Chemical evolution of a pleistocene rhyolitic center: Sierra La Primavera,Jalisco, México

The late Pleistocene caldera complex of the Sierra La Primavera, Jalisco, México, contains well-exposed lava flows and domes, ash-flow tuff, air-fall pumice, and caldera-lake sediments. All eruptive units are high-silica rhyolites, but systematic chemical differences correlate with age and eruptive mode. The caldera-producing unit, the 45-km³ Tala Tuff, is zoned from a mildly peralkaline first-erupted portion enriched in Na, Rb, Cs, Cl, F, Zn, Y, Zr, Hf, Ta, Nb, Sb, HREE, Pb, Th, and U to a metaluminous last-erupted part enriched in K, LREE, Sc, and Ti; Al, Ca, Mg, Mn, Fe, and Eu are constant within analytical errors. The earliest post-caldera lava, the south-central dome, is nearly identical to the last-erupted portion of the Tala Tuff, whereas the slightly younger north-central dome is chemically transitional from the south-central dome to later, moremafic, ring domes. This sequence of ash-flow tuff and domes represents the tapping of progressively deeper levels of a zoned magma chamber 95,000 ± 5,000 years ago. Since that time, the lavas that erupted 75,000, 60,000, and 30,000 years ago have become decreasingly peralkaline and progressively enriched only in Si, Rb, Cs, and possibly U. They represent successive eruption of the uppermost magma in the post-95,000-year magma chamber.Eruptive units of La Primavera are either aphyric or contain up to 15% phenocrysts of sodic sanidine quartz >ferrohedenbergite >fayalite>ilmenite±titanomagnetite. Whereas major-element compositions of sanidine, clinopyroxene, and fayalite phenocrysts changed only slightly between eruptive groups, concentrations of many trace elements changed by factors of 5 to 10, resulting in crystal/glass partition coefficients that differ by factors of up to 20 between successively erupted units. The extreme variations in partitioning behavior are attributed to small changes in bulk composition of the melt because major-element compositions of the phenocrysts and temperature, pressure, and oxygen fugacity of the magma all remained essentially constant.Crystal settling and incremental partial melting by themselves appear incapable of producing either the chemical gradients within the Tala Tuff magma chamber or the trends with time in the post-caldera lavas. Transport of trace metals as volatile complexes within the thermal and gravitational gradient in volatilerich but water-undersaturated magma is considered the dominant process responsible for compositional zonation in the Tala Tuff. The evolution of the post-caldera lavas with time is thought to involve the diffusive emigration of trace elements from a relatively dry magma as a decreasing proportion of network modifiers and/or a decreasing concentration of complexing ligands progressively reduced trace-metal-site availability in the silicate melt.     

Surface properties of the Orgueil meteorite: implications for the early history of solar system volatiles

Dehydration of Orgueil by stepwise calcination produced more than a tenfold change in its Kr B.E.T. surface area, which increased to 120 m²/g, then fell to 40 m²/g. This phenomenon characterizes structures of the montmorillonite type, but not other plausible constituents of Orgueil. It results from vacating of interlayer sites by H₂O molecules which are replaced by Kr until finally the sheets collapse, excluding Kr. Differential calorimetric scans of Orgueil also gave a better match for montmorillonite than for other minerals. However, a simple identification as montmorillonite conflicts with chemical analyses of Orgueil phyllosilicates.Exchangeability of H₂O in Orgueil was shown by water regain from lab air between calcination cycles and similarily of the cycles. Room temperature dehydration revealed up to 6 per cent free surface adsorbed water. High D/H ratios in CI's may result from D enrichment in OH^? groups during equilibration of dispersed phyllosilicate dust with nebula gas at temperatures ?0°C. Adsorption on the very large free and interlayer surface areas of this dust was the major mechanism by which volatiles still uncondensed at the time of gas-dust separation (including planetary primordial Ar, Kr and Xe) were incorporated into solid solar system material.     

Small-scale electrical resistivity tomography of wet fractured rocks

This paper describes a series of experiments that tested the ability of the electrical resistivity tomography (ERT) method to locate correctly wet and dry fractures in a meso-scale model. The goal was to develop a method of monitoring the flow of water through a fractured rock matrix. The model was a four by six array of limestone blocks equipped with 28 stainless steel electrodes. Dry fractures were created by placing pieces of vinyl between one or more blocks. Wet fractures were created by injecting tap water into a joint between blocks. In electrical terms, the dry fractures are resistive and the wet fractures are conductive. The quantities measured by the ERT system are current and voltage around the outside edge of the model. The raw ERT data were translated to resistivity values inside the model using a three-dimensional Occam's inversion routine. This routine was one of the key components of ERT being tested. The model presented several challenges. First, the resistivity of both the blocks and the joints was highly variable. Second, the resistive targets introduced extreme changes the software could not precisely quantify. Third, the abrupt changes inherent in a fracture system were contrary to the smoothly varying changes expected by the Occam's inversion routine. Fourth, the response of the conductive fractures was small compared to the background variability. In general, ERT was able to locate correctly resistive fractures. Problems occurred, however, when the resistive fracture was near the edges of the model or when multiple fractures were close together. In particular, ERT tended to position the fracture closer to the model center than its true location. Conductive fractures yielded much smaller responses than the resistive case. A difference-inversion method was able to correctly locate these targets.     

