Hydrological and sediment yield response to summer rainfall in a small high Arctic watershed

In cold regions, the response and related antecedent mechanisms that produce flood flows from rainfall events have received limited study. In 2007, a small watershed at Cape Bounty, Melville Island, Nunavut, was studied in detail during the melt season. Two rainfall events on June 30 and July 22, totalling 9·2 and 10·8 mm, respectively, represented significant contributions to seasonal discharge and sediment transport in a year with a low winter snowpack. The precipitation events elevated discharge and suspended sediment concentrations to twice the magnitude of the nival melt, and generated the only measurable downstream lacustrine turbidity current of the season. In two days, rainfall runoff transported 35% of the seasonal suspended sediment load, in contrast to 29% transported over the nival freshet. The magnitude and intensity of the rain events were not unusual in this setting, but the rainfall response was substantial in comparison with equivalent past events. Exceptional temperatures of July 2007 generated early, deep permafrost thaw, and ground ice melt. The resultant increase in soil moisture amplified the subsequent rainfall runoff and sediment transport response. These results demonstrate the importance of antecedent moisture conditions and the role of permafrost active layer development as an important factor in the rainfall runoff and sediment transport response to precipitation events. Copyright © 2009 John Wiley & Sons, Ltd.     

High‐resolution diatom δ18O records,from the last 150 years,reflecting changes in amount of winter precipitation in two sub‐Arctic high‐altitude lakes in the Swedish Scandes

Waters from high‐altitude alpine lakes are mainly recharged by meteoric water. Because of seasonal variations in precipitation and temperature and relatively short hydraulic residence times, most high‐altitude lakes have lake water isotopic compositions (δ¹⁸O_lake) that fluctuate due to seasonality in water balance processes. Input from snowmelt, in particular, has a significant role in determining lake water δ¹⁸O. Here we compare two high‐resolution δ¹⁸O_diatom records from lake sediments in the Swedish Scandes with instrumental data from the last century obtained from nearby meteorological stations. The time period AD 1900–1990 is characterised by an increase in winter precipitation and high winter/summer precipitation ratios and this is recorded in δ¹⁸O_diatom as decreasing trends. Lowest δ¹⁸O_diatom values and highest amount of winter precipitation are found around AD 1990 when the winter North Atlantic Oscillation index was above +2. We conclude that for the last 150 a the main factor affecting the δ¹⁸O_diatom signal in these sub‐Arctic high‐altitude lakes with short residence times has been changes in amount of winter precipitation and that δ¹⁸O_diatom derived from high‐altitude lakes in the Swedish Scandes can be used as a winter precipitation proxy. Copyright © 2010 John Wiley & Sons, Ltd.     

A tale of two species: Extirpation and range expansion during the late Quaternary in an extreme environment

Death Valley, California is today the hottest hyperarid area in the western Hemisphere with temperatures of 57 °C (134 °F) recorded. During the late Quaternary, pluvial Lake Manly covered much of the Valley and contributed to a much more moderate climate. The abrupt draining of Lake Manly in the mid-Holocene and coincident dramatic shifts in temperature and aridity exerted substantial selection pressure on organisms living in this area. Our research investigates the adaptive response of Neotoma (woodrats) to temperature change over the late Quaternary along a steep elevational and environmental gradient. By combining fieldwork, examination of museum specimens, and collection of paleomiddens, our project reconstructs the divergent evolutionary histories of animals from the valley floor and nearby mountain gradients (− 84 to > 3400 m). We report on recent paleomidden work investigating a transition zone in the Grapevine Mountains (Amargosa Range) for two species of woodrats differing significantly in size and habitat preferences: N. lepida, the desert woodrat, and N. cinerea, the bushy-tailed woodrat. Here, at the limits of these species' thermal and ecological thresholds, we demonstrate dramatic fluctuations in the range boundaries over the Holocene as climate shifted. Moreover, we find fundamental differences in the adaptive response of these two species related to the elevation of the site and local microclimate. Results indicate that although N. cinerea are currently extirpated in this area, they were ubiquitous throughout the late Pleistocene and into the middle Holocene. They adapted to climate shifts over this period by phenotypic changes in body mass, as has been demonstrated for other areas within their range; during colder episodes they were larger, and during warmer intervals, animals were smaller. Their presence may have been tied into a much more widespread historical distribution of juniper (Juniperus sp.); we document a downward displacement of approximately 1000 m relative to juniper's modern extent in the Amargosa Range. These results suggest a cooler and more mesic habitat association persisting for longer and at lower elevations than previously reported.     

