Alkaline and calc-alkaline lavas near Los Volcanes,Jalisco, Mexico: geochemical diversity and its significance in volcanic arcs

A remarkably diverse suite of lavas erupted during the late-Pliocene at the volcanic front of the western Mexican Volcanic Belt near the town of Los Volcanes, Jalisco. This region is much closer to the Middle America Trench than the main axis of Quaternary andesite-dacite stratovolcanoes, and volcanism occurred in a complex tectonic regime involving both subduction of the young Rivera Plate and transverse crustal extension of the Jalisco structural block. The variety of lava types covers a wide spectrum from highly potassic minettes and leucitites to calc-alkaline basalts and andesites which are compositionally similar to those erupted elsewhere in the Mexican Volcanic Belt. Other alkaline varieties intermediate between these extremes include absarokites, trachybasalts and trachyandesites. Phlogopite, amphibole and apatite are common phenocryst phases; whole-rock compositions show a wide range of alkali contents (e.g. K₂O of 1.0–8.6 wt.%), and typically contain >5 wt.% MgO. MgO, Ni, V and Cr show little systematic decrease with increasing SiO₂, suggesting that these lavas have evolved from primitive, mantle derived magmas with a wide range of SiO₂ contents. Strong enrichments in incompatible trace elements are observed in all of the lavas (Sr 700–5100 ppm, Ba 470–4800, Ce 22–325 ppm, Zr 90–700 ppm), as is the relative enrichment of large ion lithophile (Ba, Sr, Rb) and light rare-earth elements (La, Ce) over the high field strength elements (Ti, Zr) which is typical of magmas in volcanic arcs. This enrichment pattern suggests that these magmas come from source regions which contain incompatible element-rich phases such as phlogopite, amphibole and apatite. The petrological and geochemical features of the lavas which occur in the Los Volcanes region provide direct evidence of the extreme heterogeneity which may exist in magma source regions at convergent margins. The complex tectonic regime in western Mexico further suggests that rifting and crustal extension play an important role in the generation and successful ascent of melts from enriched regions of the sub-arc mantle.     

The magnetospheric disturbance ring current as a source for probing the deep earth electrical conductivity

Two current rings have been observed in the equatorial plane of the earth at times of high geomagnetic activity. An eastward current exists between about 2 and 3.5 earth radii (Re) distant, and a larger, more variable companion current exists between about 4 and 9 Re. These current regions are loaded during geomagnetic substorms. They decay, almost exponentially, after the cessation of the particle influx that attends the solar wind disturbance. This review focuses upon characteristics needed for intelligent use of the ring current as a source for induction probing of the earth's mantle. Considerable difficulties are found with the assumption thatDst is a ring-current index.     

TG/plus—a pyrolysis method for following maturation of oil and gas generation zones using Tmax of methane

Thermogravimetric Fourier transform infrared spectroscopy (TG-FTIR) analyses were carried out on two sets of isolated kerogens covering a wide maturity range from low mature (0.46% R_o) through the end of oil and gas generation (maximum R_{o = 5.32%}). Data onweight percent and T_max for evolution of methane, volatile tars, ethylene, SO₂, NH₃, CO₂, and CO are reported. The T_max of methane shows the most consistent response to increasing maturation in both sets of samples. Results are comparable to those of whole rocks from an Alaskan North Slope well analyzed previously. The collective data for both whole rocks and isolated kerogens shows a generally linear correlation between %R_o and T_max of methane, with the exception of R_o of about 2.0% where a dip in the curve occurs. The slope of the correlation line was steeper for the predominantly terrigenous Wilcox kerogen than for more marine Colorado kerogen or for the Alaskan North Slope whole rock samples, probably reflecting differences in the chemical nature of various kerogen sets, which is also reflected by differences in the shapes of the pyrolysis curves of SO₂, CO₂, CO, H₂O, and ethylene. These preliminary data indicate that T_max of methane is a good maturation indicator for whole rocks and isolated kerogens up to an R_o of about 4%, which includes all of the wet gas and a considerable portion of the dry gas generation zones. This correlation was also observed for samples containing migrated bitumen, where it was not possible to obtain a reliable T_max for the volatile tar (S2) peak. The more terrigenous Wilcox kerogens also showed a good correlation of the T_max of ethylene with %R_o. T_max of ammonia evolution did not correlate with maturity and occurred 100–200°C lower than previously found for whole rocks, consistent with a whole-rock source of pyrolytic ammonia for Alaskan whole rock samples. HI and OI indices were calculated in several ways and plotted to reflect kerogen type as well as both the residual oil and gas generation potential. The ratio of pyrolyzable to combustible sulfur (evolved as SO₂) was independent of maturity and showed a clear difference between the more terrigenous Wilcox kerogens and the more marine Colorado kerogens.     

