Transfer function-noise modeling and spatial interpolation to evaluate the risk of extreme (shallow) water-table levels in the Brazilian Cerrados

Water regimes in the Brazilian Cerrados are sensitive to climatological disturbances and human intervention. The risk that critical water-table levels are exceeded over long periods of time can be estimated by applying stochastic methods in modeling the dynamic relationship between water levels and driving forces such as precipitation and evapotranspiration. In this study, a transfer function-noise model, the so called PIRFICT-model, is applied to estimate the dynamic relationship between water-table depth and precipitation surplus/deficit in a watershed with a groundwater monitoring scheme in the Brazilian Cerrados. Critical limits were defined for a period in the Cerrados agricultural calendar, the end of the rainy season, when extremely shallow levels (<0.5-m depth) can pose a risk to plant health and machinery before harvesting. By simulating time-series models, the risk of exceeding critical thresholds during a continuous period of time (e.g. 10 days) is described by probability levels. These simulated probabilities were interpolated spatially using universal kriging, incorporating information related to the drainage basin from a digital elevation model. The resulting map reduced model uncertainty. Three areas were defined as presenting potential risk at the end of the rainy season. These areas deserve attention with respect to water-management and land-use planning.     

Sulfuric acid vapor and other cloud-related gases in the Venus atmosphere: Abundances inferred from observed radio opacity

Laboratory measurements of the microwave opacity of gaseous sulfuric acid under Venus atmospheric conditions indicate that it is an exceptionally strong absorber. They also suggest that its absorptivity has a surprisingly weak dependence on radio frequency, as compared with other common gaseous absorbers. Initial theoretical studies also indicate a large absorptivity and weak frequency dependence, although the measured opacity is several times the computed value, presumably due to deviations from Van Vleck-Weisskopf theory for pressures near and above about 1 atm. The absorbing characteristics of sulfuric acid vapor appear to reconcile what had been thought to be an inconsistency among measurements and deductions concerning the constituents of the atmosphere of Venus, and radio occultation, radar reflection, and radio emission measurements of its opacity. These and previous laboratory measurements of sulfur dioxide, water vapor, and carbon dioxide are used to model relative contributions to opacity as a function of height, in a way that is consistent with observations of the constituents and absorbing properties of the atmosphere. We conclude that sulfuric acid vapor is likely to be the principal microwave absorber in the 30- to 50-km-altitude range of the middle atmosphere of Venus. It would need to have a mixing ratio there of about 35 to 90 ppm if it were the sole absorber. Carbon dioxide, the predominant atmospheric gas, is the main absorber below about 30 km, while sulfur dioxide is an important but secondary absorber in both regions. Water vapor and cloud particulates appear to be only minor contributors to the total opacity. While gaseous sulfuric acid has not been directly measured in any of the in situ probe experiments (due to particular instrumental limitations), its presence at an abundance of the deduced order of magnitude is implied by these and other observations. We suggest that improved radio occultation measurements, in conjuction with high-resolution microwave emission observations and more detailed laboratory studies, could provide important data for investigating the sulfur compound chemistry in the atmosphere of Venus, and that the techniques and results may have application to the study of atmospheric conditions associated with acid rain on Earth.     

A Weichselian deglaciation model applied to the Early Permian glaciation in the northeast Karoo Basin,South Africa

It generally is assumed that the Early Permian Gondwana deglaciation in South Africa started with a collapse of the marine ice‐sheet. The northeast part of the Karoo Basin became ice‐free as a result of this collapse. The deglaciation here probably took place under temperate glacial conditions. Three glacial phases have been identified. Phase 1: the marine ice retreat of 400 km over the northeast Karoo Basin, which may have been completed over a few thousand years. The glaciers grounded in the shallower areas around the shore of the basin. Phase 2: the smaller, now mainly continental ice‐sheet here re‐stabilised and remained more or less stationary for several tens of thousand years. During this phase, between 50 and 200 m of massive glaciomarine mud with dropstones accumulated in the open, marine basin that became ice‐free during Phase 1. Isostatic uplift, as a response to the first rapid deglaciation phase, can be traced in the inland part of the region. Phase 3: the final deglaciation may have taken 10 to 20 kyr. After this time no new ice sheet was built up over southern Africa. The entire Early Permian deglaciation of the northeast Karoo Basin was completed within thousands rather than millions of years. Phases 1 and 3 had lengths similar to typical Quaternary deglaciations, whereas Phase 2 was a long, stable phase, more similar to a full Quaternary glaciation. Copyright © 2001 John Wiley & Sons, Ltd.     

