Relationship of manganese-iron oxides and associated heavy metals to grain size in stream sediments

The distribution of ammonium citrate-leachable lead, zinc and cadmium among size fractions in stream sediments is strongly influenced by the presence of hydrous Mn-Fe oxides in the form of coatings on sediment grains. Distribution curves showing leachable metals as a function of particle size are given for eight samples from streams in New York State. These show certain features in common; in particular two concentrations of metals, one in the finest fractions, and a second peak in the coarse sand and gravel fraction. The latter can be explained as a result of the increasing prevalence and thickness of oxide coatings with increasing particle size, with the oxides serving as collectors for the heavy metals. The distribution of Zn and Cd in most of the samples closely parallels that of Mn; the distribution of Pb is less regular and appears to be related to Fe in some samples and Mn in others. The concentration of metals in the coarse fractions due to oxide coatings, combined with the common occurrence of oxide deposition in streams of glaciated regions, raises the possibility of using coarse materials for geochemical surveys and environmental heavy-metal studies.     

Environmental change recorded in sediments from the Marmes rockshelter archaeological site, southeastern Washington state, USA

The Marmes Rockshelter archaeological site in southeastern Washington state contains a > 11 kyr stratigraphic record that was excavated in the 1960s but only recently analyzed in detail. We present the results of physical, chemical, and isotopic analyses of archived Marmes sediments from rockshelter, hillslope, and floodplain locations. Multiple lines of evidence including éboulis production, soil chemistry, and δ¹³C and δ¹⁸O signatures in soil organic matter and calcium carbonate suggest that relatively cool, moist conditions 10,600 to 9700 ¹⁴C yr BP were followed by relatively warm and dry conditions as early as 9000 ¹⁴C yr BP. Warm and dry conditions extended to the late Holocene, followed by a return to cooler and moister climate. The limited range of δ¹³C and δ¹⁸O values in Marmes paleosols suggests that the magnitude of moisture and temperature shifts was locally buffered in the lower Snake River Canyon but adequate to generate significant changes in sedimentation and soil formation, possibly due to nonlinear geological and pedological processes. These buffered canyon environments were well suited for establishing residential bases associated with foraging and logistical collecting strategies and may have minimized the influence of climate changes in food resource abundance.     

Crustal attenuation in the Southern Andean retroarc (38°–39°30′ S) determined from tectonic and gravimetric studies: The Lonco-Luán asthenospheric anomaly

The western retroarc of the Southern Andes between 38° and 40° S is formed by a NNW-elongated ridge not associated with stacked thrust sheets. On the contrary, during the last 4–3 Ma this ridge was affected by extensional deformation, regional uplift and related folding on a very broad scale. Receiver function analysis shows that the drainage divide area and adjacent retroarc lie over an attenuated crust. Expected crustal thickness at these latitudes is around 38 km, whereas in this part of the retroarc the thickness is less than 32 km. The causes for such attenuation have been linked to a moderate steepening of the subducted Nazca plate beneath the South American plate, which is suggested by a westward shift and narrowing of the magmatic arc during the last 4 to 5 Ma. Gravimetric studies show that the upper plate did not react homogeneously to slab steepening, but ancient sutures and lithospheric discontinuities deeply buried under Mesozoic to Cenozoic sequences in the retroarc were locally reactivated. These processes resulted in an asthenospheric anomaly that correlates at the surface with the area of Pliocene to Quaternary doming, widespread extension and three radial troughs. Two of the troughs have accommodated substantial amounts of extension, but the third was probably aborted at an early stage. Moreover, the presence of an anomalous concentration of calderas and large volcanic centers over the proposed asthenospheric anomaly, and their age distribution, may indicate minor migration of the asthenospheric anomaly between 4 and 2 Ma through the western South American plate.     

Locational components of urban and regional public policy in postwar Vietnam: The case of Ho Chi Minh city (Saigon)

Ho Chi Minh city is a primate city in which wartime dislocations have resulted in severely overcrowded conditions as well as a shrunken economic base. The new government has opted for a policy based on the rapid decanting of a large proportion of the urban population by encouraging internal migration to New Economic Zones in the hinterlands. This policy is based on a hierarchial administrative structure with agricultural productivity as its prime goal. A Green Belt policy has been articulated that uses the New Economic Zones as building blocks. The Green Belts constrain urban growth as well as provide foodstuff for the remaining urban residents. The ultimate objective of the overall policy is a balanced urban and regional environment that is in tune with Vietnam's agricultural economy.     

