Observations of nitrogen isotope fractionation in deeply embedded protostars

(Abridged) The terrestrial planets, comets, and meteorites are significantly enriched in 15N compared to the Sun and Jupiter. While the solar and jovian nitrogen isotope ratio is believed to represent the composition of the protosolar nebula, a still unidentified process has caused 15N-enrichment in the solids. Several mechanisms have been proposed to explain the variations, including chemical fractionation. However, observational results that constrain the fractionation models are scarce. While there is evidence of 15N-enrichment in prestellar cores, it is unclear how the signature evolves into the protostellar phases. Our aim is to measure the 14N/15N ratio around three nearby, embedded low-to-intermediate-mass protostars. Isotopologues of HCN and HNC were used to probe the 14N/15N ratio. A selection of H13CN, HC15N, HN13C, and H15NC transitions was observed with the APEX telescope. The 14N/15N ratios were derived from the integrated intensities assuming a standard 12C/13C ratio. The assumption of optically thin emission was verified using radiative transfer modeling and hyperfine structure fitting. Two sources, IRAS 16293A and R CrA IRS7B, show 15N-enrichment by a factor of around 1.5-2.5 in both HCN and HNC with respect to the solar composition. Solar composition cannot be excluded for the third source, OMC-3 MMS6. Furthermore, there are indications of a trend toward increasing 14N/15N ratios with increasing outer envelope temperature. The enhanced 15N abundances in HCN and HNC found in two Class~0 sources (14N/15N of 160-290) and the tentative trend toward a temperature-dependent 14N/15N ratio are consistent with the chemical fractionation scenario, but 14N/15N ratios from additional tracers are indispensable for testing the models. Spatially resolved observations are needed to distinguish between chemical fractionation and isotope-selective photochemistry.Comment: Accepted for publication in Astronomy and Astrophysics. 16 pages, 13 figure

The status of shark and ray fishery resources in the Gulf of California: applied research to improve management and conservation

Seasonal surveys were conducted during 1998–1999 in Baja California, Baja California Sur, Sonora, and Sinaloa to determine the extent and activities of artisanal elasmobranch fisheries in the Gulf of California. One hundred and forty–seven fishing sites, or camps, were documented, the majority of which (n = 83) were located in Baja California Sur. Among camps with adequate fisheries information, the great majority (85.7%) targeted elasmobranchs during some part of the year. Most small, demersal sharks and rays were landed in mixed species fisheries that also targeted demersal teleosts, but large sharks were usually targeted in directed drift gillnet or, to a lesser extent, surface longline fisheries. Artisanal fishermen were highly opportunistic, and temporally switched targets depending on the local productivity of teleost, invertebrate, and elasmobranch fishery resources. Major fisheries for small sharks ( 1.5 m, “tiburón”) were minor components of artisanal elasmobranch fisheries in Sonora and Sinaloa, but were commonly targeted during summer and early autumn in Baja California and Baja California Sur. The pelagic thresher shark (Alopias pelagicus) and silky shark (Carcharhinus falciformis) were most commonly landed in Baja California, whereas a diverse assemblage of pelagic and large coastal sharks was noted among Baja California Sur landings. Rays dominated summer landings in Baja California and Sinaloa, when elevated catch rates of the shovelnose guitarfish (Rhinobatos productus, 13.2 individuals/vessel/trip) and golden cownose ray (Rhinoptera steindachneri, 11.1 individuals/vesse/trip) primarily supported the respective fisheries. The Sonoran artisanal elasmobranch fishery was the most expansive recorded during this study, and rays (especially R. productus) dominated spring and summer landings in this state. Seasonal catch rates of small demersal sharks and rays were considerably greater in Sonora than in other surveyed states. Many tiburón populations (e.g., C. leucas, C. limbatus, C. obscurus, Galeocerdo cuvier) have likely been overfished, possibly shifting effort towards coastal populations of cazón and rays. Management recommendations, including conducting demographic analyses using available life history data, determining and protecting nursery areas, and enacting seasonal closures in areas of elasmobranch aggregation (e.g., reproduction, feeding), are proposed. Without effective, enforceable management to sustain or rebuild targeted elasmobranch populations in the Gulf of California, collapse of many fisheries is a likely outcome. (PDF contains 243 pages

