Tracing the temporal evolution of soil redistribution rates in an agricultural landscape using 239+240Pu and 10Be

Two principal groups of processes shape mass fluxes from and into a soil: vertical profile development and lateral soil redistribution. Periods having predominantly progressive soil forming processes (soil profile development) alternate with periods having predominantly regressive processes (erosion). As a result, short-term soil redistribution – years to decades – can differ substantially from long-term soil redistribution; i.e. centuries to millennia. However, the quantification of these processes is difficult and consequently their rates are poorly understood. To assess the competing roles of erosion and deposition we determined short- and long-term soil redistribution rates in a formerly glaciated area of the Uckermark, northeast Germany. We compared short-term erosion or accumulation rates using plutonium-239 and -240 (^239+240Pu) and long-term rates using both in situ and meteoric cosmogenic beryllium-10 (¹⁰Be). Three characteristic process domains have been analysed in detail: a flat landscape position having no erosion/deposition, an erosion-dominated mid-slope, and a deposition-dominated lower-slope site. We show that the short-term mass erosion and accumulation rates are about one order of magnitude higher than long-term redistribution rates. Both, in situ and meteoric ¹⁰Be provide comparable results. Depth functions, and therefore not only an average value of the topsoil, give the most meaningful rates. The long-term soil redistribution rates were in the range of −2.1 t ha^-1 yr^-1 (erosion) and +0.26 t ha^-1 yr^-1 (accumulation) whereas the short-term erosion rates indicated strong erosion of up to 25 t ha^-1 yr^-1 and accumulation of 7.6 t ha^-1 yr^-1. Our multi-isotope method identifies periods of erosion and deposition, confirming the ‘time-split approach’ of distinct different phases (progressive/regressive) in soil evolution. With such an approach, temporally-changing processes can be disentangled, which allows the identification of both the dimensions of and the increase in soil erosion due to human influence. © 2019 John Wiley & Sons, Ltd.     

Climate and relief-induced controls on the temporal variability of denudation rates in a granitic upland

How soil erosion rates evolved over the last about 100 ka and how they relate to environmental and climate variability is largely unknown. This is due to a lack of suitable archives that help to trace this evolution. We determined in situ cosmogenic beryllium-10 (¹⁰Be) along vertical landforms (tors, boulders and scarps) on the Sila Massif to unravel their local exhumation patterns to develop a surface denudation model over millennia. Due to the physical resistance of tors, their rate of exhumation may be used to derive surface and, thus, soil denudation rates over time. We derived soil denudation rates that varied in the range 0–0.40 mm yr^-1. The investigated boulders, however, appear to have experienced repositioning processes about ~20–25 ka bp and were therefore a less reliable archive. The scarps of the Sila upland showed a rapid bedrock exposure within the last 8–15 ka. Overall, the denudation rates increased steadily after 75 ka bp but remained low until about 17 ka bp . The exhumation rates indicate a denudation pulse that occurred about 17–5 ka bp . Since then the rates have continuously decreased. We identify three key factors for these developments – climate, topography and vegetation. Between 75 and 17 ka bp , climate was colder and drier than today. The rapid changes towards warmer and humid conditions at the Pleistocene–Holocene transition apparently increased denudation rates. A denser vegetation cover with time counteracted denudation. Topography also determined the extent of denudation rates in the upland regime. On slopes, denudation rates were generally higher than on planar surfaces. By determining the exhumation rates of tors and scarps, soil erosion rates could be determined over long timescales and be related to topography and particularly to climate. This is key for understanding geomorphic dynamics under current environmental settings and future climate change. © 2019 John Wiley & Sons, Ltd. © 2019 John Wiley & Sons, Ltd.     

Species selection and assessment of eco-engineering effects of seedlings for biogeomorphological landscape experiments

Landscape experiments of fluvial environments such as rivers and deltas are often conducted with live seedlings to investigate effects of biogeomorphological interactions on morphology and stratigraphy. However, such experiments have been limited to a single species, usually alfalfa (Medicago sativa), whereas important environments in nature have many different vegetation types and eco-engineering effects. Landscape experimentation would therefore benefit from a larger choice of tested plant species. For the purpose of experimental design our objective was to identify fast-germinating and fast-growing species and determine their sensitivity to flow conditions during and after settling, their maximum growth, hydraulic resistance and added bank strength. We tested germination time and seedling growth rate of 18 candidate species with readily available seeds that are fast growing and occur at waterlines, plus Medicago sativa as a control. We selected five species that germinate and develop within days and measured properties and eco-engineering effects depending on plant age and density, targeting typical experimental conditions of 0–0.3 m/s flow velocity and 0–30 mm water depth. Tested eco-engineering effects include bank strength and flow resistance. We found that Rumex hydrolapathum can represent riparian trees. The much smaller Veronica beccabunga and Lotus pedunculatus can represent grass and saltmarsh species as they grow in dense patches with high flow resistance but are readily erodible. Sorghum bicolor grows into tall, straight shoots, which add significantly to bank strength, but adds little flow resistance and may represent sparse hardwood trees. Medicago sativa also grows densely under water, suggesting a use for mangroves and perhaps peat. In stronger and deeper flows the application of all species changes accordingly. These species can now be used in a range of landscape experiments to investigate combined effects on living landscape patterns and possible facilitation between species. The testing and treatment methodology can be applied to new species and other laboratory conditions. © 2019 The Authors. Earth Surface Processes and Landforms Published by John Wiley & Sons Ltd. © 2019 The Authors Earth Surface Processes and Landforms Published by John Wiley & Sons Ltd © 2019 The Authors Earth Surface Processes and Landforms Published by John Wiley & Sons Ltd.     

