Showing papers by "University of Iceland published in 1997"

Seismic structure of the Iceland mantle plume

Abstract: Oceanic hotspots are generally accepted to be the manifestations of plumes of hot, upwelling mantle material1,2, but the nature of such flows remains enigmatic. Iceland, for example, is one of the most thoroughly investigated hotspots, yet previous seismological3–5 and geodynamic6–12 studies have been unable to constrain the width or temperature of the plume. Here we report the results of a regional broadband seismic experiment undertaken to determine the three-dimensional velocity structure of the upper mantle beneath Iceland using relative travel times of body waves from teleseismic earthquakes. Inversion solutions of the data show a cylindrical zone of low P- and S-wave velocities that extends from 100km to at least 400km depth beneath central Iceland. The radius of the low-velocity anomaly is about 150km, and its magnitude is approximately 2% for P waves and 4% for S waves, indicating that Iceland is underlain by a hot, narrow plume of upwelling mantle.

The δ18O record along the Greenland Ice Core Project deep ice core and the problem of possible Eemian climatic instability

Abstract: Over 70,000 samples from the 3029-m-long Greenland Ice Core Project (GRIP) ice core drilled on the top of the Greenland Ice Sheet (Summit) have been analyzed for δ8O. A highly detailed and continuous δ8O profile has thus been obtained and is discussed in terms of past temperatures in Greenland. We also discuss a three-core stacked annual δ8O profile for the past 917 years. The short-term (<50 years) variability of the annual δ8O signal is found to be 1‰ in the Holocene, and estimates for the coldest parts of the last glacial are 3‰ or higher. These data also provide insights into possible disturbances of the stratigraphic layering in the core which seems to be sound down to the onset of the Eemian. Spectral analysis of highly detailed sequences of the profile helps determine the smoothing of the δ8O signal, which for the Holocene ice is found to be considerably stronger than expected. We suggest this is due to a process involving diffusion of water molecules along crystal boundaries in the recrystallizing ice matrix. Deconvolution techniques were employed for restoring with great confidence the highly attenuated annual δ8O signal in the Holocene. We confirm earlier findings of dramatic temperature changes in Greenland during the last glacial cycle. Abrupt and strong climatic shifts are also found within the Eem/Sangamon Interglaciation, which is normally recorded as a period of warm and stable climate in lower latitudes. The stratigraphic continuity of the Eemian layers is consequently discussed in section 3 of this paper in terms of all pertinent data which we are not able to reconcile.

Quantum Geometry: A Statistical Field Theory Approach

Abstract: Preface 1. Introduction 2. Random walks 3. Random surfaces 4. Two-dimensional gravity 5. Monte Carlo simulations 6. Gravity in higher dimensions 7. Topological quantum field theories References Index.

Ice–volcano interaction of the 1996 Gjálp subglacial eruption, Vatnajökull, Iceland

Abstract: Volcanic eruptions under glaciers can cause dangerous floods and lahars1,2,3 and create hyaloclastite (fragmented glassy rock) mountains4,5,6,7,8. But processes such as the rate of heat transfer between ice and magma, edifice formation, and the response of the surrounding glacier are poorly understood, because of the lack of data. Here we present observations from the fissure eruption at Vatnajokull ice cap, Iceland, in October 1996. In the 13 days of the eruption 3 km3 of ice were melted and the erupted magma fragmented into glass forming a hyaloclastite ridge 6–7 km long and 200–300 m high under 500–750 m of ice. Meltwater of temperatures of 15–20 °C flowed along a narrow channel at the glacier bed into the Grimsvotn subglacial lake for five weeks, before draining in a sudden flood, or jokulhlaup. Subsidence and crevassing of the ice cap occurred over the eruptive fissure and the meltwater path, whereas elsewhere the glacier surface remained intact, suggesting that subglacial eruptions do not trigger widespread basal sliding in warm-based glaciers.

Study of a single BRCA2 mutation with high carrier frequency in a small population.

