A chemostratigraphic method to determine the end of impact‐related sedimentation at marine‐target impact craters (Chesapeake Bay,Lockne, Tvären)

Abstract– To better understand the impact cratering process and its environmental consequences at the local to global scale, it is important to know when in the geological record of an impact crater the impact‐related processes cease. In many instances, this occurs with the end of early crater modification, leaving an obvious sedimentological boundary between impactites and secular sediments. However, in marine‐target craters the transition from early crater collapse (i.e., water resurge) to postimpact sedimentation can appear gradual. With the a priori assumption that the reworked target materials of the resurge deposits have a different chemical composition to the secular sediments we use chemostratigraphy (δ¹³C_carb, %C_org, major elements) of sediments from the Chesapeake Bay, Lockne, and Tvären craters, to define this boundary. We show that the end of impact‐related sedimentation in these cases is fairly rapid, and does not necessarily coincide with a visual boundary (e.g., grain size shift). Therefore, in some cases, the boundary is more precisely determined by chemostratigraphy, especially carbonate carbon isotope variations, rather than by visual inspection. It is also shown how chemostratigraphy can confirm the age of marine‐target craters that were previously determined by biostratigraphy; by comparing postimpact carbon isotope trends with established regional trends.     

Cepheid parallaxes and the Hubble constant

Revised Hipparcos parallaxes for classical Cepheids are analysed together with 10 Hubble Space Telescope ( HST )-based parallaxes. In a reddening-free V , I relation we find that the coefficient of log  P is the same within the uncertainties in our Galaxy as in the Large Magellanic Cloud (LMC), contrary to some previous suggestions. Cepheids in the inner region of NGC 4258 with near solar metallicities confirm this result. We obtain a zero-point for the reddening-free relation and apply it to the Cepheids in galaxies used by Sandage et al. to calibrate the absolute magnitudes of Type Ia supernova (SNIa) and to derive the Hubble constant. We revise their result for H ₀ from 62 to 70 ± 5 km s⁻¹ Mpc⁻¹. The Freedman et al. value is revised from 72 to 76 ± 8 km s⁻¹ Mpc⁻¹. These results are insensitive to Cepheid metallicity corrections. The Cepheids in the inner region of NGC 4258 yield a modulus of 29.22 ± 0.03 (int.) compared with a maser-based modulus of 29.29 ± 0.15. Distance moduli for the LMC, uncorrected for any metallicity effects, are 18.52 ± 0.03 from a reddening-free relation in V , I ; 18.47 ± 0.03 from a period–luminosity relation at K ; 18.45 ± 0.04 from a period–luminosity–colour relation in J , K . Adopting a metallicity correction in V , I from Macri et al. leads to a true LMC modulus of 18.39 ± 0.05.     

Formulation of a unified constitutive model for clays and sands

This paper presents a new generalized effective stress model, referred to as MIT-S1, which is capable of predicting the rate independent, effective stress–strain–strength behaviour of uncemented soils over a wide range of confining pressures and densities. Freshly deposited sand specimens compressed from different initial formation densities approach a unique condition at high stress levels, referred to as the limiting compression curve (LCC), which is linear in a double logarithmic void ratio, e, mean effective stress space, p′. The model describes irrecoverable, plastic strains which develop throughout first loading using a simple four-parameter elasto-plastic model. The shear stiffness and strength properties of sands in the LCC regime can be normalized by the effective confining pressure and hence can be unified qualitatively, with the well-known behaviour of clays that are normally consolidated from a slurry condition along the virgin consolidation line (VCL). At lower confining pressures, the model characterizes the effects of formation density and fabric on the shear behaviour of sands through a number of key features: (a) void ratio is treated as a separate state variable in the incrementally linearized elasto-plastic formulation: (b) kinematic hardening describing the evolution of anisotropic stress–strain properties: (c) an aperture hardening function controls dilation as a function of ‘formation density’; and (d) the use of a single lemniscate-shaped yield surface with non-associated flow. These features enable the model to describe characteristic transitions from dilative to contractive shear response of sands as the confining pressure increases. This paper summarizes the procedures used to select input parameters for clays and sands, while a companion paper compares model predictions with measured data to illustrate the model capability for describing the shear behaviour of clays and sands. Copyright © 1999 John Wiley & Sons, Ltd.     

Variations of the neutralino elastic cross-section with CP-violating phases

We analyze the neutralino–nucleus elastic cross-section in the minimal supersymmetric extension of the standard model (MSSM), including contributions from CP-violating phases, using the four-fermi neutralino–quark interaction. Over a wide range of the MSSM parameter space, we show the variations in the cross-sections due to these phases. We further concentrate on the regions which are consistent with constraints from the electric dipole moment of the electron, neutron and mercury atom. In the regions we examine in detail, we find suppressions by up to a factor of two, while enhancements in the cross-sections are no greater than 10%.     

