Salt tectonics driven by differential sediment loading: stability analysis and finite-element experiments

At many continental margins, differential sediment loading on an underlying salt layer drives salt deformation and has a significant impact on the structural evolution of the basin. We use 2‐D finite‐element modelling to investigate systems in which a linear viscous salt layer underlies a frictional‐plastic overburden of laterally varying thickness. In these systems, differential pressure induces the flow of viscous salt, and the overburden experiences updip deviatoric tension and downdip compression. A thin‐sheet analytical stability criterion for the system is derived and is used to predict conditions under which the sedimentary overburden will be unstable and fail, and to estimate the initial velocities of the system. The analytical predictions are in acceptable agreement with initial velocity patterns of the numerical models. In addition to initial stability analyses, the numerical model is used to investigate the subsequent finite deformation. As the systems evolve, overburden extension and salt diapirism occur in the landward section and contractional structures develop in the seaward section. The system evolution depends on the relative widths of the salt basin and the length scale of the overburden thickness variation. In narrow salt basins, overburden deformation is localised and characterised by high strain rates, which cause the system to reach a gravitational equilibrium and salt movement to cease earlier than for wide salt basins. Sedimentation enhances salt evacuation by maintaining a differential pressure in the salt. Continued sedimentary filling of landward extensional basins suppresses landward salt diapirism. Sediment progradation leads to seaward propagation of the landward extensional structures and depocentres. At slow sediment progradation rates, the viscous flow can be faster than the sediment progradation, leading to efficient salt evacuation and salt weld formation beneath the landward section. Fast sediment progradation suppresses the viscous flow, leaving salt pillows beneath the prograding wedge.     

Soil evolution and subalpine ecosystem changes in the French Alps inferred from geochemical analysis of lacustrine sediments

This study aimed to reconstruct the history of soil development, ecosystem changes and associated erosional processes in a small mountain lacustrine basin at the decennial to millennial scale. Geochemical proxies of soil evolution were analysed in the Holocene lacustrine sediments and peats from Thyl Lake, Maurienne Valley, French Alps. Podzolization and chemical weathering processes were assessed using secondary Al- and Fe-bearing phases together with major and Rare Earth Elements (REE). The resulting proxy records, spanning ca. 4,400 years between 8.6 and 4.2 cal ka BP, indicate that progressive pedogenesis occurred after deglaciation in a relatively stable subalpine ecosystem. As shown by the associated increase in Al- and Fe-bearing phases and some REE fractions, the establishment of a mixed cembra pine ecosystem from ca. 7.2–6.5 ka BP was associated with enhanced podzolisation processes in the catchment. The progressive soil development was followed by a rapid transformation of the local environment and plant cover (the open waters of the lake were replaced by a confined peat environment) together with changes in forest fire regimes from ca. 6.8 ka BP. Depleted REE patterns, associated with low contents of secondary Al and Fe, suggest a decrease in chemical weathering and podzolization in the catchment at that time, possibly associated with local intensification of weathering and drainage processes in a relatively acidic peat environment. The higher variability of cembra pine and the increased abundance of sedge and other herbaceous plant remains in the lake sediment indicate semi-open vegetation environments from 5.7 cal ka BP onwards. Whereas fire events and plant cover appear to be significantly related, the soil processes seem primarily linked to vegetation composition, and secondarily to changes in fire regime.     

Assessment of pingo distribution and morphometry using an IfSAR derived digital surface model, western Arctic Coastal Plain, Northern Alaska

