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221.
Colin Neal Helen P. Jarvie Alison Love Margaret Neal Heather Wickham Sarah Harman 《Journal of Hydrology》2008,350(3-4):154-165
The water quality along the River Kennet, in the Thames basin of southern England, was examined in terms of the influence of point- and diffuse-nutrient inputs. The river is supplied mainly from a Cretaceous Chalk aquifer and hence the waters are of a calcium bicarbonate type. The nitrate largely comes from agricultural sources, with concentrations decreasing downstream due to plant uptake and probable denitrification. In contrast, soluble reactive phosphorus (SRP) is largely associated with sewage inputs and concentrations increase downstream in line with effluents from major towns such as Newbury and Reading. Adjacent to the river in the lower half of the catchment is the Kennet and Avon Canal and the two are in places hydrologically connected. The canal inputs may influence calcium carbonate (calcite) precipitation and increase suspended sediment and particulate phosphorus concentrations in the river. Monitoring upstream and downstream of Marlborough sewage treatment works (STW) showed that SRP concentrations in the effluent were highly variable due to variable efficiency of P stripping and still sufficiently concentrated to dominate downstream river SRP with potential impacts on stream ecology. Biological recovery in this river following P stripping at STWs is complex and controlling those spikes in SRP that are above a threshold of 100 μg l−1 may be a critical requirement. More stringent effluent targets than are currently recommended may be needed (less than 800 μg RP l−1) to achieve good ecological status in this river depending on SRP concentrations upstream. 相似文献
222.
Andrew Finlayson Emrys Phillips Ívar Örn Benediktsson Lucas K. Zoet Neal R. Iverson Jez Everest 《地球表面变化过程与地形》2019,44(1):367-380
The interaction between drumlins and overriding glacier ice is not well studied, largely due to the difficulty of identifying and accessing suitable active subglacial environments. The surge-type glacier Múlajökull, in central Iceland, overlies a known field of actively forming drumlins and therefore provides a rare opportunity to investigate the englacial structures that have developed in association with ice flow over the subglacial drumlins. In this study detailed ground-penetrating radar surveys are combined with field observations to identify clear sets of up-glacier and down-glacier dipping fractures at Múlajökull's margin. These are interpreted as conjugate shear planes or P- and R-type Reidel shears that developed and filled with saturated sediment derived from the glacier bed, during a previous surge. The fracture sets exhibit focused spatial distributions that are influenced by the subglacial topography. In particular, down-glacier dipping fractures are strongly focused over drumlin stoss slopes. These fractures, although well developed at depth, were mostly unable to transmit basal water and sediment up to the glacier surface during the surge cycle. In contrast, up-glacier dipping fractures formed over drumlin lee sides and in more gently sloping swales, and more frequently connected to the glacier surface, providing a pathway for the evacuation of basal water and water-saturated sediment. The study suggests that the subglacial drumlins under Múlajökull's margin have influenced the nature and distribution of englacial fractures, which could potentially contribute to spatial variations in basal water pressure during a surge. BGS © UKRI 2018 相似文献
223.
Jesse Abrams Erin C. Pischke Amarella Eastmond Chelsea A. Silva Cassandra Moseley 《社会与自然资源》2019,32(5):548-565
Rural communities worldwide are increasingly confronted with the simultaneous impacts of environmental change dynamics and processes of economic restructuring that diminish traditional sources of state support while shifting investments toward large-scale intensive production models. A key question in this context is how livelihoods are affected by these interacting forces of change and whether new production models can contribute to resilience at the household and community scales. Here we examine the impacts of oil palm production on marginal rural communities in Mexico that have experienced both the economic changes associated with neoliberal policy reforms and the dynamics of environmental change. We find that oil palm production can contribute to livelihood resilience when community members participate in its management and governance and when production is associated with state support, but that the kinds of neoliberal policies promoting oil palm expansion may exacerbate existing community vulnerabilities in the face of environmental change. 相似文献
224.
Understanding across-margin transport has long been recognized as crucial for wise management of our coastline and shelf waters. Issues related to sewage outfalls, nutrient and pollutant dispersal, carbon export, and shoreline sediment budgets all require an understanding of these processes. Across-margin transport of water and sediment at cuspate foreland headlands has been largely unrecognized, and the processes responsible for this export unappreciated. We examined physical process on Cape Lookout Shoal, a cape-associated shoal on the North Carolina continental shelf, through numerical modeling and field observations of near-bottom currents. The cuspate foreland setting of the northern South Atlantic Bight has been previously characterized as wave-dominated with a principal alongshore directed sediment transport and physical circulation forced by wave and wind-driven currents along the inner and mid-shelf. Our findings instead suggest that a seaward-directed, tidal-driven headland flow many play a significant role in the direction of net sediment transport on the shoal and ultimately its location and long-term maintenance. The shoal's location relative to the promontory-induced residual eddies and the region of active deposition differs from traditionally held ideas on sedimentary processes at headland-related sand banks. In addition, the headland flows may also serve as a first-order mechanism for rapidly exporting nearshore and estuarine waters to the outer-shelf. 相似文献
225.
Gary Hoffmann Eli Silver Simon Day Neal Driscoll Bruce Appelgate 《Marine Geophysical Researches》2009,30(4):229-236
Extinct volcanic islands in the Bismarck volcanic arc are fringed by well-developed coral reefs. Drowned platforms offshore
from these islands provide evidence for subsidence in the central section of the arc, north of the Finisterre Terrane–Australia
collision. Bathymetric and backscatter data collected onboard the R/V Kilo Moana in 2004 reveal regularly spaced (~200 m interval)
drowned platforms at depths as much as 1,100 m below sea level. However, the adjacent mainland coast has well documented raised
terraces indicating long-term uplift. Local subsidence may be due to cessation of magmatic activity and cooling, flexural
loading by the uplifting Finisterre Range, loading by nearby active volcanic islands, and/or sediment loading on the seafloor
north of the Finisterre Range. We present some simple models in order to test whether flexural loading can account for local
subsidence. We find that volcanic and sedimentary loading can explain the inferred relative subsidence. 相似文献
226.