Abstract: Germ-line changes in the cancer-predisposition gene BRCA2 are found in a small proportion of breast cancers. Mutations in the BRCA2 gene have been studied mainly in families with high risk of breast cancer in females, and male breast cancer also has been associated with BRCA2 mutations. The importance of germ-line BRCA2 mutations in individuals without a family history of breast cancer is unknown. The same BRCA2 mutation has been found in 16/21 Icelandic breast cancer families, indicating a founder effect. We determined the frequency of this mutation, 999del5, in 1,182 Icelanders, comprising 520 randomly selected individuals from the population and a series of 632 female breast cancer patients (61.4% of patients diagnosed during the study period) and all male breast cancer patients diagnosed during the past 40 years. We detected the 999del5 germ-line mutation in 0.6% of the population, in 7.7% of female breast cancer patients, and in 40% of males with breast cancer. The mutation was strongly associated with onset of female breast cancer at age <50 years, but its penetrance and expression are varied. A number of cancers other than breast cancer were found to be increased in relatives of mutation carriers, including those with prostate and pancreatic cancer. Furthermore, germ-line BRCA2 mutation can be present without a strong family history of breast cancer. Comparison of the age at onset for mother/daughter pairs with the 999del5 mutation and breast cancer indicates that age at onset is decreasing in the younger generation. Increase in breast cancer incidence and lower age at onset suggest a possible contributing environmental factor.

Greed is good: Approximating independent sets in sparse and bounded-degree graphs

Abstract: Theminimum-degree greedy algorithm, or Greedy for short, is a simple and well-studied method for finding independent sets in graphs. We show that it achieves a performance ratio of (Δ+2)/3 for approximating independent sets in graphs with degree bounded by Δ. The analysis yields a precise characterization of the size of the independent sets found by the algorithm as a function of the independence number, as well as a generalization of Turan's bound. We also analyze the algorithm when run in combination with a known preprocessing technique, and obtain an improved $$(2\bar d + 3)/5$$ performance ratio on graphs with average degree $$\bar d$$ , improving on the previous best $$(\bar d + 1)/2$$ of Hochbaum. Finally, we present an efficient parallel and distributed algorithm attaining the performance guarantees of Greedy.

Risk factors for malignant diseases: a cohort study on a population of 22,946 Icelanders.

Abstract: The records of a cohort of 11,580 females and 11,366 males participating in an Icelandic cardiovascular risk factor study were linked with the Icelandic Cancer Registry, identifying 1,785 males and 1,490 females who had been registered with neoplastic diseases from 1968 to 1995. The interval between the time of measurement of the variables and the diagnosis of the malignancy ranged from 4 to 27 years. The variables consisted of answers from a questionnaire on smoking and the use of hypertensive drugs and anthropometric and biochemical measurements. Cox's regression was applied to analyze the predictive power of the variables on the risk of cancer after the first examination at the Heart Preventive Clinic, Reykjavik. Univariate analyses, adjusted for age, were performed for each variable and each major site. Within each major site, multivariate regression analysis was applied for variables that were found significantly (10% level in univariate analysis) positive or negative as risk factors. The results show that smoking is the most important risk factor, negative only for endometrium. For lung cancer, the risk is twice as strong for females as it is for males, whereas for pancreas, males have a relative risk ratio of 4.5, compared with 2.4 for females. Height is a risk factor for all sites for each sex, for breast in females, and for kidney in males. Several anthropometric risk factors were studied. Some of these can describe positive or negative relative risk ratios for cancer, and their use may shed light on cancer pathogenesis. Serum cholesterol is a negative risk factor for breast cancer in females, but triglycerides are a positive risk factor for cervix cancer in females and for colon or rectum and thyroid cancer in males. Serum glucose is a positive risk factor for prostate cancer and a negative risk factor for lymphomas and leukemias.

Acute Bacterial Meningitis in Adults: A 20-Year Overview

Abstract: Background: Most clinical overviews of acute bacterial meningitis have either focused on children or all age groups combined, although the disease poses serious problems in the adult population. Objective: To study the clinical and microbiological features of adult bacterial meningitis in Iceland, as a representative of the average European or North American community. Patients and Methods: Data on a total of 132 cases in 127 patients (age, ≥16 years) who were diagnosed as having acute bacterial meningitis in Iceland during the years 1975 to 1994 were collected from patient and laboratory records. Complete hospital records were found for 119 of the 132 cases identified. Results: The annual incidence was 1.7/100 000 to 7.2/ 100 000 inhabitants (mean, 3.8/100 000). The most common causative organisms wereNeisseria meningitidis(56%),Streptococcus pneumoniae(20%),Listeria monocytogenes(6%), andHaemophilus influenzae(5%). Neisseria meningitidis caused 93% of the infections in the 16to 20-year-old age group, but it caused only 25% of the infections in patients aged 45 years or older.Listeria monocytogenescaused 14% of these cases. Cases of nosocomial and recurrent meningitis were rare. A significant underlying illness or condition was present in 39% of the patients. The mean mortality was 19.7%, and it did not change during the study period. Conclusions: In a study that involved all adult patients with bacterial meningitis in a single country for 2 decades, meningococci and pneumococci were the most frequent causative agents. However, meningococci were responsible for only one fourth of the cases among adult patients aged 45 years or older; most of these cases were caused by pneumococci andListeria. Despite modern medical developments, approximately 20% of adult patients with bacterial meningitis died. Arch Intern Med. 1997;157:425-430