Improved methods for high-precision Pb-Pb dating of extra-terrestrial materials

Dating meteoritic materials by the Pb–Pb isochron method depends on constructing linear arrays typically defined by mixtures of initial and radiogenic Pb after the removal of terrestrial contaminant Pb. The method also depends on minimizing the amount of laboratory Pb blank added to the sample during processing and analyses. With the aim to analyze smaller sample sizes and decrease processing times, we have devised a new method for the construction of isochrons using the stepwise dissolution of meteoritic materials that better defines reduced amounts of Pb blank, reduces the risk of random anomalous Pb contamination, and increases sample throughput. Samples are processed in a PFA Teflon™ pipette tip fitted with a frit inside a heated, sealed chamber that can be manually over-pressured to expel reagents directly into a PFA Teflon™ vial below. With four independent chambers, three samples can be processed simultaneously with a fourth position to assess the Pb contribution of the combined blank and spike for each step. The matched blank-spike Pb for each step provides a specific blank estimate for each step that ensures a more accurate correction for non-sample Pb and, therefore, reduces the uncertainty on each analysis. We assess the performance of this new method by reporting the results of dating a fragment of a chondrule from the well-characterized CBa chondrite Gujba and compare these results with previously published data for this meteorite. The improvements reduce the minimum sample sizes that can be successfully dated by the Pb–Pb method, an important development for size-limited materials such as small chondrules and samples returned from space missions

El estado actual de los tiburones y rayas sujectos a explotación comercial en el Golfo de California: una investigación aplicada al mejoramiento de su manejo pesquero y conservación

Se realizaron prospecciones estacionales durante 1998–1999 en Baja California, Baja California Sur, Sonora y Sinaloa para determinar la dimensión y las actividades de las pesquerías artesanales de elasmobranquios dentro del Golfo de California. Ciento cuarenta y siete campamentos o sitios pesqueros fueron documentados, la mayoría de los cuales (n= 83) fueron localizados en Baja California Sur. Entre los campamentos con información pesquera adecuada, la gran mayoría (85.7%) pescaron elasmobranquios en algún momento del año. La mayoría de los pequeños tiburones y rayas demersales fueron capturados en pesquerías multiespecíficas, las cuales también tienen como objetivo teleósteos, pero los grandes tiburones fueron capturados generalmente por pesquerías de redes agalleras a la deriva dirigidas a ellos, o en menor grado, por pesquerías de palangre de superficie. Los pescadores artesanales presentaron un comportamiento altamente oportunístico y temporalmente cambiaban de especie objetivo dependiendo de la productividad de los recursos pesqueros locales de especies de escama, invertebrados y elasmobranquios. Las mayores pesquerías de pequeños tiburones (≤1.5 m, “cazón”) se documentaron en Baja California, Sonora, y Sinaloa durante la primavera y adicionalmente durante otoño e invierno en Sonora. Los tiburones del genero Mustelus (Mustelus spp) dominaron los desembarques de cazón en los estados norteños, mientras que los tiburones martillo juveniles (Sphyrna lewini) sostuvieron la pesquería en Sinaloa. Los grandes tiburones (> 1.5 m, “tiburón”) fueron componente menor de las pesquerías artesanales de elasmobranquios en Sonora y Sinaloa, pero comúnmente fueron capturados en verano y principios de otoño en Baja California y Baja California Sur. El tiburón zorro pelágico (Alopias pelagicus) y el tiburón piloto (Carcharhinus falciformis) fueron comúnmente desembarcados en Baja California, mientras que un diverso ensamble de tiburones pelágicos y grandes costeros fueron observados en los desembarques de Baja California Sur. Las rayas dominaron los desembarques en verano en Baja California y Sinaloa, sostenidas principalmente por elevadas tasas de captura de la guitarra común (Rhinobatos productus, 13.2 individuos/embarcación/viaje) y del tecolote (Rhinoptera steindachneri, 11.1 individuos/embarcación/viaje). La pesquería de rayas de Sonora fue la más extensamente documentada durante este estudio y las rayas (especialmente R. productus) dominaron los desembarques en primavera y verano en este estado. Las tasas de captura estacionales de pequeños tiburones demersales y rayas fueron considerablemente mayores en Sonora que en los otros estados prospectados. Numerosas poblaciones de tiburón (p. ej. C. leucas, C. limbatus, C. obscurus, Galeocerdo cuvier) han sido probablemente sobreexplotados, lo que posiblemente este causando un cambio en el esfuerzo hacia las poblaciones de cazón y rayas costeros. Se proponen recomendaciones de manejo, incluyendo conducir análisis demográficos empleando la información de historias de vida disponible, localizando y protegiendo áreas de crianza e implementando temporadas de veda estacional en las áreas de mayor concentración de elasmobranquios (p. ej. alimentación, reproducción). Sin un manejo pesquero efectivo, vigilado, para sostener o reconstruir las poblaciones de elasmobranquios sujetas a explotación comercial dentro del Golfo de California, el colapso de las pesquerías es un evento probable. (PDF contains 261 pages

