Origin and provenance of spherules and magnetic grains at the Younger Dryas boundary

One or more bolide impacts are hypothesized to have triggered the Younger Dryas cooling at ∼12.9 ka. In support of this hypothesis, varying peak abundances of magnetic grains with iridium and magnetic microspherules have been reported at the Younger Dryas boundary (YDB). We show that bulk sediment and/or magnetic grains/microspherules collected from the YDB sites in Arizona, Michigan, New Mexico, New Jersey, and Ohio have ¹⁸⁷Os/¹⁸⁸Os ratios ≥1.0, similar to average upper continental crust (= 1.3), indicating a terrestrial origin of osmium (Os) in these samples. In contrast, bulk sediments from YDB sites in Belgium and Pennsylvania exhibit ¹⁸⁷Os/¹⁸⁸Os ratios <<1.0 and at face value suggest mixing with extraterrestrial Os with ¹⁸⁷Os/¹⁸⁸Os of ∼0.13. However, the Os concentration in bulk sample and magnetic grains from Belgium is 2.8 pg/g and 15 pg/g, respectively, much lower than that in average upper continental crust (=31 pg/g), indicating no meteoritic contribution. The YDB site in Pennsylvania is remarkable in yielding 2- to 5-mm diameter spherules containing minerals such as suessite (Fe-Ni silicide) that form at temperatures in excess of 2000 °C. Gross texture, mineralogy, and age of the spherules appear consistent with their formation as ejecta from an impact 12.9 ka ago. The ¹⁸⁷Os/¹⁸⁸Os ratios of the spherules and their leachates are often low, but Os in these objects is likely terrestrially derived. The rare earth element patterns and Sr and Nd isotopes of the spherules indicate that their source lies in 1.5-Ga Quebecia terrain in the Grenville Province of northeastern North America.The Younger Dryas (YD) event represents a major cooling interlude during the last deglaciation between 12.9 and 11.6 ka and is widely recorded over the mid- to high latitudes in the Northern Hemisphere with associated perturbations in the tropical regions. The abrupt onset of this event appears to have coincided with the Pleistocene megafaunal extinction in North America and Europe (1) and Clovis Paleoindian cultural modifications and population declines (2, 3). The prevalent postulated mechanism for the YD cooling is meltwater flooding and iceberg calving that released fresh water into the northeast Atlantic and/or the Arctic Oceans (4–7), resulting in a temporary shutdown of meridional overturning circulation (8). Recent modeling work (9) has added support to the idea that flooding of the Arctic via the McKenzie River could provide the “freshwater cap” that inhibited open-ocean deep convection.Firestone et al. (10) proposed an alternate hypothesis that the cooling was directly or indirectly triggered by one or more cosmic airbursts or impacts that engendered enormous environmental and biotic changes. In support of this hypothesis, they reported extraterrestrial (ET) signatures from 12 sites dating to the Younger Dryas boundary (YDB) at ∼12.9 ka (10). The ET evidence from these sites is sometimes associated with a black layer (or black mat), which is also found at about 70 Clovis-age archaeological sites (1). Directly beneath the black mat is the YDB, a relatively thin layer, which is reported to exhibit variable enrichment in Ir (10). Firestone et al. (10) also found the YDB layer to be enriched in magnetic grains with variable amounts of Ir, magnetic microspherules, fullerenes containing ET ³He, charcoal, soot, carbon spherules, and glass-like carbon. The impact hypothesis was further supported and extended by Kennett et al. (11), who reported nanodiamonds formed at high pressure at the YDB. The hypothesis is controversial, however, in part because other investigators have suggested alternate scenarios to explain the above evidential markers (12–15).To date, 184 confirmed impact structures have been identified around the world (Earth Impact Database: www.passc.net/EarthImpactDatabase/index.html). Crater structure, shock metamorphism, and a meteoritic contribution are important markers used to confirm an impact structure (16–18). However, a visible crater, breccias, and high-pressure–modified minerals remain unreported for the YDB horizon, with the exception of possibly shock-synthesized hexagonal nanodiamonds (11, 19), an observation disputed by Daulton and coworkers (12, 15).The proponents of the YDB impact hypothesis have pointed out, however, the lack of traditional impact markers in a number of widely accepted impact events (e.g., Australasian tektites, Libyan Desert glass, the Tunguska event), all suggested to have resulted from non-crater–producing airbursts (19, 20). Thus, a lack of traditional markers at the YDB may possibly be the result of one or more impactors exploding in the atmosphere or striking the Laurentide Ice Sheet (10). Recently, a submerged 4-km-wide candidate impact crater (Corossol Crater) has been discovered in the Sept Iles, Gulf of St. Lawrence, Canada and has been provisionally dated to 12.9 ka.^† Two other features, one (Charity Shoal) submerged in Lake Ontario (21) and the other (Bloody Creek) in Nova Scotia (22), may represent additional or alternate impact craters associated with the YDB event. They have not been accurately dated, but their proposed range of dates spans the YD onset.A key piece of evidence reported by Firestone et al. (10) is that of anomalously high concentrations of magnetic spherules with diameters ranging from 10 to 150 μm and magnetic grains at the YDB. Magnetic grains at several YDB sites were reported to be enriched in Ir but inferred to show a nugget effect as sometimes Ir enrichment could not be reproduced (10). The concentrations of magnetic spherules reported by Firestone et al. (10) have been confirmed by seven independent groups (20, 23) from YDB sites in Venezuela (24), Arizona (25), New Mexico, South Carolina, and Maryland (26). Another study by Surovell et al. (27) was unable to replicate either the abundances or the chronostratigraphy of the magnetic spherules reported by Firestone et al. (10). That finding, however, has been contradicted by LeCompte et al. (26), who found magnetic spherules in three sites examined by Surovell et al. (27).Studies investigating Ir enrichment in bulk sediment have also produced conflicting results. Paquay et al. (28) could not replicate the high values of Ir reported by Firestone et al. (10) at a number of locations, including Murray Springs, AZ. In contrast, Haynes et al. (29) found extremely high Ir concentrations in the YDB magnetic fraction (72 ng/g) at Murray Springs. This value is substantially higher than the 2.25 ng/g of Ir reported by Firestone et al. (10) in bulk sediment and is >3,000 times crustal abundance, exceeding concentrations found in most meteorites and impact craters. Similarly, anomalous enrichments in rare earth elements (REE) and high concentrations of both osmium (Os) and Ir in YDB sediments from Murray Springs and Lommel (Belgium) have been reported.^‡ Likewise, Ir enrichments in the YDB layer in southwest England have been reported.^§ Thus, some independent groups have confirmed YDB Ir anomalies, whereas others have not.Although anomalously high concentrations of Ir are considered to be indicators of meteoritic influxes during an ET impact on Earth (30), high Ir concentrations alone are insufficient to prove an ET contribution as they can also result from terrestrial processes. Turekian (31) argued that Os concentrations and isotope measurements should be used to detect the existence of an ET component. Being a platinum group element (PGE), Os is highly enriched in meteorites/cosmic dust and depleted in the upper continental crust. The average Os/Ir ratios of meteorites and for the upper continental crust are 1.1 and 1.4, respectively (see compilation in ref. 