From the editor     

Alan R. Gillespie 《Quaternary Research》2005,63(3):231

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Late Quaternary megafloods from Glacial Lake Atna, Southcentral Alaska, U.S.A.     

Michael Wiedmer  David R. Montgomery  Alan R. Gillespie  Harvey Greenberg 《Quaternary Research》2010,73(3):413-424

Geomorphic, stratigraphic, geotechnical, and biogeographic evidence indicate that failure of a Pleistocene ice dam between 15.5 and 26 ka generated a megaflood from Glacial Lake Atna down the Matanuska Valley. While it has long been recognized that Lake Atna occupied ≥ 9000 km² of south-central Alaska's Copper River Basin, little attention has focused on the lake's discharge locations and behaviors. Digital elevation model and geomorphic analyses suggest that progressive lowering of the lake level by decanting over spillways exposed during glacial retreat led to sequential discharges down the Matanuska, Susitna, Tok, and Copper river valleys. Lake Atna's size, ∼ 50 ka duration, and sequential connection to four major drainages likely made it a regionally important late Pleistocene freshwater refugium. We estimate a catastrophic Matanuska megaflood would have released 500–1400 km³ at a maximum rate of ≥ 3 × 10⁶ m³ s^{− 1}. Volumes for the other outlets ranged from 200 to 2600 km³ and estimated maximum discharges ranged from 0.8 to 11.3 × 10⁶ m³ s^{− 1}, making Lake Atna a serial generator of some of the largest known freshwater megafloods.  相似文献   

Lead isotope systematics of some igneous rocks from the Egyptian shield     

J.G. Gillespie  T.H. Dixon 《Precambrian Research》1983,20(1):63-77

Lead isotope data on late Precambrian igneous rocks from the eastern desert of Egypt are presented. Previous work has indicated that this igneous suite is characterized by uniformly low initial ratios of ${}^{37}\text{Sr}{}^{86}\text{Sr}$ (< 0.7035). The Pb data define three groups, loosely corresponding to age. An older tonalite to granodiorite (OTG) suite, with ages in the range 610–710 Ma, has Pb isotope characteristics similar to modern, mantle-derived oceanic mafic rocks. The age-corrected initial Pb isotope ratios of the OTG group lie near the “ocean regression line” in correlation diagrams of ${}^{206}\text{Pb}{}^{204}\text{Pb}$ vs. ${}^{207}\text{Pb}{}^{204}\text{Pb}$ and ${}^{203}\text{Pb}{}^{204}\text{Pb}$. The isotope data imply an origin for the OTG group analogous to calc-alkalic igneous rocks in modern intra-oceanic island arcs.A younger suite of K-rich plutonic rocks (570–595 Ma) has similar ${}^{206}\text{Pb}{}^{204}\text{Pb}$ and ${}^{208}\text{Pb}{}^{204}\text{Pb}$ initial ratios relative to the OTG suite, but have higher ${}^{207}\text{Pb}{}^{204}\text{Pb}$ initial ratios when calculated from measured whole-rock U and Pb contents. However, K-feldspar and galena, associated with this suite, indicate initial Pb isotope ratios close to the OTG suite. We interpret this to indicate that the younger granites could have originated by anatexis of OTG-type material, but they probably experienced an episode of metasomatism after emplacement.A suite of volcanic rocks (the Dokhan Volcanics) from the central eastern desert are intermediate between the younger granites and OTG in both age (～ 610 Ma) and Pb isotope characteristics.Limited data on initial ${}^{208}\text{Pb}{}^{204}\text{Pb}$ ratios suggest that all the Egyptian samples originated from a source with anomalously low $\text{Th}\text{Pb}$ ratios.Although the relative abundance of granitic rocks in the Egyptian Shield decreases to the south, no evidence of north-south heterogeneity in the Pb isotope composition of the respective source regions is apparent in the data. However, the westernmost sample studied, from the Aswan area, contains distinctly more radiogenic Pb relative to the mean. This location may mark the boundary between the Late Precambrian ensimatic orogen now outcropping in the Egyptian and Saudi Arabian Shields, and an older sialic craton to the west.  相似文献   

Ambient biogenic hydrocarbons and isoprene emissions from a mixed deciduous forest     

J. D. Fuentes  D. Wang  H. H. Neumann  T. J. Gillespie  G. Den Hartog  T. F. Dann 《Journal of Atmospheric Chemistry》1996,25(1):67-95

