Elevation trends and shrink–swell response of wetland soils to flooding and drying     

Donald R. Cahoon  Brian C. Perez  Bradley D. Segura  James C. Lynch   《Estuarine, Coastal and Shelf Science》2011,91(4):463-474

Given the potential for a projected acceleration in sea-level rise to impact wetland sustainability over the next century, a better understanding is needed of climate-related drivers that influence the processes controlling wetland elevation. Changes in local hydrology and groundwater conditions can cause short-term perturbations to marsh elevation trends through shrink–swell of marsh soils. To better understand the magnitude of these perturbations and their impacts on marsh elevation trends, we measured vertical accretion and elevation dynamics in microtidal marshes in Texas and Louisiana during and after the extreme drought conditions that existed there from 1998 to 2000. In a Louisiana marsh, elevation was controlled by subsurface hydrologic fluxes occurring below the root zone but above the 4 m depth (i.e., the base of the surface elevation table benchmark) that were related to regional drought and local meteorological conditions, with marsh elevation tracking water level variations closely. In Texas, a rapid decline in marsh elevation was related to severe drought conditions, which lowered local groundwater levels. Unfragmented marshes experienced smaller water level drawdowns and more rapid marsh elevation recovery than fragmented marshes. It appears that extended drawdowns lead to increased substrate consolidation making it less resilient to respond to future favorable conditions. Overall, changes in water storage lead to rapid and large short-term impacts on marsh elevation that are as much as five times greater than the long-term elevation trend, indicating the importance of long-term, high-resolution elevation data sets to understand the prolonged effects of water deficits on marsh elevation change.  相似文献   

Evidence for coelacanths in the Late Triassic (Rhaetian) of England     

Luke M. Hauser  David M. Martill 《Proceedings of the Geologists' Association. Geologists' Association》2013

The right gular plate of an indeterminate coelacanth from the Westbury Formation (Upper Triassic, Rhaetian) of Blue Anchor Point, Somerset, southwest England is reported. This occurrence represents the first convincing evidence of coelacanths from the Triassic of the United Kingdom. The new specimen suggests a fish of approximately 0.61 m length.  相似文献   

SPATIAL AND TEMPORAL DISTRIBUTIONS OF ATMOSPHERIC MOISTURE IN A SMALL MIDWESTERN CITY     

James A. Henry 《自然地理学》2013,34(2):146-159

Location, timing, and intensities of urban atmospheric moisture anomalies in the relatively small city of Lawrence, Kansas are mapped, explained, and compared with previously studied cities. Forty-five urban-rural dew point distributions were obtained during mornings, afternoons, and evenings in August, September, and October. A meteorologically-equipped auto was used to traverse an 88.5-km route through the major land uses in the city and surrounding countryside. Rural dew points exceeded urban values much more frequently than the reverse. On several dates, a reversal of the urban-rural dew point relationship occurred; in the afternoon, rural dew points were greater than urban values, but at night urban values exceeded those in rural areas. Lowest values often corresponded with the most developed sections of the city, and the central business district exerted the most consistent influence on dew points. Greatest gradients developed on the periphery of the developed area. Pattern complexity was generally at a maximum in the afternoon and was least complex during morning hours. Results compare and contrast with previous urban-rural humidity studies.  相似文献   

The structural preservation of a titanosaurid (Dinosauria: Sauropoda) vertebral ligament     

《Cretaceous Research》2016

Within the past decade exceptional preservation of original organic components have been reported from several dinosaurian families, including members of Sauropodomorpha. Here we document the partial preservation of a vertebral ligament in the dorsal and sacral series of a titanosaur. Unlike other cases of tissue preservation, this structure does not represent biomineralization of the original organic components. Histology, morphology, and comparative anatomy from extant taxa as well as the preferential placement on the vertebral column suggests that it represents the partial preservation of the nuchal ligament. While preservation of other sauropod connective tissues are known, this case represents the first reported non-biomineralized tissue from a sauropod. In consideration of the location and external and internal morphologies compared to modern vertebral ligaments, we believe this structure to be the micritic replacement of the original tissue via microbially-mediated processes. Along with the suggestive external morphology, internally we interpret some of the unusual structures to represent the remnants of the collagen and elastin fascicles that are strongly overprinted by a clotted, thrombolytic-like fabric recording microbial activity and deposition of micrite prior to lithification. In consideration of the interpreted depositional history, we theorize that post-deposition, bacteria deposited the micrite as a byproduct of metabolization. Subsequently, the recognition of this structure as a vertebral ligament, the largest of such thus documented, substantiates previous findings on the morphological attributes of sauropod vertebral ligaments.  相似文献   