如何正确使用地震的震级

震级是表示地震自身大小的一个量,是地震的基本参数之一,正确使用震级,对于科学研究、地震预报、地震应急等工作至关重要。本文从震级的发展过程、震级的单色性、震级统一的不可行性、震级转换的危害性、震级优选的科学性等方面,论述了震级的使用方法,以便技术人员和管理人员在实际工作中能够正确地使用地震的震级。     

Development of spatial regression models for predicting summer river temperatures from landscape characteristics: Implications for land and fisheries management

There is increasing demand for models that can accurately predict river temperature at the large spatial scales appropriate to river management. This paper combined summer water temperature data from a strategically designed, quality controlled network of 25 sites, with recently developed flexible spatial regression models, to understand and predict river temperature across a 3,000 km² river catchment. Minimum, mean and maximum temperatures were modelled as a function of nine potential landscape covariates that represented proxies for heat and water exchange processes. Generalised additive models were used to allow for flexible responses. Spatial structure in the river network data (local spatial variation) was accounted for by including river network smoothers. Minimum and mean temperatures decreased with increasing elevation, riparian woodland and channel gradient. Maximum temperatures increased with channel width. There was greater between‐river and between‐reach variability in all temperature metrics in lower‐order rivers indicating that increased monitoring effort should be focussed at these smaller scales. The combination of strategic network design and recently developed spatial statistical approaches employed in this study have not been used in previous studies of river temperature. The resulting catchment scale temperature models provide a valuable quantitative tool for understanding and predicting river temperature variability at the catchment scales relevant to land use planning and fisheries management and provide a template for future studies.     

Resistant rock layers amplify cosmogenically-determined erosion rates

Prior numerical modeling work has suggested that incision into sub-horizontal layered stratigraphy with variable erodibility induces non-uniform erosion rates even if base-level fall is steady and sustained. Erosion rates of cliff bands formed in the stronger rocks in a stratigraphic sequence can greatly exceed the rate of base-level fall. Where quartz in downstream sediment is sourced primarily from the stronger, cliff-forming units, erosion rates estimated from concentrations of cosmogenic beryllium-10 (¹⁰Be) in detrital sediment will reflect the locally high erosion rates in retreating cliff bands. We derive theoretical relationships for threshold hillslopes and channels described by the stream-power incision model as a quantitative guide to the potential magnitude of this amplification of ¹⁰Be-derived erosion rates above the rate of base-level fall. Our analyses predict that the degree of erosion rate amplification is a function of bedding dip and either the ratio of rock erodibility in alternating strong and weak layers in the channel network, or the ratio of cliff to intervening-slope gradient on threshold hillslopes. We test our predictions in the cliff-and-bench landscape of the Grand Staircase in southern Utah, USA. We show that detrital cosmogenic erosion rates in this landscape are significantly higher (median 300 m/Ma) than the base-level fall rate (~75 m/Ma) determined from the incision rate of a trunk stream into a ~0.6 Ma basalt flow emplaced along a 16 km reach of the channel. We infer a 3–6-fold range in rock strength from near-surface P-wave velocity measurements. The approximately four-fold difference between the median ¹⁰Be-derived erosion rate and the long-term rate of base-level fall is consistent with our model and the observation that the stronger, cliff-forming lithologies in this landscape are the primary source of quartz in detrital sediments. © 2020 John Wiley & Sons, Ltd.     