Solar neutrino measurement with the radiochemical gallium detector (GALLEX)

The GALLEX experiment for the detection of solar neutrinos by means of a radiochemical gallium detector is operated by groups from Italy, France, Germany, Israel and the USA in the Gran Sasso underground laboratory (LNGS) near L'Aquila (Italy). It consists of (i) the technical scale tank made of glass fiber reinforced polyester fabric containing 101 tonnes (54 m³) of a highly concentrated (8 moll^–1) GaCl₃ solution; (ii) a gas sparging system for desorption of GeCl₄ which has been formed by interaction of the neutrinos with gallium according to⁷¹Ga + _e ⁷¹Ge + e^– and by addition of ca. 1 mg of a stable Ge isotope; (iii) the absorption columns for concentration of GeCl₄ into a volume of 1 l of water; (iv) the laboratory scale apparatus for conversion of GeCl₄ to GeH₄ and mixing with the counting gas Xe; (v) the counter filling station, and (vi) the low level proportional counters.Contributions of possible side reactions which have to be corrected for, e.g. by cosmic muons, fast neutrons and-emitters are discussed, as well as the purification of the target solution from long-lived (t _1/2 = 271 d) cosmogenic⁶⁸Ge. A first preliminary result after one year of solar neutrino measurement is presented. This constitutes the first direct measurement of the basic proton-proton fusion reaction in the core of the sun. This result, appreciably below the predictions of the standard solar model (SSM) (132 SNU) can be interpreted, together with the results of the chlorine and KAMIOKANDE experiments either by astrophysics or by neutrino oscillations (MSW effect). The solar neutrino measurements are continuing and a calibration experiment with a⁵¹Cr source is in preparation.Presented at the 2nd UN/ESA Workshop, held in Bogotá, Colombia, 9–13 November, 1992.for the GALLEX Collaboration: P. Anselmann, W. Hampel, G. Heusser, J. Kiko, T. Kirsten, E. Pernicka, R. Plaga, U. Ronn, M. Sann, C. Schlosser, R. Wink, M. Wojcik (Max Planck Institut fur Kernphysik, Heidelberg); R. v. Ammon, K.H. Ebert, T. Fritsch, K. Hellriegel, E. Henrich, L. Stieglitz, F. Weirich (Kernforschungszentrum Karlsruhe, Institut für Heisse Chemie); M. Balata, E. Bellotti, N. Ferrari, H. Lalla, T. Stolarczyk (INFN-Laboratori Nazionali del Gran Sasso); C. Cattadori, O. Cremonesi, E. Fiorini, S. Pezzoni, L. Zanotti (Universita di Milano und INFN); F. von Feilitzsch, R. Mößbauer, U. Schanda (TU München, Garching); G. Berthomieu, E. Schatzman (Observatoire Nizza); I. Carmi, I. Dostrovsky (Weizman Institut, Rehovot); C. Bacci, P. Belli, R. Bernabei, S. d'Angelo, L. Paoluzi (Universita di Roma und INFN); S. Carbit,M. Cribier, G. Dupont, L. Gosset, J. Rich, M. Spiro, C. Tao, D. Vignaud (CEN Saclay); R.L. Hahn, F.X. Hartmann, J.K. Rowley, R.W. Stoenner, J. Weneser (Brookhaven National Laboratory).     

