Combined three-dimensional electric and seismic tomography study on the Åknes rockslide in western Norway

We present a combined 3-D geoelectric and seismic tomography study conducted on the large Åknes rockslide in western Norway. Movements on the slope are strongly influenced by water infiltration, such that the hydrogeological regime is considered as a critical factor affecting the slope stability. The aim of our combined geophysical study was to identify and visualize the main shallow tension fractures and to determine their effect on hydraulic processes by comparing the geophysical results with information from borehole logging and tracer tests. To resolve the complex subsurface conditions of the highly fractured rock mass, a three-dimensional set-up was chosen for our seismic survey. To map the water distribution within the rock mass, a pattern of nine intersecting 2-D geoelectric profiles covered the complete unstable slope. Six of them that crossed the seismic survey area were considered as a single data set in a 3-D inversion. For both methods, smoothing-constraint inversion algorithms were used, and the forward calculations and parameterizations were based on unstructured triangular meshes. A pair of parallel shallow low-velocity anomalies (< 1400 m/s) observed in the final seismic tomogram was immediately underlain by two anomalies with resistivities <13 kΩm in the resistivity tomogram. In combination with borehole logging results, the low-velocity and resistivity anomalies could be associated with the drained and water-filled part of the tension fractures, respectively. There were indications from impeller flowmeter measurements and tracer tests that such tension fractures intersected several other water-filled fractures and were responsible for distinct changes of the main groundwater flow paths.     

Holocene alpine glaciation inferred from lacustrine sediments on northeastern Baffin Island,Arctic Canada

With accelerated melting of alpine glaciers, understanding the future state of the cryosphere is critical. Because the observational record of glacier response to climate change is short, palaeo‐records of glacier change are needed. Using proglacial lake sediments, which contain continuous and datable records of past glacier activity, we investigate Holocene glacier fluctuations on northeastern Baffin Island. Basal radiocarbon ages from three lakes constrain Laurentide Ice Sheet retreat by ca. 10.5 ka. High sedimentation rates (0.03 cm a^?1) and continuous minerogenic sedimentation throughout the Holocene in proglacial lakes, in contrast to organic‐rich sediments and low sedimentation rates (0.005 cm a^?1) in neighbouring non‐glacial lakes, suggest that glaciers may have persisted in proglacial lake catchments since regional deglaciation. The presence of varves and relatively high magnetic susceptibility from 10 to 6 ka and since 2 ka in one proglacial lake suggest minimum Holocene glacier extent ca. 6–2 ka. Moraine evidence and proglacial and threshold lake sediments indicate that the maximum Holocene glacier extent occurred during the Little Ice Age. The finding that glaciers likely persisted through the Holocene is surprising, given that regional proxy records reveal summer temperatures several degrees warmer than today, and may be due to shorter ablation seasons and greater accumulation‐season precipitation. Copyright © 2009 John Wiley & Sons, Ltd.     

Can 234U–230Th dating be used to date large semi‐arid tufas? Challenges from a study in the Naukluft Mountains,Namibia

The large, extensive tufa deposits of the semi‐arid Naukluft Mountains, Namibia are potentially important palaeoenvironmental indicators in an area with few proxy records. Tufas are reliable indicators of increased moisture availability, and have been shown to be amenable to ²³⁴U–²³⁰Th dating, although two challenges are detrital contamination and open‐system behaviour. Densely cemented tufa facies are good candidates for dating, minimising these problems. We report attempts to date five densely‐cemented units, which are only found rarely within the Naukluft deposits. We applied a detailed methodology using multiple subsample analysis, measurement of insoluble residues, application of ‘isochron’ mixing lines, and attempted open‐systems modelling, alongside observations of micromorphology and cathodoluminescence in order to assess the validity of any obtained dates. Surprisingly, densely cemented tufas were found not always to be suitable for dating. Two units contained detrital contamination, which could not be corrected for using a single leachate correction or ‘isochron’ methods. Two units contained ‘excess ²³⁰Th’. This could result under a closed‐system if initial (²³⁴U/²³⁸U) was sufficiently high. Alternatively this may be the result of open‐system behaviour, and loss of uranium, or incorporation of initial unsupported ²³⁰Th, which render samples unsuitable for ²³⁴U–²³⁰Th dating. Micromorphological appearance and cathodoluminescence behaviour are used to explore these possibilities. This study exemplifies the need for careful sample selection, and highlights the importance of analysing multiple subsamples from any tufa sample. The detailed methodology applied proves to be a powerful tool for identifying the range of problems that can be encountered when selecting suitable candidate samples for successful dating. It also shows that semi‐arid tufa sequences may contain very little material suitable for dating. A reliable age of c 80 ka was obtained for a banded unit within a large fluvial barrage, with less reliable dates suggesting tufa deposition during times since >350 ka through to the late Holocene. Copyright © 2010 John Wiley & Sons, Ltd.     

