Noble gases in ureilites: cosmogenic, radiogenic, and trapped components

Abundances and isotopic compositions of Ne (in bulk samples only), Ar, Kr, and Xe have been investigated in 6 monomict, 3 polymict, and the diamond-free ureilite ALH78019 and their acid-resistant, C-rich residues. Isotopic ratios of Kr and Xe are very uniform and agree with data for ureilites from the literature. The measured ratio ³⁸Ar/³⁶Ar showed large variations due to an experimental artifact. This is shown to be connected to the pressure dependence of the instrumental mass discrimination, which for ureilites with their low abundance of ⁴⁰Ar is different from that of the usual air standard. This observation necessitates a reassessment for the recently reported ³⁶Ar excesses due to possible decay of extinct ³⁶Cl in the Efremovka meteorite.Trapped ²²Ne in the range of (1.4-2.5) × 10⁻⁸ cc STP/g is present in bulk ureilites. A Ne three-isotope plot for polymict ureilites indicates the presence of solar Ne. ²¹Ne-based cosmic ray exposure ages for the 10 ureilites studied range from 0.1 Ma (for ALH78019) to 46.8 Ma (for EET83309)All ureilites may have started with nearly the same initial elemental ratio (¹³²Xe/³⁶Ar)₀, established in the nebula during gas trapping into their carbon carrier phases (diamond, amorphous C) by ion implantation. Whereas diamonds are highly retentive, amorphous C has suffered gas loss due to parent body metamorphism. The correlation of the elemental ratios ¹³²Xe/³⁶Ar and ⁸⁴Kr/³⁶Ar along the mass fractionation line could be understood as a two-component mixture of the unaffected diamond gases and the fractionated (to varying degrees) gases from amorphous C. In this view, the initial ratio (¹³²Xe/³⁶Ar)₀ is a measure of the plasma temperature in the nebula at the formation location of the carbon phases. Its lack of correlation with Δ¹⁷O (a signature of the silicate formation location) indicates that carbon phases and silicates formed independently in the nebula, and not from a carbon-rich magmaThe elemental ratios ¹³²Xe/³⁶Ar and ⁸⁴Kr/³⁶Ar in carbon-rich acid residues show a decreasing trend with depth (inferred from carbon consumption during combustion), which can be interpreted as a consequence of the ion implantation mechanism of gas trapping that leads to greater depth of implantation for lighter mass ionThe similarity between trapped gases in phase Q in primitive chondrites and the C phases in ureilites—for both elemental and isotopic compositions—strongly suggests that phase Q might also have received its noble gases by ion implantation from the nebula. The slight differences in the elemental ratios can be explained by a plasma temperature at the location of phase Q gas loading that was about 2000 K lower than for ureilite C phases. This inference is also consistent with the finding that the trapped ratio ¹²⁹Xe/¹³²Xe (1.042 ± 0.002) in phase Q is slightly higher, compared to that of ureilite C phases (1.035 ± 0.002), as a consequence of in situ decay of ¹²⁹I, and becomes observable due to higher value of I/Xe in phase Q as a result of ion implantation at about 2000 K lower plasma temperature.     

Influence of 850 mb trajectories on acid precipitation in southwestern British Columbia

Precipitation chemistry in southwestern British Columbia has been examined to determine if the shutdown of the ASARCO smelter in Tacoma, Washington in March of 1985 had a significant effect.The smelter emitted about 300 tonnes/day of sulphur dioxide and about one tonne/day of arsenic. The smelter had a supplementary control system (SCS) which was implemented during adverse meteorological conditions to meet ambient air standards. The SCS was in operation in October 1984.Five storms from October 1984, before the shutdown, that produced precipitation over the eight-station network in southwestern British Columbia, were considered. The 850 mb back trajectories and ionic loadings in the collected precipitation samples were compared. A similar comparison was made for three storms in October 1985, after the shutdown of the smelter. An examination of cases whose trajectories passed over the smelter before and after the shutdown indicated an over 50% decrease in volume weighted average sulphate concentration in precipitation over the Gulf Islands (islands in the southern Strait of Georgia). However, when the trajectory from the northwest passes over British Columbia coastal sources, higher excess SO₄ concentrations were observed in the precipitation over the Gulf Islands, indicating a linkage between these concentrations and the British Columbia coastal sources. There was no apparent change in values at stations farthest from the Strait of Georgia with one exception. It was also noted that there was reduction in arsenic in precipitation, which could be considered as a tracer, after the shutdown of the ASARCO smelter. This paper enlarges on the above observation with comparisons of the major meteorological and chemical characteristics related to eight storms.     

Impact of atmospheric and physical forcings on biogeochemical cycling of dissolved oxygen and nutrients in the coastal Bay of Bengal

Time-series observations were conducted off Visakhapatnam, central west coast of Bay of Bengal, from October 2007 to April 2009 to examine the influence of physical and atmospheric processes on water column nutrients biogeochemistry. The thermal structure displayed inversions of 0.5 to 1.0° C during winter and were weaker in summer. The water column was vertically stratified during the entire study period and was stronger during October–November 2007 and August–December 2008 compared to other study periods. High concentrations of chlorophyll-a and nutrients were associated with the extreme atmospheric events. The strong relationship of nutrients with salinity indicates that physical processes, such as circulation, mixing and river discharge, have a significant control on phytoplankton blooms in the coastal Bay of Bengal. Phosphate seems to be a controlling nutrient during winter whereas availability of light and suspended matter limits production in summer. Formation of low oxygen conditions were observed in the bottom waters due to enhanced primary production by extreme atmospheric events; however, re-oxygenation of bottom waters through sinking of oxygen-rich surface waters by a warm core (anticyclonic) eddy led to its near recovery. This study reveals that atmospheric and physical processes have significant impacts on the water column biogeochemistry in the coastal Bay of Bengal.     

On the variation of temperature structure parameter in cloud- and clear-air during the summer monsoon season

High resolution aicraft observations of temperature were made in cloud-air and clear-air at different heights in the lower atmosphere over the Deccan Plateu, India, during the summer monsoon season (June–September) of 1976. Temperature fluctuations in the horizontal at each flight level were utilised for computing the temperature structure parameter (C _T ² ) at that level for studying the hieght dependence ofC _T ² . The results were found to be in agreement with those obtained by other investigators. Also, the height variations ofC _T ² in cloud-air and clear-air during active and weak monsoon conditions are studied. Marked differences were noticed in the behaviour ofC _T ² during active and weak monsoon conditions.     

On some future tsunamis in the Pacific Ocean

Possible tsunamis in the Pacific Ocean, especially in its northeastern part, are discussed in relation to a predicted major earthquake in the Shumagin Seismic Gap (located in the eastern part of the Aleutian Island Chain) and to a major eruption of the St. Augustine volcano in Cook Inlet, Alaska. The deep-water propagation of the tsunami generated in the Shumagin Gap is simulated through the use of a spherical polar coordinate grid of the approximate size of 14km. The tsunami generated by the St. Augustine volcano is studied through the fine mesh grid confined to the Cook Inlet only. The numerical models were calibrated against historical tsunami data. The properties of the tsunami signal are described by the maximum amplitude which occurs in the tsunami record. This allows us to single out the direction along which a maximum tsunami is to be expected.Presented at the International Conference on Natural and Man-Made Hazards in Coastal Zones, held in Ensenada, Mexico, August 1988.