The structure of the oceanic crust off southern Peru determined from an ocean bottom seismometer

An oceanic crustal model has been produced for the Nazca plate south of the Nazca Ridge prior to subduction into the Peru-Chile Trench at 18°S latitude. Consistent delays of theP_n arrivals and a discontinuity in the tau-p curve indicate a low-velocity zone at the base of the crust. Observed upper mantle velocities are low; however, the mantle velocity increases with depth, at least to 20 km, to a value of 8.5 km/s. A possible petrological cause for the low-velocity zone is partially serpentinized peridotite; however, no clear refracted shear waves were observed to constrain this interpretation.     

The determination of biomarker distributions by tandem mass spectrometry

The rapid development of biomarker geochemistry and its application to petroleum exploration problems has been due in no small part to gas chromatography-mass spectrometry (GC-MS). The recent availability of hybrid and triple stage quadrupole mass spectrometers have introduced a number of novel ways for the detection and identification of biomarkers present in extremely complex mixtures of organic compounds from various sources including crude oils and source rock extracts.A triple stage quadrupole mass spectrometer has been used in the MS/MS mode with both a gas chromatograph and direct insertion probe as inlet systems, to develop alternative methods for biomarker analyses. Selected ions formed in the ion source of the mass spectrometer are permitted to enter the collision cell where, following collision with an inert gas, the daughter ions are separated using the third quadrupole. In this manner specific parent/daughter ion relationships are utilized to monitor and resolve classes of biomarkers or individual components from within a complex mixture. Furthermore, deuterated analogues of naturally occurring biomarkers can be utilized for quantitation purposes since the parent/daughter relationship for the standard will differ, depending on the number of deuterium atoms present, even though the relative retention times are identical.The ability of MS/MS to resolve components spectrometrically as well as chromatographically greatly enhances the analytical short-column capabilities of this system. In this paper the utilization of GC-MS/MS and the direct insertion probe MS/MS to determine biomarker distributions in crude oils both qualitatively and quantitatively are discussed. The results from the study show that it is possible to use the probe data to rapidly correlate oils on the basis of their source materials and if necessary select samples for more detailed analysis by GC-MS/MS. A major advantage of the direct insertion probe method is speed of analysis although some component resolution is lost. In an effort to minimize this problem and still maintain the rapid analysis time, the use of short columns in the GC is also described. This approach provides the ability to undertake relatively rapid analyses with limited chromatographic resolution but at relatively high levels of sensitivity.     

Tridymite: Effect of hydrostatic pressure to 6 kbar on temperatures of two rapidly reversible transitions

Trajectories of two reversible phase transitions in a low-Na synthetic tridymite have been determined to 6 kbar by differential thermal analysis (DTA) in hydrostatic apparatus using Ar or CO₂. The temperature of the lower transition increases from 111 ° C at 1 bar linearly with pressure with slope 1₅ deg kbar^–1. Pressure raises the temperature of the upper transition from 157 ±2 ° or 159 ° C (independently determined) at 1 bar wit a slope of 53 deg kbar^–1, up to 0.7 kbar; for the data above that pressure, the initial slope is 64 deg kbar^–1. Above 2–1/2 kbar, the variation is linear with slope 70 deg kbar^–1. No evidence for other transitions was found at any of the apparent changes of slope. Hystereses for both transitions decreased at high pressures compared to 1-bar. Preferred values for the transition enthalpies, together with these slopes and the Clausius-Clapeyron equation, yield estimates for the volume changes at the transitions of 0.01 (lower) and 0.15 to 0.25 (upper) cm³ gfw^–1. These calculated volume changes are not consonant with many of the high temperature volumetric data on tridymites of varying origins.     

Limitations to growth of microorganisms on Uranus,Neptune, and Titan

We have examined a wide range of physical, chemical, and thermal models of the atmosphere of Uranus. In that model, which we believe maximizes favorable conditions for the support of life [Weidenschilling and Lewis, Icarus20, 465–476 (1973)], we find the probability of growth of a contaminant terrestrial microorganisms to be nil. If, as is likely, conditions are even more extreme on Neptune, the probability of contamination of both of the outer planets Uranus and Neptune is nil. The Wiedenschilling and Lewis model guarantees the presence of water droplets through the temperature range 0 to 100°C; other published models add water liquid at higher temperatures or fail to provide liquid water at all within this temperature range. In this model the heavy elements (C, N, O, etc.) are enhanced in Uranus by a factor sufficient to form a deep massive cloud layer of aqueous ammonia solution droplets. We can estimate the probability of growth with respect to the following factors: the presence of stable liquid water, convection of parcels of atmosphere to lethally hot depths, solar energy sources reaching the water layer, organic molecular production by solar ultraviolet light, ammonia concentration at the water cloud level, ionic species distribution, and concentrations at the water cloud level. The evaluation of these factors suggests that most terrestrial life as we know it would be excluded on the basis of any one of them. We know of no organism that would be adapted to all the stringent Uranus conditions simultaneously. The discovery of even a single species of Earth organism that can survive or grow under allowable outer planetary conditions would establish new principles in biology.Titan, the methane-rich moon of Saturn, may be more hospitable for terrestrial organisms than any of the other objects of the outer solar system. Even there we see formidable barriers to the growth of an Earth organism in Titan's atmosphere. We recognize that revision of our views concerning Titan must occur as more is learned about this satellite.We advocate the abandonment, in principle, of the probabilistic approach to the estimation of growth of terrestrial organisms on spacecraft, planets, and satellites in the solar system. We do not support an approach which estimates probabilities of qualitatively unknown phenomena. We recommend a strategy which involves identification and intensive study of those organisms most likely to thrive under known conditions for each of the planets respectively. (Unknown environmental conditions may be allowed to vary optimally.) Some explicit areas for Earth-based experimentation are indicated.     

