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排序方式: 共有177条查询结果,搜索用时 0 毫秒
81.
A. J. Timothy JULL Lanny R. McHARGUE Philip A. BLAND Richard C. GREENWOOD Alexander W. R. BEVAN Kyeong J. KIM Sarah E. LaMOTTA Jeffrey A. JOHNSON 《Meteoritics & planetary science》2010,45(8):1271-1283
Abstract– We have investigated the terrestrial ages, or residence times, of 78 meteorites (representing 73 discrete falls) recovered in Western Australia, and one from South Australia, using both 14C measurements and also 14C/10Be. The samples studied included two ureilites, one CK and one EL chondrite. We have included 10Be measurements from 30 meteorites, including some meteorites for which the 14C terrestrial age was previously determined. We find that the 14C/10Be terrestrial ages are more precise than 14C alone, as we can correct for shielding effects. In general, the two different age determinations age by 14C–10Be are precise to 0.5–1 ka and 14C alone within 1–2 ka. However, measurement of the 14C age alone gives good agreement with the 14C–10Be for most samples. The study of the terrestrial ages of meteorites gives us useful information concerning the storage and weathering of meteorites and the study of fall times and terrestrial age. We have compared the terrestrial ages to weathering, degree of oxidation (estimated from Mössbauer studies) and Δ17O. In this study, we found that weathering is not well correlated with terrestrial age for Nullarbor meteorites. However, there is a good correlation between degree of oxidation and Δ17O. The implications for the study of terrestrial ages and weathering from other desert environments will be discussed. 相似文献
82.
前寒武纪时期铁矿形成过程中铁物质如何迁移的研究,已取得了众多科研成果,但仍有些问题未得到很好的解决。尤其是对陆壳来源性铁矿床(非Algoma类型)的成因,仍然具有诸多争议,焦点主要集中在铁迁移问题。笔者以朝鲜半岛龙渊铁矿床为例研究了铁介质形成和运输的问题。首先通过铁矿石的地球化学研究和前人研究结果的考察,发现此铁矿床不属于Algoma类型,而是在强酸性介质条件下陆壳物质风化、移动和沉积而形成的。那么为什么会出现如此强大的酸性环境条件呢?为了揭示这一点,对在当时环境下把水的性质能够变成酸性的物质进行了热力学计算。研究结果表明,当硫化物如黄铁矿风化时,形成了能够使铁源物质风化和迁移的介质。这些结果也符合这样一个事实,即目前从富含硫化物地层淋沥的水的pH值小于3.5,并且铁含量远高于非硫化物类型的地层。本次研究结果表明,陆壳来源的铁矿床形成过程中,不能忽视硫化物的风化作用。 相似文献
83.
GUEBUEM KIM NAJID HUSSAIN THOMAS M. CHURCH 《Tellus. Series B, Chemical and physical meteorology》2000,52(1):74-80
Many researchers have reported the widespread occurrence of excess 210 Po in the global atmosphere and suggested probable sources such as resuspension of top soils, stratospheric aerosols, sea spray of the surface micro‐layer, volcanic emission, and bio‐volatile 210 Po species from the productive ocean. We have observed excess 210 Po on aerosols in the coastal atmosphere of the Chesapeake and Delaware Bays. On‐board measurements in the Chesapeake Bay atmosphere show that the increase of this excess 210 Po is dependent upon wind speed. Simultaneously measured activity ratios of 7 Be/210 Pb and 210 Pb/222 Rn argue against either higher altitude air or continental soils as the source of this excess. We hypothesize that the excess 210 Po originates mainly from surface waters either by the sea‐spray of the surface microlayer, or more likely, by gas exchange. We conclude gas exchange as the mechanism since the polonium excess increases linearly with wind speed over a threshold of 3 m s−1 (mean) similar to other gases (i.e., CO2 , SF6 , and DMS). In addition, higher 210 Po excess with lower 222 Rn is observed in on‐shore marine air at Lewes, DE. This suggests sea‐air exchange of volatile Po along with other bio‐volatile species (i.e., DMS, DMSe, and MMHg) in the coastal productive ocean during high wind speeds. 相似文献
84.