32), implying little fractionation between these elements during continental crust formation. In contrast, the Os/Ir ratio in sediments is quite variable due to the differences in redox behavior of these elements. Thus, the Os/Ir ratio of organic rich black shale and pelagic carbonates (= 3–5) is much higher than that of Fe-Mn nodules/crusts and pelagic sediment (= 0.2–0.4) (32).A difference in Os isotopic composition between meteorites and continental crustal rocks, however, makes Os isotopes a highly sensitive tracer of extraterrestrial material. Radiogenic ¹⁸⁷Os is produced from β-decay of ¹⁸⁷Re with a half-life of 42 Ga. The Os isotope ratio (¹⁸⁷Os with respect to stable and nonradiogenic isotope ¹⁸⁸Os) of various terrestrial and ET materials is thus a reflection of their Re/Os ratio and time elapsed after their formation from a primitive source. On average, ¹⁸⁷Os/¹⁸⁸Os ratios of meteorites/cosmic dust range from 0.117 to 0.128 and are similar to the primitive upper mantle (0.129 with ¹⁸⁷Re/¹⁸⁸Os = 0.36). In comparison, average ¹⁸⁷Os/¹⁸⁸Os ratio of the upper continental crust is approximately 10 times higher (1.26 with ¹⁸⁷Re/¹⁸⁸Os = 48). The Os concentration of meteorites is variable with ordinary and carbonaceous meteorites and cosmic dust ranging from 0.6 to 1 μg/g and iron meteorites ∼30 μg/g (see compilation in ref. 32). In comparison, the Os concentration of terrestrial samples is from 3 ng/g (mantle peridotite) to 10–50 pg/g (loess samples representing average upper continental crust). Low ¹⁸⁷Os/¹⁸⁸Os ratio and high Os concentration in ET material provide a substantial contrast to the upper continental material with high ¹⁸⁷Os/¹⁸⁸Os ratio and extremely low Os concentrations. Thus, ¹⁸⁷Os/¹⁸⁸Os ratios combined with Os concentrations are a robust analytical approach to determine ET contribution in samples. A number of investigations have used Os isotope composition to trace meteorite impacts throughout geological time and to determine the extent to which the meteorite component is present in target rocks or in distal ejecta (e.g., tektites) from the impact (ref. 32 and references therein).Paquay et al. (28) and others^¶ reported relatively high ¹⁸⁷Os/¹⁸⁸Os ratios in the bulk YDB sediments and concluded that PGEs in the YDB horizons in North America and Europe have a terrestrial origin. However, Firestone et al. (10) reported the highest Ir concentrations not in bulk sediments, but rather in the YDB magnetic grains. Moreover, they suggested that the magnetic grains and microspherules recovered from the YDB were most likely ET-impact ejecta. Because microspherules and magnetic grains constitute only a small fraction of YDB sediments (tens to hundreds of thousands of magnetic grains amounting to a few grams per kilogram; e.g., refs. 10, 23, 26), bulk sediment analyses can be expected to overwhelm Ir and Os values imparted by these materials. Consequently, it is essential to determine the Os abundance and isotope composition of magnetic spherules and grains to evaluate the origin and provenance of these objects.Here we examine Os abundance and isotope ratios in YDB bulk sediment samples from six locations, including Blackwater Draw, NM; Sheriden Cave, OH; Murray Springs, AZ; Gainey, MI; Melrose, PA; and Lommel, Belgium. We also investigate magnetic grains for the Gainey and Lommel sites and magnetic microspherule clusters from Newtonville, NJ and Melrose, PA. Finally, we examine the origin and provenance of large (2- to 5-mm diameter) spherules from Melrose, using their mineralogy, major element, and REE contents and Os, Nd, and Sr isotope composition. Initial data from this study have been reported previously (^¶, 33). For comparing results from other studies where only Ir is measured, we assume that ET Ir and Os should follow each other with an Os/Ir ratio of 1.1 as fractionation between Os and Ir during an impact is unlikely.     

