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91.
Revealing the natural complexity of topographic change processes through repeat surveys and decision‐tree classification
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Topographic change processes (TCPs) are the mechanisms by which a landscape is interpreted to be experiencing landform deformation, and are defined by the specific actions occurring within a contiguous, localized region that cause sediment to be either deposited or eroded. Past topographic change studies have mostly been focused at the site scale. The goal of this study was to identify and delineate spatially explicit TCP types across the valley width in a 34‐km long cobble‐gravel river at the scale of one‐tenth of the bankfull channel width over a period of seven to nine years. To accomplish this, a new procedure was developed that analyzes spatial patterns of topographic change evident from differencing two raster digital elevation models and accounting for sources of uncertainty, then identifying and classifying those changes using a decision tree framework that invokes the locations of those changes as they relate to the locations of specific geographic characteristics. Once mapped, TCP polygons were analyzed for areal patterns and volumetric rates of change. Results showed that 19 unique TCP types occurred and that they have organized but complex spatial patterns. Within this study segment, overbank storage processes occurred over the most area and displaced the most net volume of sediment, while cohesive bank retreat created the largest net change in topographic elevations. Analyses of the TCPs reveal that the regulated lower Yuba River (LYR) is not experiencing the expected combination of channel incision and floodplain deposition commonly reported below dams. Instead, the LYR is a dynamic valley that is still adjusting valley‐wide to the upstream dam with a diverse suite of processes that cause the channel and floodplains to scour and fill in concert. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
92.
Peter Brown Valerie Marchenko Danielle E. Moser Robert Weryk William Cooke 《Meteoritics & planetary science》2013,48(2):270-288
We propose that the Taurid meteor shower may contain bodies able to survive and be recovered as meteorites. We review the expected properties of meteorite‐producing fireballs, and suggest that end heights below 35 km and terminal speeds below 10 km s?1 are necessary conditions for fireballs expected to produce meteorites. Applying the meteoroid strength index (PE criteria) of Ceplecha and McCrosky (1976) to a suite of 33 photographically recorded Taurid fireballs, we find a large spread in the apparent meteoroid strengths within the stream, including some very strong meteoroids. We also examine in detail the flight behavior of a Taurid fireball (SOMN 101031) and show that it has the potential to be a (small) meteorite‐producing event. Similarly, photographic observations of a bright, potential Taurid fireball recorded in November of 1995 in Spain show that it also had meteorite‐producing characteristics, despite a very high entry velocity (33 km s?1). Finally, we note that the recent Maribo meteorite fall may have had a very high entry velocity (28 km s?1), further suggesting that survival of meteorites at Taurid‐like velocities is possible. Application of a numerical entry model also shows plausible survival of meteorites at Taurid‐like velocities, provided the initial meteoroids are fairly strong and large, both of which are characteristics found in the Taurid stream. 相似文献
93.
浙江舟山海岸带古木埋藏区铁的微生物成矿作用 总被引:1,自引:0,他引:1
以浙江舟山海岸带铁矿石为研究对象, 通过对铁矿石及其周围环境背景样品的形态学显微观察、矿物学及地球化学分析, 结果显示渗漏水沉淀铁泥中存在大量形貌与Leptothrix ochracea和Gallionella ferruginea中性铁氧化菌极为相似的微生物鞘, 该微生物可促进Fe2+的氧化和Fe3+的快速沉淀, 并且细胞最终被完全矿化后将永久保存起来.与此相对应的是: 在铁矿石内部存在大量的似球形和丝杆状的针铁矿, 并且还保留了死亡的铁细菌外鞘, 这些特征揭示该铁矿石与微生物历史活动密切相关.将现代渗漏水铁泥中铁细菌的矿化作用和铁矿石中保留的微生物活动记录相对比, 为该环境下的铁矿石生物成矿作用及其成因机制提供了良好的佐证.铁矿石的形成与古木堆积密切相关, 古木埋藏腐烂过程产生的腐植酸加剧了基岩及其周围土壤中的铁淋滤进入到潮间带, 从而为铁矿石形成提供充足的铁来源.该研究有助于更好理解和认识地史时期腐植质及微生物在铁矿床形成中的作用. 相似文献
94.
95.
Although fossil assemblages from the late Early Pleistocene are very rare in Britain, the site of Westbury Cave in Somerset, England, has the potential to address this gap. The mammal fossils recovered previously from the Siliceous Member in Westbury Cave, though few in number, have hinted at an age for the deposits that is as yet unparalleled in Britain. Here, we describe the first bona fide occurrence of Hippopotamus in the British Early Pleistocene, discovered during recent reinvestigation of the Siliceous Member. The hippo fossil indicates a refined biochronological age of ca. 1.5–1.07 Ma for the Siliceous Member and a palaeoclimate that was warm and humid, which accords well with previous palaeoenvironmental inferences. A synthesis of late Early Pleistocene hippo occurrences suggests that the Siliceous Member hippo may have been part of an early colonization of north-west Europe by these megaherbivores, possibly during MIS (Marine Oxygen Isotope Stage) 31. Alternatively, it evidences a currently cryptic northward migration during an even earlier temperate phase. In either case, the Siliceous Member is likely to represent a warm period that has not been recognized previously in the British Quaternary record. 相似文献