Seasonal connections between meteoric water and streamflow generation along a mountain headwater stream

In snowmelt-driven mountain watersheds, the hydrologic connectivity between meteoric waters and stream flow generation varies strongly with the season, reflecting variable connection to soil and groundwater storage within the watershed. This variable connectivity regulates how streamflow generation mechanisms transform the seasonal and elevational variation in oxygen and hydrogen isotopic composition (δ¹⁸O and δD) of meteoric precipitation. Thus, water isotopes in stream flow can signal immediate connectivity or more prolonged mixing, especially in high-relief mountainous catchments. We characterized δ¹⁸O and δD values in stream water along an elevational gradient in a mountain headwater catchment in southwestern Montana. Stream water isotopic compositions related most strongly to elevation between February and March, exhibiting higher δ¹⁸O and δD values with decreasing elevation. These elevational isotopic lapse rates likely reflect increased connection between stream flow and proximal snow-derived water sources heavily subject to elevational isotopic effects. These patterns disappeared during summer sampling, when consistently lower δ¹⁸O and δD values of stream water reflected contributions from snowmelt or colder rainfall, despite much higher δ¹⁸O and δD values expected in warmer seasonal rainfall. The consistently low isotopic values and absence of a trend with elevation during summer suggest lower connectivity between summer precipitation and stream flow generation as a consequence of drier soils and greater transpiration. As further evidence of intermittent seasonal connectivity between the stream and adjacent groundwaters, we observed a late-winter flush of nitrate into the stream at higher elevations, consistent with increased connection to accumulating mineralized nitrogen in riparian wetlands. This pattern was distinct from mid-summer patterns of nitrate loading at lower elevations that suggested heightened human recreational activity along the stream corridor. These observations provide insights linking stream flow generation and seasonal water storage in high elevation mountainous watersheds. Greater understanding of the connections between surface water, soil water and groundwater in these environments will help predict how the quality and quantity of mountain runoff will respond to changing climate and allow better informed water management decisions.     

Jurassic igneous rocks of the central Sanandaj–Sirjan zone (Iran) mark a propagating continental rift,not a magmatic arc

Jurassic igneous bodies of the Sanandaj–Sirjan zone (SaSZ) in SW Iran are generally considered as a magmatic arc but critical evaluation of modern geochronology, geochemistry and radiogenic isotopes challenges this conclusion. There is no evidence for sustained igneous activity along the ~1,200 km long SaSZ, as expected for a convergent plate margin; instead activity was brief at most sites and propagated NW at ~20 mm/a. Jurassic igneous rocks define a bimodal suite of gabbro‐diorite and granite. Chemical and isotopic compositions of mafic rocks indicate subcontinental lithospheric mantle sources that mostly lacked subduction‐related modifications. The arc‐like features of S‐type granites reflect massive involvement of Cadomian crust and younger sediments to generate felsic melts in response to mafic intrusions. We conclude that Jurassic SaSZ igneous activity occurred in a continental rift, not an arc. SaSZ igneous rocks do not indicate that subduction along the SW margin of Eurasia began in Jurassic time.     

Petrochronology of oxidized granulites from southern Peru

Sapphirine–quartz granulites from the Cocachacra region of the Arequipa Massif in southern Peru record early Neoproterozoic ultrahigh‐temperature metamorphism. Phase equilibrium modelling and zircon petrochronology are used to quantify timing and pressure–temperature (P–T) conditions of metamorphism. Modelling of three magnetite‐bearing sapphirine–quartz samples indicates peak temperatures of >950°C at ~0.7 GPa and a clockwise P–T evolution. Elevated concentrations of Al in orthopyroxene are also consistent with ultrahigh‐temperature conditions. Neoblastic zircon records ages of c. 1.0–0.9 Ga that are interpreted to record protracted ultrahigh‐temperature metamorphism. Th/U ratios of zircon of up to 100 reflect U‐depleted whole‐rock compositions. Concentrations of heavy rare earth elements in zircon do not show systematic trends with U–Pb age but do correlate with variable whole‐rock compositions. Very large positive Ce anomalies in zircon from two samples probably relate to strongly oxidizing conditions during neoblastic zircon crystallization. Low concentrations of Ti‐in‐zircon (<10 ppm) are interpreted to result from reduced titania activities due to the strongly oxidized nature of the granulites and the sequestration of titanium‐rich minerals away from the reaction volume. Whole‐rock compositions and oxidation state have a strong influence on the trace element composition of metamorphic zircon, which has implications for interpreting the geological significance of ages retrieved from zircon in oxidized metamorphic rocks.     

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.     

A siphon mechanism for supplying prominence mass

We examine a siphon-like mechanism for moving mass from the chromosphere to a gravitational well at the top of a magnetic loop to form a prominence. The calculations assume no apriori flow velocity at the loop base. Instead heating in the loop legs drives the flow. The prominence formation process requires two steps. First, the background heating rate must be reduced to on the order of 1 % of the initial heating rate required to maintain the coronal loop. This forms an initial condensation at the top of the loop. Second, the heating must take place only in the loop legs in order to produce a pressure differential which drives mass up into the well at the top of the loop. The heating rate in the loop must be increased once the prominence has begun to form or full prominence densities can not be achieved in a reasonable time. We conclude that this heating driven siphon-like mechanism is feasible for producing and maintaining prominences.     

Evolution of cold shock-bounded slabs

The stability and evolution of cold, shock-bounded slabs is studied using numerical hydrodynamic simulations. We confirm the analysis of Vishniac (1994) [ApJ, 428, 186], who showed that such slabs are unstable if they are perturbed by a displacement larger than their width. The growth rate of this nonlinear thin shell instability (NTSI) is found to increase with decreasing wavelength, in qualitative agreement with Vishniac's analysis. The NTSI saturates when the bending angle becomes large and the growth in the width of the slab pinches off the perturbation. After saturation, the slab remains greatly extended with an average density much less than the original slab density, supported primarily by supersonic turbulence within the slab. Linear perturbations are also found to be unstable in that they can lead to turbulent flow within the slab, although this response to linear perturbations is distinct from, and much less violent than the NTSI.Richard McCray     

Evidence in a glassy cosmic spherule from Antarctica for grazing incidence encounters with the Earth's atmosphere

Abstract— Cosmic dust accreted by the Earth can be extensively reprocessed during atmospheric encounters. The textures and compositions of reprocessed material provide important constraints by which the processes affecting extraterrestrial matter in the Earth's atmosphere can be better understood. Here we report results on an unusual Antarctic glassy cosmic spherule that demonstrates strong textural evidence for at least two grazing incidence encounters with the Earth's atmosphere prior to final reentry. The particle consists of a central glassy core with four peripheral glass lobes that transect a silicate particle rim. The texture of the particle confirms previous theoretical speculations that some high velocity, low incidence angle interplanetary particles experience numerous encounters with the Earth's atmosphere and also indicates that micrometeorites demonstrating multiple melting episodes should be interpreted with caution.