Seismicity at Fuego,Pacaya, Izalco,and San Cristobal Volcanoes,Central America, 1973–1974

Seismic data collected at four volcanoes in Central America during 1973 and 1974 indicate three sources of seismicity: regional earthquakes with hypocentral distances greater than 80 km, earthquakes within 40 km of each volcano, and seismic activity originating at the volcanoes due to eruptive processes. Regional earthquakes generated by the underthrusting and subduction of the Cocos Plate beneath the Caribbean Plate are the most prominent seismic feature in Central America. Earthquakes in the vicinity of the volcanoes occur on faults that appear to be related to volcano formation. Faulting near Fuego and Pacaya volcanoes in Guatemala is more complex due to motion on a major E-W striking transform plate boundary 40 km north of the volcanoes. Volcanic activity produces different kinds of seismic signatures. Shallow tectonic or A-type events originate on nearby faults and occur both singly and in swarms. There are typically from 0 to 6 A-type events per day withb value of about 1.3. At very shallow depths beneath Pacaya, Izalco, and San Cristobal large numbers of low-frequency or B-type events are recorded with predominant frequencies between 2.5 and 4.5 Hz and withb values of 1.7 to 2.9. The relative number of B-type events appears to be related to the eruptive states of the volcanoes; the more active volcanoes have higher levels of seismicity. At Fuego Volcano, however, low-frequency events have unusually long codas and appear to be similar to tremor. High-amplitude volcanic tremor is recorded at Fuego, Pacaya, and San Cristobal during eruptive periods. Large explosion earthquakes at Fuego are well recorded at five stations and yield information on near-surface seismic wave velocities (α=3.0±0.2 km/sec.).     

X-ray absorption contrast images of binary chemical reactions

Low-divergence synchrotron-sourced X-rays enable a radiographic imaging scheme for full characterization of binary chemical reactions and characterization by type of more complex reactions, in situ, in diamond anvil cells (DAC). Spatially resolved reactants are induced to react by laser heating of their interface. The spatially intermediate products are observed through X-ray absorption contrast. Limits to the technique include the ability to maintain controlled experiment geometry during compression and the ability to resolve chemical differences between reactants and products by X-ray absorption. The ability to make in situ observations at experimental pressure and temperature obviates the problem with quenching techniques for capturing liquid compositions in experiments with dimensions smaller than the diffusion length during quenching time. Partially molten Fe-alloy systems, of poor quenchability, are examined at DAC pressures and temperatures for relevance to Earth's core constitution and evolution. Determinations of eutectic melting in Fe–FeS match known results. Of the probable light elements that may alloy with Fe in the Earth's liquid outer core, Fe–FeS experiments show only modest quenching problems, but C and Si alloy experiments are highly vulnerable to quenching artifacts. The observed reactivity of FeS, Fe₃C, FeSi, and FeO(OH) with Fe in DAC makes the observed non-reactivity between Fe and FeO more significant, reducing the probability that oxygen alone is the major alloy in Earth's molten outer core.     

River adjustment to changes in sediment load: The effects of tin mining on the Ringarooma River,Tasmania, 1875–1984

The mining of alluvial tin in the Ringarooma basin began in 1875, reached a peak in 1900–20, and had virtually ceased by 1982. During that time 40 million m³ of mining waste were supplied to the main river, quickly replacing the natural bed material and requiring major adjustments to the channel. Based on estimates of sediment supply from more than 50 widely scattered mines and the frequency of flows capable of transporting the introduced load, the river's transport history is reconstructed using a mass-conservation model. Because of the lengthy time period (110 years) and river distance (75 km) involved, the model cannot predict detailed change but it does reproduce the main pattern of sediment movement in which successive phases of aggradation and degradation progress downstream. Peak storage is predicted in that part of the river where braiding and anastomosis are best developed. Aggradation was most rapid in the upper reaches close to major supply points, becoming slower and later with distance downstream. Channel width increased by up to 300 per cent where the valley floor was broad and braiding became relatively common. Bridges had frequently to be replaced. While bed levels were still rising in lower reaches, degradation began in upper ones, notably after 1950, and by 1984 had progressed downriver over 30 km. Rates of incision reached 0·5 m yr^?1, especially in the early 1970s when record high flows occurred. As a result of degradation the bed material became gravelly through either reexposure of the original bed or lag concentration of coarser fractions. Also a narrower unbraided channel has developed. The river is beginning to heal itself and upper reaches now have reasonably stable beds but at least another 50 years will be required for the river to cleanse its channel of mining debris.     

