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51.
Jason C. Senkbeil David A. Scott Pilar Guinazu-Walker Meganne S. Rockman 《The Professional geographer》2013,65(4):610-620
The 27 April 2011 EF4 Tuscaloosa tornado directly impacted more than 50,000 residents, causing forty-five fatalities within the city and sixty-five in total. It was a rare urban tornado with varying impacts on the three major ethnic and racial groups within the city. A hybrid survey and interview of open-ended and closed questions was conducted with 211 Tuscaloosa area residents in a two-week period after the tornado. Results indicate significant differences in risk perception, preparedness, and shelter lead time among the three ethnic and racial groups. Furthermore, results were still significant for perception after controlling for the effects of age, education, and experience. 相似文献
52.
During a 96-hour period in mid-March 1987, a series of four serial frost-thaw events occurred in a research field at the University of Michigan Matthaei Botanical Gardens. Soil temperature and electric potential were recorded at two near surface probe arrays (0, 3, 6 & 9 cm) and (0, 5, 10 & 15 cm) with a horizontal separation of approximately 1 m beneath the organic-mineral soil interface at hourly intervals. These time series revealed strong repetitive serial patterns. When the electric potential measurements were converted to a surrogate measure of electrolyte concentration, the C-index, strong coupled flow effects appeared to be present in the time series owing to the effects of surface freeze-thaw, evaporation-distillation and soil water advection. The data set shows the low frequency effects of upward soil water advection increasing the soil temperature and modulating the electrolyte concentration. These observations also demonstrate that soil freezing processes have two distinct regimes dominated by (1) early soil water advection toward the frost front from below and (2) downward expulsion of electrolyte-rich water as ice fills the pore space. These two processes of soil water advection and expulsion augmented by local evaporation produce two distinct near surface C-index signatures of serial diurnal frost. The signature depends on the direction of the vertical electrolyte gradient near the surface. [Key words: soil, frost-thaw, soil water, soil temperature.] 相似文献
53.
Fluvial process and the establishment of bottomland trees 总被引:1,自引:0,他引:1
The effects of river regulation on bottomland tree communities in western North America have generated substantial concern because of the important habitat and aesthetic values of these communities. Consideration of such effects in water management decisions has been hampered by the apparent variability of responses of bottomland tree communities to flow alteration. When the relation between streamflow and tree establishment is placed in a geomorphic context, however, much of that variability is explained, and prediction of changes in the tree community is improved.The relation between streamflow and establishment of bottomland trees is conditioned by the dominant fluvial process or processes acting along a stream. For successful establishment, cottonwoods, poplars, and willows require bare, moist surfaces protected from disturbance. Channel narrowing, channel meandering, and flood deposition promote different spatial and temporal patterns of establishment. During channel narrowing, the site requirements are met on portions of the bed abandoned by the stream, and establishment is associated with a period of low flow lasting one to several years. During channel meandering, the requirements are met on point bars following moderate or higher peak flows. Following flood deposition, the requirements are met on flood deposits ;high above the channel bed. Flood deposition can occur along most streams, but where a channel is constrained by a narrow valley, this process may be the only mechanism that can produce a bare, moist surface high enough to be safe from future disturbance. Because of differences in local bedrock, tributary influence, or geologic history, two nearby reaches of the same stream may be dominated by different fluvial processes and have different spatial and temporal patterns of trees. We illustrate this phenomenon with examples from forests of plains cottonwood (Populus deltoides ssp. monilifera) along meandering and constrained reaches of the Missouri River in Montana. 相似文献
54.
