Understanding fen hydrology across multiple scales

Fens, which are among the most biodiverse of wetland types in the USA, typically occur in glacial landscapes characterized by geo‐morphologic variability at multiple spatial scales. As a result, the hydrologic systems that sustain fens are complex and not well understood. Traditional approaches for characterizing such systems use simplifying assumptions that cannot adequately capture the impact of variability in geology and topography. In this study, a hierarchical, multi‐scale groundwater modelling approach coupled with a geologic model is used to understand the hydrology of a fen in Michigan. This approach uses high‐resolution data to simulate the multi‐scale topographic and hydrologic framework and lithologic data from more than 8500 boreholes in a statewide water well database to capture the complex geology. A hierarchy of dynamically linked models is developed that simulates groundwater flow at all scales of interest and to delineate the areas that contribute groundwater to the fen. The results show the fen receiving groundwater from multiple sources: an adjacent wetland, local recharge, a nearby lake and a regional groundwater mound. Water from the regional mound flows to an intermediate source before reaching the fen, forming a ‘cascading’ connection, while other sources provide water through ‘direct’ connections. The regional mound is also the source of water to other fens, streams and lakes in this area, thus creating a large, interconnected hydrologic system that sustains the entire ecosystem. In order to sustainably manage such systems, conservation efforts must include both site‐based protection and management, as well as regional protection and management of groundwater source areas. Copyright © 2016 John Wiley & Sons, Ltd.     

A note on the howard-Malkus-Whitehead floating heat sources

Abstract

Some deficiencies in a recent paper by Howard, Malkus and Whitehead are examined. The problem is reformulated in terms of an integro-differential equation, from which both asymptotic and numerical solutions are obtained.     

Laser argon dating of melt breccias from the Siljan impact structure,Sweden: Implications for a possible relationship to Late Devonian extinction events

Abstract— In earlier studies, the 65‐75 km diameter Siljan impact structure in Sweden has been linked to the Late Devonian mass extinction event. The Siljan impact event has previously been dated by K‐Ar and Ar‐Ar chronology at 342‐368 Ma, with the commonly quoted age being 362.7 ± 2.2 Ma (2 s?, recalculated using currently accepted decay constants). Until recently, the accepted age for the Frasnian/Famennian boundary and associated extinction event was 364 Ma, which is within error limits of this earlier Siljan age. Here we report new Ar‐Ar ages extracted by laser spot and laser step heating techniques for several melt breccia samples from Siljan (interpreted to be impact melt breccia). The analytical results show some scatter, which is greater in samples with more extensive alteration; these samples generally yield younger ages. The two samples with the least alteration yield the most reproducible weighted mean ages: one yielded a laser spot age of 377.2 ± 2.5 Ma (95% confidence limits) and the other yielded both a laser spot age of 376.1 ± 2.8 Ma (95% confidence limits) and a laser stepped heating plateau age over 70.6% ³⁹Ar release of 377.5 ± 2.4 Ma (2 s?). Our conservative estimate for the age of Siljan is 377 ± 2 Ma (95% confidence limits), which is significantly different from both the previously accepted age for the Frasnian/Famennian (F/F) boundary and the previously quoted age of Siljan. However, the age of the F/F boundary has recently been revised to 374.5 ± 2.6 Ma by the International Commission for Stratigraphy, which is, within error, the same as our new age. However, the currently available age data are not proof that there was a connection between the Siljan impact event and the F/F boundary extinction. This new result highlights the dual problems of dating meteorite impacts where fine‐grained melt rocks are often all that can be isotopically dated, and constraining the absolute age of biostratigraphic boundaries, which can only be constrained by age extrapolation. Further work is required to develop and improve the terrestrial impact age record and test whether or not the terrestrial impact flux increased significantly at certain times, perhaps resulting in major extinction events in Earth's biostratigraphic record.     

Estimation of instantaneous peak flows from maximum mean daily flows using the HBV hydrological model

The record length and quality of instantaneous peak flows (IPFs) have a great influence on flood design, but these high resolution flow data are not always available. The primary aim of this study is to compare different strategies to derive frequency distributions of IPFs using the Hydrologiska Byråns Vattenbalansavdelning (HBV) hydrologic model. The model is operated on a daily and an hourly time step for 18 catchments in the Aller‐Leine basin, Germany. Subsequently, general extreme value (GEV) distributions are fitted to the simulated annual series of daily and hourly extreme flows. The resulting maximum mean daily flow (MDF) quantiles from daily simulations are transferred into IPF quantiles using a multiple regression model, which enables a direct comparison with the simulated hourly quantiles. As long climate records with a high temporal resolution are not available, the hourly simulations require a disaggregation of the daily rainfall. Additionally, two calibrations strategies are applied: (1) a calibration on flow statistics; (2) a calibration on hydrographs. The results show that: (1) the multiple regression model is capable of predicting IPFs with the simulated MDFs; (2) both daily simulations with post‐correction of flows and hourly simulations with pre‐processing of precipitation enable a reasonable estimation of IPFs; (3) the best results are achieved using disaggregated rainfall for hourly modelling with calibration on flow statistics; and (4) if the IPF observations are not sufficient for model calibration on flow statistics, the transfer of MDFs via multiple regressions is a good alternative for estimating IPFs. Copyright © 2015 John Wiley & Sons, Ltd.     