Two-feldspar geothermometry in granulite facies metamorphic rocks

A geothermometric technique based on equilibria between coexisting plagioclase and alkali feldspar was applied to quartzo-feldspathic granulites from Salvador, BA, Brazil. The conditions of metamorphism were determined to be in the range 750 ° C–800 ° C, 4–8 Kb, by comparison with experimental data on the stabilities of sapphirine, phlogopite and other minerals occurring in the associated rocks. Selected feldspar data gives temperatures near, but slightly below, this range. Several variants of the Wood and Banno model, as well as an empirical two-pyroxene geothermometer, were also tested and found to give temperatures which were apparently 50 °–100 ° high. The solubility of Al₂O₃ in orthopyroxene indicates temperatures which are about 200 ° to high, suggesting that Fe in the natural assemblages significantly changes relationships observed experimentally in MgO-Al₂O₃-SiO₂ systems.     

High outflow channels on Mars indicate Hesperian recharge at low latitudes and the presence of Canyon Lakes

A number of martian outflow channels were carved by discharges from large dilational fault zones. These channels were sourced by groundwater, not surface water, and when observed on high-standing plateaus they provide indicators of elevated paleo-groundwater levels. We identify three outflow channels of Hesperian age that issued from a 750-km-long fault zone extending from Candor Chasma to Ganges Chasma. Two of these channels, Allegheny Vallis and Walla Walla Vallis, have sources >2500 m above the topographic datum, too high to be explained by discharge from a global aquifer that was recharged solely in the south polar region. The indicated groundwater levels likely required regional sources of recharge at low latitudes. The floodwaters that erupted from Ophir Cavus to form Allegheny Vallis encountered two ridges that restricted the flow, forming temporary lakes. The flow probably breached or overtopped these obstructions quickly, catastrophically draining the lakes and carving several scablands. After the last obstacle had been breached, a single main channel formed that captured all subsequent flow. We performed hydrologic analyses of this intermediate phase of the flooding, prior to incision of the channel to its present depth. Using floodwater depths of 30-60 m, we calculated flow velocities of 6-15 m s⁻¹ and discharges in the range of . Locally higher flow velocities and discharges likely occurred when the transient lakes were drained. Variable erosion at the channel and scabland crossing of MOLA pass 10644 suggests that the upper 25-30 m may consist of poorly consolidated surface materials underlain by more cohesive bedrock. We infer that an ice-covered lake with a surface elevation >2500 m probably existed in eastern Candor Chasma because this canyon is intersected by the Ophir Catenae fault system from which Allegheny Vallis and Walla Walla Vallis originated. We introduce a new hydrology concept for Mars in which the groundwater system was augmented by recharge from canyon lakes that were formed when water was released by catastrophic melting of former ice sheets in Tharsis by effusions of flood basalts. This model could help to reconcile the expected presence of a thick cryosphere during the Hesperian with the abundant evidence for groundwater as a source for some of the circum-Chryse outflow channels.     

Garnet polycrystals and the significance of clustered crystallization

Polycrystalline garnets are common in metamorphic rocks and may form as a result of close spacing of nuclei (if clustering is early) or impingement of larger grains (if clustering occurs later in the growth history). The timing of clustering relative to garnet growth is relevant to understanding the formation and evolution of porphyroblasts and evaluating the significance (if any) of clustering. Electron backscattered diffraction (EBSD) analysis of garnet-bearing metamorphic rocks reveals the presence of polycrystalline garnet in nine localities examined in this study: the northern Appalachians (Vermont, Maine, New York, USA); North American Cordillera (North Cascades Range, Washington; Snake Range, Nevada, USA); western Rocky Mountains (British Columbia, Canada); southern Menderes Massif (Turkey); Santander Massif (Colombia); and the Sanandaj–Sirjan zone (Hamadan, Iran). In some samples, polycrystals comprise ~20–30% of garnets analyzed, and chemical and textural evidence suggests that early coalescence of garnet polycrystals is common. Some early-coalescing polycrystals exhibit growth zoning that is concentric about the geometric center of the polycrystal. In thin section, these garnets may be undetectable as polycrystals based on morphology or zoning. In some polycrystals, zoning is unrelated to the location of internal grain boundaries; in others, Fe–Mn–Mg zoning has a different pattern than that of Ca; zoning patterns may vary on the scale of a single thin section. In addition, some polycrystals are characterized by high-angle misorientation boundaries that may be in special (non-random) orientations, an observation that indicates that these polycrystals are not random clusters of grains. The presence of internal grain boundaries may affect diffusion pathways and length scales, and may facilitate communication of porphyroblast interiors with matrix phases, thereby influencing reaction history of the rock and the composition/zoning of garnet.     

