Local isotropy and anisotropy in the sheared and heated atmospheric surface layer

Longitudinal velocity and temperature measurements above a uniform dry lakebed were used to investigate sources of eddy-motion anisotropy within the inertial subrange. Rather than simply test the adequacy of locally isotropic relations, we investigated directly the sources of anisotropy. These sources, in a daytime desert-like climate, include: (1) direct interaction between the large-scale and small-scale eddy motion, and (2) thermal effects on the small-scale eddy motion. In order to explore these two anisotropy sources, we developed statistical measures that are sensitive to such interactions. It was found that the large-scale/small-scale interaction was significant in the inertial subrange up to 3 decades below the production scale, thus reducing the validity of the local isotropy assumption. The anisotropy generated by thermal effects was also significant and comparable in magnitude to the former anisotropy source. However, this thermal anisotropy was opposite in sign and tended to counteract the anisotropy generated by the large-scale/smallscale interaction. The thermal anisotropy was attributed to organized ramp-like patterns in the temperature measurements. The impact of this anisotropy cancellation on the dynamics of inertial subrange eddy motion was also considered. For that purpose, the Kolmogorov-Obukhov structure function equation, as derived from the Navier-Stokes equations for locally isotropic turbulence, was employed. The Kolmogorov-Obukhov structure function equation in conjunction with Obukhov's constant skewness closure hypothesis reproduced the measured second- and third-order structure functions. Obukhov's constant skewness closure scheme, which is also based on the local isotropy assumption, was verified and was found to be in good agreement with the measurements. The accepted 0.4 constant skewness value derived from grid turbulence experiments overestimated our measurements. A suggested 0.26 constant skewness value, which we derived from Kolmogorov's constant, was found to be adequate.     

Absarokites from the western Mexican Volcanic Belt: constraints on mantle wedge conditions

We have investigated the near liquidus phase relations of a primitive absarokite from the Mascota region in western Mexico. Sample M.102 contains ~11.6 wt% MgO, Mg#=0.73 and the lava contains Fo₉₀ olivine phenocrysts, indicating near equilibrium with the mantle. High-pressure experiments on a synthetic analogue of the absarokite composition containing low and high H₂O abundances of (~2 and ~5 wt%, respectively) were performed in a piston cylinder apparatus over the pressure range of 1.2 to 2.0 GPa. The composition containing ~2 wt% H₂O is multiply saturated with olivine and orthopyroxene at 1.6 GPa and 1,400 °C. At the same pressure, clinopyroxene appears 30 °C below the liquidus. At an H₂O content of ~5 wt% the multiple saturation with olivine and orthopyroxene occurs at 1.7 GPa and 1,300 °C. Assuming a batch-melting process, we suggest that the primitive absarokite was segregated from a depleted lherzolite or harzburgite residue at ~50 km, placing the depth of origin well within the mantle wedge beneath the Jalisco Block. A low degree (<5 %wt%) batch-melt of an original metasomatized depleted lherzolite or harzburgite source would contain the observed trace element abundances found in M.102. The liquidus phase relations are not consistent with the presence of non-peridotitic veins at the depth of last equilibration. Therefore, we propose that the Mascota absarokites segregated at an apparent melt fraction of less than 5% from a depleted peridotitic source. Melting first began at a greater depth as a small degree H₂O- and trace element- rich melt of a metasomatized peridotite that ascended into the overlying wedge and re-equilibrated with shallower, hotter mantle.Editorial responsibility: J. Hoefs     

Organic micropollutants in marine plastics debris from the open ocean and remote and urban beaches

To understand the spatial variation in concentrations and compositions of organic micropollutants in marine plastic debris and their sources, we analyzed plastic fragments (∼10 mm) from the open ocean and from remote and urban beaches. Polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), dichloro-diphenyl-trichloroethane and its metabolites (DDTs), polybrominated diphenyl ethers (PBDEs), alkylphenols and bisphenol A were detected in the fragments at concentrations from 1 to 10,000 ng/g. Concentrations showed large piece-to-piece variability. Hydrophobic organic compounds such as PCBs and PAHs were sorbed from seawater to the plastic fragments. PCBs are most probably derived from legacy pollution. PAHs showed a petrogenic signature, suggesting the sorption of PAHs from oil slicks. Nonylphenol, bisphenol A, and PBDEs came mainly from additives and were detected at high concentrations in some fragments both from remote and urban beaches and the open ocean.     

