Multiple-methods investigation of recharge at a humid-region fractured rock site,Pennsylvania, USA

Lysimeter-percolate and well-hydrograph analyses were combined to evaluate recharge for the Masser Recharge Site (central Pennsylvania, USA). In humid regions, aquifer recharge through an unconfined low-porosity fractured-rock aquifer can cause large magnitude water-table fluctuations over short time scales. The unsaturated hydraulic characteristics of the subsurface porous media control the magnitude and timing of these fluctuations. Data from multiple sets of lysimeters at the site show a highly seasonal pattern of percolate and exhibit variability due to both installation factors and hydraulic property heterogeneity. Individual event analysis of well hydrograph data reveals the primary influences on water-table response, namely rainfall depth, rainfall intensity, and initial water-table depth. Spatial and seasonal variability in well response is also evident. A new approach for calculating recharge from continuous water-table elevation records using a master recession curve (MRC) is demonstrated. The recharge estimated by the MRC approach when assuming a constant specific yield is seasonal to a lesser degree than the recharge estimate resulting from the lysimeter analysis. Partial reconciliation of the two recharge estimates is achieved by considering a conceptual model of flow processes in the highly-heterogeneous underlying fractured porous medium.     

Analysis of the rock accretions in the lower pecos region of southwest texas

Calcium oxalate (whewellite) was found to be the primary component of crusts on limestone in the dry rock shelters throughout the Lower Pecos region of Texas. This material forms a translucent patina that covers the pictographs in this area. Evidence from analyses using SEM/EDS, FTIR, XRD, Raman spectroscopy, and AMS is presented that suggests the oxalate-rich crusts were produced by metabolic activity of lichen or fungi on or near the surface of the limestone substrate. The paucity of hyphae and microbes in samples studied using SEM may indicate that the organisms responsible for the production of the oxalate are no longer present on the shelter walls. Radiocarbon ages of three oxalate samples range from 2100 to 5570 years B. P., indicating that the crust may be used for obtaining chronological information on the rock art.     

Do Stable Isotope Data from Calcrete Record Late Pleistocene Monsoonal Climate Variation in the Thar Desert of India?

Late Pleistocene terrestrial climate records in India may be preserved in oxygen and carbon stable isotopes in pedogenic calcrete. Petrography shows that calcrete nodules in Quaternary sediments of the Thar Desert in Rajasthan are pedogenic, with little evidence for postpedogenic alteration. The calcrete occurs in four laterally persistent and one nonpersistent eolian units, separated by colluvial gravel. Thermoluminescence and infrared- and green-light-stimulated luminescence of host quartz and feldspar grains gave age brackets for persistent eolian units I–IV of ca. 70,000–60,000, ca. 60,000–55,000, ca. 55,000–43,000, and ca. 43,000–25,000 yr, respectively. The youngest eolian unit (V) is <10,000 yr old and contains no calcrete. Stable oxygen isotope compositions of calcretes in most of eolian unit I, in the upper part of eolian unit IV, and in the nonpersistent eolian unit, range between −4.6 and −2.1‰ PDB. These values, up to 4.4‰ greater than values from eolian units II and III, are interpreted as representing nonmonsoonal¹⁸O-enriched “normal continental” waters during climatic phases when the monsoon weakened or failed. Conversely, 25,000–60,000-yr-old calcretes (eolian units II and III) probably formed under monsoonal conditions. The two periods of weakened monsoon are consistent with other paleoclimatic data from India and may represent widespread aridity on the Indian subcontinent during isotope stages 2 and 4. The total variation in δ¹³C is 1.7‰ (0.0–1.7‰), and δ¹³C covaries positively and linearly with δ¹⁸O. δ¹³C values are highest when δ¹⁸O values indicate the most arid climatic conditions. This is best explained by expansion of C₄grasses at the expense of C₃plants at low latitudes during glacial periods when atmosphericpCO₂was lowered. C₄dominance was overridingly influenced by global change in atmosphericpCO₂despite the lowered summer rainfall.     