Quantifying erosional equilibrium across a slowly eroding,soil mantled landscape

The textbook concept of an equilibrium landscape, which posits that soil production and erosion are balanced and equal channel incision, is rarely quantified for natural systems. In contrast to mountainous, rapidly eroding terrain, low relief and slow-eroding landscapes are poorly studied despite being widespread and densely inhabited. We use three field sites along a climosequence in South Africa to quantify very slow (2-5 m/My) soil production rates that do not vary across hillslopes or with climate. We show these rates to be indistinguishable from spatially invariant catchment-average erosion rates while soil depth and chemical weathering increase strongly with rainfall across our sites. Our analyses imply landscape-scale equilibrium although the dominant means of denudation varies from physical weathering in dry climates to chemical weathering in wet climates. In the two wetter sites, chemical weathering is so significant that clay translocates both vertically in soil columns and horizontally down hillslope catenas, resulting in particle size variation and the accumulation of buried stone lines at the clay-rich depth. We infer hundred-thousand-year residence times of these stone lines and suggest that bioturbation by termites plays a key role in exhuming sediment into the mobile soil layer from significant depths below the clay layer. Our results suggest how tradeoffs in physical and chemical weathering, potentially modulated by biological processes, shape slowly eroding, equilibrium landscapes. © 2019 John Wiley & Sons, Ltd.     

Coastal geomorphic chronosequences across broad spatiotemporal scales. Metrical observations from the Cape Verde hotspot

The study of the coastal landscapes of hotspot oceanic islands through comprehensive structural metrics and ecological estimators represents an opportunity to explore geomorphological transformations and broad spatiotemporal scale features of coastal evolution. As part of this approach, a new metrical comparative analysis is presented in this study, comprising four islands in different evolutionary stages. They belong to the Cape Verde archipelago, which forms a double insular chain in which an east–west gradient in age and evolution is particularly evident across the southern chain. A space-for-time (SFT) substitution approach is applied to the coasts of (1) Fogo, in the shield stage; (2) Santiago, in the early post-erosional stage; (3) São Vicente, in the advanced post-erosional stage; and (4) Boa Vista, in the last erosional stage. From the obtained spatial distributions and frequencies of landforms, the coastal landscapes of these islands are compared in relation to their (i) geomorphic composition, using similarity indices (Whittaker, β_w, Sorensen, C_s) and nestedness estimators (NOFD, WNODF), (ii) geomorphic abundance, using morpho-assembling densities (D_gm), and (iii) geomorphic diversity, using six alpha-diversity indices (Richness, S, Menhinick, D_MN, Simpson, D, Shannon, H', Berger-Parker, d, and Brillouin, HB). An advanced geomorphological taxonomy is implemented for areas with limited open-access data, including a set of planform features captured through scale-frequency decomposition. Photographic, cartographic and fieldwork data are used for landform identification at 1200 random sampling points, empirically determined by a bootstrap method. The results show a chronological ordering of the compared variables and a possible co-evolution towards an increase in organizational geomorphic complexity of coastal systems at broad space-timescales. The method proposed in this study can contribute, from a metrical perspective, to finding new long-term evolutionary features and constitutes an advance in the development of an integrated model of coastal evolution in oceanic islands. © 2019 John Wiley & Sons, Ltd.     

工业用地转型下地理想象与地方意义构建 ——广州广钢新城案例

在城市化进程中工业用地转型背景下,选择广钢新城为案例进行实证分析,从地理想象的视角,探讨周边村民、旧广钢职工以及新进入者等主体对广钢新城地方意义和文化内涵进行解构和重构的过程,并进一步解析地方建构与身份认同的联系。研究发现：1）由于主观性和空间实践体验的差异,不同社会群体对于广钢的地方意义有着截然不同的想象;2）周边村民、广钢旧厂职工集体记忆的嬗变,反映了空间形态、社会结构变化与在地群体的想象构建以及身份认同的耦合关系;3）新进入者的地方身份认同建立在排除在地群体的地方文化想象之上,二者对于广钢新城的地方意义和身份认同是断裂的,2个社会群体对于地方意义的感知与认同在未来还需要进一步的交流与融合;4）在社会话语构建的文化冲击以及权力和资本的支配下,外界群体逐渐产生对广钢新城地方文化意义的多元想象。广钢片区的大规模空间重塑以及由此衍生的地方意义重构说明经济、社会和文化因素对地方建构具有直接而深刻的影响。     

川西林盘文化景观基因识别与提取

文化景观是文化地理特征的集中体现,川西林盘文化景观基因识别与提取的研究是对林盘本质的认知和解读。根据文化景观的关键特征,基于文献与实地考察,综合运用问卷与半结构式访谈等方式进行调查,借助SPSS软件进行数据统计和处理,客观地明晰了林盘文化景观的结构组成;并在此基础上引入“景观基因”理论,以川西林盘的文化景观为研究对象,从遗传信息的表达,以及基因所属位列、功能性和重要度2个层面进行了相应的分类,进一步阐释了识别与提取的方法与流程。同时,在土地利用数据、卫星影像及地理信息数据电子地图等资料的辅助下,以新津、温江、崇州林盘群系与鱼子孔、龚林盘和胥家院子聚落单元为例,进行了文化景观基因的识别,归纳梳理出其共性基因、个性基因,并提取出民居、渠系、林地等主体基因,菜地、井等胞质基因、石笼、平桥等附着基因和治水工程、移民文化等调节基因。