Retinal vessel dilatation and elongation precedes diabetic macular oedema.

Abstract: Aims/background—Retinal vessel dilatation is a well known phenomenon in diabetes. In this study, the theory of whether excessive changes in diameter and length of retinal vessels occur in the development of diabetic macular oedema was tested, supporting a hypothesis that the development of diabetic macular oedema may be linked to hydrostatic pressure changes described in Starling’s law. Methods—From fundus photographs of diabetic patients attending a regular eye screening programme, the diameter and segment length of retinal vessels were measured in three retinopathy groups (12 patients each) with diabetic macular oedema (DMO), background retinopathy and no retinopathy, over a period of approximately 4 years, ending at the time of diagnosis of diabetic macular oedema in the DMO group. Results—A statistically significant dilatation and elongation of retinal arterioles, venules, and their macular branches was found before the diagnosis of macular oedema in the DMO group. No significant changes were found in the other two groups. Conclusion—It is suggested that Starling’s law applies to the formation of oedema in the retina as in other tissues. (Br J Ophthalmol 1997;81:274‐278)

Prevalence of pain combinations and overall pain A study of headache, stomach pain and back pain among school-children

Abstract: The prevalence of pain combinations among school children is addressed in view of earlier findings of high pain prevalence in this population. The study considers the prevelance of combinations of three common pains — headache, stomach pain and back pain — in a random national sample of 2173 Icelandic 11–12 and 15–16 year-old school children. According to the results from this study there is a 78.2% prevalence of monthly pain. One or more instances of weekly pain is experienced by 40.4% of the children, and 15.6% experience two or three pains weekly. Relevance of gender is greater when dealing with combinations of monthly than weekly pain. Combinations of weekly pains are more gender related than single weekly pains. Girls have significantly more frequent overall pain than boys. The distribution of pain combinations varies by age but not overall pain. The study yields important information about the extent of the overall problem of pain among school children and indicates the need to further study the cor...

Effect of Cyclodextrins on Topical Drug Delivery to the Eye

Abstract: Cyclodextrins are oligosaccharides which form a new group of pharmaceutical excipients. Cyclodextrins have been added to aqueous eye drop preparations to solubilize lipophilic water-insoluble drug, to increase the chemical stability of drugs, or to reduce local drug irritation in the eye. Hydrophilic cyclodextrins, such as 2-hydroxypropyl-β-cyclodextrin, have been shown to be nontoxic to the eye and are generally well tolerated in aqueous eye drop formulations. However, improper formulation of aqueous cyclodextrin containing eye drop solutions can reduce the topical availability of the drug molecule. This paper reviews the effects of cyclodextrins and aqueous cyclodextrin eye drop formulations on the ocular bioavailability of drugs.

Hybrid consensus theoretic classification

Abstract: Hybrid classification methods based on consensus from several data sources are considered. Each data source is at first treated separately and modeled using statistical methods. Then weighting mechanisms are used to control the influence of each data source in the combined classification. The weights are optimized in order to improve the combined classification accuracies. Both linear and nonlinear optimization methods are considered and used in classification of two multisource remote sensing and geographic data sets. A nonlinear method which utilizes a neural network gives excellent experimental results. The hybrid statistical/neural method outperforms all other methods in terms of test accuracies in the experiments.

In vitro susceptibility of Helicobacter pylori to protolichesterinic acid from the lichen Cetraria islandica.

Abstract: With reference to the traditional use of Cetraria islandica (Iceland moss) for relief of gastric and duodenal ulcer, plant extracts were screened for in vitro activity against Helicobacter pylori. (+)-Protolichesterinic acid, an aliphatic alpha-methylene-gamma-lactone, was identified as an active component. The MIC range of protolichesterinic acid, in free as well as salt form, was 16 to 64 micrograms/ml.