Pingos are circular to elongate ice-cored mounds that form by injection and freezing of pressurized water in near-surface permafrost. Here we use a digital surface model (DSM) derived from an airborne Interferometric Synthetic Aperture Radar (IfSAR) system to assess the distribution and morphometry of pingos within a 40,000 km² area on the western Arctic Coastal Plain of northern Alaska. We have identified 1247 pingo forms in the study region, ranging in height from 2 to 21 m, with a mean height of 4.6 m. Pingos in this region are of hydrostatic origin, with 98% located within 995 drained lake basins, most of which are underlain by thick eolian sand deposits. The highest pingo density (0.18 km^− 2) occurs where streams have reworked these deposits. Morphometric analyses indicate that most pingos are small to medium in size (< 200 m diameter), gently to moderately sloping (< 30°), circular to slightly elongate (mean circularity index of 0.88), and of relatively low height (2 to 5 m). However, 57 pingos stand higher than 10 m, 26 have a maximum slope greater than 30°, and 42 are larger than 200 m in diameter. Comparison with a legacy pingo dataset based on 1950s stereo-pair photography indicates that 66 may have partially or completely collapsed over the last half-century. However, we mapped over 400 pingos not identified in the legacy dataset, and identified only three higher than 2 m to have formed between ca. 1955 and ca. 2005, indicating that caution should be taken when comparing contemporary and legacy datasets derived by different techniques. This comprehensive database of pingo location and morphometry based on an IfSAR DSM may prove useful for land and resource managers as well as aid in the identification of pingo-like features on Mars.     

Paleolimnology of Lake Tanganyika, East Africa, over the past 100 kyr

New sediment core data from a unique slow-sedimentation rate site in Lake Tanganyika contain a much longer and continuous record of limnological response to climate change than have been previously observed in equatorial regions of central Africa. The new core site was first located through an extensive seismic reflection survey over the Kavala Island Ridge (KIR), a sedimented basement high that separates the Kigoma and Kalemie Basins in Lake Tanganyika.Proxy analyses of paleoclimate response carried out on core T97-52V include paleomagnetic and index properties, TOC and isotopic analyses of organic carbon, and diatom and biogenic silica analyses. A robust age model based on 11 radiocarbon (AMS) dates indicates a linear, continuous sedimentation rate nearly an order of magnitude slower here compared to other core sites around the lake. This age model indicates continuous sedimentation over the past 79 k yr, and a basal age in excess of 100 k yr.The results of the proxy analyses for the past 20 k yr are comparable to previous studies focused on that interval in Lake Tanganyika, and show that the lake was about 350 m lower than present at the Last Glacial Maximum (LGM). Repetitive peaks in TOC and corresponding drops in ¹³C over the past 79 k yr indicate periods of high productivity and mixing above the T97-52V core site, probably due to cooler and perhaps windier conditions. From 80 through 58 k yr the ¹³C values are relatively negative (–26 to –28 l) suggesting predominance of algal contributions to bottom sediments at this site during this time. Following this interval there is a shift to higher values of ¹³C, indicating a possible shift to C-4 pathway-dominated grassland-type vegetation in the catchment, and indicating cooler, dryer conditions from 55 k yr through the LGM. Two seismic sequence boundaries are observed at shallow stratigraphic levels in the seismic reflection data, and the upper boundary correlates to a major discontinuity near the base of T97-52V. We interpret these discontinuities to reflect major, prolonged drops in lake level below the core site (393 m), with the lower boundary correlating to marine oxygen isotope Stage 6. This suggests that the previous glacial period was considerably cooler and more arid in the equatorial tropics than was the last glacial period.     

Stream Temperature Spatial Variability Reflects Geomorphology,Hydrology, and Microclimate: Navarro River Watershed,California

Stream temperatures are critical to coldwater fish and vary with microclimate, geomorphology, and hydrology, including influx of groundwater. Spatial variability of stream temperatures was examined at reach and watershed scales within the 816 km² Navarro River watershed in California. Field monitoring and numerical modeling illustrate that stream temperatures were highest at sites with high solar incidence (low shading and wide streams), long travel times, and low discharge. Microclimate helps explain deviation from the general pattern of streams warming with increasing drainage area. Reach-scale field observations of channel width and groundwater influx explain variation in stream temperatures not captured by watershed-scale models.     