Monomineral universal clinopyroxene and garnet barometers for peridotitic,eclogitic and basaltic systems

New versions of the universal Jd-Di exchange clinopyroxene barometer for peridotites,pyroxenites and eclogites,and also garnet barometer for eclogites and peridotites were developed.They were checked using large experimental data sets for eciogitic(~530) and peridotitic systems(650).The precision of the universal Cpx barometer for peridotites based on Jd-Di exchange is close to Cr-Tschermalite method produced by Nimis and Taylor(2000).Cpx barometer was transformed by the substitution of major multiplier for K_D by the equations dependent from Al-Na-Fe.Obtained equation in combination with the thermometer of Nimis and Taylor(2000) allow to reconstruct position of the magma feeder systems of the alkali basaltic magma within the mantle diapirs in modern platforms like in Vitim plateau and other Southern Siberia localities and several localities worldwide showing good agreement of pressure ranges for black and green suites.These equations allow construct PTX diagrams for the kimberlite localities in Siberia and worldwide calculating simultaneously the PT parameters for different groups of mantle rocks.They give very good results for the concentrates from kimberlite lamproites and placers with mantle minerals.They are useful for PT estimates for diamond inclusions.The positions of eclogite groups in mantle sections are similar to those determined with new Gar—Cpx barometer produced by C.Beyer et al.(2015).The Fe rich eclogites commonly trace the boundary between the lower upper parts of subcontinental lithospheric mantle(SCLM) at 3-4 CPa marking pyroxenite eclogites layer.Ca-rich eclogites and especially grospydires in SCLM beneath Precambrian kimberlites occurs near pyroxenite layer but in younger mantle sections they became common in the lower parts.The diamondiferous Mg Cr-less group eclogites referring to the ancient island arc complexes are also common in the middle part of mantle sections and near 5-6 GPa.Commonly eclogites in lower apart of mantle sections are remelted and trace the high temperature convective branch.The Mg-and Fe-rich pyroxenites also show the extending in pressure trends which suggest the anatexic melting under the influence of volatiles or under the interaction with plums.     

Sulfur isotope and fluid inclusion geochemistry of metamorphic Cu–Au vein deposits,central Cobar area,NSW, Australia

Vein-type, structurally controlled Cu–Au mineralisation hosted by turbidites of late Silurian to earliest Devonian age, forms an important resource close to the eastern margin of the Cobar Basin. Here we report 103 new sulfur isotope analyses, together with homogenisation temperatures and salinity data for 545 primary two-phase fluid inclusions for the mineralised zones from the central area of the Cobar mining district. A range in δ³⁴S values from 3.8 to 11.2‰ (average 7.9‰) is present. Sulfur is likely to have been derived from rock sulfur/sulfide in basin and basement rocks, but there may be an additional connate-derived component. Homogenisation temperatures (T_h) for inclusion fluids trapped in quartz and minor calcite veins range from near 150°C to 397°C. Fluid inclusions are characterised by a low CO₂ content and low, but variable salinities (2.1–9.1 wt% NaCl equivalent). Generations of inclusion fluids correspond to six paragenetic stages of vein quartz deposition and recrystallisation at the Chesney mine. Primary fluid inclusions in the first two stages were subjected to re-equilibration resulting from increased confining pressure. Their T_h range (151–317°C) is considered a minimum for the temperature of these fluids. Sulfide and gold deposition at Chesney appears to be related to fluids of moderately high T_h (range 270–397°C) associated with the final paragenetic stage. T_h for the ore-related fluids may be close to the solvus of the H₂O–NaCl–CO₂ system and hence near trapping temperatures. Late-stage entry of a hot, moderately saline ore-forming fluid is implicated as the origin of the Cu–Au mineralisation. However, comparison with geochemical data for the vein-style Zn–Pb–Ag deposits at Cobar demonstrates that differences in metal content for individual zones cannot be attributed to major differences in fluid temperature or salinity. Rather, these differences are probably due to variations in source-rock reservoirs and structural pathways along which the ore-forming fluids moved.     

On climate change and cultural geography: farming on the Lizard Peninsula, Cornwall, UK

Climate change as a global problem has moved relatively swiftly into high profile political debates over the last 20?years or so, with a concomitant diffusion from the natural sciences into the social sciences. The study of the human dimensions of climate change has been growing in momentum through research which attempts to describe, evaluate, quantify and model perceptions of climate change, understand more about risk and assess the construction of policy. Cultural geographers?? concerns with the construction of knowledge, the workings of social relations in space and the politics and poetics of place-based identities provide a lens through which personal, collective and institutional responses to climate change can be evaluated using critical and interpretative methodologies. Adopting a cultural geography approach, this paper examines how climate change as a particular environmental discourse is constructed through memory, observation and conversation, as well as materialised in farming practices on the Lizard Peninsula, Cornwall, UK.     

