Stable carbon isotope changes during artificial charring of propagules

Charred organic remains are ubiquitous in the archaeological and fossil record and are often used to interpret past environments and climate. This study focuses on the physical and chemical alteration that takes place during heating (i.e. charring). Modifications to the internal and external morphology were noted alongside the change in molecular and stable carbon isotope signature. Molecular analyses were undertaken using direct temperature resolved mass spectrometry and the stable carbon isotopes determined using isotope ratio mass spectrometry. The results of this study document a general enrichment in ¹³C/¹²C composition of charred material which could reflect the changes observed in both the molecular composition and the relative proportions of the molecules formed. These results indicate that spurious results might be inferred when comparing the stable carbon isotope signature of charred/charcoalified material with uncharred organic matter     

Geophysical models for the tectonic framework of the Lake Vostok region, East Antarctica

Aerogeophysical and seismological data from a geophysical survey in the interior of East Antarctica were used to develop a conceptual tectonic model for the Lake Vostok region. The model is constrained using three independent data sets: magnetic, seismic, and gravimetric. A distinct change in the aeromagnetic anomaly character across Lake Vostok defines a crustal boundary. Depth to magnetic basement estimates image a 400-km-wide and more than 10-km-deep sedimentary basin west of the lake. Analysis of teleseismic earthquakes suggests a relatively thin crust beneath Lake Vostok consistent with predictions from kinematic and flexural gravity modelling. Magnetic, gravity, and subglacial topography data reveal a tectonic boundary within East Antarctica. Based on our kinematic and flexural gravity modelling, this tectonic boundary appears to be the result of thrust sheet emplacement onto an earlier passive continental margin. No data presently exist to date directly either the timing of passive margin formation or the subsequent shortening phase. The preserved thrust sheet thickness is related to the thickness of the passive margin crust. Because a significant amount of time is required to erode the thrust sheet topography, we suggest that these tectonic events are Proterozoic in age. Minor normal reactivation of the thrust sheet offers a simple mechanism to explain the formation of the Lake Vostok Basin. A low level of seismicity exists in the vicinity of this tectonic boundary. The existence of a crustal boundary in the Antarctic interior provides new constraints on the Proterozoic architecture of the East Antarctic craton.     

Sub-ice geology inland of the Transantarctic Mountains in light of new aerogeophysical data

The Transantarctic Mountains are a major geologic boundary that bisects the Antarctic continent, separating the low-lying, tectonically active terrains of West Antarctica from the East Antarctic craton. A new comprehensive aerogeophysical data set, extending 1150 km from the Ross Sea into the interior of East Antarctica provides insights into the complex structure inland of the Transantarctic Mountains. Geophysical maps, compiled from 21 000 km of gravity, magnetic and subglacial topography data, outline the boundaries of several geologic and tectonic segments within the survey area. The coherent pattern in magnetic data and mesa topography suggests a subglacial extent of the Transantarctic Mountains 400–500 km inland the last exposed rock outcrops. We estimate the maximum thickness of a potential sediment infill in the Wilkes Subglacial Basin to be less than 1 km, based on gravity modeling and source depth estimates from magnetic data. The coherent nature of the potential field and topography data, together with the northwest–southeast trends, define the Adventure Subglacial Trench and the Resolution Subglacial Highlands as a tectonic unit. The crustal structure and the strong similarity of the observed gravity with fold-and-thrust belts suggest a compressional scenario for the origin of the Adventure Subglacial Trench and the Resolution Subglacial Highlands. The complexity and apparent structural control of the Wilkes Subglacial Basin raise the issue of what influence pre-existing structures may have played in the formation of the Transantarctic Mountains system. The previous hypothesis of a thermal boundary beneath the mountains is difficult to reconcile with our new gravity data. The apparent difficulties to match our new data with certain key aspects of previous models suggests that a reassessment of the existing uplift models is necessary. We have modeled the prominent gravity anomaly over the Transantarctic Mountains with thicker crust.     

The July 2007 rock and ice avalanches at Mount Steele, St. Elias Mountains, Yukon, Canada

A large rock and ice avalanche occurred on the north face of Mount Steele, southwest Yukon Territory, Canada, on July 24, 2007. In the days and weeks preceding the landslide, several smaller avalanches initiated from the same slope. The ice and rock debris traveled a maximum horizontal distance 5.76 km with a maximum vertical descent of 2,160 m, leaving a deposit 3.66 km² in area on Steele Glacier. The seismic magnitude estimated from long-period surface waves (M _s) is 5.2. Modeling of the waveforms suggests an estimated duration of approximately 100 s and an average velocity of between 35 and 65 m/s. This landslide is one of 18 large rock avalanches known to have occurred since 1899 on slopes adjacent to glaciers in western Canada. We describe the setting, reconstruct the event chronology and present a preliminary characterization of the Mount Steele ice and rock avalanches based on field reconnaissance, analysis of seismic records and an airborne LiDAR survey. We also present the results of a successful dynamic simulation for the July 24 event.     

Geomorphology,hydrology, and aquatic vegetation drive seasonal hyporheic flow patterns across a gravel‐dominated floodplain

Across 1·7 km² of the Umatilla River floodplain (Oregon, USA), we investigated the influences of an ephemeral tributary and perennial ‘spring channel’ (fed only by upwelling groundwater) on hyporheic hydrology. We derived maps of winter and summer water‐table elevations from data collected at 46 monitoring wells and 19 stage gauges and used resulting maps to infer groundwater flow direction. Groundwater flow direction varied seasonally across the floodplain and was influenced by main channel stage, flooding, the tributary creek, and the location and direction of hyporheic exchange in the spring channel. Hyporheic exchange in the spring channel was evaluated with a geochemical mixing model, which confirmed patterns of floodplain groundwater movement inferred from water‐table maps and showed that the spring channel was fed predominantly by hyporheic water from the floodplain aquifer (87% during winter, 80% during summer), with its remaining flow supplied by upslope groundwater from the adjacent catchment aquifer. Summertime growth of aquatic macrophytes in the spring channel also influenced patterns of hyporheic exchange and groundwater flow direction in the alluvial aquifer by increasing flow resistance in the spring channel, locally raising surface water stage and adjacent water‐table elevation, and thereby altering the slope of the water‐table in the hyporheic zone. The Umatilla River floodplain is larger than most sites where hyporheic hydrology has been investigated in detail. Yet, our results corroborate other research that has identified off‐channel geomorphic features as important drivers of hyporheic hydrology, including previously published modeling efforts from a similar river and field observations from smaller streams. Copyright © 2007 John Wiley & Sons, Ltd.     

Gravitational focusing and shielding of meteoroid streams

The gravitational attraction of planets can cause significant perturbation of the trajectories of meteoroids. The resulting deflection can result in significant enhancement of the flux of meteoric particles in the neighbourhood of the planet. We give an analytical method for calculating the relative flux of stream meteoroids in the vicinity of a planet. We include the effect of shielding of certain regions that are not accessible to the meteoroids that have impacted the planet. We compare our results with those of Divine et al. and although we confirm the accuracy of their trajectory equations, the fluxes predicted by their calculations are not consistent with integrated fluxes over a planet predicted by the classic work of Öpik. Our method yields predictions for the integrated flux enhancement factor that are identical to Öpik's. We present the results of these calculations and find that in all cases, tail-like regions of enhanced meteoroid flux appear downstream of the planet, with very large enhancements possible in the case of the giant planets for all probable stream velocities.