Experiments were conducted during the growing season of 1993 at a mixed deciduous forest in southern Ontario, Canada to investigate the atmospheric abundance of hydrocarbons from phytogenic origins, and to measure emission rates from foliage of deciduous trees. The most abundant phytogenic chemical species found in the ambient air were isoprene and the monoterpenes -pinene and -pinene. Prior to leaf-bud break during spring, ambient hydrocarbon mixing ratios above the forest remained barely above instrument detection limit (20 parts per trillion), but they became abundant during the latter part of the growing season. Peak isoprene mixing ratios reached nearly 10 parts per billion (ppbv) during mid-growing season while maximum monoterpene mixing ratios were close to 2 ppbv. Both isoprene and monoterpene mixing ratios exhibited marked diurnal variations. Typical isoprene mixing ratios were highest during mid-afternoon and were lowest during nighttime. Peak isoprene mixing ratios coincided with maximum canopy temperature. The diurnal pattern of ambient isoprene mixing ratio was closely linked to the local emissions from foliage. Isoprene emission rates from foliage were measured by enclosing branches of trees inside environment-controlled cuvette systems and measuring the gas mixing ratio difference between cuvette inlet and outlet airstream. Isoprene emissions depended on tree species, foliage ontogeny, and environmental factors such as foliage temperature and intercepted photosynthetically active radiation (PAR). For instance, young (<1 month old) aspen leaves released approximately 80 times less isoprene than mature (>3 months old) leaves. During the latter part of the growing season the amount of carbon released back to the atmosphere as isoprene by big-tooth and trembling aspen leaves accounted for approximately 2% of the photosynthetically fixed carbon. Significant isoprene mixing ratio gradients existed between the forest crown and at twice canopy height above the ground. The gradient diffusion approach coupled with similarity theory was used to estimate canopy isoprene flux densities. These canopy fluxes compared favorably with values obtained from a multilayered canopy model that utilized locally measured plant microclimate, biomass distribution and leaf isoprene emission rate data. Modeled isoprene fluxes were approximately 30% higher compared to measured fluxes. Further comparisons between measured and modeled canopy biogenic hydrocarbon flux densities are required to assess uncertainties in modeling systems that provide inventories of biogenic hydrocarbons.  相似文献   

Evidence for Holocene megafloods down the Tsangpo River gorge, southeastern Tibet   总被引：1，自引：0，他引：1  

David R. Montgomery  Bernard Hallet  Noah Finnegan  Alan Gillespie 《Quaternary Research》2004,62(2):201-207

Lacustrine and alluvial terraces and sediments record the extent of at least two Holocene glacially dammed lakes immediately upstream of the Tsangpo River gorge at the eastern syntaxis of the Himalaya. The larger lake covered 2835 km², with a maximum depth of 680 m and contained an estimated 832 km³ of water; the smaller lake contained an estimated 80 km³ of water. Radiocarbon dating of wood and charcoal yielded conventional radiocarbon ages of 8860 ± 40 and 9870 ± 50 ¹⁴C yr B.P. for the higher set of lake terraces, and 1220 ± 40 and 1660 ± 40 ¹⁴C yr B.P. for sediments from the lower terraces. Catastrophic failure of the glacial dams that impounded the lakes would have released outburst floods down the gorge of the Tsangpo River with estimated peak discharges of up to 1 to 5 × 10⁶ m³ s⁻¹. The erosive potential represented by the unit stream power calculated for the head of the gorge during such a catastrophic lake breakout indicates that post-glacial megafloods down the Tsangpo River were likely among the most erosive events in recent Earth history.  相似文献   

The geomorphic and climatic significance of rock glaciers     

Eric J. Steig  Douglas H. Clark  Noel Potter  Jr.  Alan R. Gillespie 《Geografiska Annaler: Series A, Physical Geography》1998,80(3-4):173-174

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Adjustments to calculated Quaternary displacement rates across the Fish Springs fault, Owens Valley Fault Zone, California     

Stephen J. Martel  T. Mark Harrison  Alan Gillespie 《Quaternary Research》1989,32(3)

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A comparison of bowen ratio and eddy correlation sensible and latent heat flux measurements above deciduous forest     

Alan G. Barr  K. M. King  T. J. Gillespie  G. Den Hartog  H. H. Neumann 《Boundary-Layer Meteorology》1994,71(1-2):21-41

Sensible and latent heat flux densities (H and E) were measured above a mature, 18 m deciduous forest during July and August, 1988, using the Bowen ratio-energy balance (BREB) and eddy correlation (EC) methods. EC estimates ofH and E underestimated day-time surface available energy by 11%. EC also partitioned available energy differently than BREB. for/L<0.0, EC favouredH and BREB favoured E. Practical and theoretical limitations of the BREB and EC methods above forests are discussed. The most plausible causes for the failure of EC to close the surface energy balance are a low frequency loss of flux and the failure of a single point measurement to account for the spatial dispersive flux. The most plausible causes of the EC-BREB energy partitioning anomaly are the invalidity of the BREB similarity assumption and the violation of flux-gradient diffusion assumptions in the near-field diffusion region.  相似文献   

运用生物地理学理论与遥感影像寻找本·拉登     

Thomas W. Gillespie  John A. Agnew  Erika Mariano  Scott Mossler  Nolan Jones  MattBraughton  Jorge Gonzalez 《遥感学报》2011,15(4):869-874

本·拉登到底在哪里？这是我们这个时代最重要的政治问题之一。在这篇文章中,我们依据反映生物分布和灭绝规律的生物地理学理论(距离衰减理论、岛屿生物地理理论、本·拉登个人数据),结合遥感数据(陆地卫星 ETM+、航天雷达地形测图、QuickBird),从全球、区域、局部三个空间尺度分析并最终指出本·拉登当前最可能的所在地。这是第一次运用科学手段来确定本·拉登居所。该方法具有可重复性：一旦美国情报部门提供新的信息,便可通过该方法重新定位本·拉登当前最有可能的藏身场所。  相似文献   

Reply to the comments of Reger, Lowell, and Evenson on “Late Quaternary megafloods from Glacial Lake Atna, Southcentral Alaska, U.S.A.”     

Michael Wiedmer  David R. Montgomery  Alan R. Gillespie  Harvey Greenberg 《Quaternary Research》2011,75(1):303-304

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