Apophyllite (001) surface alteration in aqueous solutions studied by HAFM

Depending on pH and temperature, two different types of surface reactions occur on the apophyllite (001) surface in aqueous HCl-solutions at temperatures from 20 to 130 °C. At low pH, laterally spreading hillocks cover the surface. The hillocks are softer than the pristine surface, chemical analysis shows a depletion in Ca + K, and the spreading velocity of hillocks depends on pH. This indicates a change in chemical bond strength, non-stoichiometric dissolution and a mechanism involving protons. External disturbances such as the AFM scanning tip cause the upper surface layers to peel off revealing that the active sites of hillock formation are between the silicate layers of apophyllite. The observed process can therefore be described by a penetrative ion-replacement reaction which proceeds well below the surface monolayer. By this ion-replacement, the silicate layers eventually become destabilised. The observed reaction, therefore, is equivalent to an incongruent dissolution process. Despite structural similarities, this process is only superficially similar to the ion-exchange occurring in clay minerals or zeolites. In these minerals, the structural backbone is not destabilized. At a more neutral pH and high temperatures, step retreat and etch pit formation can be observed on the apophyllite (001) surface thus indicating a more congruent dissolution mechanism.     

Landslide and Tsunami 21 November 2000 in Paatuut,West Greenland

A large landslide occurred November 21, 2000 at Paatuut, facing the Vaigat Strait onthe west coast of Greenland. 90 million m³ (260 million tons) of mainly basalticmaterial slid very rapidly (average velocity 140 km/h) down from 1,000–1,400 maltitude. Approximately 30 million m³ (87 million tons) entered the sea, creatinga tsunami with an run-up height of 50 m close to the landslide and 28 m at Qullissat,an abandoned mining town opposite Paatuut across the 20 km wide Vaigat strait. Theevent was recorded seismically, allowing the duration of the slide to be estimated tocirca 80 s and also allowing an estimate of the surface-wave magnitude of the slideof 2.3. Terrain models based on stereographic photographs before and after the slidemade it possible to determine the amount of material removed, and the manner ofre-deposition. Simple calculations of the tsunami travel times are in good correspondencewith the reports from the closest populated village, Saqqaq, 40 km from Paatuut, whererefracted energy from the tsunami destroyed a number of boats. Landslides are notuncommon in the area, due to the geology with dense basaltic rocks overlying poorlyconsolidated sedimentary rocks, but the size of the Paatuut slide is unusual. Based onthe observations it is likely at least 500 years since an event with a tsunami of similarproportions occurred. The triggering of the Paatuut slide is interpreted to be caused byweather conditions in the days prior to the slide, where re-freezing melt water inpre-existing cracks could have caused failure of the steep mountain side.     

The geochemical controls on vent fluids from the Lucky Strike vent field, Mid-Atlantic Ridge

Hydrothermal vent fluids were collected from the Lucky Strike site at 37°17′N on the Mid-Atlantic Ridge in both 1993 and 1996. Seven vents were sampled with the DSV Alvin in 1993 and six vents were sampled in 1996 using the ROV Jason during the LUSTRE '96 Cruise. As three of the vents were sampled in both 1993 and in 1996, a time series of vent fluid chemistry is also reported. Measured temperatures ranged from 202 to 333°C at the 1618–1726 m depth of the vent field, which is located on Lucky Strike Seamount. These fluids are either equal to or less than the local bottom seawater in chlorinity. While the range in fluid compositions at Lucky Strike is generally within that observed elsewhere, the unusual aspects of the fluid chemistries are the relatively high pH and low Fe, Mn, Li and Zn. We attribute this, as well as an usually low Sr/Ca ratio, to reaction with a highly altered substrate. The high Si and Cu contents suggest a deep, as well as hot, source for these fluids. The fluid compositions therefore suggest formation by super-critical phase separation at a depth not less than 1300 m below the seafloor, and reaction with a relatively oxic, and previously altered, substrate. There is temporal variability in some of the vent fluid compositions as Li, K, Ca and Fe concentrations have increased in some of the vents, as has the Fe/Mn (molar) ratio, although the chlorinities have remained essentially constant from 1993 to 1996. While there is not a simple relationship between vent fluid compositions (or temperatures) and distance from the lava lake at the summit of the seamount, the vent fluids from many of the vents can be shown to be related to others, often at distances >200 m. The most southeasterly vents (Eiffel Tower and the Marker/Mounds vents) are distinct in chlorinity and other chemical parameters from the rest of the vents, although closely related to each other within the southeastern area. Similarly all of the vents not in this one area, appear closely related to each other. This suggests one or two source fluids for many of the vents, as is also inferred to be the case at TAG, but which is in contrast to observations on faster spreading ridges. This may suggest inherently different plumbing for hydrothermal systems at slower versus faster spreading ridges.     