A Comparative Study of Five Reference Materials and the Lombard Meteorite for the Determination of the Platinum‐Group Elements and Gold by LA‐ICP‐MS

A range of independently characterised reference materials (RMs) for LA‐ICP‐MS, used for the determination of the platinum‐group elements (PGE) and Au in a sulfide matrix, were analysed and compared: 8b, PGE‐A, NiS‐3, Po727‐T1, Po724‐T and the Lombard meteorite. The newly developed RM NiS‐3 was used as the RM for the calibration of all LA‐ICP‐MS analyses and the measured concentrations of the other RMs compared against their published concentrations. This data were also used to assess the consistency of concentrations calibrated against the different RMs. It was found that Po727‐T1 and 8b produced results that were comparable, within uncertainty, for all elements. Po727‐T1 also produced consistent results with NiS‐3 for all elements. All other RMs showed differences for some elements, especially Ru in Po724‐T, and Os, Ir and Au in PGE‐A. The homogeneity of the PGE and Au in each RM was assessed, by comparing the precision of multiple LA‐ICP‐MS spot analyses with the average uncertainty of the signal. Po724‐T, Po727‐T1 and the Lombard meteorite were found to be homogeneous for all elements, but 8b, PGE‐A and NiS‐3 were heterogeneous for some elements. This is the first direct comparison between a range of independently characterised PGE and Au LA‐ICP‐MS RMs.     

Li and B isotopic variations in an Allende CAI: Evidence for the in situ decay of short-lived Be and for the possible presence of the short-lived nuclide Be in the early solar system