Magma accumulation rates and thermal histories of plutons of the Sierra Nevada batholith,CA 总被引:2,自引:0,他引:2
Jesse W. Davis Drew S. Coleman John T. Gracely Richard Gaschnig Michael Stearns 《Contributions to Mineralogy and Petrology》2012,163(3):449-465
Zircon U–Pb geochronology results indicate that the John Muir Intrusive Suite of the central Sierra Nevada batholith, California,
was assembled over a period of at least 12 Ma between 96 and 84 Ma. Bulk mineral thermochronology (U–Pb zircon and titanite,
40Ar/39Ar hornblende and biotite) of rocks from multiple plutons comprising the Muir suite indicates rapid cooling through titanite
and hornblende closure following intrusion and subsequent slow cooling through biotite closure. Assembly of intrusive suites
in the Sierra Nevada and elsewhere over millions of years favors growth by incremental intrusion. Estimated long-term pluton
assembly rates for the John Muir Intrusive Suite are on the order of 0.001 km3 a−1 which is inconsistent with the rapid magma fluxes that are necessary to form large-volume magma chambers capable of producing
caldera-forming eruptions. If large shallow crustal magma chambers do not typically develop during assembly of large zoned
intrusive suites, it is doubtful that the intrusive suites represent cumulates left behind following caldera-forming eruptions. 相似文献
227.
To understand the evolution of a disaster, we propose a Framework for Assessing Crisis in a System Environment (FACSE). FACSE is set in a multi-system environment, containing the human system as well as the various natural and technological systems that interact with people. We take a lifecycle perspective, via which we quantify rhythms of life exhibited in multiple systems, across different scales, at different times. The lifecycle perspective also implies a relative approach in that rhythms of life during time t can be compared against those during t-1. We illustrate how rhythms of life in the human system can be measured at different scales. We propose a new concept??the degree of disaster, which is a composite score that encompasses the various measurements of rhythms of life from multiple systems, across different scales. We conclude the paper by discussing the potential offered by FACSE in disaster research as well as the limitations. 相似文献
228.
Tom J. Coulthard Jeff C. Neal Paul D. Bates Jorge Ramirez Gustavo A. M. de Almeida Greg R. Hancock 《地球表面变化过程与地形》2013,38(15):1897-1906
Landscape evolution models (LEMs) simulate the geomorphic development of river basins over long time periods and large space scales (100s–1000s of years, 100s of km2). Due to these scales they have been developed with simple steady flow models that enable long time steps (e.g. years) to be modelled, but not shorter term hydrodynamic effects (e.g. the passage of a flood wave). Nonsteady flow models that incorporate these hydrodynamic effects typically require far shorter time steps (seconds or less) and use more expensive numerical solutions hindering their inclusion in LEMs. The recently developed LISFLOOD‐FP simplified 2D flow model addresses this issue by solving a reduced form of the shallow water equations using a very simple numerical scheme, thus generating a significant increase in computational efficiency over previous hydrodynamic methods. This leads to potential convergence of computational cost between LEMs and hydrodynamic models, and presents an opportunity to combine such schemes. This paper outlines how two such models (the LEM CAESAR and the hydrodynamic model LISFLOOD‐FP) were merged to create the new CAESAR‐Lisflood model, and through a series of preliminary tests shows that using a hydrodynamic model to route flow in an LEM affords many advantages. The new model is fast, computationally efficient and has a stronger physical basis than a previous version of the CAESAR model. For the first time it allows hydrodynamic effects (tidal flows, lake filling, alluvial fans blocking valley floor) to be represented in an LEM, as well as producing noticeably different results to steady flow models. This suggests that the simplification of using steady flow in existing LEMs may bias their findings significantly. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
229.
230.
Jesse C. Vermaire Marie-Hélène Greffard Émilie Saulnier-Talbot Irene Gregory-Eaves 《Journal of Paleolimnology》2013,50(4):447-456
Submerged macrophyte abundance strongly influences aquatic ecosystems. Because of a lack of monitoring data, however, the long-term dynamics of such aquatic plants are poorly understood. Increasingly, paleolimnologists use changes in subfossil algae and invertebrates to infer past submerged macrophyte dynamics and assess how human activities have altered this important primary producer component of aquatic ecosystems. We evaluated the sensitivity of subfossil diatom and chironomid assemblages to historically documented changes in macrophyte abundance in Chenango Lake, New York, USA, where macrophyte cover has been monitored since 1978. We also tested the ability of a semi-quantitative diatom-based macrophyte-abundance inference model to detect the pronounced macrophyte decline that was observed between 1993 and 2001. Diatoms responded to the recent loss of macrophytes, with a decline in the relative abundance of macrophyte-associated taxa. Estimates of macrophyte abundance fluctuated according to the diatom-based inference model. Chironomid changes were coherent with the diatom-inferred macrophyte zones. The largest shifts in subfossil assemblages occurred before the start of the monitoring record and coincided with construction of a ~4.3-m-high dam on the lake, which substantially expanded the littoral habitat. Even in heavily managed systems, large reductions in macrophyte abundance can be detected with paleolimnological approaches. 相似文献