Context and climate change: an integrated assessment for Barrow, Alaska

An intensive approach to Barrow, Alaska’s adaptations to climate change and variability during recent decades suggests reconsideration of the interconnected roles of science, policy, and decision-making structures. First, profound uncertainties are inherent in unique interactions among the many natural and human factors affecting Barrow’s vulnerability. Science cannot significantly reduce these uncertainties through extensive approaches, but intensive approaches can reconstruct and update local trends, clarify the underlying dynamics, and harvest experience for policy purposes. Second, sound policies to reduce Barrow’s vulnerability to coastal erosion and flooding must incorporate these profound uncertainties and the multiple values of the community. Minimizing vulnerability to climate change is only one of the community’s interests, and must compete with other interests for limited time, attention, funds and other resources. Third, the community itself is in the best position to understand its own context, to decide on sound policies, and to take responsibility for those decisions. In short, local context matters in science, policy, and decision-making structures for adaptation to climate change and variability. Overall, cognitive constraints may be the most important human dimension of climate change. Factoring the global problem into more tractable local problems would make the most of our cognitive capacity.     

A Method for Estimating Field-Scale Mass Transfer Rate Parameters and Assessing Aquifer Cleanup Times

A general method for estimating ground-water solute mass transfer rate parameters from field test data is presented. The method entails matching solute concentration and hydraulic head data collected during the recovery phase of a pumping test through application of a simulation-regression technique. Estimation of hydraulic conductivity and mass transfer rate parameter values is performed by fitting model simulations to the data. Parameter estimates are utilized to assess cleanup times for pump-and-treat aquifer remediation scenarios. Uncertainty in the cleanup time estimate is evaluated using statistical information obtained with the parameter estimation technique. Application of the method is demonstrated using a hypothetical ground-water flow and solute transport system. Simulations of field testing, parameter estimation, and remedial time frames are performed to evaluate the usefulness of the method. Sets of random noise that signify potential field and laboratory measurement errors are combined with the hypothetical data to provide rigorous testing of the method. Field tests are simulated using ranges of values for data noise, the mass transfer rate parameters, the test pumping rates, and the duration of recovery monitoring to evaluate their respective influence on parameter and cleanup time estimates. The demonstration indicates the method is capable of yielding accurate estimates of the solute mass transfer rate parameters. When the parameter values for the hypothetical system are well estimated, cleanup time predictions are shown to be more accurate than when calculated using the local equilibrium assumption.     

Revised age of proximal deposits in the Zagros foreland basin and implications for Cenozoic evolution of the High Zagros

The regionally extensive, coarse-grained Bakhtiyari Formation represents the youngest synorogenic fill in the Zagros foreland basin of Iran. The Bakhtiyari is present throughout the Zagros fold-thrust belt and consists of conglomerate with subordinate sandstone and marl. The formation is up to 3000 m thick and was deposited in foredeep and wedge-top depocenters flanked by fold-thrust structures. Although the Bakhtiyari concordantly overlies Miocene deposits in foreland regions, an angular unconformity above tilted Paleozoic to Miocene rocks is expressed in the hinterland (High Zagros).

The Bakhtiyari Formation has been widely considered to be a regional sheet of Pliocene–Pleistocene conglomerate deposited during and after major late Miocene–Pliocene shortening. It is further believed that rapid fold growth and Bakhtiyari deposition commenced simultaneously across the fold-thrust belt, with limited migration from hinterland (NE) to foreland (SW). Thus, the Bakhtiyari is generally interpreted as an unmistakable time indicator for shortening and surface uplift across the Zagros. However, new structural and stratigraphic data show that the most-proximal Bakhtiyari exposures, in the High Zagros south of Shahr-kord, were deposited during the early Miocene and probably Oligocene. In this locality, a coarse-grained Bakhtiyari succession several hundred meters thick contains gray marl, limestone, and sandstone with diagnostic marine pelecypod, gastropod, coral, and coralline algae fossils. Foraminiferal and palynological species indicate deposition during early Miocene time. However, the lower Miocene marine interval lies in angular unconformity above ~ 150 m of Bakhtiyari conglomerate that, in turn, unconformably caps an Oligocene marine sequence. These relationships attest to syndepositional deformation and suggest that the oldest Bakhtiyari conglomerate could be Oligocene in age.

The new age information constrains the timing of initial foreland-basin development and proximal Bakhtiyari deposition in the Zagros hinterland. These findings reveal that structural evolution of the High Zagros was underway by early Miocene and probably Oligocene time, earlier than commonly envisioned. The age of the Bakhtiyari Formation in the High Zagros contrasts significantly with the Pliocene–Quaternary Bakhtiyari deposits near the modern deformation front, suggesting a long-term (> 20 Myr) advance of deformation toward the foreland.     