Omission of Nitrous Oxide during Anesthesia Reduces the Incidence of Postoperative Nausea and Vomiting: A Meta-Analysis

Background: Postoperative nausea and vomiting are important causes of morbidity after general anesthesia. Nitrous oxide has been implicated as an emetogenic agent in many studies. However, several other trials have failed to sustain this claim. The authors tried to resolve this issue through a meta-analysis of randomized controlled trials comparing the incidence of postoperative nausea and vomiting after anesthesia with or without nitrous oxide.

Methods: Of 37 published studies retrieved by a search of articles indexed on the MEDLINE database from 1966 to 1994, 24 studies (26 trials) with distinct nitrous-oxide and non-nitrous oxide groups were eligible for the meta-analysis. The pooled odds ratio and relative risk were calculated. Post hoc subgroup analysis was also performed to qualify the result.

Results: The pooled odds ratio was 0.63 (0.53 to 0.75). Omission of nitrous oxide reduced the risk for postoperative nausea and vomiting by 28% (18% to 37%). In the subgroup analysis, the maximal effect of omission of nitrous oxide was seen in female patients. In patients undergoing abdominal surgery and general surgical procedures, the effect of omission of nitrous oxide, although in the same direction, was not significant.     

Anesthesia for percutaneous transcatheter closure of perimembranous ventricular septal defect

OBJECTIVE: To review the anesthetic management for percutaneous transcatheter closure of perimembranous ventricular septal defect (VSD) with an Amplatzer asymmetric occluder device and to highlight the hemodynamic effects and potential complications associated with its delivery. DESIGN: Retrospective review of prospectively collected data. SETTING: University-affiliated teaching hospital. PARTICIPANTS: Nine consecutive children undergoing elective percutaneous transcatheter closure of perimembranous VSD. INTERVENTIONS: General anesthesia with sevoflurane for cardiac catheterization and percutaneous transcatheter device placement. MEASUREMENTS AND MAIN RESULTS: Ten anesthetics were delivered in 9 children ages 23 to 65 months with perimembranous VSD for attempted placement of an Amplatzer asymmetric device. The device was successfully placed in 7 patients. In 1 patient the device embolized to the right femoral artery, and was retrieved with a bioptome. Fluoroscopy time (59.8 +/- 17.24 min) was prolonged compared to that in other studies of placement of this device. All patients had episodes of arrhythmia and hemodynamic disturbance. Arrhythmias ranged from atrial or ventricular ectopic events to various degrees of atrioventricular block. Complete heart block occurred during the procedure in 1 patient and after the procedure in another patient. Hypotensive episodes occurred in 7 patients, and were attributed to arrhythmias in 5 patients and hypovolemia in 2 patients. Two patients were given blood transfusions after the procedure because they had signs of hypovolemia and a greater than 10% decrease in hemoglobin levels. CONCLUSIONS: Anesthesia for perimembranous VSD occluder placement is associated with hemodynamic instability, arrhythmias, prolonged procedure times, and inevitable and sometimes substantial blood loss.     

Bacteriological profile in chronic dacryocystitis

A study was conducted to see the bacteriological profile in 172 cases of chronic dacryocystitis attending eye outpatients department of Midnapore Medical College and Hospital and NRS Medical College and Hospital, Kolkata. The patients underwent clinical examination of the eyes and lacrimal system, with particular attention to drainage system. The culture and sensitivity test was done from the material, which regurgitated through the punctum following pressure on sac. Out of 172 cases, 66(38.4%) were found with epiphora, 44 cases (25.9%) of epiphora with mass, 8 cases (4.6%) with epiphora mass with cellulitis and in 54 (31.4%) with chronic conjunctivitis. Bactriologically, 142 patients (82.5%) were revealed with pure culture, mixed culture in 18 cases (10.5%) and no growth of organism in 12 cases (7.0%). Staphylococcus aureus was found primary pathogen in disease process. Candida albicans were seen in 4 cases (2.3%).     

