Collapse and evolution of flattened star clusters     

C. M. Boily †  C. J. Clarke  S. D. Murray 《Monthly notices of the Royal Astronomical Society》1999,302(2):399-412

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Estimating external magnetic field differences at high geomagnetic latitudes from a single station          下载免费PDF全文

Ciarán D. Beggan  Laurence Billingham  Ellen Clarke 《Geophysical Prospecting》2018,66(6):1227-1240

Providing an accurate estimate of the magnetic field on the Earth's surface at a location distant from an observatory has useful scientific and commercial applications, such as in repeat station data reduction, space weather nowcasting or aeromagnetic surveying. While the correlation of measurements between nearby magnetic observatories at low and mid‐latitudes is good, at high geomagnetic latitudes () the external field differences between observatories increase rapidly with distance, even during relatively low magnetic activity. Thus, it is of interest to describe how the differences (or errors) in external magnetic field extrapolation from a single observatory grow with distance from its location. These differences are modulated by local time, seasonal and solar cycle variations, as well as geomagnetic activity, giving a complex temporal and spatial relationship. A straightforward way to describe the differences are via confidence intervals for the extrapolated values with respect to distance. To compute the confidence intervals associated with extrapolation of the external field at varying distances from an observatory, we used 695 station‐years of overlapping minute‐mean data from 37 observatories and variometers at high latitudes from which we removed the main and crustal fields to isolate unmodelled signals. From this data set, the pairwise differences were analysed to quantify the variation during a range of time epochs and separation distances. We estimate the 68.3%, 95.4% and 99.7% confidence levels (equivalent to the 1σ, 2σ and 3σ Gaussian error bounds) from these differences for all components. We find that there is always a small non‐zero bias that we ascribe to instrumentation and local crustal field induction effects. The computed confidence intervals are typically twice as large in the north–south direction compared to the east‐west direction and smaller during the solstice months compared to the equinoxes.  相似文献   

Particle‐size evidence of barrier estuary regime as a new proxy for ENSO climate variability          下载免费PDF全文

David W. Clarke  John F. Boyle  Andrew J. Plater 《地球表面变化过程与地形》2017,42(10):1520-1534

We investigate a new proxy for ENSO climate variability based on particle‐size data from long‐term, coastal sediment records preserved in a barrier estuary setting. Corresponding ~4–8 year periodicities identified from Wavelet analysis of particle‐size data from Pescadero Marsh in Central Coast California and rainfall data from San Francisco reflect established ENSO periodicity, as further evidenced in the Multivariate ENSO Index (MEI), and thus confirms an important ENSO control on both precipitation and barrier regime variability. Despite the fact that barrier estuary mean particle size is influenced by coastal erosion, precipitation and streamflow, balanced against barrier morphology and volume, it is encouraging that considerable correspondence can also be observed in the time series of MEI, regional rainfall and site‐based mean particle size over the period 1871–2008. This correspondence is, however, weakened after c.1970 by temporal variation in sedimentation rate and event‐based deposition. These confounding effects are more likely when: (i) accommodation space may be a limiting factor; and (ii) particularly strong El Niños, e.g. 1982/1983 and 1997/1998, deposit discrete >cm‐thick units during winter storms. The efficacy of the sediment record of climate variability appears not to be compromised by location within the back‐barrier setting, but it is limited to those El Niños that lead to barrier breakdown. For wider application of this particle size index of ENSO variability, it is important to establish a well‐resolved chronology and to sample the record at the appropriate interval to characterize deposition at a sub‐annual scale. Further, the sample site must be selected to limit the influence of decreasing accommodation space through time (infilling) and event‐based deposition. It is concluded that particle‐size data from back‐barrier sediment records have proven potential for preserving evidence of sub‐decadal climate variability, allowing researchers to explore temporal and spatial patterns in phenomena such as ENSO. Copyright © 2017 John Wiley & Sons, Ltd.  相似文献   

Long‐term sediment yield from a small catchment in southern Brazil affected by land use and soil management changes          下载免费PDF全文