Scott A. Reinemann David F. Porinchu Mae Sexauer Gustin Bryan G. Mark 《Journal of Paleolimnology》2014,52(4):405-418
The geochemistry of lake sediments was used to identify anthropogenic factors influencing aquatic ecosystems of sub-alpine lakes in the western United States during the past century. Sediment cores were recovered from six high-elevation lakes in the central Great Basin of the United States. The proxies utilized to examine the degree of recent anthropogenic environmental change include spheroidal carbonaceous particle (SCP), mercury (Hg), and sediment organic content estimated using loss-on-ignition. Chronologies for the sediment cores, developed using 210Pb, indicate the cores span the twentieth century. Mercury flux varied between lakes but all exhibited increasing fluxes during the mid-twentieth century. The mean ratio of modern (post-A.D. 1985) to preindustrial (pre-A.D. 1880) Hg flux was 5.2, which is comparable to the results from previous studies conducted in western North America. Peak SCP flux for all lakes occurred between approximately A.D. 1940 and A.D. 1970, after which time the SCP flux was greatly reduced. The reduction in SCP input is likely due to better controls on combustion sources. Measured Hg concentrations and calculated sedimentation rates suggest atmospheric Hg flux increased in the early 1900s, from A.D. 1920 to A.D. 1990, and at present. Atmospheric deposition is the primary source of the anthropogenic inputs of Hg and SCPs to these high elevation lakes. The input of SCPs, which is largely driven by regional sources, has declined with the implementation of national pollution control regulations. Mercury deposition in the Great Basin has most likely been influenced more by regional inputs. 相似文献
55.
Jessica D. Tomkins Scott F. Lamoureux Dermot Antoniades Warwick F. Vincent 《Journal of Paleolimnology》2009,41(1):225-242
Sediment aggregates (“sedimentary pellets”) within the sedimentary record of Lake A (83°00′ N, 75°30′ W), Ellesmere Island,
Canada, are used to construct a 1000 year proxy record of ice-cover extent and dynamics on this perennially ice-covered, High
Arctic lake. These pellets are interpreted to form during fall or early winter when littoral sediment adheres to ice forming
around the lake’s periphery or during summer through the development of anchor ice. The sediment likely collects in ice interstices
and is concentrated in the upper ice layers through summer surface ice melt and winter basal ice growth. The pellets remain
frozen in the ice until a summer or series of summers with reduced ice cover allows for their deposition across the lake basin.
Sedimentary pellet frequency within multiple sediment cores is used to develop a chronology of ice-cover fluctuations. This
proxy ice-cover record is largely corroborated by a record of unusual sedimentation in Lake A involving iron-rich, dark-orange
to red laminae overlying more diffuse laminae with a lighter hue. This sediment sequence is hypothesized to represent years
with reduced ice cover through increased chemocline ventilation and iron deposition. During the past millennium, the most
notable period of inferred reduced ice cover is ca. 1891 AD to present. Another period of ice cover mobility is suggested
ca. 1582–1774 AD, while persistent ice cover is inferred during the 1800s and prior to 1582 AD. The proxy ice-cover record
corresponds well with most regional melt-season proxy temperature and paleoecological records, especially during the 1800s
and 1900s.
相似文献
Jessica D. TomkinsEmail: |
56.
57.
58.
Petrogenesis and paleotectonic history of the Wild Bight Group,an Ordovician rifted island arc in central Newfoundland 总被引:1,自引:0,他引:1
H. Scott Swinden G. A. Jenner B. J. Fryer J. Hertogen J. C. Roddick 《Contributions to Mineralogy and Petrology》1990,105(2):219-241
The Wild Bight Group (WBG) is a sequence of early and middle Ordovician volcanic, subvolcanic and epiclastic rocks, part of the Dunnage Tectonostratigraphic Zone of the Newfoundland Appalachians. A detailed geochemical and Nd-isotopic study of the volcanic and subvolcanic rocks has been carried out to determine the geochemical characteristics of the rocks, interpret their palcotectonic environments and constrain their petrogenetic history. The lower and central stratigraphic levels of the WBG contain mafic volcanic rocks with island-arc geochemical signatures, including LREE-enriched are tholeiites with
Nd(t)
=-0.1 to +2.2 (type A-I), LREE-depleted arc tholeiites with
Nd(t)
=+5.6 to +7.1 (type A-II) and an unusual suite of strongly incompatible-element depleted tholeiites in which
Nd(t)
ranges from-0.9 to +4.6 and is negatively correlated with147Sm/144Nd (type A-III). High-silica, low-K rhyolites occur locally in the central part of the stratigraphy, associated with mafic rocks of arc affinity, and have