Effect of ellagic acid on hepatic and pulmonary xenobiotic metabolism in mice: studies on the mechanism of its anticarcinogenic action

Our recent studies have shown that ellagic acid, a naturallyoccurring dietary plant phenol, protects BALB/c mice against3-methylcholanthrene-induced skin tumorigenesis. To furtherelucidate the mechanism of the antineoplastic action of ellagicacid its effect on hepatic and pulmonary benzo[a]pyrene (BP)metabolism, cytochrome P-450-dependent monooxygenases and glutathioneS-transferase activities were studied in BALB/c mice. Chronicoral feeding of the compound in drinking water (0.3 mg/1 for16 weeks) or acute intraperitoneal administration (50 mg/kgfor five consecutive days) of ellagic acid resulted in 20–25%decreases in hepatic and pulmonary cytochrome P-450 levels.Hepatic and pulmonary aryl hydrocarbon hydroxylase and 7-ethoxycoumarinO-deethylase activities in both groups of ellagic acid-treatedanimals were 33–52% and 28–43% lower than theirrespective non-ellagic acid-treated controls. Hepatic as wellas pulmonary aminopyrine N-demethylase and epoxide hydrolaseactivities were unchanged in both groups of ellagic acid-treatedmice. Hepatic glutathione S-transferase activity towards BP-4,5-oxide or l-chloro-2, 4-dinitrobenzene as substrates was foundto be enhanced 51 – 79% and 38–58% in both groupsof animals. H.p.l.c. analysis of organic solvent-soluble metabolitesof BP by liver and lung microsomes indicated a substantial inhibitionof diol formation (including BP-7, 8-diol), as well as of phenolsand quinones. In liver, these inhibitory effects were more pronouncedafter oral feeding than after intraperitoneal administration.Our results indicate that both acute and chronic administrationof ellagic acid inhibits BP metabolism and/or enhances glutathioneS-transferase activity. Thus the modulation of polycyclic aromatichydrocarbon metabolism by ellagic acid may be related to theanticarcinogenic effects of this compound.     

Inflammatory response to cardiac surgery and strategies to overcome it

A general activation of the immune system is observed during any operative procedure as a physiological response to the surgical trauma. Cardiopulmonary bypass may directly activate the inflammatory response by three distinct mechanisms: direct 'contact activation' of the immune system following exposure of blood to the foreign surfaces, ischaemia-reperfusion injury to vital organs and systemic endotoxaemia resulting from gut translocation of endotoxin. The inflammatory response depends upon recruitment and activation of inflammatory cells. The cellular immune response, in particular polymorphonuclear cell-endothelial adhesion, leads to widespread endothelial damage and dysfunction. Increased oxygen derived free radical activity represents a risk for myocardial and pulmonary complications. The clinical consequences of the stress response vary from a mild generalised transient response, termed the 'systemic inflammatory response syndrome,' to life threatening organ dysfunction. The introduction of the 'off-pump' coronary artery bypass graft surgery has now made it possible to differentiate the influence of cardiopulmonary bypass and surgical access on different modalities of the immune response. 'Off-pump' cardiac surgery has been found to trigger inflammatory response, lesser than 'on-pump' cardiac surgery. Researches are directed towards understanding this complex interplay of humoral and cellular mediators and develop strategies to limit the resultant organ dysfunction. Current literature on the various mediators of this inflammatory response, the role of surgical stress, the pathogenesis of the organ damage and strategies to limit / overcome this response are reviewed.     

Primary melanocytic tumors of the central nervous system: a neuroradiological and clinicopathological study of five cases and brief review of literature

Primary melanocytic tumors of the central nervous system (CNS) are uncommon lesions. These lesions arise from the melanocytes located within leptomeninges and include diffuse melanocytosis and meningeal melanomatosis (seen in neurocutaneous melanosis), melanocytoma, and malignant melanoma. To study, the clinical course, neuroradiological features, morphology and immunohistochemistry of primary melanocytic tumor of CNS. Demographic, clinical and surgico-pathologic findings of five patients with melanocytic tumors seen between 1996 and 2003 were studied. In this study, five cases of primary melanocytic tumors have been reported: four cases of malignant melanoma and one case of melanocytoma. Three of the 5 cases were intracranial and 2 were spinal. The mean age in the present study was 26 years. Presenting features varied according to the location. Primary melanocytic tumor of CNS are rare. Whenever possible, complete surgical excision is the best treatment.