排序方式： 共有5条查询结果，搜索用时 15 毫秒
1
1.
Karlyn S. Westover Sherilyn C. Fritz Tatyana A. Blyakharchuk Herbert E. Wright 《Journal of Paleolimnology》2006,35(3):519-541
The sedimentary diatom records of three shallow lakes in the Altai Mountains, southern Siberia, were examined to assess the
nature and timing of Holocene environmental changes. Few paleoenvironmental records, especially reconstructions not based
on pollen, have been reported from this region. The lakes differ in elevation, annual precipitation, and catchment vegetation.
Diatom assemblages in all lakes were dominated for the entire period of record by small benthic species of Pseudostaurosira Williams & Round, Staurosira Ehrenberg, and Staurosirella Williams & Round. Planktonic taxa only occur in very low abundances (<5%). The most diverse diatom flora was found in Dzhangyskol,
which is situated at the lowest elevation within a forested catchment. A lack of detailed information on the ecological preferences
of the dominant taxa and the complexity of environmental drivers make direct interpretation of the diatom record difficult.
However, other proxies suggest that dramatic shifts in dominance between Staurosira elliptica and Staurosirella pinnata in Grusha Ozero reflect millennial-scale variability in climate. Together, chironomids and diatoms provide evidence of a
cooling possibly correlative to the Younger Dryas Stade and subsequent early-Holocene warming consistent with pollen evidence
of afforestation, which also is likely linked to increased humidity. By ~6000 cal year BP, the transition to a cooler, more
continental climate had begun. The diatom record of Akkol shows significantly less variation in diatom community composition,
but biogenic silica accumulation rates, a proxy for diatom productivity, appear to reflect climatic variability driven by
insolation trends over the past 8000 years. Long-term variability in Dzhangyskol is not clearly linked to climate. 相似文献
2.
For the past five years the Millsaps Sorbent Laboratory has been actively engaged in developing standards for initial and long-term oil spill remedial technologies. As a voting member of the American Society of Testing Materials (ASTM) F-20 Committee, Canadian General Standards Board, and the US Coast Guard Sorbents Task Force, the laboratory has been engaged in developing useful, pragmatic protocols for various chemical and physical sorbent and filtration technologies driven by the deadlines imposed by the Oil Pollution Act of 1990 (OPA 90). The open border approach to certification of technologies and products promulgated by the US/Canadian Free Trade Agreement has placed the US users and producers of such products and systems in a unique and tenuous position. Canadian standards and goals are grandfathered into the United States under this agreement and products have official US government certification based on Canadian regulations. This situation is unfavorable to the US domestic environment and economy for several specific scenarios. Included in these scenarios are: abundant warmwater zones and inland waters of the US versus Canada, the basic chemical variation between Canadian and US crude oils, the different generally accepted remediation technologies in the US versus Canadian, and the technology validation procedures prior to purchase inherent to both countries. 相似文献
3.
Rixt de Jong Christian Kamenik Karlyn Westover Martin Grosjean 《Journal of Paleolimnology》2013,50(4):519-533
Relatively little is known about past cold-season temperature variability in high-Alpine regions because of a lack of natural cold-season temperature proxies as well as under-representation of high-altitude sites in meteorological, early-instrumental and documentary data sources. Recent studies have shown that chrysophyte stomatocysts, or simply cysts (sub-fossil algal remains of Chrysophyceae and Synurophyceae), are among the very few natural proxies that can be used to reconstruct cold-season temperatures. This study presents a quantitative, high-resolution (5-year), cold-season (Oct–May) temperature reconstruction based on sub-fossil chrysophyte stomatocysts in the annually laminated (varved) sediments of high-Alpine Lake Silvaplana, SE Switzerland (1,789 m a.s.l.), since AD 1500. We first explore the method used to translate an ecologically meaningful variable based on a biological proxy into a simple climate variable. A transfer function was applied to reconstruct the ‘date of spring mixing’ from cyst assemblages. Next, statistical regression models were tested to convert the reconstructed ‘dates of spring mixing’ into cold-season surface air temperatures with associated errors. The strengths and weaknesses of this approach are thoroughly tested. One much-debated, basic assumption for reconstructions (‘stationarity’), which states that only the environmental variable of interest has influenced cyst assemblages and the influence of confounding variables is negligible over time, is addressed in detail. Our inferences show that past cold-season air-temperature fluctuations were substantial and larger than those of other temperature reconstructions for Europe and the Alpine region. Interestingly, in this study, recent cold-season temperatures only just exceed those of previous, multi-decadal warm phases since AD 1500. These findings highlight the importance of local studies to assess natural climate variability at high altitudes. 相似文献
4.
Daniel G. Gavin Andrew C.G. Henderson Karlyn S. Westover Sherilyn C. Fritz Ian R. Walker Melanie J. Leng Feng Sheng Hu 《Quaternary Science Reviews》2011,30(9-10):1243-1255
Several abrupt climate events during the Holocene, including the widely documented oscillation at 8.2 thousand years before present (ka), are attributed to changes in the North Atlantic thermohaline circulation. Additional mechanisms, such as interactions between atmospheric circulation, ice-sheet dynamics, and the influence of solar irradiance, also have been proposed to explain abrupt climatic events, but evidence remains elusive. This study presents evidence from multi-proxy analyses on the Holocene sediments of Eleanor Lake, interior British Columbia. Climatic inferences from our decadal-resolution record of biogenic silica (BSi) abundance are supported by changes in diatom and pollen assemblages from the same core and correlations with existing regional climate records. The BSi record reveals abrupt and persistent climatic shifts at 10.2, 9.3, and 8.5 ka, the latter two of which are coeval with major collapses of the Laurentide Ice Sheet. The record also reveals a short-term cooling at 8.2 ka that is distinct from the 8.5 ka event and similar in magnitude to several other late-Holocene coolings. BSi is correlated with solar-irradiance indices (r = 0.43–0.61), but the correlation is opposite in sign to that expected from direct solar forcing and weakens after 8 ka. Possible mechanisms causing the abrupt and persistent climate changes of the early Holocene include 1) sudden losses of ice and proglacial lake extent, causing a shift in the meridional structure of atmospheric circulation, 2) a possible link between solar minima and El Niño-like conditions that are correlated with warm spring temperature in interior British Columbia, and 3) the influence of solar irradiance variability on the position of the polar jet, possibly via effects on the strength of the glacial anticyclone. 相似文献
5.
Theresa N. Westover 《The Professional geographer》1985,37(4):410-420
While recent research provides a general model of the causes and consequences of fear of crime, little is known about how the relationships among these model components may vary from one setting to another. Garofalo's fear of crime model is used to explore the extent and impact of fear of crime in recreational settings. Two hundred sixty-eight visitors to three Midwestern district level parks were interviewed on-site using a structured questionnaire. Results generally support the model and demonstrate a link between perception and behavior related to fear of crime. Respondents who felt unsafe in the park alone also tended to report avoidance behavior. Females were more likely than males to report both fear and avoidance. Direct experience with park crime and incivility, perceptions of other park visitors, and familiarity with the park were not strongly associated with either feelings of safety or reported avoidance. 相似文献
1