof plagioclase, which nucleates heterogeneously on the pyroxenephenocrysts. The more rapidly cooled rocks attained greaterdegrees of supercooling in both stages, hence greater nucleationrates and smaller crystals.     

Environmental change during the Allerød and Younger Dryas reconstructed from Swiss tree-ring data

Annually resolved tree-ring width variations and radiocarbon ages were measured from a collection of 120 Lateglacial pine stumps excavated on the Swiss Plateau. These data – representing the oldest absolutely dated wood samples worldwide – extend the absolute tree-ring chronology from Central Europe by 183 years back to 12 593 cal. yr BP (10 644 cal. yr BC). They also yield a 1420-year floating chronology covering the entire Allerød and the early Younger Dryas (14 170–12 750 cal. yr BP). Radiocarbon data suggest a 250-year jump in the ¹⁴C reservoir correction around the time of the Allerød to Younger Dryas transition, although calendric dating of the floating chronology – by filling a ∼150 year gap – is necessary for confirmation. Various subgroups, based on the year of germination, were used to assess temporal changes in growth characteristics along the Allerød to Younger Dryas transition. Comparison of these Lateglacial data with a reference data set of living and historic pines from the Swiss Valais (AD 940–2000) revealed differences in both growth trend and level. The generally slower Lateglacial growth was likely influenced by higher geomorphic activity and severe climatic conditions. After removal of the biological age-trend, a strong common signal found in the tree-ring data suggests some skill in estimating interannual to multidecadal Lateglacial climatic variations.     

The Kapoeta howardite: Implications for the regolith evolution of the howardite-eucrite-diogenite parent body

Abstract— A large hand sample and numerous polished thin sections, made from the hand sample, of the Kapoeta howardite and its many diverse lithic clasts were studied in detail by optical microscopy and electron microprobe techniques in an attempt to understand the surface processes that operated on the howardite-eucrite-diogenite (HED) parent body (most likely the asteroid 4 Vesta). Four unique, unusually large clasts, designated A (mafic breccia), B (granoblastic eucrite), D (howardite) and H (melt-coated breccia), were selected for detailed study (modal analysis, mineral microprobe analysis, and noble gas measurements). Petrographic studies reveal that Kapoeta consists of a fine-grained matrix made mostly of minute pyroxene and plagioclase fragments, into which are embedded numerous different lithic and mineral clasts of highly variable sizes. The lithic clasts include pyroxene-plagioclase (eucrite), orthopyroxenite (diogenite), howardite, impact-melt, metal-sulfide-rich, and carbonaceous chondrite clasts. The howardite clasts include examples of lithic clasts that constitute breccias-within-breccias, suggesting that at least two regolith generations are represented in the Kapoeta sample we studied. The clast assemblage suggests that repeated shock lithification was an important process during regolith evolution. Noble gas analyses of clast samples fall into two populations: (a) solar-gas-rich clasts H (rim only) and D and (b) clasts A and B, which are essentially free of solar gases. The concentrations of solar noble gases in the two matrix samples differ by a factor of ～40. It appears that clast D is a true regolith breccia within the Kapoeta howardite (breccia-within-breccia), while clast H is a regolith breccia that has been significantly impact reworked. Our data indicate that the Kapoeta howardite is an extraordinarily heterogeneous rock in modal mineral and lithic clast abundances, grain size distributions, solar-wind noble gas concentrations and cosmic-ray exposure ages. These results illustrate the repetitive nature of impact comminution and lithification in the regolith of the HED parent body.     

Late-glacial varved clays in the Bornholm Basin and Hanö Bay

Distribution, thickness, lithology and stratigraphy of varved clays are discussed. The sediments were deposited in the Baltic Ice Lake during the Late Weichselian.