341 research outputs found
A atremia
No full textUm medico de New-York, o dr. Neftel, publica nos Virch. Arch. observações suas sobre um novo estado morbido que recebeu aquele nome
The Dilemma of the Rapid Variations in CO<sub>2</sub> in Greenland Ice Cores (Abstract)
No full textStauffer and others (1984) observed in the Greenland ice core from Dye 3 CO2 variations between low values in the range of 180-200 ppm volume and higher values in the range of 240-260 ppm volume, over a period tentatively dated to 30 000-40 000 years B.P. The low values are observed in samples with low δ18O, high particle and chemical impurity contents, representing precipitation from periods of cold climate; the higher CO2 concentrations are measured in samples which reflect warm climatic conditions because of their high δ18O and low impurity contents. The change from one climatic state to the other seems to occur within less than a century. This unexpected observation has received considerable attention from the scientific community. First, the question was posed whether the rapid CO2 variations do indeed reflect atmospheric CO2-concentration changes or whether they are caused by the interaction of gases with chemical constituents in the ice matrix. Secondly, carbon-cycle modellers searched for a mechanism in the carbon system which could produce such rapid changes in atmospheric CO2. Here we try to answer the first question.Arguments in favour of an atmospheric origin of the CO2 variations are: CO2 measurements on an ice core from Siple Station, Antarctica, show a monotonous increase over the past 250 years, from values around 280 ppm volume to values overlapping closely with the direct atmospheric data which have been obtained since 1958 (Neftel and others 1985). Thus a consistent picture of the anthropogenic CO2 increase has been obtained which is compatible with estimates of the man-made CO2 emissions and with the carbon-cycle calculations of the airborne fraction of these emissions.In addition, convincing CO2-concentration data have been obtained, from two ice cores from Greenland and four ice cores from Antarctica, for the transition from low (180-200 ppm volume) late-Wisconsin values to the higher (260-300 ppm volume) Holocene values. In spite of considerable differences in temperature, fabrics and chemical properties, the Wisconsin CO2-concentration values for ice cores from different locations lie in a narrow band. The Holocene values show a greater scatter; in particular, the values obtained for the Dye 3 core are significantly higher than those for the other cores. This can be explained by a relatively high temperature, which leads to melting in summer. In refrozen melt layers, CO2 is enriched in comparison with the other air gases. On the basis of these observations we conclude that melt layers are the main factor which can lead to considerable deviations in the CO2 concentration of the air occluded in ice cores from that of the atmosphere at ice formation.To test if the melt-layer effect was the cause of the rapid changes in CO2 concentration which were observed in the Dye 3 core, increments of the Camp Century core from north Greenland were measured for CO2; they show similarly rapid Wisconsin δ18O and CO2 changes, though they are less abrupt than in the Dye 3 core. At Camp Century the present mean annual temperature is 4.5 °C lower than at Dye 3, and during the Wisconsin the temperature difference was even greater, as indicated by the δ18O values. Because even for Holocene ice in the Camp Century core the melt-layer effect on the CO2 concentration is relatively small, we exclude any similar significant effect on the Wisconsin ice. On the basis of the Greenland ice-core information alone, therefore, we have no indication that the rapid Wisconsin CO2 variations do not represent an atmospheric phenomenon.Arguments against an atmospheric origin for the rapid CO2 variations stem from detailed analyses of the ice core from Byrd Station, Antarctica. Observation of the increase in anthropogenic atmospheric CO2 in both hemispheres indicates that within a few years CO2 becomes well mixed throughout the atmosphere. Thus the CO2 variations observed in the Greenland core should also appear in the Antarctic core increments which represent corresponding periods. A thorough CO2 study was performed on a Byrd Station core fraction which, according to a tentative time-scale (personal communication from C.U. Hammer, 1987), corresponds to the time interval 5000-55 000 years B.P. (Fig. 1). In what follows, we briefly discuss the data in the 25 000–55 000 years B.P. time interval. A first look reveals that all the data lie in a band between 180 and 220 ppm volume; no values are observed in the 240 and 260 ppm volume band. There are periods showing trends, e.g. from c. 210 ppm volume around 40 000 years B.P. to c. 185 ppm volume around 32 000 years B.P., and periods of strong scattering of the data, e.g. between 41 000 and 44 000 years B.P., with values in the 185-215 ppm volume band. At Byrd Station the annual accumulation is 0.16 m of water equivalent, compared to 0.50 m of water equivalent at Dye 3. This means that at Byrd Station, both the difference between the age of the surrounding ice and the mean age of the air, and the width of the age distribution of the gases, are larger than at Dye 3. In addition, the Byrd Station core does not show the rapid δ18O changes which indicate warm periods with relatively high accumulation. Thus one might argue that in the Byrd Station core the CO2 changes are dampened, due to a significantly wider age distribution of the gases in the ice.This effect was modelled by Neftel and others (in press). They applied a deconvolution procedure in order to reinstall the range of possible atmospheric CO2 concentrations (see Fig. 2). This procedure results in a somewhat wider band of possible CO2 concentrations; in some periods it is 170-230 ppm volume. But it does not produce the possible high values in the range observed in the Greenland ice cores. All this new information leads to the following ambiguous conclusions:The new CO2 data for the Camp Century ice core show rapid Wisconsin CO2 changes similar to those in the Dye 3 data, in spite of the differences in the temperatures and the physical and chemical properties of the two ice cores. However, similar CO2 changes do not appear in the Byrd Station ice core (from Antarctica) in the core fraction thought to represent the corresponding time interval.There are three possible ways to resolve the apparent discrepancy:(1) In the Wisconsin warm-period ice, CO2 is artificially enriched, due to a process which has not yet been identified.