Timing and Extent of Late Quaternary Paleolakes in the Trans-Pecos Closed Basin, West Texas and South-Central New Mexico

The Trans-Pecos Closed Basin is a hydrographically closed region covering 20,000 km²centered on Salt Basin, 160 km east of El Paso, Texas. Geomorphic and limnetic evidence have been used to identify four major highstands for Lake King during the last glacial maximum (LGM). Additional geomorphic features from a second, recently identified, paleolake, Lake Sacramento, have been found in the Beargrass subbasin, a nested subbasin approximately 75 km northwest of Salt Basin. Radiocarbon ages of the organic material in Lake King sediments date four abrupt climate changes and rapid lacustrine transgressions during the LGM with a quasi-periodicity of 2000 yr. Geomorphic evidence in the Beargrass subbasin identifies lake cycles contemporaneous with those in Lake King. The dates for these transgressions correlate with the dates of freshening events identified by researchers in paleolake basins elsewhere in New Mexico. The quasi-periodicity of the events approximates that of Dansgaard–Oeschger events identified from Greenland ice cores. The contemporaneity of the Trans-Pecos transgressions with transgressive events in other basins in the region suggests that paleolakes in the region were in phase with respect to abrupt climate changes during the latter stages of the LGM.     

Application of Compositional Techniques in the Field of Crystal Chemistry: A Case Study of Luzonite, a Sn-Bearing Mineral

Out-of-equilibrium crystallization often produces complex compositional variability in minerals, generating zoning and other mixing phenomena. The appropriate microchemical characterization of the resulting out-of-equilibrium patterns is of critical importance in understanding the overall physical and chemical properties of the host crystalline phases. In this framework, the modeling of compositional changes assumes a fundamental role. However, when compositional data are used, their management with standard exploratory, statistical, graphical, and numerical tools may give misleading results attributable to the phenomenon of induced correlations. To avoid these problems, methods able to extract compositional data from their constrained space (the simplex) in order to apply standard statistics, have to be adopted. As an alternative, the use of tools having properties able to work in the simplex geometry has to be considered. A luzonite single crystal (ideal composition, Cu₃AsS₄) exhibiting concentric and sector zoning was studied using electron probe microanalysis in order to understand the mechanisms which give rise to chemical variability and conditions in the developing environment. Compositional variations were determined by collecting data along three different transects. The major and minor elements (Cu, As, S, Fe, Sb, Sn) were analyzed with the aim of characterizing their patterns of association in the crystal and, hence, crystal evolution. The whole covariance structure as well as the chemical relationships between the successive zones was investigated by means of compositional methods, considering both data transformation and the stay in the simplex approach. Results indicate that the crystal grew under quiescent conditions, where chemical control was primarily exercised by the mineral’s surface and only minor effects were due to changes in the composition of the surrounding fluid. Consequently, an oscillatory uptake of chemical components occurred in which a competition between famatinite-like (Cu₃SbS₄) and kuramite-like (Cu₃SnS₄) domains characterized the As-poor zones.     

Interannual variability in glacier contribution to runoff from a high‐elevation Andean catchment: understanding the role of debris cover in glacier hydrology

We present a field‐data rich modelling analysis to reconstruct the climatic forcing, glacier response, and runoff generation from a high‐elevation catchment in central Chile over the period 2000–2015 to provide insights into the differing contributions of debris‐covered and debris‐free glaciers under current and future changing climatic conditions. Model simulations with the physically based glacio‐hydrological model TOPKAPI‐ETH reveal a period of neutral or slightly positive mass balance between 2000 and 2010, followed by a transition to increasingly large annual mass losses, associated with a recent mega drought. Mass losses commence earlier, and are more severe, for a heavily debris‐covered glacier, most likely due to its strong dependence on snow avalanche accumulation, which has declined in recent years. Catchment runoff shows a marked decreasing trend over the study period, but with high interannual variability directly linked to winter snow accumulation, and high contribution from ice melt in dry periods and drought conditions. The study demonstrates the importance of incorporating local‐scale processes such as snow avalanche accumulation and spatially variable debris thickness, in understanding the responses of different glacier types to climate change. We highlight the increased dependency of runoff from high Andean catchments on the diminishing resource of glacier ice during dry years.     

Chronology of the impact of Quaternary climate change on continental environments in the Arabian Peninsula

The remains of former lakes show that in the past the Arabian Peninsula experienced much wetter conditions than today. The last of these humid periods dates to about 10 to 5.5 ka ago. The chronological framework for an earlier humid phase, radiocarbon dated to some 35–20 ka, is inconsistent with evidence from other records from the region. Possibly, these ages are significantly underestimating the true depositional age due to methodological problems. The earliest phase of dune accumulation known so far is dated to the penultimate glaciation maximum of the mid latitudes (ca. 150 ka). Subsequently, dune accumulation occurred around 110 ka, 65 ka and 20 ka ago. All these phases concur with rapid drops in global sea level that caused a drying out of the Persian Gulf basin and of the shelf of the Oman coast. In contrast to some previous interpretations, it is concluded here that aeolian deposition has been limited by sediment supply and not by preservation potential.     