Nd(t)
=+4.7 to +5.4. The upper stratigraphic levels of the WBG dominantly contain rocks with non-arc geochemical signatures, including alkalic basalts with
Nd(t)
=+4.6 to +5.5 (type N-I), strongly LREE- and incompatible element-enriched tholeiites that are transitional between alkalic and non-alkalic rocks with
Nd(t)
=+4.4 to +7.0 (type N-II) and rocks with flat to slightly LREE-enriched patterns and
Nd(t)
=+5.1 to +7.4 (type N-III). Rocks with non-arc and arc signatures are locally interbedded near the stratigraphic type of the WBG. Nd-isotopic data in the type A-I and A-II rocks are generally compatible with mixing/partial melting models involving depleted mantle, variably contaminated by a subducted crustally-derived sediment. The petrogenesis of type A-III rocks must involve source mixing and multi-stage partial melting, but the details are not clear. The geochemistry and Nd isotope data for types N-I, N-II and N-III rocks are compatible with petrogenetic models involving variable partial melting of a source similar to that postulated for modern oceanic island basalts. Comparison of the WBG with modern analogues suggests a 3-stage developmental model: stage 1) island-arc volcanism (eruption of type mafic volcancs); stage 2) arc-rifting (continued eruption of type A-I, A-I, eruption of types A-II and A-III mafic volcanics and high-silica, low-K rhyolites); and stage 3) back-arc basin volcanism (continued minor eruption of type A-I basalts, eruption of types N-I, N-II, N-III basalts). Stages 1 and 2 volcanism involved partial melting of subduction contaminated mantle, while stage 3 volcanism utilized depleted-mantle sources not affected by the subducting slab. This model provides a basis for interpreting coeval sequences in central Newfoundland and a comparative framework for some early Paleozoic oceanic volcanic sequences elsewhere in the Appalachian orogen. 相似文献
59.
A study of the isotopic composition of plankton from Woods Hole Harbor was conducted to investigate seasonal variation in carbon and nitrogen stable isotopes in a shallow coastal environment. Stable isotopic ratios of carbon and nitrogen both showed temporal variation on the scale of weeks to months, with heaviest (most positive) values in summer to fall for both isotopes. Particulate organic matter (POM) δ13C values were highest (?19‰ to ?21‰) in August to November and lower (?21‰ to ?25‰) at other times of the year, while δ13N-POM values were highest (9.5‰ to 12‰) in March to September and lower (7.5‰ to 9.5‰) at other times of the year. Stable isotopic values were significantly correlated with temperature, DI13C, and C∶N ratios, but not with [DIC], [POC], [PN], [chlorophyll], or the taxonomic composition of the phytoplankton. There was no direct evidence of allochthonous inputs of carbon and nitrogen to the system. Woods Hole δ13C values were virtually identical to Georges Bank plankton values; similar POC: Chlorophyll and C∶N ratios in the two systems further suggest that Woods Hole Harbor is principally a marine system. The high δ13C values of net plankton (>20 μm) during summer and early fall are consistent with a smaller degree of photosynthetic isotopic fractionation at that time, related to temperature and/or [CO2(aq)]. This pattern was not seen, however, in total POM. Plankton δ13N values were higher in Woods Hole Harbor than on Georges Bank, especially during warmer periods, possibly due to high rates of nitrification and organic matter recycling in Woods Hole waters. Relatively wide ranges of stable isotopic values from both Woods Hole Harbor and Georges Bank suggest that seasonality should be considered when attempting to establish endmember C and N isotopic values for temperate marine plankton. Preliminary results from size-fractionated samples suggest that cyanobacteria may fractionate carbon isotopes to a greater degree than net phytoplankton. 相似文献
60.
Salinity stratification in a river-dominated estuary 总被引:1,自引:0,他引:1
Analysis of salinity data from Mobile Bay indicates that stratification-destratification events within this broad, shallow estuary are not uncommon. These events are related to the strength of the winds, through their influence on wave generation and subsequent bottom drag coefficient increases, and to the strength of river discharge. They do not appear to be due to the strength of tidal currents, as has been observed elsewhere. Furthermore, river flow appears to be the dominant control, the winds being important only in the absence of large freshwater discharges. The annual spring freshet can flush most of the salt from the bay. During other times of the year the relative strengths of river discharge and wind stress change the bay from highly stratified to nearly homogeneous and back on a variety of time scales ranging from daily to seasonal. 相似文献