KIM AARIS-SØRENSEN RONNIE LILJEGREN 《Boreas: An International Journal of Quaternary Research》2004,33(1):61-73
This article presents new data on the Late Pleistocene giant deer, Megaloceros giganteus (Blumenbach), describing its distribution in time and space, geographical and sexual variation and general biology. Twenty‐three south Scandinavian fossils found in situ in lacustrine sediments or redeposited in glaciofluvial material form the basis of this investigation. Fourteen C dates show that the giant deer inhabited southern Scandinavia in the ice‐free late Middle Weichselian from c. 40000 to 28000 BP (the Sandnes Interstadial) and again in the Late Weichselian from c. 12000 to 10700 BP (Older Dryas, Allerød and early Younger Dryas Chronozones), corresponding to a calibrated range from c. 14300 to 12400 cal. yr BP. Osteometric analyses show that the Scandinavian giant deer belonged to the upper size range of the lateglacial Irish population and that a marked sexual dimorphism existed, the males being 10–11% larger than the females. Investigations furthermore point at an antler cycle similar to that among extant northern cervids, and subsequently at a rutting season in autumn. The skeletal remains also prove the occurrence of twin delivery and the possibility of reaching an ontogenetic age of at least 23 years. During both occurrences the Scandinavian giant deer population was part of the northernmost distribution of the species in Europe and the palaeogeographical settings and palaeoenvironmental conditions of the two periods show striking similarities. Clearly, the giant deer were able to colonize and survive in a landscape dominated by grasses and sedges with scattered shrubs and dwarf shrubs. They came as close as 200–250 km to the ice front and their distribution included coastal areas along a cold sea with drifting icebergs. They were present in the area at least from March until November. However, the pure arctic conditions created during the early phase of the Younger Dryas event led to a new local extinction around 10700 14C yr BP. This was the beginning of a total Eurasian extinction which, at least in Europe, was completed before the Pleistocene/Holocene transition. 相似文献
85.
This study presents an exploratory analysis aimed at improving understanding
of the variability of Hong Kong air quality associated with different
climate conditions. Significantly negative correlations were found when
Nino 3 led particulate matter ≤10 μm PM10) and NO2
by 2--3 months over the Hong Kong territory, while the other pollutants
(e.g., O3, SO2) showed modest correlations. A significant
decreasing trend in visibility was observed during the autumn and winter,
which has potential implications for the air-quality degradation and the
endangerment of human health in Hong Kong. In an El Nino summer, the
visibility was relatively better, while visibility in other seasons was
diminished. On the other hand, in La Nina events, significant changes
occurred in visibility in winter and autumn. Air pollution indices were less
sensitive to the South China Summer Monsoon (SCSM), but a relatively high
correlation existed between the East Asian Winter Monsoon (EAWM) and air
pollutants. Rainfall was lower during most of the strong EAWM years compared
to the weak years. This result suggests that the pollutants that accumulate
in Hong Kong are not easy to wash out, so concentrations remain at a higher
level. Finally, based on the conditional Air Pollution Index (API) risk
assessment, site-specific vulnerabilities were analyzed to facilitate the
development of the air-quality warning systems in Hong Kong. 相似文献
86.
87.
Radon-222 in boundary layer and free tropospheric continental outflow events at three ACE-Asia sites
88.
89.
Correlation assessment and monitoring of the potential pollutants in the surface sediments of Pyeongchang River, Korea 总被引:1,自引:0,他引:1
The study provides a baseline for the assessment of the organic and inorganic pollution specially, heavy metal contamination in the surface sediments of Pyeongchang River,South Korea.The assessment of the study areas was done with respect to metal pollution load,ecological risk and geoaccumulated risk.Based upon the used indices,a priority index(Pindex) was used to rank the utmost contaminated sites.Though the concentrations of mercury in all sediments were below the guideline, the significant enriched contamination was observed by all applied indices.As expected,the values of pollution load index(PLI),ecological risk index(RI) and geoaccumulation risk index(Igeo) demonstrated lower heavy metal contamination in upstream areas compared to the downstream. Admittedly,sediments were unpolluted to slightly-polluted according to PLI while high to extremely high ecological risks were observed in several sediment samples.Furthermore,all the samples were uncontaminated as per Igeo.After simplification of Igeo,the Pindex,showed the utmost contaminated sediments with a value of 2.537.Notably,protective measures should be taken to the highly contaminated areas which are prioritized by Pindex Admittedly,the maximum concentrations of total organic carbon,total nitrogen,inorganic nitrogen,total phosphorous,inorganic phosphorous,calcium, magnesium,sodium and potassium were significantly observed as 7.8×104,3,185,36,1,032,3.7, 1,5163,2,881,669 and 4,076mg/kg accordingly. 相似文献
90.
The Dabic-Sulu orogenic belt in central-eastern China is considered as a high-pressure and ultrahighpressure metamorphic belt that demensions are comparatively large,and formed as a result of the colli... 相似文献