The quaternary glacial history of the Lahul Himalaya,northern India

This paper presents the first glacial chronology for the Lahul Himalaya, Northern India. The oldest glaciation, the Chandra Glacial Stage, is represented by glacially eroded benches at altitudes greater than 4300 m above sea-level. This glaciation was probably of a broad valley type. The second glaciation, the Batal Glacial Stage, is represented by highly weathered and dissected lateral moraines, which are present along the Chandra valley and some of its tributaries. This was an extensive valley glaciation. The third major glaciation, the Kulti Glacial Stage, is represented by well-preserved moraines in the main tributary valleys of the Chandra valley. This represents a less extensive valley glaciation. Two minor glacial advances, the Sonapani I and II, are represented by small sharp-crested moraines, which are within a few hundred metres or few kilometres of the present-day glaciers. The change in style and extent of glaciation is attributed to an increase in aridity throughout the Quaternary, due either to global climatic change or uplift of the Pir Panjal mountains to the south of Lahul, which restricted the northward penetration of the south Asian summer monsoon. © 1996 John Wiley & Sons, Ltd.     

What the Sunspot Record Tells Us About Space Climate

The records concerning the number, sizes, and positions of sunspots provide a direct means of characterizing solar activity over nearly 400 years. Sunspot numbers are strongly correlated with modern measures of solar activity including: 10.7-cm radio flux, total irradiance, X-ray flares, sunspot area, the baseline level of geomagnetic activity, and the flux of galactic cosmic rays. The Group Sunspot Number provides information on 27 sunspot cycles, far more than any of the modern measures of solar activity, and enough to provide important details about long-term variations in solar activity or “Space Climate.” The sunspot record shows: 1) sunspot cycles have periods of 131± 14 months with a normal distribution; 2) sunspot cycles are asymmetric with a fast rise and slow decline; 3) the rise time from minimum to maximum decreases with cycle amplitude; 4) large amplitude cycles are preceded by short period cycles; 5) large amplitude cycles are preceded by high minima; 6) although the two hemispheres remain linked in phase, there are significant asymmetries in the activity in each hemisphere; 7) the rate at which the active latitudes drift toward the equator is anti-correlated with the cycle period; 8) the rate at which the active latitudes drift toward the equator is positively correlated with the amplitude of the cycle after the next; 9) there has been a significant secular increase in the amplitudes of the sunspot cycles since the end of the Maunder Minimum (1715); and 10) there is weak evidence for a quasi-periodic variation in the sunspot cycle amplitudes with a period of about 90 years. These characteristics indicate that the next solar cycle should have a maximum smoothed sunspot number of about 145 ± 30 in 2010 while the following cycle should have a maximum of about 70 ± 30 in 2023.     

Biomediation of calcrete at the gold anomaly of the Barns prospect,Gawler Craton,South Australia

Anomalously high Au concentrations (2.5 to 50 ppb) in regolith carbonate accumulations, such as calcrete and calcareous sands in aeolian sand dunes overlying Au mineralisation of the Gawler Craton, South Australia, show a marked covariance of Au with K, Mg and most notably Ca. This relationship appears to be linked to the authigenic formation of smectites and carbonates within the aeolian dunes in the region. However, little is known about the processes involved in the formation of carbonates under semi-arid and arid conditions. In this study the geochemical properties of aeolian dunes along several depth profiles of 2 to 4 m are investigated in order to assess the relationship between Au mobility and calcrete formation. In the profiles a strongly systematic relationship between Au and the increasing Ca–Mg contents at depth highlights the close association between the enrichment of Au in the calcrete and the underlying hydrothermal mineralisation. Intense calcrete formation and concurrent Au enrichment also occurs in the vicinity of roots penetrating the dune. Thin section petrography and cathodoluminescence show that most of the calcrete in the regolith profiles is micritic; some sparic crystallites have also been identified. To demonstrate the presence of microbial processes that may mediate the formation of calcrete, samples from a depth profile in the dune were taken under sterile conditions. After amendment with urea and incubation of up to 24 h, up to 18 mg/l of NH₄⁺ were detected in near surface samples. At depth of 2.3 m 1 mg/l NH₄⁺ were detected compared to a control that contained below 0.05 mg/l NH₄⁺. These results suggest that the genesis of calcrete and pedogenic carbonate in the area may be partly biologically mediated via processes such as the metabolic breakdown of urea by resident microbiota which generates a pH and pCO₂ environment conducive to the precipitation of carbonate. In the process of urea breakdown organic Au complexes such as Au-amino acid complexes may become destabilised in solution and Au may be co-precipitated, resulting in the fine, non-particulate distribution of Au throughout the micritic calcrete carbonate. In conclusion, this study suggests a coupled mechanism of biologically mediated and inorganic mechanisms that lead to the formation of Au-in-calcrete anomalies.     

Distribution and effectiveness of nivation in Mediterranean mountains: Peñalara (Spain)

Snow accumulation is responsible for geomorphic and biogeographic processes taking place in the southern sector of the Peñalara massif in central Spain (40°51′N, 3°57′W; max. altitude 2428 m at Pico de Peñalara). This work compares the intensity of nivation on the eastern slope, leeward of the prevailing westerly winds and heavily eroded by glacial activity during the Pleistocene, to that of the western slope on the windward side, unaffected by glacial erosion and completely covered by a thick weathering mantle. On the eastern slope, nivation is effective only where the weathering mantle is exposed or on morainic formations. It does not occur on the landforms derived from glacial erosion. In contrast, the western side shows almost no evidence of snow action except where catastrophic mass movements have altered the regularity of the slope. During the post-glacial epoch, nivation cirques formed in the scars left by mass wasting. In the last 30 years, spring temperatures have increased and this activity has diminished. The pattern of evolution observed at Peñalara can be extrapolated to other Mediterranean mountains with similar characteristics such as marginal glacial activity during the Pleistocene, unconsolidated formations on the summits caused by chemical weathering, and dry, hot summers that can increase the effectiveness of nivation.