Modelling and observation of hedgerow transpiration effect on water balance components at the hillslope scale in Brittany

Vegetation has a major influence on the water and energy balance of the earth's surface. In the last century, human activities have modified land use, inducing a consequent change in albedo and potential evapotranspiration. Linear vegetation structures (hedgerows, shelterbelts, open woodland, etc) were particularly abundant but have declined considerably over the past several decades. In this context, it is important to quantify their effect on water and energy balance both on a global scale (climate change and weather prediction) and on a local scale (soil column, hillslope and watershed). The main objective of this study was to quantify the effect of hedgerows on the water cycle by evaluating spatial and temporal variations of water balance components of a hillslope crossed by a hedgerow. Water flow simulation was performed using Hydrus‐2D to emphasize the importance of transpiration in the water balance and to evaluate water extraction from groundwater. Model validation was performed by comparing simulated and observed soil matrix potentials and groundwater levels. Hedgerow transpiration was calculated from sap flow measurements of four trees. Water balance components calculated with a one‐dimensional water balance equation were compared with simulations. Simulation runs with and without tree root uptake underlined the effect of hedgerow transpiration, increasing capillary rise and decreasing drainage. Results demonstrated that the spatial and temporal variability of water balance components was related to the hedgerow presence as well as to the meteorological context. The relations between transpiration, groundwater proximity and soil‐water availability determined the way in which water balance components were affected. Increased capillary rise and decreased drainage near hedges were related to the high transpiration of trees identified in this study. Transpiration reached twice the potential evapotranspiration when groundwater level and precipitation amounts were high. Water balance analysis showed that transpiration was a substantial component, representing 40% of total water output. These results may offer support for improving hydrological models by including the effect of land use and land cover on hydrological processes. Copyright © 2012 John Wiley & Sons, Ltd.     

Extreme events as drivers of early human behaviour in Africa? The case for variability,not catastrophic drought

Extreme Late Quaternary climatic events, sometimes of considerable continental extent, are being proposed as major contributors to ancestral human behaviour, particularly migration, in Africa. Most recently, a catastrophic drought in the Afro‐Asian monsoon region has been proposed for 16 000–17 000 years ago, driven by global impacts of the Heinrich event 1 (H1), with potentially significant consequences for Palaeolithic cultures. We provide a new analysis of the assertion and find, on examination of a wide set of palaeoenvironmental records, that the scale and extent of the proposed drought is not supported. While some parts of the African tropics, close to the equator, do appear dry at this time, data for the tropics as a whole suggest markedly variable terrestrial conditions, with some environmental systems experiencing very positive hydrological excursions during H1. We contend that in the quest for evidence of climate drivers of ancestral human behaviour, the variability associated with spatially and temporally complex climatic conditions is a significant factor in itself. Copyright © 2011 John Wiley & Sons, Ltd.     

