40Ar/39Ar and K–Ar geochronology of magmatic and hydrothermal events in a classic low-suphidation epithermal bonanza deposit: El Peñon, northern Chile

The epithermal El Peñon gold–silver deposit consists of quartz–adularia veins emplaced within a late Upper Paleocene rhyolitic dome complex, located in the Paleocene–Lower Eocene Au–Ag belt of northern Chile. Detailed K–Ar and ⁴⁰Ar/³⁹Ar geochronology on volcano–plutonic rocks and hydrothermal minerals were carried out to constrain magmatic and hydrothermal events. The Paleocene to Lower Eocene magmatism in the El Peñon area is confined to a rhomb-shaped basin, which was controlled by N–S trending normal faults and both NE- and NW-trending transtensional fault systems. The earliest products of the basin-filling sequences comprise of Middle to Upper Paleocene (～59–55 Ma) welded rhyolitic ignimbrites and andesitic to dacitic lavas, with occasional dacitic dome complexes. Later, rhyolitic and dacitic dome complexes (～55–52 Ma) represent the waning stages of volcanism during the latest Upper Paleocene and the earliest Eocene. Lower Eocene porphyry intrusives (～48–43 Ma) mark the end of the magmatism in the basin and a change to a compressive tectonomagmatic regime. ⁴⁰Ar/³⁹Ar geochronology of hydrothermal adularia from the El Peñon deposit yields ages between 51.0±0.6 and 53.1±0.5 Ma. These results suggest that mineralization occurred slightly after the emplacement of the El Peñon rhyolitic dome at 54.5±0.6 Ma (⁴⁰Ar/³⁹Ar age) and was closely tied to later dacitic–rhyodacitic bodies of 52 to 53 Ma (K–Ar ages), probably as short-lived pulses related to single volcanic events.     

The role of interbasin groundwater transfers in geologically complex terranes,demonstrated by the Great Basin in the western United States

In the Great Basin, USA, bedrock interbasin flow is conceptualized as the mechanism by which large groundwater fluxes flow through multiple basins and intervening mountains. Interbasin flow is propounded based on: (1) water budget imbalances, (2) potential differences between basins, (3) stable isotope evidence, and (4) modeling studies. However, water budgets are too imprecise to discern interbasin transfers and potential differences may exist with or without interbasin fluxes. Potentiometric maps are dependent on conceptual underpinnings, leading to possible false inferences regarding interbasin transfers. Isotopic evidence is prone to non-unique interpretation and may be confounded by the effects of climate change. Structural and stratigraphic considerations in a geologically complex region like the Great Basin should produce compartmentalization, where increasing aquifer size increases the odds of segmentation along a given flow path. Initial conceptual hypotheses should explain flow with local recharge and short flow paths. Where bedrock interbasin flow is suspected, it is most likely controlled by diversion of water into the damage zones of normal faults, where fault cores act as barriers. Large-scale bedrock interbasin flow where fluxes must transect multiple basins, ranges, and faults at high angles should be the conceptual model of last resort.     

Contrasting definitions for the term ‘karst aquifer’

It is generally considered that karst aquifers have distinctly different properties from other bedrock aquifers. A search of the literature found five definitions that have been proposed to differentiate karst aquifers from non-karstic aquifers. The five definitions are based upon the presence of solution channel networks, hydraulic conductivities >10^?6 m/s, karst landscapes, channels with turbulent flow, and caves. The percentage of unconfined carbonate aquifers that would classify as ‘karst’ ranges from <1 to >50%.     

Mapping Soils,Vegetation, and Landforms: An Integrative Physical Geography Field Experience*

Students in a graduate seminar at Michigan State University produced a series of detailed vegetation, soils, and landform maps of a 1.5‐square‐mile (3.9 km²) study area in southwest Lower Michigan. The learning outcomes (maps) and skill development objectives (sampling strategies and various GIS applications) of this field‐intensive mapping experience were driven by the assumption that students learn and understand relationships among physical landscape variables better by mapping them than they would in a classroom‐based experience. The group‐based, problem‐solving format was also intended to foster collaboration and camaraderie. The study area lies within a complex, interlobate moraine. Fieldwork involved mapping in groups of two or three, as well as soil and vegetation sampling. Spatial data products assembled and used in the project included topographic maps, a digital elevation model (DEM), aerial photographs, and NRCS (National Resource Conservation Service) soil maps. Most of the soils are dry and sandy, with the main differentiating characteristic being the amount of, and depth to, subsurface clay bands (lamellae) or gravelly zones. The presettlement (early 1830s) vegetation of the area was oak forest, oak savanna, and black oak “barrens.” Upland sites currently support closed forests of white, black, and red oak, with a red maple, dogwood, and sassafras understory. Ecological data suggest that these oak forests will, barring major disturbance, become increasingly dominated by red maple. This group‐based, problem‐solving approach to physical geography education has several advantages over traditional classroom‐based teaching and could also be successfully applied in other, field‐related disciplines.     