Chemical structure of the deep atmosphere of Jupiter

We report here the equilibrium abundances calculated for a system of over 500 compounds of 27 selected elements along a nominal Jupiter adiabat. Several species predicted to be of negligible abundance in the visible upper troposphere if chemical equilibrium is exactly attained are found to be potential tracers of rapid vertical motions. Vertical mixing of certain species, especially CO, PH₃, AsH₃, GeS, and GeH₄, may provide detectable quantities of these species near the visible cloudtops due to quenching and incomplete equilibration of the rapidly rising, rapidly cooling gas. Observational prospects for detecting such tracers of deep circulation are discussed in the light of the spectroscopic detection of CO in the 5-μm window on Jupiter and the confirmation of PH₃ on both Jupiter and Saturn.     

Urban encounters and sexual health among gay and bisexual immigrant men: perspectives from the settlement and aids service sectors

Gay men's health researchers in North America have recently attended to sexual and mental health issues affecting ethnic minority men, many of whom are also immigrants. Most of this work is grounded in epidemiological models that focus on relationships between individual HIV risk and sexual behaviors. Consequently, they the frame the sexual health of gay and bisexual immigrants as the product of cultural issues (for example, family and religious homophobia, lack of health education) or gay community issues (prevalence of drug use and casual sex) that lead to self‐devaluation, depression, and unprotected sex with multiple partners. Few studies, however, examine these phenomena through the lens of migration and resettlement. Using the narratives from twelve in‐depth interviews with settlement and AIDS Service Organization (ASO) workers in Toronto, Ottawa, and London, Ontario, Canada, this article examines four types of post‐migration urban encounters that influence sexual health: negotiations of resettlement‐related stress, encounters with the urban gay community, encounters with the online gay community, and encounters with sexual health promotion itself. The findings suggest that these encounters are important intervening events that mediate the relationship between the attributes of the immigrant and his sexual health behaviors and outcomes.     

Effects of logging intensity on structure and composition of a broadleaf-Korean pine mixed forest on Changbai Mountains,northeast China

In order to identify a harvesting model which is beneficial for broadleaf-Korean pine mixed forest(BKF) sustainability, we investigated four types of harvested stands which have been logged with intensities of 0(T0, control), 15%(T1, low intensity), 35%(T2, moderate intensity), and 100%(T3, clear-cutting), and examined the impacts of logging intensity on composition and structure of these stands. Results showed that there were no significant differences between T0 and T1 for all structural characteristics, except for density of seeding and large trees. The mean diameter at breast height(DBH, 1.3 m above the ground), stem density and basal area of large trees in T2 were significantly lower than in T0, while the density of seedlings and saplings were significantly higher in T2 than in T0. Structural characteristics in T3 were entirely different from T0. Dominant tree species in primary BKF comprised 93%, 85%, 45% and 10% of the total basal area in T0, T1, T2 and T3, respectively. Three community similarity indices, the Jaccard′s similarity coefficient(CJ); the Morisita-Horn index(CMH); and the Bray-Curtis index(CN), were the highest for T0 and T1, followed by T0 and T2, and T0 and T3, in generally. These results suggest that effects of harvesting on forest composition and structure are related to logging intensities. Low intensity harvesting is conductive to preserving forest structure and composition, allowing it to recover in a short time period. The regime characterized by low logging intensity and short rotations appears to be a sustainable harvesting method for BKF on the Changbai Mountains.     

LASCO observations of the coronal rotation

The near-rigid rotation of the corona above the differential rotation of the photosphere has important implications for the form of the global coronal magnetic field. The magnetic reconfiguring associated with the shear region where the rigidly-rotating coronal field lines interface with the differentially-rotating photospheric field lines could provide an important energy source for coronal heating. We present data on coronal rotation as a function of altitude provided by the Large Angle Spectrometric Coronagraph (LASCO) instrument aboard the Solar and Heliospheric Observatory (SOHO) spacecraft. LASCO comprises of three coronagraphs (C1, C2, and C3) with nested fields-of-view spanning 1.1 R to 30 R. An asymmetry in brightness, both of the Fexiv emission line corona and of the broad-band electron scattered corona, has been observed to be stable over at least a one-year period spanning May 1996 to May 1997. This feature has presented a tracer for the coronal rotation and allowed period estimates to be made to beyond 15R, up to 5 times further than previously recorded for the white-light corona. The difficulty in determining the extent of differential motion in the outer corona is demonstrated and latitudinally averaged rates formed and determined as a function of distance from the Sun. The altitude extent of the low latitude closed coronal field region is inferred from the determined rotation periods which is important to the ability of the solar atmosphere to retain energetic particles. For the inner green line corona (<2 R) we determine a synodic rotation period of (27.4±0.1) days, whereas, for the outer white- light corona, (>2.5 R) we determine a rotation period of (27.7±0.1) days.