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141.
Benthic community responses to thin-layer disposal of dredged material were assessed at three sites in Mississippi Sound, USA. Community composition differed between disposal and reference sites (ANOSIM) for each disposal event. Oweniid sand worms, Amphinomid fire worms, and brittle stars (Ophiuriodea) contributed the most to community distinctions between disposal and reference sites (SIMPER), with higher abundances at the disposal sites. Total infaunal abundance was similar to pre-disposal and reference conditions within 3-10 months of thin-layer disposal. Distinctions in community composition between disposal and reference sites (nMDS) persisted throughout the 16-month study period, but were less pronounced at the site where sediment composition was unchanged by disposal. Size distributions of some taxa (e.g., gastropod and hemichordate) suggest adults recolonized the newly deposited sediments either through vertical migration or lateral immigration from adjacent areas. Thin-layer disposal offers a viable alternative to conventional open-water disposal practices and warrants further exploration for other areas with deeper bathymetries and different hydrodynamic regimes. 相似文献
142.
Here, we present two high-resolution records of macroscopic charcoal from high-elevation lake sites in the Sierra Nevada, California, and evaluate the synchroneity of fire response for east- and west-side subalpine forests during the past 9200 yr. Charcoal influx was low between 11,200 and 8000 cal yr BP when vegetation consisted of sparse Pinus-dominated forest and montane chaparral shrubs. High charcoal influx after ∼ 8000 cal yr BP marks the arrival of Tsuga mertensiana and Abies magnifica, and a higher-than-present treeline that persisted into the mid-Holocene. Coeval decreases in fire episode frequency coincide with neoglacial advances and lower treeline in the Sierra Nevada after 3800 cal yr BP. Independent fire response occurs between 9200 and 5000 cal yr BP, and significant synchrony at 100- to 1000-yr timescales emerges between 5000 cal yr BP and the present, especially during the last 2500 yr. Indistinguishable fire-return interval distributions and synchronous fires show that climatic control of fire became increasingly important during the late Holocene. Fires after 1200 cal yr BP are often synchronous and corroborate with inferred droughts. Holocene fire activity in the high Sierra Nevada is driven by changes in climate linked to insolation and appears to be sensitive to the dynamics of the El Niño-Southern Oscillation. 相似文献
143.
Pradeep Adhikari Yang Hong Kimberly R. Douglas Dalia Bach Kirschbaum Jonathan Gourley Robert Adler G. Robert Brakenridge 《Natural Hazards》2010,55(2):405-422
Floods have profound impacts on populations worldwide in terms of both loss of life and property. A global inventory of floods
is an important tool for quantifying the spatial and temporal distribution of floods and for evaluating global flood prediction
models. Several global hazard inventories currently exist; however, their utility for spatiotemporal analysis of global floods
is limited. The existing flood catalogs either fail to record the geospatial area over which the flood impacted or restrict
the types of flood events included in the database according to a set of criteria, limiting the scope of the inventory. To
improve upon existing databases, and make it more comprehensive, we have compiled a digitized Global Flood Inventory (GFI)
for the period 1998–2008 which also geo-references each flood event by latitude and longitude. This technical report presents
the methodology used to compile the GFI and preliminary findings on the spatial and temporal distributions of the flooding
events that are contained in the inventory. 相似文献
144.
L. C. Radke P. W. Ford I. T. Webster I. Atkinson G. Douglas K. Oubelkheir J. Li B. Robson B. Brooke 《Aquatic Geochemistry》2010,16(1):1-29
The Fitzroy River delivers large amounts of nutrients and fine sediments to Keppel Bay (contiguous with the Great Barrier
Reef Lagoon) during intermittent flow events. This study explores sources, forms and transformations of nutrients in Keppel
Bay, and develops a functional process zonation that integrates seabed geochemistry and water column nutrient characteristics
which are controlled by suspended sediment. The water column and seabed properties were investigated over two dry seasons,
with supplementary core incubations taken to measure carbon decomposition rates and nutrient fluxes. Keppel Bay can be divided
into three zones, the: zone of maximum resuspension (ZMR); coastal transitional zone (CTZ); and blue water zone (BWZ). Mineralisation
of predominantly terrestrial organic matter occurs in the ZMR where nutrient uptake by phytoplankton is light limited. The
CTZ and BWZ had higher light penetration and phytoplankton growth was likely limited by N and P, respectively. The identified
zones conform to the bathymetry and hydrodynamic characteristics of the bay, allowing for the development of an integrated
conceptual model accounting for the benthic and pelagic biogeochemical processes. Recognition of these different zones shows
that considerable variation in benthic and water column properties is possible within a small system with the bathymetric
and hydrodynamic characteristics of the fluidized bed reactor. 相似文献
145.