Invertebrate traces in pseudo–coprolites from the upper Cretaceous Marília Formation (Bauru Group), Minas Gerais State,Brazil     

《Cretaceous Research》2016

Pseudo–coprolites are inorganic structures often confused with fossil faeces. The absence of some diagnostic features, such as inclusions, coprofabrics, grain adhesion, and defined shape, suffices to disregard these structures as coprolites. Herein we revise the so–called “coprolites” from the Serra da Galga Member of the Marília Formation (Maastrichtian of Bauru Group, Paraná Basin), at “Ponto 1 do Price” locality near the town of Peirópolis (Uberaba municipality, Minas Gerais State, Brazil) and conclude that they are, in fact, pseudo–coprolites related to calcretes. These data also agree with the geological setting of “Ponto 1 do Price”, composed mainly of coarse sandstones and conglomerates, in which these pseudo–coprolites were found. In addition, some of these specimens exhibit superficial traces, here described as a new boring ichnospecies, Asthenopodichnium fallax isp. nov., produced by invertebrates in Late Cretaceous fresh–water settings of Brazil.  相似文献   

Concentrations and annual fluxes of sediment‐associated chemical constituents from conterminous US coastal rivers using bed sediment data     

Arthur J. Horowitz  Verlin C. Stephens  Kent A. Elrick  James J. Smith 《水文研究》2012,26(7):1090-1114

Coastal rivers represent a significant pathway for the delivery of natural and anthropogenic sediment‐associated chemical constituents to the Atlantic, Pacific and Gulf of Mexico coasts of the conterminous USA. This study entails an accounting segment using published average annual suspended sediment fluxes with published sediment‐associated chemical constituent concentrations for (1) baseline, (2) land‐use distributions, (3) population density, and (4) worldwide means to estimate concentrations/annual fluxes for trace/major elements and total phosphorus, total organic and inorganic carbon, total nitrogen, and sulphur, for 131 coastal river basins. In addition, it entails a sampling and subsequent chemical analysis segment that provides a level of ‘ground truth’ for the calculated values, as well as generating baselines for sediment‐associated concentrations/fluxes against which future changes can be evaluated. Currently, between 260 and 270 Mt of suspended sediment are discharged annually from the conterminous USA; about 69% is discharged from Gulf rivers (n = 36), about 24% from Pacific rivers (n = 42), and about 7% from Atlantic rivers (n = 54). Elevated sediment‐associated chemical concentrations relative to baseline levels occur in the reverse order of sediment discharges: Atlantic rivers (49%) > Pacific rivers (40%) > Gulf rivers (23%). Elevated trace element concentrations (e.g. Cu, Hg, Pb, Zn) frequently occur in association with present/former industrial areas and/or urban centres, particularly along the northeast Atlantic coast. Elevated carbon and nutrient concentrations occur along both the Atlantic and Gulf coasts but are dominated by rivers in the urban northeast and by southeastern and Gulf coast (Florida) ‘blackwater’ streams. Elevated Ca, Mg, K, and Na distributions tend to reflect local petrology, whereas elevated Ti, S, Fe, and Al concentrations are ubiquitous, possibly because they have substantial natural as well as anthropogenic sources. Almost all the elevated sediment‐associated chemical concentrations found in conterminous US coastal rivers are lower than worldwide averages. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

A 15 000‐year record of climate change in northern New Mexico,USA, inferred from isotopic and elemental contents of bog sediments     

Luz M. Cisneros‐Dozal  Jeffrey M. Heikoop  Julianna Fessenden  R. Scott Anderson  Philip A. Meyers  Craig D. Allen  Marcey Hess  Toti Larson  George Perkins  Michael Rearick 《第四纪科学杂志》2010,25(6):1001-1007