Chemostratigraphy of the Tamengo Formation (Corumbá Group,Brazil): A contribution to the calibration of the Ediacaran carbon-isotope curve

The Corumbá Group, cropping out in the southern Paraguay Belt in Brazil, is one of the most complete Ediacaran sedimentary archives of palaeogeographic, climatic, biogeochemical and biotic evolution in southwestern Gondwana. The unit hosts a rich fossil record, including acritarchs, vendotaenids (Vendotaenia, Eoholynia), soft-bodied metazoans (Corumbella) and skeletal fossils (Cloudina, Titanotheca). The Tamengo Formation, made up mainly of limestones and marls, provides a rich bio- and chemostratigraphic record. Several outcrops, formerly assigned to the Cuiabá Group, are here included in the Tamengo Formation on the basis of lithological and chemostratigraphical criteria. High-resolution carbon isotopic analyses are reported for the Tamengo Formation, showing (from base to top): (1) a positive δ¹³C excursion to +4‰ PDB above post-glacial negative values, (2) a negative excursion to −3.5‰ associated with a marked regression and subsequent transgression, (3) a positive excursion to +5.5‰, and (4) a plateau characterized by δ¹³C around +3‰. A U-Pb SHRIMP zircon age of an ash bed interbedded in the upper part of the δ¹³C positive plateau yielded 543 ± 3 Ma, which is considered as the depositional age ( Babinski et al., 2008a). The positive plateau in the upper Tamengo Formation and the preceding positive excursion are ubiquitous features in several successions worldwide, including the Nama Group (Namibia), the Dengying Formation (South China) and the Nafun and Ara groups (Oman). This plateau is constrained between 542 and 551 Ma, thus consistent with the age of the upper Tamengo Formation. The negative excursion of the lower Tamengo Formation may be correlated to the Shuram–Wonoka negative anomaly, although δ¹³C values do not fall beyond −3.5‰ in the Brazilian sections. Sedimentary breccias occur just beneath this negative excursion in the lower Tamengo Formation. One possible interpretation of the origin of these breccias is a glacioeustatic sea-level fall, but a tectonic interpretation cannot be completely ruled out.     

Disc precession in the M31 dipping X-ray binary Bo 158?

We present results from three XMM–Newton observations of the M31 low mass X-ray binary (LMXB) XMMU J004314.4+410726.3 (Bo 158), spaced over 3 d in 2004 July. Bo 158 was the first dipping LMXB to be discovered in M31. Periodic intensity dips were previously seen to occur on a 2.78-h period, due to absorption in material that is raised out of the plane of the accretion disc. The report of these observations stated that the dip depth was anticorrelated with source intensity. In light of the 2004 XMM–Newton observations of Bo 158, we suggest that the dip variation is due to precession of the accretion disc. This is to be expected in LMXBs with a mass ratio ≲0.3 (period ≲4 h), as the disc reaches the 3:1 resonance with the binary companion, causing elongation and precession of the disc. A smoothed particle hydrodynamics simulation of the disc in this system shows retrograde rotation of a disc warp on a period of  ∼11 P _orb  , and prograde disc precession on a period of  29 ± 1 P _orb  . This is consistent with the observed variation in the depth of the dips. We find that the dipping behaviour is most likely to be modified by the disc precession, hence we predict that the dipping behaviour repeats on an  81 ± 3 h  cycle.