The concentrations and isotopic compositions of lithium, beryllium, and boron, analyzed in situ by ion microprobe in 66 spots of a type B1 Ca-Al-rich inclusion (CAI 3529-41) from the Allende meteorite, are reported. Large variations are observed for both the Li and the B isotopic ratios with ⁷Li/⁶Li ranging from 9.2 ± 0.22 to 12.22 ± 0.43 (a ≈250‰ range in δ⁷Li values) and ¹⁰B/¹¹B ranging from 0.2468 ± 0.0057 to 0.4189 ± 0.0493 (a 410‰ range in δ¹¹B values). The very low Li concentrations (<1 ppb) observed in several anorthite and fassaite grains require that a correction for the contribution of spallogenic Li produced during irradiation of the Allende meteoroid by galactic cosmic rays (GCR) be made (after this correction ⁷Li/⁶Li ranges from 9.2 ± 0.22 to 13.44 ± 0.56, i.e., a ≈350‰ range in δ⁷Li values). In 3529-41, the ¹⁰B/¹¹B ratios are positively correlated with ⁹Be/¹¹B in a manner indicating the in situ decay of short-lived ¹⁰Be (half-life = 1.5 Ma) with a ¹⁰Be/⁹Be ratio at the time of formation of the CAI of 8.8 ± 0.6 × 10⁻⁴, which is in agreement with previous findings [McKeegan, K.D., Chaussidon, M., Robert, F., 2000. Incorporation of short-lived ¹⁰Be in a calcium-aluminum-rich inclusion from the Allende meteorite. Science289, 1334-1337]. The present detailed investigation demonstrates that only minor perturbations of the ¹⁰Be-¹⁰B system are present in 3529-41, contrary to the ²⁶Al/²⁶Mg system for which numerous examples of isotopic redistribution following crystallization were observed [Podosek, F.A., Zinner, E.K., MacPherson, G.J., Lundberg, L.L., Brannon, J.C., Fahey, A.J., 1991. Correlated study of initial ⁸⁷Sr/⁸⁶Sr and Al-Mg systematics and petrologic properties in a suite of refractory inclusions from the Allende meteorite. Geochim. Cosmochim. Acta55, 1083-1110]. Petrographically based criteria were developed to identify within the 66 analyzed spots in 3529-41, those where post-magmatic perturbation of the Li and Be distributions occurred. Li and Be concentrations measured in different analytical spots are compared with those predicted by using experimentally determined partition coefficients according to a model of closed-system crystallization of the CAI melt. These criteria show that 56% of the spots in melilite, 38% in anorthite, and 8% in fassaite suffered post-crystallization perturbations of Li and/or Be distributions. In the remaining spots, which do not show obvious indication of redistribution of Li or Be, the ⁷Li/⁶Li isotopic variations (corrected for GCR exposure) are positively correlated with⁹Be/⁶Li suggesting the in situ decay of now-extinct ⁷Be. The derived isochron implies that at the time of its formation, the CAI melt had a ⁷Be/⁹Be ratio of 0.0061 ± 0.0013 and a ⁷Li/⁶Li ratio of 11.49 ± 0.13. In contrast, all the spots in 3529-41, which do show evidence for post-magmatic redistribution of Li and Be, have relatively constant ⁷Li/⁶Li, averaging 11.72 ± 0.56, which is consistent with mass balance calculations for Li isotopic homogenization in the CAI after the decay of ⁷Be. The incorporation of live ⁷Be in 3529-41 requires, because of the very short half-life of this nuclide (53 days), that it be produced essentially contemporaneously with the formation of the CAI. Therefore, the irradiation processes responsible for production of ⁷Be must have occurred within the solar accretion disk. Calculations developed in the framework of the x-wind model [Gounelle, M., Shu, F.H., Shang, H., Glassgold, A.E., Rehm, E.K., Lee, T., 2004. The origin of short-lived radionuclides and early Solar System irradiation (abstract). Lunar Planet. Sci.35, 1829] reproduce the ⁷Be and ¹⁰Be abundances observed in 3529-41. The correlated presence of ⁷Be and ¹⁰Be in 3529-41 is thus a strong argument that ¹⁰Be, which is observed rather ubiquitously in CAIs, is also a product of irradiation in the early solar system, as might be a significant fraction of other short-lived radionuclides observed in early solar system materials.     

Worst case scenario as stakeholder decision support: a 5- to 6-m sea level rise in the Rhone delta, France

Risk policy and public attitudes appear disconnected from research predicting warmer climate partially due to human activity. To step out of this stalled situation, a worst case scenario of a 5- to 6-m sea level rise (SLR) induced by the collapse of the WAIS and occurring during the period 2030–2130 is constructed and applied to the Rhone delta. Physical and socio-economic scenarios developed with data from the Rhone delta context are developed and submitted to stakeholders for a day-long workshop. Group process analysis shows a high level of trust and cooperation mobilized to face the 5–6 m SLR issue, despite potentially diverging interests. Two sets of recommendations stem from the scenario workshop. A conservative “wait and see” option is decided when the risk of the WAIS collapse is announced in 2030. After WAIS collapse generates an effective 1 m SLR rise by 2050, decisions are taken for total retreat and rendering of the Rhone delta to its hydrological function. The transposition of these results into present-day policy decisions could be considered. The methodology developed here could be applied to other risk objects and situations, and serve for policy exercises and crisis prevention.     

