Geochemical evidence for the origin of late Quaternary loess, Seward Peninsula, Alaska

Loess is the most widespread surficial deposit in the state of Alaska. Although loess of last glacial age is common in mid-continental North America, records of last glacial loess in Alaska have been elusive. Here we report a record of last glacial loess on the Seward Peninsula, along with studies of a transect of loess localities also thought to date to the last glacial period. Highly diverse hypotheses on the origin of loess on the Seward Peninsula include local volcanic rocks and mountain ranges to the north and south, as well as offshore deposits. We present particle size and mineralogical and geochemical data that indicate that the most likely sources for loess in this part of eastern Beringia are glaciogenic silt particles of the Kobuk River and Noatak River, both of which drain the Brooks Range to the northeast. Northeasterly paleowinds were therefore responsible for loess on the Seward Peninsula. Such winds probably occurred during late summer or early autumn, when river discharge was at a minimum but temperatures were still above freezing. Northeasterly winds during the last glacial period are in good agreement with a recent paleoclimate model.黄土是阿拉斯加州分布最广泛的地表沉积物。尽管末次冰期黄土在北美大陆中部广为分布，但阿拉斯加地区的末次冰期黄土记录却一直较为匮乏。本研究报道了苏厄德半岛（Seward Peninsula）的末次冰期黄土记录，并对沿剖面分布的、同样被认为形成于末次冰期的多个黄土测点开展了研究。关于苏厄德半岛黄土的成因，目前已提出诸多不同假说，其潜在物源包括南北两侧的本地火山岩与山脉，以及近海沉积物。本研究提供的粒度、矿物学与地球化学数据表明，白令海东部该区域黄土的最可能物源为科伯克河（Kobuk River）与诺阿塔克河（Noatak River）的冰成因粉砂颗粒，两条河流均发源于东北部的布鲁克斯山脉（Brooks Range）。因此，东北向古风是苏厄德半岛黄土形成的动力条件。这类古风大概率出现于夏末或初秋时段，此时河流径流量降至最低，但气温仍高于冰点。末次冰期的东北向古风与最新的古气候模型结果高度吻合。