Jean P.G. Minella  Gustavo H. Merten  Claúdia A.P. Barros  Rafael Ramon  Alexandre Schlesner  Robin T. Clarke  Michele Moro  Leandro Dalbianco 《水文研究》2018,32(2):200-211

Sediments produced from eroding cultivated land can cause on‐site and off‐site effects that cause considerable economic and social impacts. Despite the importance of soil conservation practices (SCP) for the control of soil erosion and improvements in soil hydrological functions, limited information is available regarding the effects of SCP on sediment yield (SY) at the catchment scale. This study aimed to investigate the long‐term relationships between SY and land use, soil management, and rainfall in a small catchment. To determine the effects of anthropogenic and climatic factors on SY, rainfall, streamflow, and suspended sediment concentration were monitored at 10‐min intervals for 14 years (2002–2016), and the land use and soil management changes were surveyed annually. Using a statistical procedure to separate the SY effects of climate, land use, and soil management, we observed pronounced temporal effects of land use and soil management changes on SY. During the first 2 years (2002–2004), the land was predominantly cultivated with tobacco under a traditional tillage system (no cover crops and ploughed soil) using animal traction. In that period, the SY reached approximately 400 t·km^?2·year^?1. From 2005 to 2009, a soil conservation programme introduced conservation tillage and winter cover crops in the catchment area, which lowered the SY to 50 t·km^?2·year^?1. In the final period (2010–2016), the SCP were partially abandoned by farmers, and reforested areas increased, resulting in an SY of 150 t·km^?2·year^?1. This study also discusses the factors associated with the failure to continue using SCP, including structural support and farmer attitudes.  相似文献   

What controls submarine channel development and the morphology of deltas entering deep-water fjords?     

Jenny A. Gales  Peter J. Talling  Matthieu J.B. Cartigny  John Hughes Clarke  Gwyn Lintern  Cooper Stacey  Michael A. Clare 《地球表面变化过程与地形》2019,44(2):535-551

River deltas and associated turbidity current systems produce some of the largest and most rapid sediment accumulations on our planet. These systems bury globally significant volumes of organic carbon and determine the runout distance of potentially hazardous sediment flows and the shape of their deposits. Here we seek to understand the main factors that determine the morphology of turbidity current systems linked to deltas in fjords, and why some locations have well developed submarine channels while others do not. Deltas and associated turbidity current systems are analysed initially in five fjord systems from British Columbia in Canada, and then more widely. This provides the basis for a general classification of delta and turbidity current system types, where rivers enter relatively deep (>200 m) water. Fjord-delta area is found to be strongly bimodal. Avalanching of coarse-grained bedload delivered by steep mountainous rivers produces small Gilbert-type fan deltas, whose steep gradient (11°–25°) approaches the sediment's angle of repose. Bigger fjord-head deltas are associated with much larger and finer-grained rivers. These deltas have much lower gradients (1.5°–10°) that decrease offshore in a near exponential fashion. The lengths of turbidity current channels are highly variable, even in settings fed by rivers with similar discharges. This may be due to resetting of channel systems by delta-top channel avulsions or major offshore landslides, as well as the amount and rate of sediment supplied to the delta front by rivers. © 2018 John Wiley & Sons, Ltd.  相似文献   

Soil creep: problems raised by a 23 year study in Australia     

M. F. Clarke  M. A. J. Williams  T. Stokes 《地球表面变化过程与地形》1999,24(2):151-175

In October 1965 and February 1966, 55 Young-pits were installed in tropical Northern Territory (NT) and temperate New South Wales (NSW). Pits were monitored in 1968, 1971 and 1974; also, for the NT only, in 1988. In each region, half of the pits are on weathered granite, and half on sandstone. Local relief is 30m or less, and slopes are up to 20°. Annual rainfall is evenly distributed in the NSW sites (800 mm a⁻¹), but is confined to the five to six month wet season in NT (1200 mm a⁻¹). Six pits suffered external disturbance and so were not analysed. Analysis of 160 rods in 49 undisturbed pits shows: (1) vectorial movement generally not downslope parallel to the ground surface, but dominated by a vertically downward component; (2) significant uphill and vertically upward components of movement for many rods; (3) a weak correlation between total movement and sine of slope; (4) rapid movement during 1965-68, and slow movement thereafter; (5) significantly higher creep rates on the NT granites than on all other sites, perhaps because mound-building termites are especially active there. We conclude that our data do not support soil creep models which assume that all movement is downslope and slope-parallel. Repeated long-term measurements are essential to distinguish long-term creep rates from the short-term effects of disturbance. Copyright © 1999 John Wiley & Sons, Ltd.  相似文献   