Chironomid‐inferred Late‐glacial air temperatures at Whitrig Bog,Southeast Scotland

This paper presents the first chironomid‐inferred mean July air temperature reconstruction for the Late‐glacial in Britain. The reconstruction suggests that the thermal maximum occurred early in the interstadial, with temperatures reaching about 12°C. There was then a gradual downward trend to about 11°C, punctuated by four distinct cold oscillations of varying intensity. At the beginning of the Younger Dryas, mean July temperatures fell to about 7.5°C but gradually increased to about 9°C before a rapid rise at the onset of the Holocene. The chironomid‐inferred temperature curve agrees closely, both in general trends and in detail, with the GRIP ice‐core oxygen‐isotope curve. The reconstructed temperatures are 2–4°C lower than coleopteran‐inferred temperatures but are closer to those inferred from plant macrofossils and glacial equilibrium‐line altitudes during the Younger Dryas. Copyright © 2000 John Wiley & Sons, Ltd.     

A review of dimethylsulfoxide in aquatic environments

Abstract

Dimethylsulfoxide (DMSO) is an ubiquitous, albeit poorly understood, component of the marine sulfur cycle. Conventionally, the accepted formation pathways are the photochemical and microbial oxidation of dimethylsulfide (DMS). The principal loss mechanism is thought to be via microbial transformation, either consumption or reduction to DMS. The interactions between DMSO and DMS are likely to be important in controlling sea surface concentrations of DMS, and thus DMSO could influence the role played by DMS in global climate regulation. This review examines current knowledge of the distribution of DMSO in aquatic environments and the possible link between DMSO, DMS and global climate control. Mechanisms for the formation and loss of DMSO are also considered in addition to some of the factors influencing these processes. The review also considers that DMSO may be biosynthesized by phytoplankton, representing a non‐DMS source for DMSO, and that DMSO can undergo photochemical oxidation, a potential loss mechanism for DMSO in the marine environment.     

Elevational Dependence of Air Temperature Variability and Trends in British Columbia's Cariboo Mountains, 1950–2010

Pristine mountain environments are more sensitive to climate change than other land surfaces. Climatic variations in mountainous terrain are still poorly understood. Previous studies revealed inconsistent findings on the elevational dependence of warming in the mountains. In this study, the trends and elevational dependence of air temperature in the Cariboo Mountains Region (CMR) of British Columbia are explored using a surface air temperature dataset with a spatial resolution of five arc minutes over the 1950–2010 period. A Mann-Kendall test is performed for evaluation of trends and their significance. In recent decades the CMR has been warming at a faster rate than regional and global warming. The minimum air temperature trend shows significant amplified warming at higher elevations. The snow–albedo feedback and changes in cloud cover over the CMR may possibly be the major physical mechanisms responsible for these trends. The implications of such changes on the endangered mountain caribou and water resources of the area are also discussed.     

Lunar Magma Ocean crystallization revisited: Bulk composition, early cumulate mineralogy, and the source regions of the highlands Mg-suite

Crystallization of the Lunar Magma Ocean (LMO) has been numerically modeled and its products inferred from sample observations, but it has never been fully tested experimentally. This study is a reexamination of the LMO hypothesis by means of the first experimental simulation of lunar differentiation. Two end-member bulk Moon compositions are considered: one enriched in refractory lithophile elements relative to Earth and one with no such enrichment. A “two-stage” model of magma ocean crystallization based on geophysical constraints is simulated and features early crystal suspension and equilibrium crystallization followed by fractional crystallization of the residual magma ocean. An initially entirely molten Moon is assumed. Part 1 of this study, presented here, focuses on stage 1 of this model and considers the early cumulates formed by equilibrium crystallization, differences in mantle mineralogy resulting from different bulk Moon compositions, and implications for the source regions of the highlands Mg-suite.Refractory element enriched bulk Moon compositions produce a deep mantle that contains garnet and trace Cr-spinel in addition to low-Ca pyroxene and olivine. In contrast, compositions without refractory element enrichment produce a deep dunitic mantle with low-Ca pyroxene but without an aluminous phase. The differences in bulk composition are magnified in the residual melt; the residual LMO from the refractory element enriched composition will likely produce plagioclase and ilmenite earlier and in greater quantities. Both compositions produce Mg-rich early cumulate piles that extend from the core-mantle boundary to ∼355 km depth, if 50% equilibrium crystallization and whole Moon melting are assumed. These early LMO cumulates provide good fits for the source regions for a component of the high-Mg^∗, Ni- and Co-poor parental magmas of the Mg-suite cumulates, if certain conditions are called upon. The olivine in early LMO cumulates produced by either bulk Moon composition is far too rich in Cr to be reasonable for the source regions of the Mg-suite, meaning either core formation in the presence of S and/or C must be invoked to deplete the LMO and the crystallizing olivine in Cr, or that current estimates of the bulk lunar Cr content are too high. We infer that melts meeting the criteria of the Mg-suite parents could be produced from early LMO cumulates by solid state KREEP and plagioclase hybridization near the base of the crust and subsequent partial melting. Additionally, we propose a revised model for Mg-suite petrogenesis.     

Preliminary estimates of intertidal limestone erosion,one tree island,southern great barrier reef,australia

Estimations of surface lowering of 0.2-3.8 mm a^?1 were gained from measurements of pedestal heights under sedentary organisms. The role of erosion by chitons, estimated at 0.2-2.9 mm a^?1 for home scars and 0.2-0.7 mm a^?1 for grazing, and other bioerosive organisms, is briefly discussed.