RbSr total rock isotope studies on Precambrian charnockitic gneisses from South Norway: evidence for isochron resetting during a low-grade metamorphic-deformational event

The quartzo-feldspathic charnockitic orthogneisses within the Bamble sector of the so-called Sveconorwegian (1.2–0.9 b.y.) zone are highly fractionated in K and Rb such that they comprise two chemically contrasting zones — one highly K, Rb-deficient and the other with values of the same order as upper crustal lithologies.Eight series of samples, each collected from single outcrops, have yielded Rb-Sr total rock apparent ages in two distinct groups, at ～1540 and ～1060 m.y. Outcrops in both the K-deficient and normal-K suites have produced examples of each age. The older age relates to the high-grade charnockite event, and the younger to a superimposed low-grade event which occurred at the same time as the intrusion of undeformed granite sheets and pegmatite dikes; one of the granites has yielded an isochron age of 1063 ± 20 m.y. The low-grade event involved only the partial alteration of orthopyroxenes to chlorite and/or serpentine, coupled with some corrosion of biotite; the alterations were initiated along narrow, irregularly spaced, cracks and it was their development which facilitated open system behaviour of the total rock isotopic systems at some localities. The degree of rehomogenisation is a function of the intensity of the secondary alterations.Confirmation of resetting at ～1060 m.y. is given by four mineral + host rock isochrons all yielding ages within error of the age for the intrusive granite; two of these are from outcrops where the rocks retain the older ～1540-m.y. age.The secondary total rock isotopic homogenisation cannot be explained adequately by Rb mobility or by simple mixing with a fluid having its own initial⁸⁷Sr/⁸⁶Sr composition. The primary mineralogy may have determined whether individual localities and/or samples suffered net increases or net decreases in⁸⁷Sr/⁸⁶Sr.An important implication of the results is that in this, or any similar geological situation, there would be a very real possibility of drawing erroneous conclusions from regionally-collected samples, particularly if the full significance of the later, relatively minor P-T event remained undetected and/or the scale of isotopic (re-)homogenisation, were unknown. It is only because of the methods adopted that it can be stated that there is no isotopic evidence for a high grade Sveconorwegian (Grenvillian) event in this part of southern Norway.     

Rhessi and Trace Observations of the 21 April 2002 x1.5 Flare

Solar Physics - Observations of the X1.5 flare on 21 April 2002 are reviewed using the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) and the Transition Region and Coronal Explorer...     

Mineralogy of iron sulfides in CM1 and CI1 lithologies of the Kaidun breccia: Records of extreme to intense hydrothermal alteration

The polymict Kaidun microbreccia contains lithologies of C‐type chondrites with euhedral iron sulfide crystals of hydrothermal origin. Our FIB‐TEM study reveals that acicular sulfides in a CM1 lithology are composed of Fe‐rich pyrrhotite with nonintegral vacancy superstructures (NC‐pyrrhotite), troilite, and pentlandite, all showing distinct exsolution textures. Based on phase relations in the Fe‐Ni‐S system, we constrain the temperature of formation of the originally homogeneous monosulfide solid solution to the range of 100–300 °C. In some crystals the exsolution of pentlandite and the microtextural equilibration was incomplete, probably due to rapid cooling. We use thermodynamic modeling to constrain the physicochemical conditions of the extreme hydrothermal alteration in this lithology. Unless the CM1 lithology was sourced from a large depth in the parent body (internal pressure >85 bar) or the temperatures were in the lower range of the interval determined, the water was likely present as vapor. Previously described light δ³⁴S compositions of sulfides in Kaidun's CM1 lithology are likely due to the loss of ³⁴S‐enriched H₂S during boiling. Platy sulfide crystals in an adjacent, intensely altered CI1 lithology are composed of Fe‐poor, monoclinic 4C‐pyrrhotite and NC‐pyrrhotite and probably formed at lower temperatures and higher fS₂ relative to the CM1 lithology. However, a better understanding of the stability of Fe‐poor pyrrhotites at temperatures below 300 °C is required to better constrain these conditions.     

