Argon diffusion domains in K-feldspar II: kinetic properties of MH-10

Viewing K-feldspars as containing a discrete distribution of diffusion domain sizes reconciles otherwise disconsonant features common in their ⁴⁰Ar/³⁹Ar age spectra and Arrhenius plots but raises a fundamental question. What are the features in K-feldspar that endow it with this behavior? We report here the results of two different kinds of experiments that help isolate the nature of the responsible diffusion properties. To assess the thermal stability of the diffusion domains during laboratory treatment, MH-10 K-feldspar was step-heated to 850°C, removed from the furnace and split. One split was reirradiated and the other returned to the furnace and completely degassed. Following re-irradiation, the original heating schedule was used to degas the second aliquot. Apart from the first 5% of gas released, the diffusion properties show little change relative to the original result but, it appears, the physical character of a portion of the smallest domain has been altered. Results of duplicate step-heating experiments of samples treated at 750°C, 950°C and 1100°C prior to irradiation are consistent with the conclusions of the double irradiation experiment. In a second series of experiments, sized aggregates of MH-10 K-feldspar were analyzed by the ⁴⁰Ar/³⁹Ar step-heating method. The resultant log(r/r _o) plots reveal that the largest domain is annihilated when the particle size is reduced to about 50 μm. From this result we infer that the largest diffusion domain size is between 60 and about 130 μm in diameter. This estimate, together with knowledge of the relative domain size distribution obtained from modeling the log(r/r _o) plot, sets the size of the smallest domain to be less than about 1 μm. Microstructural examination of MH-10 K-feldspar identifies sub-grain features that correspond in size to our independent estimates for the largest and smallest diffusion domains. These results strongly support the view that low-temperature K-feldspars contain a distribution of diffusion length scales that are well approximated as discrete domain sizes and that laboratory heating below the onset of melting does not significatly affect the ability to obtain thermal reconstructions from the ⁴⁰Ar/³⁹Ar systematics.     

Challenges in urban water management in the United States

The growing intensity of use of water in the urban environment of the United States is posing significant challenges for its supply, utilization, and protection. The development of traditional water sources is becoming more difficult, and water suppliers are turning to conservation and reuse as alternatives. Price disincentives and better water use management are being utilized to attempt to deal with limited capacity for distribution of water in periods of high demand. Urban runoff presents significant localized flooding problems. Management of floods and floodplains is given focus under the National Flood Insurance Act of 1968, which has as its goal the deterrence of development in flood-prone areas. Water quality goals, being developed and pursued under the Safe Drinking Water Act of 1974, are becoming more encompassing as the breadth of pollutants identified in the urban environment expands. Wastewater control strategies developed under the Federal Water Pollution Control Act of 1972 are expanding their emphasis upon nonpoint source pollution, as opposed to traditional emphasis upon point source pollution. Integrated management of the water resource will become increasingly necessary to adequately address water problems in the urban environment. State and local responsibilities for urban water management and control will likely increase.     

Progress in isotope paleohydrology using lake sediment cellulose

Recent advances in sample preparation techniques and mass spectrometry have fostered more routine oxygen isotope analysis of aquatic cellulose in lake sediment cores, a proxy for lake water oxygen isotope history. These methodological developments have significantly increased the feasibility of incorporating this approach into high-resolution, multi-site, and multi-proxy studies, which are frequently necessary to answer complex hydrological, hydroecological and hydroclimatic questions requiring a paleoenvironmental perspective. Direct translation of lake sediment aquatic cellulose oxygen isotope composition into lake water oxygen isotope composition offers appreciable opportunity for quantitative paleohydrological reconstructions, as evidenced by studies conducted over the past 15 years that span Holocene and pre-historical timescales.     

The gas content and buoyancy of strombolian ash plumes

Plinian plumes erupt with a bulk density greater than that of air, and depend upon air entrainment during their gas-thrust phase to become buoyant; if entrainment is insufficient, the column collapses into a potentially deadly pyroclastic flow. This study shows that strombolian ash plumes can be erupted in an initially buoyant state due to their extremely high initial gas content, and in such cases are thus impervious to column collapse. The high gas content is a consequence of decoupled gas rise in the conduit, in which particles are ultimately incidental. The relations between conduit gas flow, eruption style and plume density are explored here for strombolian scenarios and contrasted with conventional wisdom derived from plinian eruptions. Considering the inherent relation between gas content and initial plume density together with detailed measurements of plume velocities can help unravel ambiguities surrounding conduit processes, eruption styles and hazards at poorly understood volcanoes. Analysis of plume dynamics at Santiaguito volcano, Guatemala adds further support for a model involving decoupled gas rise in the conduit.     

Transient Behaviour of Tropospheric Ozone Precursors in a Global 3-D CTM and Their Indirect Greenhouse Effects

The global three-dimensional Lagrangian chemistry-transport model STOCHEM has been used to follow the changes in the tropospheric distributions of the two major radiatively-active trace gases, methane and tropospheric ozone, following the emission of pulses of the short-lived tropospheric ozone precursor species, methane, carbon monoxide, NO_x and hydrogen. The radiative impacts of NO_x emissionswere dependent on the location chosen for the emission pulse, whether at the surface or in the upper troposphere or whether in the northern or southern hemispheres. Global warming potentials were derived for each of the short-lived tropospheric ozone precursor species by integrating the methane and tropospheric ozone responses over a 100 year time horizon. Indirect radiative forcing due to methane and tropospheric ozone changes appear to be significant for all of the tropospheric ozone precursor species studied. Whereas the radiative forcing from methane changes is likely to be dominated by methane emissions, that from tropospheric ozone changes is controlled by all the tropospheric ozone precursor gases, particularly NO_xemissions. The indirect radiative forcing impacts of tropospheric ozone changes may be large enough such that ozone precursors should be considered in the basket of trace gases through which policy-makers aim to combat global climate change.     

Use of deuterated water as a conservative artificial groundwater tracer

Conservative tracers are necessary to obtain groundwater transport velocities at the field scale. Deuterated water is an effective tracer for this purpose due to its similarity to water, chemical stability, non-reactivity, ease of handling and sampling, relatively neutral buoyancy, and reasonable price. Reliable detection limits of 0.1 mg deuterium/L may be obtained in field tests. A field example is presented in which deuterated water, bromide, and pentafluorobenzoic acid are used as groundwater tracers. Deuterated water appeared to be transported conservatively, producing almost identical breakthrough curves as that of other soluble tracers. Electronic Publication