Elemental (C, N, Pb) and isotopic (δ¹³C, δ¹⁵N) measurements of cored sediment from a small bog in northern New Mexico reveal changes in climate during the Late Pleistocene and Holocene. Abrupt increases in Pb concentration and δ¹³C values ca. 14 420 cal. YBP indicate significant runoff to the shallow lake that existed at that time. Weathering and transport of local volcanic rocks resulted in the delivery of Pb‐bearing minerals to the basin, while a ¹³C‐enriched terrestrial vegetation source increased the δ¹³C values of the sedimentary material. Wet conditions developed over a 300 a period and lasted for a few hundred years. The Younger Dryas period (ca. 12 700–11 500 cal. YBP) caused a reduction in terrestrial productivity reflected in decreasing C/N values, δ¹⁵N values consistently greater than 0‰ and low organic content. By contrast, aquatic productivity increased during the second half of this period, evidenced by increasing δ¹³C values at the time of highest abundance of algae. Dry conditions ca. 8 000–6 000 cal. YBP were characterised by low organic carbon content and high Pb concentrations, the latter suggesting enhanced erosion and aeolian transport of volcanic rock. The range in δ¹³C, δ¹⁵N and C/N values in the sedimentary record fall within the range of modern plants, except during the periods of runoff and drought. The sedimentary record provides evidence of natural climate variability in northern New Mexico, including short‐ (multi‐centennial) and long‐(millennial) term episodes during the Late Pleistocene and Holocene. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

Variability of the apparent quantum efficiency of CO photoproduction in the Gulf of Maine and Northwest Atlantic   总被引：1，自引：0，他引：1  

Lori A. Ziolkowski  William L. Miller   《Marine Chemistry》2007,105(3-4):258-270

The photochemical oxidation of colored, dissolved organic matter (CDOM) is important for carbon cycling in the ocean. This oxidation process produces a number of products, including carbon monoxide (CO). While the photochemical production efficiency of CO (apparent quantum yield, AQY, defined in terms of CDOM absorbance) has been reported to be similar for many water types, a full evaluation of the observed natural variability in CO AQY requires additional study. Here we use a polychromatic irradiation system to determine twenty AQY spectra at sea on fresh samples ranging from the near coastal waters of the Gulf of Maine to the offshore waters of the Northwest Atlantic. Despite the geographic variability of these marine samples the AQY of CO production in the Gulf of Maine and Northwest Atlantic exhibited only a small degree of variability, none of which was not correlated with measured environmental parameters. Consequently, a single aggregate AQY spectrum _λ = e^{(−(9.134+0.0425(λ−290)))+e(−(11.316+0.0142(λ−290)))} was found to adequately represent the entire data set. Significantly, the accuracy of an AQY spectrum determined using this multispectral/statistical technique was confirmed with data obtained from a monochromatic irradiation technique on a single open ocean sample. Taken together, the AQY spectra determined in this study were similar in magnitude and shape to those previously published for marine samples and, overall, were somewhat lower than those previously reported for freshwater studies.  相似文献   

美国水文气象科研业务考察纪实     

除多 《高原山地气象研究》2012,32(1):91-96

NOAA和USGS是美国主要从事气象和水文业务与科研的两大联邦政府部门,随着NASA将科学任务重点从太空科学转向地球科学,除了继续开展卫星观测技术和遥感数据的开发和应用之外还关注水文气象领域的研究.作为国际山地中心主持的“兴都-库什-喜马拉雅地区卫星遥感降水估算”项目的内容之一,本人于2011年5～6月参加了由国际山地中心(ICIMOD)组织的赴美交叉学习活动,先后参观和考察了NOAA和USGS的主要部门和其他一些相关单位,与NASA科学家进行了广泛的交流.本文简要介绍了这些单位开展的水文气象业务和科研工作、卫星遥感应用情况以及学习访问期间的感想与体会,对提高我们目前的相关业务和科研工作以及体制机制的创新方面有一定的借鉴意义.  相似文献   

交通规划与融资机制的演变——美国高速公路百年史回顾   总被引：2，自引：0，他引：2  

李玉涛  荣朝和 《地理研究》2012,31(5):922-930

回顾美国高速公路发展的百年历史,探讨交通规划、融资机制及其二者之间的相互关系。通过对比不同时期的规划理念,发现早期规划已经考虑了交通与土地利用的关系,中间的州际高速时期则注重公路自身能力提高,后期则更加强调综合协调。从财政受益原则角度对高速公路的财税政策进行了比较分析,发现早期的物业税与后来的燃油税在融资功能与激励功能两方面存在很大差异;并从财政转移支付角度阐述了联邦政府的交通资金分配政策变化对规划理念及其实施效果的影响。研究美国公路发展历史,对于理解研究不同尺度空间概念具有重要的地理学意义,同时对我国交通政策规划和空间管治亦具有借鉴作用。  相似文献