Paléo-bactéries et minéralisations plombo-zincifères du Bled Zelfane (tunisie centrale)

Résumé Le Bled Zelfane est un synclinal mio-pliocène sur la bordure septentriono-orientale de l'ile de Kasserine. L'ile de Kasserine est une plate-forme devenue insulaire à partir du Crétacé supérieur et restée émergée jusqu'au Miocène moyen. Ce môle a été soumis apparemment pendant sa période continentale à un climat oxydant de type méditerranéen relativement sec. On note une certaine diversification dans la nature des dépôts en zones basses (secteur de Kasserine) et en zones hautes (secteur du Bled Zelfane). En zones basses, la sécheresse climatique a favorisé la formation de dépôts de gypse, d'argiles rouges et de calcrêtes. En zones hautes, une érosion par lessivage acide a créé des dépressions karstiques dans les calcaires du Crétacé supérieur (Aïn Khamouda) et dans les calcaires du Maëstrichtien (Sidi Mabrouk et Guémata). Le remplissage des cavités karstiques (poches, crevasses) s'est fait avec de l'argile blanche (halloysite), contenant des micro-cristaux de galène, avec de la calamine et des oxydes de fer. Les conditions climatiques étaient trop oxydantes pour que le zinc se dépose autrement que sous forme de calamine. Elles expliquent aussi la présence de paléo-algobactéries (aérobies) sur la calamine. La présence de micro-cristaux de galène dans l'halloysite serait liée à un sulfuretum et non à des conditions réductrices réelles. Il semble en effet que dans ces karsts du Bled Zelfane une flore bactérienne très riche a joué un rôle essentiel dans le piégeage d'éléments lourds et notamment dans la genèse de galènes supergènes et dans la cristallisation de la calamine. Une minéralisation, elle aussi plombo-zincifère, de type stratiforme, s'observe à la base de Miocène inférieur et montre de même des bactéries fossiles à foison.

---

Bled Zelfane is a Mio-Pliocene syncline on the NE edge of the palaeo-island of Kasserine, a platform which became insular at the beginning of the Upper Cretaceous and remained emergent until the Middle Miocene. During its long emergence this platform was affected by an oxidising, relatively dry, Mediterranaean-type climate. There exists a certain diversity in the sedimentary accumulations in both the lower (Kasserine area) and upper (Bled Zelfane area) sectors of this palaeo-relief. In the lower sectors the dry climate has favourised gypsum, red clays and calcrete formation. In the higher sectors erosion and leaching by acid waters has created karstic depressions in the Upper Cretaceous (Aïn Khamouda) and in the limestones of Maestrichtian (Sidi Mabrouk and Guémata). The karst cavities (pockets and fissures) are filled with white, halloysite clays containing micro-crystals of galena associated with calamine (Zn) and iron oxides. The climatic conditions were too oxidising to permit the formation of Zn other than in the form of calamine. The dry climate is also an explanation for the presence of aerobic palaeo-algobacteria on this calamine. The presence of micro-crystals of galena within the halloysite is probably related to a sulfuretum and not to effective reducing conditions. It would seem that the presence of a rich bacterial flora within the karst at Bled Zelfane has played a significant role in the stabilisation of heavy elements, and especially the supergene genesis of the galena. A Lead/Zinc mineralisation also exists at the base of the Miocene. This latter is diffused through the cement in conglomerates and sandstones.

     

Impact melting of the Cachari eucrite 3.0 Gy ago

Both the host phase and glass veins of the Cachari eucrite have been analyzed by microprobe and neutron activation analysis for their chemical compositions and by mass spectrometry for their ³⁹Ar-⁴⁰Ar gas retention ages. Cachari is chemically similar to other non-cumulate eucrites. The vesicular glass veins vary from pure glass, to devitrified glass, to areas that are substantially crystalline. The glassy areas have nearly the same concentrations of major and trace elements as the unmelted portions of Cachari, but some differences, probably due to preferential dissolution, occur along melt contacts. The glass formed by shock melting of Cachari host or of rock identical to it. ³⁹Ar-⁴⁰Ar data for the host and glass suggest distinctly different ages of 3.04 ±.07 Gy and 3.47 ±.04 Gy, respectively. The time of glass formation, which may also be the time of brecciation, is most likely given by the 3.0 Gy age of the host. The higher age for the glass is interpreted to represent incomplete Ar degassing during the 3.0 Gy event due to the greater resistance to Ar diffusion shown by the glass compared to the host. Event ages significantly younger than 4.5 Gy have now been determined for several eucrites and howardites and suggest a long dynamic regolith history for the parent body.