Collaborative Research: Resolving the Enigma of Late Quaternary Loess on the Great Plains

黄土本质上是大气粉尘的堆积，厚厚的黄土沉积物提供了粉尘运输和沉积的长期记录。这一记录在过去、现在和未来的环境变化研究中越来越重要。沙尘生产受气候变化的强烈影响，但对气候也有反馈效应。在过去的25000年里，大平原中部的黄土记录了异常快速的沙尘沉积，大大超过了世界其他地方主要沙漠沙尘源顺风处的沉积速率。这些高速率的大平原黄土堆积挑战了我们目前对主要天然沙尘源及其过去活动的理解。这个项目将检验两个可能有助于解释大平原黄土快速沉积的假设。一种假说认为，当先前在中原地区积累的河流和湖泊沉积物形成大型沙丘场时，就发生了快速的黄土沉积。如果这些沉积物在过去的25000年里被风重新加工过，它们就会被分类为沙丘沙和细粒尘埃。这一假设将通过使用光学测年来比较沙丘场形成和快速尘埃沉积的时间，并通过比较黄土的成分与提出的源物质来验证。第二种假说认为，大平原上的粉尘沉积主要集中在源区附近，导致了非常高的沉积速率，其程度远高于其他干旱地区。如此高的近源沉积率可能是由于异常粗糙的粉尘以及粉尘源和沉降点之间非常稳定的边界造成的。本文将通过比较大平原和黄土高原黄土沉降速率和粒径的顺风趋势来验证这一假设。该项目的这一部分将加强在大平原和中国工作的研究人员之间的思想和技术交流，这两个地区都有厚厚的、广泛的黄土沉积，提供了长期环境变化的宝贵记录。

Loess is essentially an accumulation of atmospheric dust, and thick loess deposits provide a long-term record of dust transport and deposition. This record is of growing importance in research on past, present, and future environmental change. Dust production is strongly influenced by climatic change, but also has feedback effects on climate. Loess in the central Great Plains records episodes of extraordinarily rapid dust deposition over the past 25,000 years, greatly exceeding deposition rates downwind of major desert dust sources elsewhere in the world. These high rates of Great Plains loess accumulation challenge our current understanding of major natural dust sources and their past activity. This project will test two hypotheses that may help explain rapid loess deposition on the Great Plains. One hypothesis states that episodes of rapid loess deposition occurred when large dune fields formed from stream and lake sediments that had previously accumulated in the central Plains. If those deposits were reworked by the wind during the past 25,000 years, they would have been sorted into dune sand and fine-grained dust. This hypothesis will be tested by using optical dating to compare the timing of dune field formation and rapid dust deposition, and by comparing the composition of the loess to the proposed source materials. The second hypothesis proposes that dust deposition in the Great Plains was concentrated near source areas, leading to very high deposition rates, to a much greater degree than in other dry regions. Such high near-source deposition rates would result from unusually coarse dust and a very stable boundary between dust sources and sinks. This hypothesis will be tested by comparing downwind trends of loess deposition rate and particle size in the Great Plains and the Loess Plateau of northern China. This part of the project will enhance interchange of ideas and techniques between researchers working in the Great Plains and China, both regions with thick, extensive loess deposits that provide valuable records of long-term environmental change.