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131.
Richard L. Reynolds Joseph G. Rosenbaum Peter van Metre Michele Tuttle Edward Callender Alan Goldin 《Journal of Paleolimnology》1999,21(2):193-206
Combined magnetic and geochemical studies were conducted on sediments from White Rock Lake, a reservoir in suburban Dallas (USA), to investigate how land use has affected sediment and water quality since the reservoir was filled in 1912. The chronology of a 167-cm-long core is constrained by the recognition of the pre-reservoir surface and by 137Cs results. In the reservoir sediments, magnetic susceptibility (MS) and isothermal remanent magnetization (IRM) are largely carried by detrital titanomagnetite that originally formed in igneous rocks. Titanomagnetite and associated hematite are the dominant iron oxides in a sample from the surficial deposit in the watershed but are absent in the underlying Austin Chalk. Therefore, these minerals were transported by wind into the watershed. After about 1960, systematic decreases in Ti, Fe, and Al suggest diminished input of detrital Fe-Ti oxides from the surficial deposits. MS and IRM remain constant over this interval, however, implying compensation by an increase in strongly magnetic material derived from human activity. Anthropogenic magnetite in rust and ferrite spherules (from fly ash?) are more common in sediment deposited after about 1970 than before and may account for the constant magnetization despite the implied decrease in detrital Fe-Ti oxides. An unexpected finding is the presence of authigenic greigite (Fe3S4), the abundance of which is at least partly controlled by climate. Greigite is common in sediments that predate about 1975, with zones of concentration indicated by relatively high IRM/MS. High greigite contents in sediment deposited during the early to mid-1950s and during the mid-1930s correspond to several-year periods of below-average precipitation and drought from historical records. Relatively long water-residence times in the reservoir during these periods may have led to elevated levels of sulfate available for bacterial sulfate reduction. The sulfate was probably derived via the oxidation of pyrite that is common in the underlying Austin Chalk. These results provide a basis for the paleoenvironmental interpretation of greigite occurrence in older lake sediments. The results also indicate that greigite formed rapidly and imply that it can be preserved in the amounts produced over a short time span (in this lake, only a few years). This finding thus suggests that, in some lacustrine settings, greigite is capable of recording paleomagnetic secular variation. 相似文献
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133.
烟台附近海岸风沙地貌的初步研究 总被引:5,自引:0,他引:5
在野外调查基础上,对烟台附近海岸风沙分布、风沙地貌类型及地貌发育过程进行了探讨。指出全新世海岸变迁是海岸风沙地貌发育的基础,6000aB.P.以来有5个风沙堆积期,前4个风沙堆积期与4个海平面下降期一致,第5风沙堆积期与小冰期一致。 相似文献
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135.
文章对秦安中新世黄土-古土壤序列中黄土层样品的常量、微量和稀土元素地球化学特征进行研究,并与西峰第四纪黄土和晚中新世-上新世的风成三趾马红土样品进行对比。结果表明,不同时代的风尘堆积具有大致相似的元素配分模式,并与上地壳平均成分高度接近,表明它们的物质均来自很广阔的物源区,经过多次循环和远距离搬运,并在搬运过程中实现了多种矿物成分高度混合,是远源风尘物质的典型特征。秦安中新世黄土与西峰第四纪黄土相比,常量元素组配特征也有一些区别,主要表现为K,Fe,Mg的轻微富集、Na元素的轻微亏损及相对较高的烧失量,与中新世黄土较细的粒度和相对温湿的气候条件有关。与世界上一些物源较近较小的黄土相比,我国北方不同时代的风尘堆积均具有Cs相对富集、Zr和Hf相对亏损的特征,与粉尘远距离传输过程中的物质分异有关,是沙漠黄土的特点。 相似文献