Managing human disturbance of wildlife in coastal areas

Human disturbance of wildlife is an under‐recognised and under‐regulated problem. This article discusses traditional approaches to conservation management in protecting wildlife from disturbance in the context of the New Zealand coastal environment and threatened birds. Limitations and challenges are identified and alternative actions proposed. The key problems are deficiencies in regulation of species disturbance, lack of definition of thresholds of harm that contemplate rarity and conservation status, insufficient comprehensive wildlife conservation planning and the need for innovative planning methods that address species mobility, permeable boundaries and environmental dynamism. Regulatory controls including enforcing setbacks/approach distances through either extended species protection or 'mobile habitat' protection are recommended.     

On the thermal structure of Uranus

Uranus has an effective temperature close to the solar equilibrium value and undoubtedly a thermal inversion of at least 140 K at a pressure of ～3 dyncm^?2. With the inversion and the thermal opacity provided by a HeH₂ mixture in a ratio close to solar abundance, acceptable agreement can be achieved with the available infrared observations. The cause of the inversion is, however, uncertain. The use of the HeH₂ opacity for Uranus is justified by the excellent agreement of the frequency variation of that opacity with the thermal spectrum of Jupiter.     

Evaluating lacustrine groundwater discharge to a large glacial lake using regional scale radon-222 surveys and groundwater modelling

Lacustrine groundwater discharge (LGD) can be an important pathway for delivering pollutants to lakes but this pathway is often poorly characterized. Evaluating the potential impact of LGD on lake water quality requires understanding the magnitude and spatial variability of LGD, as well as understanding the age and flow paths of the discharging groundwater (e.g., recharge area, groundwater flow paths, and travel times). This study first compares LGD rates along two ~40 km shoreline lengths of a large glacial lake, Lake Simcoe, Canada, that were independently estimated via a radon-222 (²²²Rn) field survey and via regional scale groundwater-surface water modelling. Backward particle tracking analysis is then used to examine the age and flow paths of the LGD and thereby assess the potential for the LGD to deliver anthropogenic pollutants to the lake. The field and modelling results compare well with respect to the magnitude and spatial variability of LGD. However, the comparison highlights the need for well-defined hydrogeological characterization if regional scale models are to be applied for LGD estimation. The particle tracking analysis indicates large variation in the groundwater flow path lengths and travels times (>1000 years to <50 years) for LGD along the shoreline. This illustrates that the LGD along different shoreline areas has varying potential to deliver anthropogenic pollutants to the lake. The study findings demonstrate the benefits of comparing independent field measured and model-simulated LGD estimates, and moreover suggest that it may be possible, in some cases, to use existing regional scale groundwater-surface water models, purpose-built for other water resource and quality objectives, to conduct preliminary evaluation of LGD contributions to lakes. Preliminary model-based evaluation would enable field efforts aiming to quantify and manage LGD to be better targeted rather than relying solely on regional scale field techniques that are often highly resource intensive.     