(2) In the Byrd Station core, the CO2 changes during the Wisconsin are attenuated to a much higher degree than was assumed in the model calculations by Neftel and others (in press).(3) The increments in both the Greenland and Byrd Station cores which have been investigated do not represent overlapping time intervals, perhaps due to rheological irregularities in the Greenland ice sheet close to bedrock (Staffelbach and others 1988, this volume).With the existing core material it is difficult to resolve this important problem. The dilemma of the rapid Wisconsin CO2, variations is one of the many important arguments for the recovery of new deep cores both in Greenland and in Antarctica.</jats:p
Theoretical and practical limitations of the acetylene inhibition technique to determine total denitrification losses
No full textThe loss of N2 from intensively managed agro-ecosystems is an important part of the N budget. Flux monitoring of N2 emissions at the field scale, e.g., by eddy correlation or aerodynamic gradient method, is impossible due to the large atmospheric N2 background (78%). The acetylene (C2H2) inhibition technique (AIT) is a rather simple and frequently used, albeit imperfect, method to determine N2 losses from intact soil cores. In principle, AIT allows an estimation of total denitrification at high temporal resolution and on small spatial scales, with limited workload and costs involved. To investigate its potential and limitations, a laboratory system with two different detection systems (photoacoustic IR spectroscopy and gas chromatography) is presented, which allowed simultaneous measurements of up to 7 intact soil cores in air-tight glass tubes in a temperature controlled cabinet (adjusted to field conditions) with automated C2H2 injection. A survey of total denitrification losses (N2 + N2O) over 1.5 yr in soil cores from an intensively managed, cut grassland system in central Switzerland supports previous reports on severe limitations of the AIT, which precluded reliable estimates of total denitrification losses. Further, the unavoidable sampling and transfer of soil samples to the laboratory causes unpredictable deviations from the denitrification activity in the field
Evidence for a 50% increase in H202 over the past 200 years from a Greenland ice core
No full textHydrogen peroxide has attracted increasing attention from atmospheric chemists over the past decade, because in the gas phase it acts as a reservoir species of OH radicals (1,2), and in the aqueous phase it plays a key role in the oxidation of SO2 to H2SO4 in clouds (3). It is also known to have an adverse effect on trees and plants (4). In 1984, hydrogen peroxide was identified as one of the dominant trace species in polar ice (5), introducing the possibility of constructing a record of atmospheric hydrogen peroxide concentrations from ice-core data. Here we present such a record for the past 700 years from Summit, Central Greenland. We find that hydrogen peroxide concentrations have increased by 50% over the past 200 years, with most of the increase occurring in the past 20 years, indicating that human activities may be responsible for such a dramatic change
Measurements of hydrogen peroxide in polar ice samples
No full textHydrogen peroxide, a powerful oxidant, is believed to be a key component in the oxidation of SO2 to H2SO4 in clouds1. The first quantitative H2O2 measurements in snow, rain, hoarfrost and fog were reported in 1874 (ref. 2), however, systematic investigations of H2O2 concentrations in precipitation and hydrometeors began only a few years ago3,4. We report here measurements of hydrogen peroxide in polar ice samples. To our knowledge, chemically-reactive species have not been previously analysed in ice core samples. Our measurements show that H2O2 is a dominant trace compound present in clouds over remote and clean areas
What have we learned from the ice cores about the atmospheric changes in the concentrations of nitrous oxide, hydrogen peroxide and other trace species?
No full textCO2 measurements on 1-cm3 ice samples with an IR laserspectrometer (IRLS) combined with a new dry extraction device
No full textA new dry gas extraction and analysis method for small (1 cm3) ice samples is presented. The extraction device, cooled to −20°C, contains two movable steel needle matrices for crushing the ice. During the crushing process the gas escaping from the ice sample is continuously analyzed for CO2 with an infrared laserspectrometer. This method enables a fast measurement (few minutes) of the CO2 concentration in the air bubbles with high spatial resolution in the ice core and a minimum potential contamination. An important CO2 contamination source due to an interaction of water vapour with surfaces is shortly discussed.
The reproducibility of extraction and analysis is ±2%. We analyzed CO2 concentrations in the trapped air from different ice cores originating from the Greenland ice sheet and from Antarctica
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