Estimating economic losses of midrise reinforced concrete shear wall buildings in sedimentary basins by combining empirical and simulated seismic hazard characterizations

Studies of recorded ground motions and simulations have shown that deep sedimentary basins can greatly increase the intensity of earthquake ground motions within a period range of approximately 1–4 s, but the economic impacts of basin effects are uncertain. This paper estimates key economic indicators of seismic performance, expressed in terms of earthquake‐induced repair costs, using empirical and simulated seismic hazard characterizations that account for the effects of basins. The methodology used is general, but the estimates are made for a series of eight‐ to 24‐story residential reinforced concrete shear wall archetype buildings in Seattle, WA, whose design neglects basin effects. All buildings are designed to comply with code‐minimum requirements (i.e., reference archetypes), as well as a series of design enhancements, which include (a) increasing design forces, (b) decreasing drift limits, and (c) a combination of these strategies. As an additional reference point, a performance‐based design is also assessed. The performance of the archetype buildings is evaluated for the seismic hazard level in Seattle according to the 2018 National Seismic Hazard Model (2018 NSHM), which explicitly considers basin effects. Inclusion of basin effects results in an average threefold increase in annualized losses for all archetypes. Incorporating physics‐based ground motion simulations to represent the large‐magnitude Cascadia subduction interface earthquake contribution to the hazard results in a further increase of 22% relative to the 2018 NSHM. The most effective of the design strategies considered combines a 25% increase in strength with a reduction in drift limits to 1.5%.     

Dating carbonate rocks with in-situ produced cosmogenic Be: Why it often fails

Surface exposure dating of carbonate rocks using cosmogenic ¹⁰Be is problematic. We have performed step-wise leaching of calcite-rich samples in order to investigate the reasons for this. Results on different grain size fractions clearly indicate the source of atmospheric ¹⁰Be is clay. We demonstrate that partial-leaching procedures, which result in moderate pH levels will not release ¹⁰Be (in-situ produced or atmospheric) due to the instant re-absorption on grain surfaces. By contrast, under strongly acidic conditions, all absorbed ¹⁰Be is leached from aluminosilicates giving abnormally high ¹⁰Be concentrations and consequently exposure ages that are too old. Dating is only possible if samples do not contain any clay minerals or if they can be removed prior to carbonate dissolution.     

A review of inverse methods in seismic site characterization

Journal of Seismology - Seismic site characterization attempts to quantify seismic wave behavior at a specific location based on near-surface geophysical properties, for the purpose of mitigating...     

Very slow erosion rates and landscape preservation across the southwestern slope of the Ladakh Range,India

Erosion rates are key to quantifying the timescales over which different topographic and geomorphic domains develop in mountain landscapes. Geomorphic and terrestrial cosmogenic nuclide (TCN) methods were used to determine erosion rates of the arid, tectonically quiescent Ladakh Range, northern India. Five different geomorphic domains are identified and erosion rates are determined for three of the domains using TCN ¹⁰Be concentrations. Along the range divide between 5600 and 5700 m above sea level (asl), bedrock tors in the periglacial domain are eroding at 5.0 ± 0.5 to 13.1 ± 1.2 meters per million years (m/m.y.)., principally by frost shattering. At lower elevation in the unglaciated domain, erosion rates for tributary catchments vary between 0.8 ± 0.1 and 2.0 ± 0.3 m/m.y. Bedrock along interfluvial ridge crests between 3900 and 5100 m asl that separate these tributary catchments yield erosion rates <0.7 ± 0.1 m/m.y. and the dominant form of bedrock erosion is chemical weathering and grusification. Erosion rates are fastest where glaciers conditioned hillslopes above 5100 m asl by over‐steepening slopes and glacial debris is being evacuated by the fluvial network. For range divide tors, the long‐term duration of the erosion rate is considered to be 40–120 ky. By evaluating measured ¹⁰Be concentrations in tors along a model ¹⁰Be production curve, an average of ~24 cm is lost instantaneously every ~40 ky. Small (<4 km²) unglaciated tributary catchments and their interfluve bedrock have received very little precipitation since ~300 ka and the long‐term duration of their erosion rates is 300–750 ky and >850 ky, respectively. These results highlight the persistence of very slow erosion in different geomorphic domains across the southwestern slope of the Ladakh Range, which on the scale of the orogen records spatial changes in the locus of deformation and the development of an orogenic rain shadow north of the Greater Himalaya. Copyright © 2014 John Wiley & Sons, Ltd.