Crustal deformation from 1992 to 1995 at the Mid-Atlantic Ridge, Southwest Iceland, mapped by satellite radar interferometry

Abstract: Satellite radar interferometry observations of the Reykjanes Peninsula oblique rift in southwest Iceland show that the Reykjanes central volcano subsided at an average rate of up to 13 millimeters per year from 1992 to 1995 in response to use of its geothermal field. Interferograms spanning up to 3.12 years also include signatures of plate spreading and indicate that the plate boundary is locked at a depth of about 5 kilometers. Below that depth, the plate movements are accommodated by continuous ductile deformation, which is not fully balanced by inflow of magma from depth, causing subsidence of the plate boundary of about 6.5 millimeters per year.

Readjustment of the Krafla Spreading Segment to crustal rifting measured by satellite radar interferometry

Abstract: Readjustment of the Krafla spreading segment on the Mid-Atlantic Ridge in Iceland, after a rifting episode from 1975 to 1984, is detected by radar interferometry. Crustal deformation from 1992 to 1995 is dominated by ∼24 mm/year subsidence above a shallow magma chamber at Krafla, superimposed on ∼7 mm/year along-axis subsidence of the spreading segment relative to its flanks. The deformation is caused by cooling contraction at ∼3 km depth and ductile flow of material away from the spreading axis, at a rate decreasing with time.

A spectrum of mutations in the second gene for autosomal dominant polycystic kidney disease (PKD2)

Abstract: Recently the second gene for autosomal dominant poly-cystic kidney disease (ADPKD), located on chromosome 4q21-q22, has been cloned and characterized. The gene encodes an integral membrane protein, polycystin-2, that shows amino acid similarity to the PKD1 gene product and to the family of voltage-activated calcium (and sodium) channels. We have systematically screened the gene for mutations by single-strand conformation-poly-morphism analysis in 35 families with the second type of ADPKD and have identified 20 mutations. So far, most mutations found seem to be unique and occur throughout the gene, without any evidence of clustering. In addition to small deletions, insertions, and substitutions leading to premature translation stops, one amino acid substitution and five possible splice-site mutations have been found. These findings suggest that the first step toward cyst formation in PKD2 patients is the loss of one functional copy of polycystin-2.

An International Study of Social Engagement Among Nursing Home Residents

Abstract: Method: data using the Resident Assessment Instrument (RAI) from nursing home populations in five countries (Denmark, Iceland, Italy, Japan, USA) were assembled from 396 277 residents. The distribution of a new quality of life measure, 'social engagement', embedded in the RAI and found to be reliable and valid in the USA, was examined and compared in the international samples. Results: in all five countries' nursing home populations engagement was highest among residents with adequate functioning in activities of daily living (ADL) and cognition, but the level of social engagement differed considerably by country among residents with poor ADL functioning, who had adequate cognition. The lowest scores were in Italy and Japan. The amount of time residents spend in activities stratified by ADL and cognition reveal the same pattern cross-culturally—cognitively impaired residents are least actively involved. Conclusions: the Minimum Data Set measure of social engagement is stable across types of residents and across nations and can serve as a marker of nursing home quality.

The preparation of concentrates of eicosapentaenoic acid and docosahexaenoic acid by lipase-catalyzed transesterification of fish oil with ethanol

Abstract: The objective of this study was to investigate the use of lipases as catalysts for producing concentrates of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) from fish oil as an alternative to conventional chemical procedures. Transesterification of fish oil with ethanol was conducted under anhydrous solvent-free conditions with a stoichiometric amount of ethanol. Among the 17 lipases tested, the results showed that Pseudomonas lipases had the highest activity toward the saturated and monounsaturated fatty acids in the fish oil, much lower activity toward EPA and DHA and, at the same time, good tolerance toward the anhydrous alcoholic conditions. With 10 wt% of lipase, based on weight of the fish oil triacylglycerol substrate (15% EPA and 9% DHA initial content), a 50% conversion into ethyl esters was obtained in 24 h at 20°C, in which time the bulk of the saturated and monounsaturated fatty acids reacted, leaving the long-chain n-3 polyunsaturated fatty acids unreacted in the residual mixture as mono-, di-, and triacylglycerols. This mixture comprised approximately 50% EPA+DHA. Total recovery of DHA and EPA was high, over 80% for DHA and more than 90% for EPA. The observed fatty acid selectivity, favoring DHA as a substrate, was most unusual because most lipases favor EPA.