Many previous investigations of mean streamwater transit times (MTT) have been limited by an inability to quantify the MTT dynamics. Here, we draw on (1) a linear relation (r 2 = 0.97) between groundwater 3H/3He ages and dissolved silica (Si) concentrations, combined with (2) predicted streamwater Si concentrations from a multiple-regression relation (R 2 = 0.87) to estimate MTT at 5-min intervals for a 23-year time series of streamflow [water year (WY) 1986 through 2008] at the Panola Mountain Research Watershed, Georgia. The time-based average MTT derived from the 5-min data was ~8.4 ± 2.9 years and the volume-weighted (VW) MTT was ~4.7 years for the study period, reflecting the importance of younger runoff water during high flow. The 5-min MTTs are normally distributed and ranged from 0 to 15 years. Monthly VW MTTs averaged 7.0 ± 3.3 years and ranged from 4 to 6 years during winter and 8–10 years during summer. The annual VW MTTs averaged 5.6 ± 2.0 years and ranged from ~5 years during wet years (2003 and 2005) to >10 years during dry years (2002 and 2008). Stormflows are composed of much younger water than baseflows, and although stormflow only occurs ~17 % of the time, this runoff fraction contributed 39 % of the runoff during the 23-year study period. Combining the 23-year VW MTT (including stormflow) with the annual average baseflow for the period (~212 mm) indicates that active groundwater storage is ~1,000 mm. However, the groundwater storage ranged from 1,040 to 1,950 mm using WY baseflow and WY VW MTT. The approach described herein may be applicable to other watersheds underlain by granitoid bedrock, where weathering is the dominant control on Si concentrations in soils, groundwater, and streamwater. 相似文献
146.
We report high-resolution macroscopic charcoal, pollen and sedimentological data for Agua Caliente, a freshwater lagoon located in southern Belize, and infer a late Holocene record of human land-use/climate interactions for the nearby prehistoric Maya center of Uxbenká. Land-use activities spanning the initial clearance of forests for agriculture through the drought-linked Maya collapse and continuing into the historic recolonization of the region are all reflected in the record. Human land alteration in association with swidden agriculture is evident early in the record during the Middle Preclassic starting ca. 2600 cal yr BP. Fire slowly tapered off during the Late and Terminal Classic, consistent with the gradual political demise and depopulation of the Uxbenká polity sometime between ca. 1150 and 950 cal yr BP, during a period of multiple droughts evident in a nearby speleothem record. Fire activity was at its lowest during the Maya Postclassic ca. 950–430 cal yr BP, but rose consistent with increasing recolonization of the region between ca. 430 cal yr BP and present. These data suggest that this environmental record provides both a proxy for 2800 years of cultural change, including colonization, growth, decline, and reorganization of regional populations, and an independent confirmation of recent paleoclimate reconstructions from the same region. 相似文献
147.
148.
Douglas S. Hall Gregory W. Henry Dietmar Böhme Peter A. Brooks Sandy Chang Ales Dolzan George L. Fortier Robert E. Fried Russell M. Genet Bruce S. Grim James Hannon Darrel B. Hoff Kevin Krisciunas Howard J. Landis Howard P. Louth Larry P. Lovell Paul Nielsen Bobby E. Powell Harry D. Powell Don Pray Thomas R. Renner Charles W. Rogers Stephen Shervais Douglas M. Slauson Samuel Slote Harold J. Stelzer Arthur J. Stokes Jack C. Troeger Louis C. Turner Norman F. Wasson Kenneth W. Zeigler 《Journal of Astrophysics and Astronomy》1991,12(4):281-287
We present photoelectric photometry of λ And never before published, obtained between February 1982 and December 1990 at 29
different observatories. Then we combine it with all other photometry available to us (previously published, contained in
the I.A.U. Commission 27 Archives, and obtained with the Vanderbilt 16-inch automatic telescope but not yet published), to
yield a 14.8-year data base. Analysis reveals a long-term cycle in mean brightness, with a full range of 0m.15 and a period of 11.4 ± 0.4 years. Because most of our new photometry was concentrated in the 1983-84 observing season,
we analyze that one well-defined light curve with a two-spot model. Spot A keeps a 0m.04 amplitude throughout four rotation cycles whereas the amplitude of spot B diminishes from 0m.09 down almost to 0m.03. The spot rotation periods were 55d.9 ± 0d.6 and 520d.8 ± 1d.0, respectively. 相似文献
149.
150.
Douglas J. Cant 《Basin Research》1991,3(2):51-62
Geometric analysis shows that the angle of migration of coastal sedimentary facies is a function of the relative sea-level change and the thickness of sediment deposited or eroded. The angle of facies migration compared to the slopes on the sediment surface determines the degree of facies preservation and stratigraphic relationships to the surrounding facies. Vertical facies successions generated by radial migration of environments show a great deal of variety because the sediment surface in both marine and non-marine areas is concave-up. Both regressive and transgressive sequences with non-erosive marine-nonmarine contacts can be generated. Transgression at a slightly lower angle can form a ravinement surface cut on non-marine deposits with onlapping barrier sands or shallow marine deposits. Regression with relative sea-level drop generates a minor erosion surface with baselapping isolated shoreline deposits. Disequilibrium conditions occur when sea level varies at a rate exceeding the ability of the system to supply or redistribute sediment, with resulting changes in surficial slopes. Onlapping and downlapping stratal relationships across erosion surfaces result because of differences in slopes between marine and non-marine environments. These discontinuities are generally less than one degree, but could possibly be recognized on high quality multichannel seismic lines. Most of these discontinuities are probably not regionally extensive enough to be regarded as sequence boundaries. Tectonic tilting or differential subsidence of strata during depositional hiatuses is necessary to generate true regional unconformities or sequence boundaries. Where facies climb with respect to horizontal, erosion surfaces produced only by this migration may cut across lithostratigraphic units at higher angles, up to 3 or 4 degrees. Low-angle erosion surfaces relevant to the scales of sequence stratigraphic studies may result only from facies migration, even during a period of relative sea-level rise. 相似文献