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61.
Although the Upper Volga has been the focus of extensive research for several decades, its origin and evolution remain uncertain. According to the most popular model, the Upper Volga drainage network formed at the end of the last deglaciation (MIS 2); before that, the river basin was occupied by a large MIS2 proglacial lake. In this study we test this hypothesis by luminescence dating several fluvial terraces in the Upper Volga valley. Despite the expected late-MIS 2 age, our results show that quartz is in saturation, and the feldspar pIRIR290 signal gives much older dates than expected (∼300–∼500 ka). We argue that, in most sections, feldspar was very likely to be well bleached prior to deposition. Thus, if the ages of sedimentary structures are overestimated, this could only result from sediment transport in darkness. It is widely accepted that the MIS 6 glaciation was the last to cover the Upper Volga basin, and so such conditions would be most easily attributed to a subglacial environment. But this explanation is confounded by the absence of till on top of most sections. The alternative explanation, that the ages accurately reflect last deposition, calls into question the well-established MIS 6 border on the Russian plain, and we conclude that further studies are required to resolve this inconsistency. 相似文献
62.
定量分析了2006年6月、2007年8月和2008年2月采自南海北部大亚湾海域的共8科、13属、23种170个石珊瑚样品的共生虫黄藻密度,探讨了石珊瑚共生虫黄藻密度的季节变化及其与珊瑚白化的关系.结果显示,所有珊瑚种属的共生虫黄藻密度都显示出明显的季节性波动,总体上夏季低、冬季高(约为夏季的2倍),是海表水温和太阳辐射协同作用的结果.夏季大规模的珊瑚白化(热白化)可能是珊瑚共生虫黄藻密度逐渐降低(排出)到一定阈值的外观表征,而非突发的生态现象;冬季珊瑚白化(冷白化)则可能是极端低温直接致珊瑚死亡,进而快速排出虫黄藻的突发现象;高共生虫黄藻密度对冬季低温乃至极端低温条件下的珊瑚生存起到一定的保护作用. 相似文献
63.
近年来,受全球气候变化及极端天气的影响,全球范围频繁出现珊瑚礁白化现象,遥感SST已广泛应用于珊瑚白化的监测和预警中。海南岛近岸及北部湾海域的珊瑚礁近年也倍受白化压力,然而,遥感SST在海南岛近岸及北部湾海域珊瑚白化预警中的适用性尚不清楚。为此,本研究比较了3种常用的遥感SST数据(OISST、GHRSST和CoralTemp)在研究海域的异同及其在夏季珊瑚白化预警中的适用性。结果表明,GHRSST的SST最高、CoralTemp次之、OISST最低,OISST的偏差在2013年以前比较明显,GHRSST和CoralTemp在2003年之后比较接近;与浮标的现场观测值相比,2006—2020年CoralTemp的平均偏差和均方根误差分别为0.03℃和0.92℃,GHRSST为0.08℃和0.96℃,OISST为-0.25℃和1.21℃,CoralTemp在研究区域内更为准确;3种遥感SST计算的DHW与使用现场SST计算的趋势一致,CoralTemp的结果更为接近现场SST。因此,3种遥感SST资料中,CoralTemp相对来说更适用于研究海域。 相似文献
64.
分别采用盐酸和草酸为浸出酸,对吉林省长白地区的瓷石进行了酸浸除铁漂白试验。对比了盐酸和草酸的酸浸除铁漂白的效果,结果发现用草酸比用盐酸除铁漂白效果好。草酸的浓度、矿浆浓度、浸出时间、浸出温度等因素对除铁漂白效果有一定影响。矿浆质量分数以15%为宜,随着草酸浓度的增加、浸出时间的延长、浸出温度的升高,瓷石的除铁漂白效果明显提高。最佳的除铁漂白方案为:草酸水溶液质量分数为l.00%,矿浆质量分数为15%,浸出反应时间为150min,浸出反应温度为60℃。经除铁漂白处理,使瓷石中铁的质量分数由原来的O.64%降为O.20%,瓷石白度由原来的68.2%提高到了83.7%,最高可达84%。 相似文献
65.