The evolution of low-metallicity asymptotic giant branch stars and the formation of carbon-enhanced metal-poor stars

We investigate the behaviour of asymptotic giant branch (AGB) stars between metallicities   Z = 10⁻⁴  and 10⁻⁸. We determine which stars undergo an episode of flash-driven mixing, where protons are ingested into the intershell convection zone, as they enter the thermally pulsing AGB phase and which undergo third dredge-up. We find that flash-driven mixing does not occur above a metallicity of   Z = 10⁻⁵  for any mass of star and that stars above  2 M_⊙  do not experience this phenomenon at any metallicity. We find carbon ingestion (CI), the mixing of carbon into the tail of hydrogen-burning region, occurs in the mass range  2 M_⊙  to around  4 M_⊙  . We suggest that CI may be a weak version of the flash-driven mechanism. We also investigate the effects of convective overshooting on the behaviour of these objects. Our models struggle to explain the frequency of Carbon-Enhanced Metal-Poor (CEMP) stars that have both significant carbon and nitrogen enhancement. Carbon can be enhanced through flash-driven mixing, CI or just third dredge-up. Nitrogen can be enhanced through hot bottom burning and the occurrence of hot dredge-up also converts carbon into nitrogen. The C/N ratio may be a good indicator of the mass of the primary AGB stars.     

The Planar Shape of Rock Joints

Knowing the planar shape of discontinuities is important when characterizing discontinuities in a rock mass. However, the real discontinuity shape is rarely known, since the rock mass is usually inaccessible in three dimensions. Information on discontinuity shape is limited and often open to more than one interpretation. This paper discusses the planar shape of rock joints, the most common discontinuities in rock. First, a brief literature review about the shape of joints is presented, including some information on joint-surface morphology, inferences from observed trace lengths on different sampling planes, information based on experimental studies, and joint shapes assumed by different researchers. This review shows that joints not affected by adjacent geological structures such as bedding boundaries or pre-existing fractures tend to be elliptical (or approximately circular but rarely). Joints affected by or intersecting such geological structures tend to be rectangular. Then, using the general stereological relationship between trace length distributions and joint size distributions developed by Zhang et al. (Geotechnique 52(6):419–433, 2002) for elliptical joints, the effect of sampling plane orientation on trace lengths is investigated. This study explains why the average trace lengths of non-equidimensional (elliptical or similar polygonal) joints on two sampling planes can be about equal and thus the conclusion that rock joints are equidimensional (circular) drawn from the fact that the average trace lengths on two sampling planes are approximately equal can be wrong. Finally, methods for characterizing the shape and size of joints (elliptical or rectangular) from trace length data are recommended, and the appropriateness of using elliptical joint shapes to represent polygonal, especially rectangular, joints is discussed.     

Eckfeld Maar: Window into an Eocene Terrestrial Habitat in Central Europe

<正>To mark the occasion of the 175th anniversary of the Rheinische Naturforschende Gesellschaft in 2009 and of the centennial of the Mainz Natural History Museum in 2010,we present a short account of our present knowledge of the Eckfeld Maar after 20 years of continuous research.This paper does not attempt to include all of the detailed results on the geology of the Eckfeld site or its biota.To date,nearly 250 papers and books have been published since the start of our project.An up-to-date list of these publications can be found at www.eckfeldermaar. de.     

Rift induced delamination of mantle lithosphere and crustal uplift: a new mechanism for explaining Rwenzori Mountains’ extreme elevation?

With heights of 4–5 km, the topography of Rwenzori Mountains, a large horst of old crustal rocks located inside a young passive rift system, poses the question “Why are the Rwenzori Mountains so high?”. The Cenozoic Western Rift branch of the East African Rift System is situated within the Late Proterozoic mobile belts between the Archean Tanzania Craton and Congo Craton. The special geological setting of the massif at a rift node encircled by the ends of the northern Western Rift segments of Lake Albert and Lake Edward suggests that the mechanism responsible for the high elevation of the Rwenzoris is related to the rifting process. Our hypothesis is based on the propagation of the rift tips, surrounding the stiff old lithosphere at Rwenzori region, thereby triggering the delamination of the cold and dense mantle lithosphere (ML) root by reducing viscosity and strength of the undermost lower crust. As a result, this unloading induces fast isostatic pop-up of the less dense crustal Rwenzori block. We term this RID—“rift induced delamination of Mantle Lithosphere”. The physical consistency of the RID hypothesis is tested numerically. Viscous flow of 2D models is approximated by a Finite Difference Method with markers in an Eulerian formulation. The equations of conservation of mass, momentum and energy are solved for a multi-component system. Based on laboratory data of appropriate rock samples, a temperature-, pressure- and stress-dependent rheology is assumed. Assuming a simple starting model with a locally heated ML, the ML block between the weakened zones becomes unstable and sinks into the asthenosphere, while the overlying continental crust rises up. Thus, RID seems to be a viable mechanism to explain geodynamically the extreme uplift. Important conditions are a thermal anomaly within the ML, a ductile lower crust with visco-plastic rheology allowing significant strength reduction and lateral density variations. The special situation of a two-sided rifting or offset rift segments to decouple the ML laterally from the surrounding continental lithosphere seems to be most decisive. Further support for the RID mechanism may come from additional crustal thickness and an extensive stress field. Some parameters, such as the excess temperature and yield stress, are very sensitive, small changes determine whether delamination takes place or not.