Io: Volcanic thermal sources and global heat flow

We have examined thermal emission from 240 active or recently-active volcanic features on Io and quantified the magnitude and distribution of their volcanic heat flow during the Galileo epoch. We use spacecraft data and a geological map of Io to derive an estimate of the maximum possible contribution from small dark areas not detected as thermally active but which nevertheless appear to be sites of recent volcanic activity. We utilize a trend analysis to extrapolate from the smallest detectable volcanic heat sources to these smallest mapped dark areas. Including the additional heat from estimates for “outburst” eruptions and for a multitude of very small (“myriad”) hot spots, we account for ～62 × 10¹² W (～59 ± 7% of Io’s total thermal emission). Loki Patera contributes, on average, 9.6 × 10¹² W (～9.1 ± 1%). All dark paterae contribute 45.3 × 10¹² W (～43 ± 5%). Although dark flow fields cover a much larger area than dark paterae, they contribute only 5.6 × 10¹² W (～5.3 ± 0.6%). Bright paterae contribute ～2.6 × 10¹² W (～2.5 ± 0.3%). Outburst eruption phases and very small hot spots contribute no more than ～4% of Io’s total thermal emission: this is probably a maximum value. About 50% of Io’s volcanic heat flow emanates from only 1.2% of Io’s surface. Of Io’s heat flow, 41 ± 7.0% remains unaccounted for in terms of identified sources. Globally, volcanic heat flow is not uniformly distributed. Power output per unit surface area is slightly biased towards mid-latitudes, although there is a stronger bias toward the northern hemisphere when Loki Patera is included. There is a slight favoring of the northern hemisphere for outbursts where locations were well constrained. Globally, we find peaks in thermal emission at ～315°W and ～105°W (using 30° bins). There is a minimum in thermal emission at around 200°W (almost at the anti-jovian longitude) which is a significant regional difference. These peaks and troughs suggest a shift to the east from predicted global heat flow patterns resulting from tidal heating in an asthenosphere. Global volcanic heat flow is dominated by thermal emission from paterae, especially from Loki Patera (312°W, 12°N). Thermal emission from dark flows maximises between 165°W and 225°W. Finally, it is possible that a multitude of very small hot spots, smaller than the present angular resolution detection limits, and/or cooler, secondary volcanic processes involving sulphurous compounds, may be responsible for at least part of the heat flow that is not associated with known sources. Such activity should be sought out during the next mission to Io.     

Late Pleistocene ice export events into the Arctic Ocean from the M'Clure Strait Ice Stream, Canadian Arctic Archipelago

Rapidly-flowing sectors of an ice sheet (ice streams) can play an important role in abrupt climate change through the delivery of icebergs and meltwater and the subsequent disruption of ocean thermohaline circulation (e.g., the North Atlantic's Heinrich events). Recently, several cores have been raised from the Arctic Ocean which document the existence of massive ice export events during the Late Pleistocene and whose provenance has been linked to source regions in the Canadian Arctic Archipelago. In this paper, satellite imagery is used to map glacial geomorphology in the vicinity of Victoria Island, Banks Island and Prince of Wales Island (Canadian Arctic) in order to reconstruct ice flow patterns in the highly complex glacial landscape. A total of 88 discrete flow-sets are mapped and of these, 13 exhibit the characteristic geomorphology of palaeo-ice streams (i.e., parallel patterns of large, highly elongated mega-scale glacial lineations forming a convergent flow pattern with abrupt lateral margins). Previous studies by other workers and cross-cutting relationships indicate that the majority of these ice streams are relatively young and operated during or immediately prior to deglaciation. Our new mapping, however, documents a large (> 700 km long; 110 km wide) and relatively old ice stream imprint centred in M'Clintock Channel and converging into Viscount Melville Sound. A trough mouth fan located on the continental shelf suggests that it extended along M'Clure Strait and was grounded at the shelf edge. The location of the M'Clure Strait Ice Stream exactly matches the source area of 4 (possibly 5) major ice export events recorded in core PS1230 raised from Fram Strait, the major ice exit for the Arctic Ocean. These ice export events occur at 12.9, 15.6, 22 and 29.8 ka (¹⁴C yr BP) and we argue that they record vigorous episodes of activity of the M'Clure Strait Ice Stream. The timing of these events is remarkably similar to the North Atlantic's Heinrich events and we take this as evidence that the M'Clure Strait Ice Stream was also activated around the same time. This may hold important implications for the cause of the North Atlantic's Heinrich events and hints at the possibility of a pan-ice sheet response.