Large‐scale deformation in the India‐Asia collision constrained by earthquakes and topography

Analysis of earthquake focal mechanisms allows division of the India‐Asia collision into kinematic domains that strongly correlate with topography. These kinematic domains indicate strain partitioning dominated by oblique slip deformation. The Kunlun and south Tibetan fault systems mark discontinuities in the strain field and bound the high, flat topography of the plateau which deforms by transtension. The northern and southern margins of Tibet deform by transpression or contraction and are topographically steep. Correlations between seismicity and topography are due to Mohr–Coulomb wedge mechanics at the northern and southern plateau margins which produce naturally steep surface slopes, whereas the flat interior and eastern margin of the Tibetan Plateau is underlain by viscous crust which supports subdued topography further muted by Cenozoic basin fill. These data indicate that the long wavelength topography of the India‐Asia collision is controlled by seismically caused surface displacements which are linked to deep crustal deformation mechanics.     

Multiphase lattice Boltzmann simulations for porous media applications

Over the last two decades, lattice Boltzmann methods have become an increasingly popular tool to compute the flow in complex geometries such as porous media. In addition to single phase simulations allowing, for example, a precise quantification of the permeability of a porous sample, a number of extensions to the lattice Boltzmann method are available which allow to study multiphase and multicomponent flows on a pore scale level. In this article, we give an extensive overview on a number of these diffuse interface models and discuss their advantages and disadvantages. Furthermore, we shortly report on multiphase flows containing solid particles, as well as implementation details and optimization issues.     

An automated routing methodology to enable direct rainfall in high resolution shallow water models

Recent high profile flood events have highlighted the need for hydraulic models capable of simulating pluvial flooding in urban areas. This paper presents a constant velocity rainfall routing scheme that provides this ability within the LISFLOOD‐FP hydraulic modelling code. The scheme operates in place of the shallow water equations within cells where the water depth is below a user‐defined threshold, enabling rainfall‐derived water to be moved from elevated features such as buildings or curbstones without causing instabilities in the solution whilst also yielding a reduction in the overall computational cost of the simulation. Benchmarking against commercial modelling packages using a pluvial and point‐source test case demonstrates that the scheme does not impede the ability of LISFLOOD‐FP to match both predicted depths and velocities of full shallow water models. The stability of the scheme in conditions unsuitable for traditional two‐dimensional hydraulic models is then demonstrated using a pluvial test case over a complex urban digital elevation model containing buildings. Deterministic single‐parameter sensitivity analyses undertaken using this test case show limited sensitivity of predicted water depths to both the chosen routing speed within a physically plausible range and values of the depth threshold parameter below 10 mm. Local instabilities can occur in the solution if the depth threshold is >10 mm, but such values are not required even when simulating extreme rainfall rates. The scheme yields a reduction in model runtime of ~25% due to the reduced number of cells for which the hydrodynamic equations have to be solved. Copyright © 2012 John Wiley & Sons, Ltd.     

Avoidance behaviour of freshwater fish and shrimp exposed to ammonia and low dissolved oxygen separately and in combination

Abstract

The responses of common smelt (Retropinna retropinna Richardson), inanga (Galaxias maculatus Jenyns), common bully (Gobiomorphus cotidianus McDowall), and shrimp (Paratya curvirostris Heller) to ammonia and low dissolved oxygen (DO), separately and in combination, were measured in a fluvarium. Only common smelt avoided high or low ammonia (c. 8.5 and 2.0 g m^?3 NH3, respectively) and low DO (c. 2 g m^?3) alone and in combination. Shrimp avoided high ammonia and low DO+ammonia but not low DO on its own. Inanga and bullies showed no significant negative response to either contaminant; rather, inanga were strongly attracted to low ammonia and bullies to low DO. These results parallel those from toxicity experiments that show common smelt and shrimp are usually among the most sensitive native species to various contaminants. The consistent and appropriate avoidance behaviour shown by common smelt, in particular, suggests this species would be a good indicator organism for assessing the health of lowland waterways.