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Pleistocene stratigraphy, glacial limits and paleoenvironments of White River and Silver Creek, southwest Yukon

机译:育空西南部的怀特河和银溪的更新世地层,冰川界限和古环境

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摘要

Quaternary glacial and non-glacial sediment exposed at White River and Silver Creek provide a record of environmental change in southwest Yukon for much of the late-Middle to Late Pleistocene. Eighteen sites at White River, located beyond the marine oxygen isotope stage (MIS) 2 glacial limit, contain thick accumulations of till, loess, peat, gravel and glaciolacustrine silt and clay, with tephras, paleosols, plant and insect macrofossils and large mammal fossils. Radiocarbon ages and eleven tephra beds constrain two tills to MIS 4 and 6. These tills correlate to the Gladstone and Reid glaciations and represent the penultimate and maximum all-time limits of the St. Elias lobe of the northern Cordilleran Ice Sheet. Two peat beds located between these tills indicate that interglacial conditions existed in the area during MIS 5e and 5a. Pond sediment deposited during mid-MIS 5 suggests that the sites were covered by an open birch tundra at this time. The MIS 3/2 transition was marked by a treeless, dry steppe-tundra populated by mammoth, horse and bison.The eleven Silver Creek sites, located ~200 km up-ice, contain a similar record of glacial and non-glacial sediment. Infrared-stimulated luminescence (IRSL) and radiocarbon dating constrain the glacial deposits at these sites to MIS 2, 4, either MIS 7 or 6, and to two Early to Middle Pleistocene, Pre-Reid glaciations. Tilting of glaciolacustrine beds of up to 1.9 mm/yr may be from uplift along the Denali fault since MIS 7. Pollen and macrofossils analyses from overlying MIS 3-aged sediment suggest that the environment was dominated by herbs and forbs, with few shrubs and almost no tree pollen at this time. Combined, the White River and Silver Creek sites contain a record of glacial and non-glacial conditions in southwest Yukon since the Middle Pleistocene.The glacial limits in southwest Yukon are markedly different from those in central Yukon. In southwest Yukon, the glacial limits are closely-spaced and were more extensive in the Middle to Late Pleistocene than in the Late Pliocene and Early Pleistocene. In central Yukon, glacial limits are separated by up to 300 km and were most extensive in the latest Pliocene and Early Pleistocene. This suggests that different forcing mechanisms controlled the extents of the St. Elias and Selwyn lobes during successive glaciations. Boundary conditions such as varying substrates, topography, moisture pathways and atmospheric circulation likely had a greater affect than tectonics and sea level on these glacial limits throughout the Plio-Pleistocene.
机译:在白河和银溪暴露的第四纪冰川和非冰川沉积物提供了育空西南部中后期至更新世大部分时间环境变化的记录。怀特河的18个地点位于海洋氧同位素阶段(MIS)2冰期界限之外,包含耕作,黄土,泥炭,砾石和冰川湖淤泥和粘土的厚积聚体,以及特弗拉斯,古土壤,植物和昆虫的大化石以及大型哺乳动物化石。放射性碳年龄和11个特非拉床限制了MIS 4和MIS的两个耕作。这些耕作与Gladstone和Reid冰川相关,代表了北部Cordilleran冰原的St. Elias叶片的倒数第二个和最大的历史极限。位于这些耕作之间的两个泥炭床表明在MIS 5e和5a期间该地区存在冰间条件。 MIS 5中期沉积的池塘沉积物表明,这些地点此时被开阔的白桦苔原所覆盖。 MIS 3/2过渡期以无树,干燥的草原苔原为标志,该草原上有猛ma象,马和野牛。11个银溪遗址位于冰上约200公里处,也有类似的冰川和非冰川沉积物记录。红外激发发光(IRSL)和放射性碳年代测定法将这些位置的冰川沉积物限制在MIS 2、4,MIS 7或6以及两次早期至中更新世前里德冰川。自MIS 7以来,沿Denali断层抬升的冰川湖床可能倾斜达1.9 mm / yr。对MIS 3年龄上覆沉积物的花粉和大型化石分析表明,该环境主要由草药和草形成,灌木很少,几乎没有。目前没有树花粉。自中更新世以来,白河和银河遗址相结合,记录了育空西南部西南地区的冰川和非冰川状况。育空西南部的冰川界限与育空中部明显不同。在育空地区的西南部,冰川界限是近距离的,并且在中更新世至晚更新世时期比在晚更新世和早更新世时期更为广泛。在育空地区中部,冰川界限相距达300公里,在最新的上新世和早更新世中最为广泛。这表明在连续的冰川期间,不同的强迫机制控制着圣埃利亚斯和塞尔温裂片的范围。边界条件,例如不同的底物,地形,水分路径和大气环流,在整个上新世以来这些冰川界限上的影响可能要比构造学和海平面更大。

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    Turner Derek Glen;

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