Submarine origin for the Neoproterozoic Wonoka canyons,South Australia

An examination of the deeply incised Ediacaran Wonoka canyons in the Adelaide Geosyncline (most recently interpreted as subaerial valleys) demonstrates their submarine origin, and confirms them as some of the best examples of ancient outcropping submarine canyons in the world. The entire canyon-fill succession is interpreted to be of deep-water (below wave base) origin, consisting of calcareous shale and siltstone together with a variety of mass-flow deposits including turbidites, grain flows and debris flows. The canyon fill lacks definitive shallow-water structures (e.g. mud cracks, fenestral fabrics or wave ripples) at all stratigraphic levels. Canyon-lining carbonate crusts that have previously been interpreted as non-marine calcretes or tufas (and used to suggest a non-marine origin for the canyons) are argued to be of deep-water, marine, microbial origin. Extremely negative carbon isotope values from the canyon-fill and canyon-lining crusts have a primary marine origin. Previously interpreted deepening upward trends in the canyon fill (used as evidence of a subaerial erosion episode followed by drowning) are suggested to be fining upward trends, caused by the transition from canyon cutting to canyon filling, with the majority of the fill being of deep-water slope origin. The basal conglomeratic canyon-fill sediments represent the last vestiges of the high-energy, deep-water, canyon-erosion environment in which the incisions formed. A deep-water origin for the canyons is consistent with all previous stratigraphic observations of the Wonoka canyons, including the conspicuous lack of regional unconformities in the lower Wonoka Formation, and their emanation from the deep-water facies of the Wonoka Formation. A submarine canyon origin also removes the need for extreme (~ 1 km) relative sea level fluctuation and associated problems (i.e. an enclosed basin with Messinian-style evaporative drawdown or thermal uplift above a migrating mantle plume) required by the subaerial valley hypotheses.     

Response of the Solar Atmosphere to the Emergence of ‘Serpentine’ Magnetic Field

Active region magnetic flux that emerges to the photosphere from below will show complexity in the structure, with many small-scale fragmented features appearing in between the main bipole and then disappearing. Some fragments seen will be absorbed into the main polarities and others seem to cancel with opposite magnetic field. In this paper we investigate the response of the corona to the behaviour of these small fragments and whether energy through reconnection will be transported into the corona. In order to investigate this we analyse data from the Hinode space mission during flux emergence on 1?–?2 December 2006. At the initial stages of flux emergence several small-scale enhancements (of only a few pixels size) are seen in the coronal line widths and diffuse coronal emission exists. The magnetic flux emerges as a fragmented structure, and coronal loops appear above these structures or close to them. These loops are large-scale structures – most small-scale features predominantly stay within the chromosphere or at the edges of the flux emergence. The most distinctive feature in the Doppler velocity is a strong ring of coronal outflows around the edge of the emerging flux region on the eastern side which is either due to reconnection or compression of the structure. This feature lasts for many hours and is seen in many wavelengths. We discuss the implications of this feature in terms of the onset of persistent outflows from an active region that could contribute to the slow solar wind.     

Quaternary tephra marker beds and their potential for palaeoenvironmental reconstruction on Chatham Island,east of New Zealand,southwest Pacific Ocean

Tephras provide one of the most reliable methods of time control and synchronisation within Quaternary sequences. We report on the identification of two widespread rhyolitic tephras – the Kawakawa and Rangitawa tephras – preserved in extensive peat deposits on Chatham Island ～900 km east of New Zealand. The tephras, both products of supereruptions from the Taupo Volcanic Zone, occur as pale, fine‐ash dominated layers typically 10–150 mm thick. Mineralogically they are dominated by rhyolitic glass, together with subordinate amounts of quartz, feldspar, hypersthene, hornblende, Fe–Ti oxides and zircon. Phlogopite/biotite was identified additionally in Rangitawa Tephra. Ages for each tephra were obtained via mineralogical and major element glass composition‐based correlation with well‐dated equivalent deposits on mainland New Zealand, and we also obtained a new zircon fission‐track age for Rangitawa Tephra (350 ± 50 ka) on Chatham Island. Both tephras were erupted at critical times for palaeoenvironmental reconstructions in the New Zealand region: the Kawakawa at ca. 27 cal. ka, near the beginning of the ‘extended’ LGM early in marine isotope stage (MIS) 2; and the Rangitawa at ca. 350 ka near the end of MIS 10. The time constraints provided by the tephras demonstrate that Chatham Island peats contain long‐distance pollen derived from mainland New Zealand, which provides a reliable proxy for identifying glacial–interglacial climate conditions, in this case during the MIS 11–10 and MIS 2–1 cycles. The two tephras thus provide important chronostratigraphic tie‐points that facilitate correlation and synchronisation not only across the Quaternary deposits of the Chatham Islands group but also with climatically significant terrestrial and marine records in the wider New Zealand region. Copyright © 2010 John Wiley & Sons, Ltd.