Major and trace-element composition and pressure–temperature evolution of rock-buffered fluids in low-grade accretionary-wedge metasediments, Central Alps

The chemical composition of fluid inclusions in quartz crystals from Alpine fissure veins was determined by combination of microthermometry, Raman spectroscopy, and LA-ICPMS analysis. The veins are hosted in carbonate-bearing, organic-rich, low-grade metamorphic metapelites of the Bündnerschiefer of the eastern Central Alps (Switzerland). This strongly deformed tectonic unit is interpreted as a partly subducted accretionary wedge, on the basis of widespread carpholite assemblages that were later overprinted by lower greenschist facies metamorphism. Veins and their host rocks from two locations were studied to compare several indicators for the conditions during metamorphism, including illite crystallinity, graphite thermometry, stability of mineral assemblages, chlorite thermometry, fluid inclusion solute thermometry, and fluid inclusion isochores. Fluid inclusions are aqueous two-phase with 3.7–4.0 wt% equivalent NaCl at Thusis and 1.6–1.7 wt% at Schiers. Reproducible concentrations of Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, B, Al, Mn, Cu, Zn, Pb, As, Sb, Cl, Br, and S could be determined for 97 fluid inclusion assemblages. Fluid and mineral geothermometry consistently indicate temperatures of 320 ± 20 °C for the host rocks at Thusis and of 250 ± 30 °C at Schiers. Combining fluid inclusion isochores with independent geothermometers results in pressure estimates of 2.8–3.8 kbar for Thusis, and of 3.3–3.4 kbar for Schiers. Pressure–temperature estimates are confirmed by pseudosection modeling. Fluid compositions and petrological modeling consistently demonstrate that chemical fluid-rock equilibrium was attained during vein formation, indicating that the fluids originated locally by metamorphic dehydration during near-isothermal decompression in a rock-buffered system.     

Fire in the virgin forests of the Boundary Waters Canoe Area,Minnesota

Fire largely determined the composition and structure of the presettlement vegetation of the Boundary Waters Canoe Area as well as the vegetation mosaic on the landscape and the habitat patterns for wildlife. It also influenced nutrient cycles, and energy pathways, and helped maintain the diversity, productivity, and long-term stability of the ecosystem. Thus the whole ecosystem was fire-dependent.At least some overstory elements in virtually all forest stands still date from regeneration that followed one or more fires since 1595 A.D. The average interval between significant fire years was about 4 yr in presettlement times, but shortened to 2 yr from 1868 to 1910 during settlement. However, 83% of the area burned before the beginning of suppression programs resulted from just nine fire periods: 1894, 1875, 1863–1964, 1824, 1801, 1755–1959, 1727, 1692, 1681. The average interval between these major fire years was 26 yr. Most present virgin forests date from regeneration that followed fires in these years. Significant areas were also regenerated by fires in 1903, 1910, 1936, and 1971. Most major fire years occurred during prolonged summer droughts of subcontinental extent, such as those of 1864, 1910, and 1936. Many fires were man-caused, but lightning ignitions were also common. Lightning alone is probably a sufficient source of ignitions to guarantee that older stands burned before attaining climax. Dry matter accumulations, spruce budworm outbreaks, blowdowns, and other interactions related to time since fire increase the probability that old stands will burn. Vegetation patterns on the landscape were influenced by such natural firebreaks as lakes, streams, wetlands, and moist slopes. Red and white pine are most common on islands, and to the east, northeast, or southeast of such firebreaks. Jack pine, aspen-birch, and sprout hardwood forests are most common on large uplands distant from or west of such firebreaks.A Natural Fire Rotation of about 100 yr prevailed in presettlement times, but many red and white pine stands remained largely intact for 150–350 yr, and some jack pine and aspen-birch forests probably burned at intervals of 50 yr or less. There is paleoecological evidence that fire was an ecosystem factor before European man arrived, and even before early man migrated to North America. Probably few areas ever attained the postulated fir-spruce-cedar-birch climax in postglacial times. To understand the dynamics of fire-dependent ecosystems fire must be studied as an integral part of the system. The search for stable communities that might develop without fire is futile and avoids the real challenge of understanding nature on her own terms.To restore the natural ecosystem of the Canoe Area fire should soon be reintroduced through a program of prescribed fires and monitored lightning fires. Failing this, major unnatural, perhaps unpredictable, changes in the ecosystem will occur.     

Maps of recent vertical crustal movements in Romania: Similarities and differences

A new map of recent crustal vertical movements (RCVM) in Romania, on the basis of high-precision geodetical measurements by the repeated levelling method, had become necessary firstly as a consequence of the great seismic event of March 4, 1977 with foci under the Vrancea Region (the most important seismic zone in the country) and secondly because many years had passed since the previous edition.Certainly, there are similitudes and differences between the two editions, and the authors will try to put these into evidence and explain them.The general view of the major geological units of Romania in both maps is the same: the mountain regions being zones of more or less continuous uplift, the plains showing subsidence movements and the plateau regions presenting a relative stability. Some modifications became apparent owing to the improvement of the national geodetical network on which the repeated levelling measurements were performed and following the choice of the Harbour of Constanţa as the fundamental point.     

Decoupling the mechanical role of pore liquids in soils to liquid viscous effect and solid–liquid adhesion effect

Acta Geotechnica - This paper presents a novel conceptual approach for evaluating the mechanical effect of pore liquids on the overall geotechnical behavior. The approach is based on empiric...