The Yungul carbonatite dykes associated with the epithermal fluorite deposit at Speewah, Kimberley, Australia: carbon and oxygen isotope constraints on their origin

The Yungul carbonatite dykes at Speewah in the Kimberley region of Western Australia were emplaced along a north-trending splay from the northeast-trending Greenvale Fault located at the western boundary of the Halls Creek Orogen. The Yungul carbonatite dykes intrude a thick composite sill of the Palaeoproterozoic Hart Dolerite (~1,790 Ma), consisting of tholeiitic dolerite and gabbro with its felsic differentiates that form the Yilingbun granophyres and associated granites. The carbonatite dykes consist of massive, calcite carbonatite that host very coarse, pegmatitic veins and pods of calcite, and have largely replaced (carbonatitized) and fenitized the country rock Hart Dolerite suite in a zone up to 150 m wide. Dykes of red-brown siliceous fluidized-breccia and epithermal-textured veins consisting of bladed quartz, adularia and fluorite are closely associated with the carbonatite dykes. The Yungul carbonatites are closely associated with fluorite occurrences with resources currently reported as 6.7 Mt at 24.6% CaF₂. The precise age of the Yungul carbonatite is not known, although it is believed to be post early Cambrian. The total REE content of the Yungul carbonatite is low (174.0–492.8 ppm; La/Yb 2.28–10.74) and thus atypical for calciocarbonatite. Chondrite-normalized REE patterns for the carbonatite are relatively flat compared to average calciocarbonatite, and show small negative Eu anomalies. These unusual geochemical features may have been acquired from the Hart Dolerite suite during emplacement of the carbonatite, a process that involved extensive replacement and fenitization of country rocks. Carbon and oxygen isotope compositions of massive calcite carbonatite and the coarse calcite veins and pods from the carbonatite suggest a deep-seated origin. The C and O isotope compositions show an overall positive correlation that can be attributed to both magmatic and magmatic-hydrothermal processes in their evolution. The magmatic δ¹³C-δ¹⁸O trend is also indicative of crustal contamination and/or low-temperature water/rock exchange. The carbon isotopic compositions have δ¹³C values that range from about ?5.2‰ to ?6.3‰ that support a mantle-derived origin for the Yungul carbonatites and are consistent with earlier conclusions based on whole-rock geochemistry and radiogenic isotopes studies.     

Characteristics of the lithospheric mantle beneath East Serbia inferred from ultramafic xenoliths in Palaeogene basanites

Mantle xenoliths from Paleogene basanites of East Serbia were studied using EMP and LA-ICP-MS techniques in order to better understand mantle characteristics in this region. Five different mantle lithologies have been distinguished: a dunite/harzburgite/lherzolite (D/HZ/L) group, clinopyroxene-rich lherzolites (Cpx-L), clinopyroxene megacrysts (Cpx-M), spinel-rich olivine websterites (OWB₁) and spinel-poor olivine websterites (OWB₂). D/HZ/L xenoliths are the most common and represent normal mantle composed of typical anhydrous spinel peridotites with well equilibrated, unzoned silicates characterized by high Mg# s. Negative correlations between Mg# and TiO₂, Al₂O₃ and CaO wt% in clinopyroxenes (cpx) and orthopyroxenes (opx) and the Cr–Al trend in spinel (sp) suggest depletion via extraction of basaltic melts. The modal composition of D/HZ/L xenoliths and unusual low-Al opx suggest that the lithospheric mantle underneath East Serbia is more depleted than normal European lithosphere. D/HZ/L xenoliths contain numerous pockets and veins filled by Cr-rich cpx, Ti-rich spinel, altered glass, apatite and rare ilmenite and phlogopite. Petrographic observations, supported by major element contents in sp and cpx, and modelling using trace element contents in cpx, indicate that the pockets and veins formed from infiltration of alkaline melts and reaction with peridotite wall-rock causing opx and spinel replacement. The same alkaline melt-related metasomatism gave rise to the Cpx-L and OWB₁ mantle xenoliths and Cpx-M xenocrysts. Trace element contents of cpx in these xenoliths show a distinctively concave downwards REE pattern with a HFSE depletion, very similar to cpx megacrysts from the Pannonian Basin and to vein cpx from Eifel. In contrast, the OWB₂ xenoliths show evidence of precipitation from subduction-related mafic to ultramafic melts, as inferred from their opx-rich lithology and unusual Cr-rich spinels. They are probably related to subduction magmatism during the Late Cretaceous.Milivoje Jovanovi: deceased in April 2004     

A geochemical method for removing the effect of tephra on lake diatom oxygen isotope records

The analysis of oxygen isotope ratios (δ¹⁸O) in biogenic silica (e.g. diatoms, sponge spicules, phytoliths) is an increasingly utilised palaeoclimatic proxy for lakes and oceans, providing an alternative to carbonate δ¹⁸O. Currently, the problems associated with the cleaning and extraction of pure diatom silica, especially from lake sediments, may compromise the precision associated with δ¹⁸O_diatom records. This is because, despite improvements in cleaning techniques, it is still difficult to remove silica-containing contaminants (e.g. tephra, silts and clays) from biogenic silica. Where it has proved difficult to remove all the silica bearing contaminants from a sample, mass balance calculations involving point counting have been used, but these are inherently inaccurate as they only consider the surface area of the contaminant, rather than volume. The new method described here assesses the volume of contamination in each sample of biogenic silica by using major and trace element geochemistry, allowing the δ¹⁸O of the contamination to be removed from the biogenic silica δ¹⁸O value by linear mass balance. In this case, diatom silica was contaminated with tephra, however the method should also be applicable to other contaminants such as silts and clays.