Protostellar collapse: rotation and disk formation

We present some important conclusions from recent calculations pertaining to the collapse of rotating molecular cloud cores with axial symmetry, corresponding to evolution of young stellar objects through classes 0 and begin of class I. Three main issues have been addressed: (1) The typical timescale for building up a preplanetary disk - once more it turned out that it is of the order of one free-fall time which is decisively shorter than the widely assumed timescale related to the so-called 'inside-out collapse'; (2) Redistribution of angular momentum and the accompanying dissipation of kinetic (rotational) energy - together these processes govern the mechanical and thermal evolution of the protostellar core to a large extent; (3) The origin of calcium-aluminium-rich inclusions (CAIs) - due to the specific pattern of the accretion flow, material that has undergone substantial chemical and mineralogical modifications in the hot (exceeding 900 K) interior of the protostellar core may have a good chance to be advectively transported outward into the cooler remote parts (beyond 4 AU, say) of the growing disk and to survive there until it is incorporated into a meteoritic body.Comment: 4 pages, 4 figure

Evolution of realized Eltonian niches across Rajidae species

The notion that closely related species resemble each other in ecological niche space (i.e., phylogenetic dependence) has been a long-standing, contentious paradigm in evolutionary biology, the incidence of which is important for predicting the ecosystem-level effects of species loss. Despite being examined across a multitude of terrestrial taxa, many aspects of niche conservatism have yet to be explored in marine species, especially for characteristics related to resource use and trophic behavior (Eltonian niche characteristics, ENCs). We combined ENCs derived from stable isotope ratios at assemblage- and species-levels with phylogenetic comparative methods, to test the hypotheses that benthic marine fishes (1) exhibit similar assemblage-wide ENCs regardless of geographic location and (2) display phylogenetically dependent ENCs at the species level. We used a 12-species sub-set of the monophyletic group Rajidae sampled from three independent assemblages (Central California, Gulf of Alaska, and Northwest Atlantic), which span two ocean basins. Assemblage-level ENCs implied low trophic diversity and high evenness, suggesting that Rajidae assemblages may exhibit a well-defined trophic role, a trend consistent regardless of geographic location. At the species level, we found evidence for phylogenetic dependence of ENCs relating to trophic diversity (i.e., isotopic niche width; SEAc). Whether individuals can be considered functional equivalents across assemblages is hard to ascertain because we did not detect a significant phylogenetic signal for ENCs relating to trophic function (e.g., trophic position). Thus, additional, complimentary approaches are required to further examine the phylogenetic dependence of species functionality. Our approach illustrates the potential of stable isotope-derived niche characteristics to provide insight on macroecological processes occurring across evolutionary time, which could help predict how assemblages may respond to the effects of species loss

Selective resuscitation in premature twins: an ethical analysis

Selective resuscitation refers to the practice of providing resuscitative efforts to one or some (but not all) infants born in the setting of multiple gestation. When one fetus is known to have a severe anomaly or severe growth restriction, parents are sometimes offered this option. In the setting of extreme prematurity, in the absence of an anomaly or severe growth restriction, parents are generally expected to make one unified decision for all the infants involved. The introduction of the Outcome Estimator, a tool that provides the ability to make individual outcome predictions for each fetus in a multiple gestation at borderline gestational age, based on contributing variables such as weight and gender, has led to the ethical dilemma of whether parents in this setting should also be offered the option of selective resuscitation. No convincing ethical argument for denying the parents the right to decide for each individual infant is apparent

Problems in obtaining precise and accurate Sr isotope analysis from geological materials using laser ablation MC-ICPMS

This paper reviews the problems encountered in eleven studies of Sr isotope analysis using laser ablation multicollector inductively coupled plasma mass spectrometry (LA-MC-ICPMS) in the period 1995–2006. This technique has been shown to have great potential, but the accuracy and precision are limited by: (1) large instrumental mass discrimination, (2) laser-induced isotopic and elemental fractionations and (3) molecular interferences. The most important isobaric interferences are Kr and Rb, whereas Ca dimer/argides and doubly charged rare earth elements (REE) are limited to sample materials which contain substantial amounts of these elements. With modern laser (193 nm) and MC-ICPMS equipment, minerals with >500 ppm Sr content can be analysed with a precision of better than 100 ppm and a spatial resolution (spot size) of approximately 100 μm. The LA MC-ICPMS analysis of 87Sr/86Sr of both carbonate material and plagioclase is successful in all reported studies, although the higher 84Sr/86Sr ratios do suggest in some cases an influence of Ca dimer and/or argides. High Rb/Sr (>0.01) materials have been successfully analysed by carefully measuring the 85Rb/87Rb in standard material and by applying the standard-sample bracketing method for accurate Rb corrections. However, published LA-MC-ICPMS data on clinopyroxene, apatite and sphene records differences when compared with 87Sr/86Sr measured by thermal ionisation mass spectrometry (TIMS) and solution MC-ICPMS. This suggests that further studies are required to ensure that the most optimal correction methods are applied for all isobaric interferences