The application of optically stimulated luminescence (OSL) dating within glacial settings can be limited where sediments have not had their OSL signal fully reset by sunlight exposure. Heterogeneous bleaching can result in age overestimations, and although it is recognized that certain depositional settings are more likely to have experienced sufficient sunlight exposure to bleach the OSL signal, no comprehensive study has empirically investigated the processes of sediment bleaching, or the variability in bleaching between deposits of the same type and within the same glacial catchment. A suite of modern glacial and glaciofluvial sediments from Fåbergstølsdalen, southern Norway, have been analysed to explore the controls that sedimentary processes and depositional setting have on bleaching of the OSL signal of quartz. There is considerable variability in the residual OSL ages of similar modern deposits, which reflects high sensitivity of the OSL signal to sediment source, sedimentary process, transport distance and depositional setting. Overdispersion values are greatest for the sediments which have been most heterogeneously bleached and these sediments have the lowest residual ages. Sampling strategies that incorporate sufficient consideration of the depositional framework of sample settings can minimize the effects of unbleached residuals on OSL age determinations. 相似文献
66.
Using the concept of bleaching in optical dating, a new index of sediment sample bleaching percentage (BLP‐2) was developed and applied to evaluate sand grain transport from riverine to deep‐marine environments. As bleached grains in modern sediments have no optically stimulated luminescence (OSL)/infrared stimulated luminescence (IRSL) signal, bleached and unbleached feldspar grains are distinguished by IRSL intensity. The BLP‐2 distribution of present deposits around the Kumano area, on the Pacific coast of central Japan, suggests that sand grains in surface turbidites obtained from the bottom of the Kumano Trough are of flood/storm origin rather than seismogenic origin. The distribution of BLP‐2 tentatively suggests sand grain erosion–transport–depositional processes; for example, origin and transport agencies of shelf sand, and influence of coastal erosion on the beach deposit. Although the present BLP analysis is not yet supported by a rigorous statistical test, it is useful to distinguish recent deposition and remobilization of sand grains. Furthermore, if the depositional age and the luminescence age of sand grains are accurately estimated, sand grain transport processes of old (late Quaternary) sediments may be estimated by the methodology similar to that of the present study. 相似文献
67.
68.
69.
Potential use of Ti-center in ESR dating of fluvial sediment 总被引:1,自引:0,他引:1
H. Tissoux C. Falgures P. Voinchet S. Toyoda J.J. Bahain J. Desprie 《Quaternary Geochronology》2007,2(1-4):367-372
The titanium centers of quartz (Ti–Li and Ti–H) were used to date fluvial sediments, together with the aluminum-center (Al). The response of these centers to the light from a solar simulator suggests that these centers are sensitive only to the UV component of the sunlight. Ti–Li and Ti–H proved to be totally bleachable by the sun and the necessary time was, respectively, 520 and 240 h. The multiple-center method was then applied on several alluvial sediments of the Creuse Valley (France). Equivalent doses and ages were calculated and Ti–Li center proved to be suitable for dating Middle Pleistocene fluvial terraces sediment. The use of both Ti–Li- and Al-centers is also recommended to determine if the sediment was well bleached at the time of deposition. 相似文献
70.
David C.W. Sanderson Paul Bishop Miriam Stark Sally Alexander Dan Penny 《Quaternary Geochronology》2007,2(1-4):322-329
Work to apply luminescence dating to archaeological sites in the Lower Mekong Delta has continued with a programme aimed at dating ancient canal sediments and brick monuments in the vicinity of ancient city of Angkor Borei. Following the successful application of OSL dating to the Paris 2 canal near Angkor Borei further fieldwork and analysis has been undertaken. The infill and substrate of the larger Paris 4 canal connecting Angkor Borei to Oc Eo, some 80 km to the south in Vietnam has been sampled and subjected to luminescence analysis. Field spectroscopy and underwater bleaching experiments were also conducted in the Baray and Angkor Borei in 2004. The results show that both illumination intensities and spectral distributions are severely altered by as little as 1.5 m of turbid water, and that OSL bleaching rates for both quartz and feldspars are reduced. Since quartz resetting is heavily dependant on the UV components in daylight, which have preferentially attenuated the effects of turbid water on OSL zeroing rates are especially marked. The new data from the Paris 4 canal, which has been dated by OSL to be between the first millenium BC and the late first millenium AD are significant to understanding the archaeological development of the Fu Nan state in the Lower Mekong Delta, and the sequence of development of the canal network linking inland agrarian sites and coastal trading centres. 相似文献