The role of peri‐glacial active layer development in determining soil‐regolith thickness across a Triassic sandstone outcrop in the UK

This paper examines the weathering processes that have combined to produce the distribution of soil‐regolith (SR) thickness across the Triassic Sherwood Sandstone Group outcrop (750 km²) in Nottinghamshire, UK. Archive borehole logs (n = 282) taken across the outcrop showed that SR thickness had mean and median depths of ~1·8 and 1·5 m, respectively. Cores were taken from a forested site to depths ~3 m for geochemical analysis. At this site the SR thickness was ~1·7 m. Analysis of the loss of elements, compared to bedrock using mass balance calculations (τ) showed that all the calcite and gypsum cement had been removed to depths of >3 m. Thus the major difference between the SR and the underlying saprolite was that the former exists as loose sand as opposed to a semi‐durable rock. Scanning electron microscopy (SEM) analysis of core samples suggested that the non‐durable rock or saprolite had greater cementation of clay particles. We propose that the mechanism through which the clay cement (and other interlocking grain bonds) was eased apart was through freeze–thaw processes associated with the summer ‘active layer development (ALD)’ during the last glacial activity in the UK. We tested this theory by developing a Monte Carlo simulation based on a simplified version of the Stefan equation. Current Arctic datasets of air and ground temperatures were obtained to provide reasonable starting conditions for input variables. These were combined with known data for thermal conductivity, bulk density and moisture content of the Sherwood Sandstone regolith. Model predictions (n = 1000) of the distribution of SR thickness accurately reflect the observed distribution thickness from the borehole logs. This is strong evidence that freeze–thaw and ‘ALD’ processes are major factors in determining the thickness of SR across this outcrop. British Geological Survey © NERC 2012     

Spatial patterns of soil water balance on intensively cultivated hillslopes in a semi‐arid environment: the impact of rock fragments and soil thickness

During past decades, a diverse system of subsistence agriculture in south‐east Spain (annual rainfall of less than 300 mm) has been overturned in favour of large‐scale plantations of almond trees without consideration for topography and related spatial patterns in soil hydrological properties. The objective of this paper is to investigate the spatial pattern in soil physical properties induced by this cultivation system, and to highlight its impact on the water balance. Soil properties were recorded along hillslopes with shallow soils developed on slates and greywackes in the upper part of the Guadelentin drainage basin (Murcia region). Frequent tillage of these almond plantations covering entire hillslopes has resulted in denudation by tillage erosion on the topographic convexities, as well as transport of rock fragments and fine earth along the slopes. These processes have created a systematic spatial pattern of soil thickness and rock fragment content: shallow and stony soils on the topographic convexities and deep soils with a rock fragment mulch in the concavities at the foot of the slopes. At the same time, a negative relationship between rock fragment content and fine earth bulk density was observed. The impact of this spatial pattern in soil properties on the water balance was evaluated using the PATTERN one‐dimensional hydrological and plant growth model. The model simulates the water balance of soil profiles covering the observed variation in soil thickness, stoniness and bulk density. The model results indicate that the highest rates of infiltration, evaporation and drainage, as well as the lowest rates of overland flow are restricted to shallow soils on the hilltops. In contrast, the deeper soils in the valley bottoms produce a more stable moisture regime than shallower soils, which tend to saturate and dry out quickly. These model results are in agreement with the spatial patterns of almond productivity: an asymptotic increase with soil thickness. Copyright © 2000 John Wiley & Sons, Ltd.     

Run‐off processes from mountains to foothills: The role of soil stratigraphy and structure in influencing run‐off characteristics across high to low relief landscapes

The critical zone features that control run‐off generation, specifically at the regional watershed scale, are not well understood. Here, we addressed this knowledge gap by quantitatively and conceptually linking regional watershed‐scale run‐off regimes with critical zone structure and climate gradients across two physiographic provinces in the Southeastern United States. We characterized long‐term (~20 years) discharge and precipitation regimes for 73 watersheds with United States Geological Survey in‐stream gaging stations across the Appalachian Mountain and Piedmont physiographic provinces of North Carolina. Watersheds included in this analysis had <10% developed land and ranged in size from 14.1–4,390 km². Thirty‐four watersheds were located in the Piedmont physiographic province, which is typically classified as a low relief landscape with deep, highly weathered soils and regolith. Thirty‐nine watersheds were located in the Appalachian Mountain physiographic province, which is typically classified as a steeper landscape with highly weathered, but shallower soils and regolith. From the United States Geological Survey daily mean run‐off time series, we calculated annual and seasonal baseflow indices (BFI), minimum, mean, and maximum daily run‐off, and Pearson's correlation coefficients between precipitation and baseflow. Our results showed that Appalachian Mountain watersheds systematically had higher minimum daily flows and BFI values. Piedmont watersheds displayed much larger deviations from mean annual BFI in response to year‐to‐year variability in precipitation. A series of linear regression models between 21 landscape metrics and annual BFIs showed non‐linear and complex terrestrial–hydrological relationships across the two provinces. From these results, we discuss how distinct features of critical zone architecture, with specific focus on soil depth and stratigraphy, may be dominating the regulation of hydrological processes and run‐off regimes across these provinces.     

2010年墨西哥Baja Mw7.2地震与中国玉树Mw6.9地震强地震动特征的对比研究

2010年4月4日墨西哥Baja地区发生Mw7.2地震,2人遇难;同年4月14日中国青海省南部玉树地区发生Mw6.9地震,截至2010年4月25日,已造成2220人遇难.有报道指出,玉树地震矩震级小于Baja地震,人员伤亡却远大于后者,主要原因在于玉树地区抗震设防标准低、建筑物抗震性能差.地震造成破坏程度的大小并非仅仅取决于矩震级的大小,而同时与其释放的地震波辐射能及发震后造成的强地面运动的大小有关.玉树地震释放的地震波辐射能约相当于Baja地震的10倍,目前玉树地震尚无实测的强震记录.针对玉树地震和Baja地震建立动态复合震源模型,分别模拟基岩上及浅层速度结构(V30,地下30 m平均剪切波速)下近断层区域的强地面运动.结果表明,基岩上及V30下玉树地震近断层区域强地面运动整体约相当于Baja地震的2倍.因此,玉树地震造成发震区域内建筑物损毁程度及人员伤亡情况均严重于Baja地震,重要原因之一在于其地震波辐射能大,且强地面运动较强.本文中所应用的动态复合震源模型,在地震矩守恒和地震波辐射能守恒的条件约束下,可以作为地震发生后补充强地面运动数据的有效手段之一.