Geothermal constraints on the hydrological regime of the TAG active hydrothermal mound, inferred from long-term monitoring

During August 1994 to March 1995, a period that included ODP Leg 158 drilling, bottom-water and sub-bottom temperatures were continuously logged by a long-term temperature monitoring system ‘Daibutsu’ at the base of the central black-smoker complex (CBC) and within the low heat flow zone at the TAG hydrothermal mound on the Mid-Atlantic Ridge. The temperature of hydrothermal fluid at CBC was also measured with a small high-temperature probe ‘Hobo’. Bottom-water temperature variations measured with Daibutsu at both sites have predominant semi-diurnal periods, causing the sub-bottom temperatures to fluctuate at these periods with reduced amplitudes and phase delays at sub-bottom depths. Seawater entrainment into the mound has been previously suggested at the low heat flow zone. We quantitatively evaluate the seawater entrainment rate at both sites from a one-dimensional numerical model, combined with a heat conduction model for the semi-diurnal variations. The entrainment rate of seawater at the base of CBC is estimated as 1.3±0.5×10⁻⁵ m/s, at least from August 17 to 30, 1994. On the other hand, the seawater entrainment rate at the low heat flow zone was undetected by long-term temperature monitoring at shallow sub-bottom depth. Nevertheless an increase in heat flow observed at the low heat flow zone during ODP drilling can be interpreted as a decrease in the entrainment rate of seawater. Before ODP Leg 158, Daibutsu measured three sub-bottom temperature anomalies at the base of CBC not derived from bottom-water temperature variations and Hobo also detected a CBC fluid temperature anomaly, indicating some natural changes in fluid flow within the mound. Daibutsu and Hobo also measured temperature anomalies during and after drilling at the ODP TAG-1 area. The Hobo temperature anomalies are inferred to have occurred when the cold fluid entrained through the drill holes at TAG-1 site reached or cooled the main fluid path to CBC. The entrained seawater through the drill holes appears to have contributed to dissolution and precipitation of anhydrite within the mound and perhaps affected the local permeability structure inside the mound. The temperature anomalies measured with Daibutsu at the base of CBC may have been induced by the change in the fluid flow pattern as a result of such permeability changes within the mound.     

A composite trans-Atlantic heat flow profile between 20°S and 35°S

Sixty new measurements together with published heat flow data in the South Atlantic between 20°S and 35°S latitude have been analyzed. Heat flux is greater through the eastern Mid-Atlantic Ridge flank and basin than their counterparts on the west but the standard deviation or spatial variation is greater on the west, contrary to expectation based on sediment thickness. The variance in the data indicates that this asymmetry in mean heat flux is statistically significant at 87% confidence level. A pair of ridge-flank minima appear in a composite trans-Atlantic profile of heat flux versus sea-floor age, suggesting hydrothermal circulation in the young oceanic crust. The Walvis Ridge has a mean excess heat flux of 28 mW m^?2 (0.7 μcal cm^?2 sec^?1) above the surrounding Cape and Angola Basins, and decreases along the ridge towards the northeast. Consistent with the apparent asymmetric distribution in the South Atlantic, it is also significantly higher than that of the Rio Grande Rise. We hypothesize that the trend and larger mean heat flux of the Walvis Ridge is best explained by a hot-spot origin, perhaps combined with higher radioactivity in the crust. However, the morphologic and heat flow differences between the Walvis Ridge and Rio Grande Rise suggest that these features have different geologic histories.     

Über die Variabilität zweier mariner Oscillatorien aus der Sektion «Margaritiferae»

Aquatic Sciences - Es wurde die Variabilität der Dimensionen zweier mariner Oscillatorien aus der Adria (Rovinj, Istrien) verglichen und ihre taxonomische Zugehörigkeit diskutiert. Die...