Center of the Iceland hotspot experiences volcanic unrest

Abstract: A volcanic eruption beneath the Vatnajokull ice cap in central Iceland (Figure 1) began on September 30,1996, along a 7-km-long fissure between the volcanoes Bardarbunga and Grimsvotn. The eruption continued for 13 days and produced ˜0.5 km3 of basaltic andesite. Meltwater from the eruption site flowed into the caldera lake of the Grimsvotn volcano, where it accumulated beneath a floating ice shelf. The lake's ice dam was lifted off the glacier bed on November 4, and in the next two days more than 3 km3 of water drained out beneath the glacier and flushed down to the south coast's alluvial plain, causing extensive flooding and damage to transportation and communication systems.

Prevalence of malocclusion in 6-year-old Icelandic children.

Abstract: The prevalence of different malocclusion features was investigated in 396 6-year-old Icelandic children, using the epidemiologic registration method described by Bjork et al. in 1964. Girls were ahead of boys with regard to dental stage (P<0.01). One or more permanent teeth were congenitally missing in 5% of the children. Postnormal occlusion was found in 27% of the boys and in 31% of the girls, and prenormal occlusion was found in 6% and 5%, respectively. Straight terminal plane at the second deciduous molars was found in individuals with either normal or postnormal occlusion. Thus, it can be misleading to use the relation of the terminal planes as a measurement of the sagittal relation between the jaws. The prevalence of hypodontia was much lower than has been reported previously for Icelandic children.

A population-based study of prognosis of ruptured cerebral aneurysm: mortality and recurrence of subarachnoid hemorrhage.

Abstract: Objective To determine the risk of recurrent subarachnoid hemorrhage (SAH), mortality, and relative survivorship following SAH caused by ruptured cerebral aneurysm (RCA). Design / Methods The 86 individuals with a first diagnosis of SAH caused by RCA in Iceland from 1958 through 1968 were followed a minimum of 24 years (range, 24 to 32.5 years) or until death to determine mortality and to identify those with recurrent SAH. Results Thirty-eight patients (44%) died within 30 days of the index event. Two additional (both comatose from onset of ictus) died in the following month. There were no deaths between 3 and 6 months after onset. Based upon the age/gender-specific person years of observation for the population of Iceland, the Standardized Mortality Ratio for the 44 surgically treated patients surviving 6 months was 1.3. Those neurologically normal or with only mild impairment at 6 months had no identified increase in mortality. Excess mortality that was limited to individuals with severe neurologic deficit at 6 months could be identified through the first 10 years after onset. Two 6-month survivors experienced recurrent SAH. Conclusions Early mortality is high (47%) in patients with RCA. In patients operated on for RCA and surviving 6 months, recurrent SAH occurred in 5%. Survivorship among patients surgically treated and neurologically normal or with mild residual neurologic impairment at 6 months following presentation was similar to that expected in the general population. Survivorship among patients surgically treated who had severe residual neurologic impairment at 6 months was significantly reduced.

Solubility of water and carbon dioxide in an icelandite at 1400°C and 10 kilobars

Abstract: The concentrations of water and carbon dissolved in an icelandite glass quenched from 1400 °C and 10 kbar were measured using Fourier transform infra-red spectroscopy and elemental analyses of carbon and hydrogen. Only carbon dioxide and water were observed in the fluid phase as analysed after quenching with a qudrupole mass analyser. The mole fraction of carbon dioxide in the fluid phase ranged from 0.36 to 0.95. Carbon is dissolved as carbonate except at the highest CO2 fluid fugacity, where a small amount of molecular CO2 is observed. Dissolved carbon in the glasses, calculated as CO2, remained constant at approximately 1 wt %, in spite of the different CO2 fluid fugacities. Water was dissolved as molecular water and as hydroxyl groups, the hydroxyl concentration in the quenched glasses remaining almost constant over the whole interval, whereas the molecular water dissolves in accordance with Henry's law. Molecular water peaks at 5200␣cm−1 and 1630 cm−1, the hydroxyl peak at 4500␣cm−1, and the carbonate peaks at 1400 cm−1–1550 cm−1 have been calibrated using elemental analyses of C and H in the quenched glasses. As molecular water decreases in the melt the higher wavenumber carbonate peak is observed to move towards the molecular water peak at 1630 cm−1 causing a split of the carbonate peaks, ranging from 45 cm−1 to 100 cm−1.