Europe adapts to climate change: Comparing National Adaptation Strategies

For the last two decades, European climate policy has focused almost exclusively on mitigation of climate change. It was only well after the turn of the century, with impacts of climate change increasingly being observed, that adaptation was added to the policy agenda and EU Member States started to develop National Adaptation Strategies (NASs). This paper reviews seven National Adaptation Strategies that were either formally adopted or under development by Member States at the end of 2008. The strategies are analysed under the following six themes. Firstly, the factors motivating and facilitating the development of a national adaptation strategy. Secondly, the scientific and technical support needed for the development and implementation of such a strategy. Thirdly, the role of the strategy in information, communication and awareness-raising of the adaptation issue. Fourthly, new or existing forms of multi-level governance to implement the proposed actions. Fifthly, how the strategy addresses integration and coordination with other policy domains. Finally, how the strategy suggests the implementation and how the strategy is evaluated. The paper notes that the role of National Adaptation Strategies in the wider governance of adaptation differs between countries but clearly benchmarks a new political commitment to adaptation at national policy levels. However, we also find that in most cases approaches for implementing and evaluating the strategies are yet to be defined. The paper concludes that even though the strategies show great resemblance in terms of topics, methods and approaches, there are many institutional challenges, including multi-level governance and policy integration issues, which can act as considerable barriers in future policy implementation.     

Sedimentary archives of the French Atlantic coast (inner Bay of Vilaine,south Brittany): Depositional history and late Holocene climatic and environmental signals

The late Holocene is of particular interest to our understanding of the evolution of coastal sedimentary systems because this period encompasses warmer and cooler periods, and rising sea level in northern Europe. Based on an approach combining AMS ¹⁴C, sedimentological and rock magnetic analyses on sediment cores complemented with seismic data collected in the macrotidal Bay of Vilaine (south Brittany), we document the depositional history of the inner bay coeval to the mid- to late-Holocene transgression in south Brittany. Correlation between sedimentary archives revealed the main sedimentary infilling phases during the last 6000 years. Four units (U1–U4) are recognized in the coastal sediment wedge of the system, corresponding to the stepwise marine invasion of the bay. We show that (1) marine inundation, due to the steep morphology of the bedrock, is diachronous between distal and proximal records. A time lag of ～1000 years is inferred over a distance of less than 5 km; (2) in the outer areas, the sedimentation has been condensed since 3000 years; (3) proximal estuarine archives offer the best record of sedimentary processes covering the last 2000 years, including the Medieval Warm Period (MWP).Correlations in proximal records in the Bay of Vilaine assess the connection between coastal sedimentary dynamics, climatic conditions and anthropogenic activities during the MWP. We match the preservation of clay deposits to increased river-borne suspended matter transported to the estuary probably as a result of accelerated land-use development (higher soil erosion) in the catchment area between ca. 880 and 1050 AD. Because the preservation of estuarine sedimentary successions is favoured when coastal wave sediment reworking is minimal, it is proposed that the prevailing climatic regime in south Brittany during the MWP likely resembled to that of the preferred negative phase of the North Atlantic Oscillation (NAO). Our data are fairly consistent with other late Holocene records from northern Europe including the Atlantic seaboard. However, they outline the difficulty in interpreting climatic and anthropogenic signatures in coastal sedimentary records where high-resolution chronologies required to unravel their respective influences are still missing.     

Oceanic fine structure

Abstract

Measurements by free fall instruments, in the San Diego Trough, the Florida Current, and the central Pacific, reveal the detailed structure of the vertical component of the oceanic temperature gradient. The temperature changes are concentrated into regions on the order of a meter thick wherein the measured gradients are often more than ten times the average gradient. The horizontal extent of the regions of high gradient is greater than 750 meters in the seasonal thermocline off San Diego, but is only a few hundred meters at depths greater than 400 meters.

Fine scale measurements show that the layers of high gradient consist of even finer fluctuations in gradient which are only a few centimeters thick. Time scales of the thinnest of these regions of high gradient are of the order of five minutes. The data also yields an estimate of the entropy generation. According to the results of an idealized model relating entropy generation to the turbulent heat transport, only 240 to 700 ergs per cm.² per sec were transported in a 25 meter vertical section measured in the San Diego Trough. This value compared with 3600 ergs per cm.² per sec estimated from the mean gradient and an eddy coefficient of 1 cm.² per sec.