Controls on salt marsh accretion: A test in salt marshes of Eastern Canada

We used¹³⁷Cs-dating to determine vertical accretion rates of 15 salt marshes on the Bay of Fundy, the Gulf of St. Lawrence, and the Atlantic coast of Nova Scotia. Accretion rates are compared to a number of factors assumed to influence vertical marsh accretion: rates of relative sea-level rise, climatic parameters (average daily temperatures and degree days) and latitude (related to insolation and day length), sediment characteristics (organic matter inventory, bulk, mineral, and organic matter density), distance of the core site from the nearest source of tidal waters, and the tidal range. Uniques to our study is a consideration of climatic parameters and latitude, which should influence organic matter production, and thus vertical accretion rates. Significant predictors of accretion rates (in order of importance) were found to be organic matter inventory, distance from a creek, and range of mean tides. Contrary to conclusions from previous studies, we found that accretion rates decreased with increasing tidal range, probably because we considered a wider span of tidal ranges, from micro- to macrotidal. Although four marshes with low organic matter inventories also show a deficit in accretion with respect to relative sea-level rise, organic matter is not limiting in two-thirds of the marshes studied, despite shorter growing seasons.     

Transport of salt and suspended sediments in a curving channel of a coastal plain estuary: Satilla River, GA

This study describes the transport of salt and suspended sediment in a curving reach of a shallow mesotidal coastal plain estuary. Circulation data revealed a subtidal upstream bottom flow during neap tide, indicating the presence of a gravitational circulation mode throughout the channel. During spring tide, landward bottom flow weakened considerably at the upstream end of the channel and changed to seaward in the middle and downstream areas of the reach, suggesting the importance of tidal pumping. Salt flux near-bottom was landward at both ends of the channel during neap tide. At spring, however, the salt flux diverged along the bottom of the thalweg suggesting that tidal pumping caused a transfer of salt vertically and laterally into the intertidal zone. Thus, landward flux of salt is maintained even in the presence of subtidal seaward flow along the bottom at the downstream end of the channel.Landward bottom stress is greater than seaward stress, preferentially transporting suspended sediments upstream. Compared with salt, however, the weight of the suspended sediments causes less upward transfer of sediments into the intertidal zone. Flood flow carried more suspended sediments landward at the upstream end compared with the downstream end. We speculate that secondary flow in the curving channel picks up increasing amounts of suspended sediments along the sides during flood and adds them to the axial flow in the thalweg. Since the landward flow along the bottom of the thalweg weakens and even reverses during spring tide, there appears to be a complex re-circulation system for sediments re-suspended in curving channels that complicates the picture of a net transport of sediments landward.     

Indigenous peoples and food entitlement losses under the impact of externally-induced change

Three issues related to hunger, famine and food security are highlighted by the problems faced by indigenous peoples in many parts of the world. They affect not only the indigenous groups themselves, but also the potential for improved and increased food availability for the rest of humankind. First, indigenous peoples themselves continue to be subjected to the shock impact of outsiders in ways that remove their traditional access to land and other resources, especially those that provide customary sources of foods. The shock is therefore very negative in nutritional as well as the better-known cultural and health terms. Second, the destruction and disruption of many indigenous peoples is reducing the pool of knowledge (ethnoschience) available concerning the biosphere and natural resources, which is of widespread value to humanity. Thirdly, this compounds the already well-known shock impact of the invaders on those natural resources through environmental destruction and habitat alteration. These issues have a potential impact on global sustainability that is far greater than might be suggested by the small numbers of indigenous peoples involved. The paper argues for priority to be given to the links between hunger and food problems, the human rights of indigenous peoples, and environmental destruction.An earlier verison of this paper was published in Hans-Georg Bohle (ed.).World of Pain and Hunger: geographical perspectives on famine and food security. Saarbrücken/Fort Lauderdale: Breitenbach Publishers, Freiburg Series in Development Studies, 1 (1994)I would like to acknowledge the support of the ESRC (UK), the IGU, the University of Greenwich, and the Henry R. Luce Food Programme at Hampshire College (USA) for enabling me to attend the IGU Workshop at which this was presented. This paper has benefitted from the helpful criticisms of members of that workshop, and of other colleagues in Britain and The Netherlands; I remain responsible for its opinions and any errors.