The new hominid skeleton from Sterkfontein,South Africa: age and preliminary assessment     

Timothy C. Partridge  John Shaw  David Heslop  Ronald J. Clarke 《第四纪科学杂志》1999,14(4):293-298

A new hominid skeleton from Sterkfontein Member 2 attaches to foot bones recovered from loose blocks during the 1980s and first described in 1995. Several flowstone horizons are present above and below the skeleton and have given clear palaeomagnetic signatures. Five changes in magnetic polarity have been identified; when constrained by the available biostratigraphy, this sequence can be placed confidently between 3.22 and 3.58 Ma. Interpolation of sedimentation rates over the small intervals between reversals allows this range to be reduced to 3.30–3.33 Ma. The skeleton is thus the oldest yet discovered and is considered to belong to a species of Australopithecus other than africanus. Copyright © 1999 John Wiley & Sons, Ltd.  相似文献   

Micro-level analysis of childhood obesity, diet, physical activity, residential socioeconomic and social capital variables: where are the obesogenic environments in Leeds?     

K L Procter  G P Clarke  J K Ransley  J Cade 《Area》2008,40(3):323-340

This paper describes global (whole of Leeds) and local (super output area) analyses of the relationship between childhood obesity and many 'obesogenic environment' variables, such as deprivation, urbanisation, access to local amenities, and perceived local safety, as well as dietary and physical activity behaviours. The analyses identify the covariates with the strongest relationships with obesity, and highlight variation in these relationships across Leeds, thus identifying 'at-risk' populations. This paper seeks to demonstrate the importance of analysis at the micro-level in order to provide health planners with additional information with which to tailor interventions and health policies to prevent childhood obesity.  相似文献   

Examining access to GP services in rural Ireland using microsimulation analysis     

Karyn Morrissey  Graham Clarke  Dimitris Ballas  Stephen Hynes  Cathal O'Donoghue 《Area》2008,40(3):354-364

This paper analyses the effect accessibility has on General Practitioner (GP) utilisation rates at the sub-national level for Ireland. Specifically, the aim of this paper is to estimate whether there is an urban–rural differential in GP utilisation rates. We do this by simulating micro-level healthcare data. Using this synthetic data, simple logit models are employed to estimate the likelihood that individuals in different jurisdictions will attend a GP surgery. These individual logit estimates are then inputted into a spatial interaction model to highlight areas with low GP accessibility given their health status. The policy implications of these results are discussed in relation to both the healthcare literature and current Irish healthcare policy.  相似文献   

Exploring the marine biotic index (AMBI): variations on a theme by Ángel Borja     

Richard M. Warwick  K. Robert Clarke  Paul J. Somerfield 《Marine pollution bulletin》2010,60(4):554-559

The results of manipulating the input data to AMBI are explored using various transformations of numerical species abundance (NAMBI), biomass (BAMBI) and production (PAMBI) from a variety of stations on the NE Atlantic shelf at which the pollution/disturbance status is known. There is a close agreement between the proportions of species in the five AMBI ecological groups and a phylum level meta-analysis axis of increasing environmental impact. All AMBI measures provide a better monotonic relationship with the impact axis than do traditional species diversity measures, which show higher diversity at intermediate levels of disturbance. A marginally better relationship with the impact axis of the meta-analysis is achieved by a moderate (square root) transformation of the data. ‘Production’ data (an appropriate combination of abundance and biomass information) provide only a marginal improvement on abundance data, but are ecologically and functionally much more relevant. Severe transformation of the data, culminating in presence/absence, degrades the relationship with the impact axis, but if only simple species lists are available then these may still be useable in making an environmental assessment.  相似文献