Venus cloud microphysics

Because sulfuric acid does not wet sulfur, composite drops in the atmosphere of Venus cannot have sulfur “cores,” but must instead have sulfur coats. Both components then communicate with the vapor phase. Drops that are fully coated with sulfur are immune to coalescence; this sets a limit to growth that may explain “Mode 3” particles. The sulfur coating is probably responsible for the anomalously low refractive indices derived from entry-probe nephelometer data. There appears to be about an order of magnitude less elemental sulfur than sulfuric acid in the clouds.     

Streambank dewatering for increased stability

Streambank erosion is often the dominant source of sediment leaving modified watersheds. Mass failure of high, steep banks is one of the most serious forms of streambank erosion. The risk of a given bank experiencing mass failure is a function of bank height, angle, and soil strength, which is governed by soil moisture. Two methods for bank dewatering were tested in adjacent sections of streambank bordering a deeply incised channel in northern Mississippi: a low‐cost pump system and subsurface horizontal drains. Pore water pressures (both positive and negative pressures, or matric suction) were continuously monitored for 2 years at the pumped site, at an adjacent untreated control section, and for 1 year at the site stabilized with horizontal drains. Resulting data were used to calculate a time series of the factor of safety using a computer model. Over the course of two wet seasons, average bank retreats for the control and pumped plots were 0·43 and 0·21 m, respectively. More limited monitoring revealed that the site with passive drains retreated about 0·23 m. At the pumped site pore water pressure was 3–4 kPa lower than at the control site during the most critical periods. Accordingly, computed factors of safety were above the failure threshold at the pumped site, but fell below unity at the control site on 11 occasions over the period of observation. Similarly, the drained site displayed generally lower pore water pressure and higher safety factors except for two events when drains were evidently overwhelmed with the volume of local surface and subsurface flows. These results suggest, but do not prove, that bank dewatering promoted lower rates of bank retreat and higher levels of stability since the three sites had slight differences in soils, geometry and boundary conditions. Initial cost of the dewatering treatments were significantly less than orthodox bank stabilization measures, but operation and maintenance requirements may be greater. Published in 2009 by John Wiley & Sons, Ltd.     

Near-infrared properties of i-drop galaxies in the Hubble Ultra Deep Field

We analyse near-infrared Hubble Space Telescope ( HST )/Near-Infrared Camera and Multi-Object Spectrometer F 110 W ( J ) and F 160 W ( H ) band photometry of a sample of 27 i '-drop candidate   z ≃ 6  galaxies in the central region of the HST /Advanced Camera for Surveys Ultra Deep Field . The infrared colours of the 20 objects not affected by near neighbours are consistent with a high-redshift interpretation. This suggests that the low-redshift contamination of this i '-drop sample is smaller than that observed at brighter magnitudes, where values of 10–40 per cent have been reported. The J – H colours are consistent with a slope flat in   f_ν ( f_λ ∝λ⁻²)  , as would be expected for an unreddened starburst. However, there is evidence for a marginally bluer spectral slope  ( f_λ ∝λ^−2.2)  , which is perhaps indicative of an extremely young starburst (∼10 Myr old) or a top heavy initial mass function and little dust. The low levels of contamination, median photometric redshift of   z ∼ 6.0  and blue spectral slope, inferred using the near-infrared data, support the validity of the assumptions in our earlier work in estimating the star formation rates, and that the majority of the i -drop candidates galaxies lie at   z ∼ 6  .     

Space weathering of small Koronis family members

The space weathering process and its implications for the relationships between S- and Q-type asteroids and ordinary chondrite meteorites is an often debated topic in asteroid science. Q-type asteroids have been shown to display the best spectral match to ordinary chondrites (McFadden, L.A., Gaffey, M.J., McCord, T.B. [1985]. Science 229, 160–163). While the Q-types and ordinary chondrites share some spectral features with S-type asteroids, the S-types have significantly redder spectral slopes than the Q-types in visible and near-infrared wavelengths. This reddening of spectral slope is attributed to the effects of space weathering on the observed surface composition. The analysis by Binzel et al. (Binzel, R.P., Rivkin, A.S., Stuart, J.S., Harris, A.W., Bus, S.J., Burbine, T.H. [2004]. Icarus 170, 259–294) provided a missing link between the Q- and S-type bodies in near-Earth space by showing a reddening of spectral slope in objects from 0.1 to 5 km that corresponded to a transition from Q-type to S-type asteroid spectra, implying that size, and therefore surface age, is related to the relationship between S- and Q-types. The existence of Q-type asteroids in the main-belt was not confirmed until Mothé-Diniz and Nesvorny (Mothé-Diniz, T., Nesvorny, D. [2008]. Astron. Astrophys. 486, L9–L12) found them in young S-type clusters. The young age of these families suggest that the unweathered surface could date to the formation of the family. This leads to the question of whether older S-type main-belt families can contain Q-type objects and display evidence of a transition from Q- to S-type. To answer this question we have carried out a photometric survey of the Koronis family using the Kitt Peak 2.1 m telescope. This provides a unique opportunity to compare the effects of the space weathering process on potentially ordinary chondrite-like bodies within a population of identical initial conditions. We find a trend in spectral slope for objects 1–5 km that shows the transition from Q- to S-type in the main-belt. This data set will prove crucial to our understanding of the space weathering process and its relevant timescales.