EAGER - feasibility study in reconstructing Holocene drought history from sediment cores in Fallen Leaf Lake, CA

美国西部的研究表明，过去10-13000年(全新世)干旱模式的深刻、长期变化对生态系统和人类产生了历史性和史前影响。这笔赠款将根据内华达山脉北部的沉积物岩芯建立一个长期的干旱记录，这是一个气候敏感的地区。该项目将侧重于分水岭的高山部分，在那里，冬季降水不足的影响比末端湖盆更直接地可测量。这些影响包括干旱期间湖泊水位降低、湖泊温度变暖、热层结强度和深度的变化以及生物结构的改变。特别是，湖泊沉积物中的硅藻化石群落预计会随着干旱的反应而变化，并可能被用作气候变化和长期干旱的指标。太浩湖分水岭落叶湖的沉积物中可能存在极好的硅藻记录，可用作古气候指标，预计分辨率可达数十年至百年。2010年拍摄的岩心初步年龄模型显示，开发详细的年代学很有希望。这些岩心包含了追溯到13300年前的连续记录，平均沉积速率为1.4毫米/年。该团队将测试亚高山湖泊沉积物中的硅藻是否可以作为冬季降水变化的指标，以及硅藻、沉积学和选定的地球化学指标的变化是否与中世纪气候异常和其他全新世长期干旱期间的干旱条件有关。这个湖是一个很好的选择，因为它记录了由降水驱动的大湖水位变化，并且该地区的水平衡和对干旱的水文反应是可以测量的。初步数据表明，与数百年干旱相关的硅藻物种数量发生了变化。该研究小组将通过观察离散样品的有机地球化学(有机质和碳、氮同位素的百分比)、复合特定地球化学和花粉分析来研究硅藻对干旱条件的反应。此外，该团队还将进行高分辨率(0.2-0.5 mm)的生物成因二氧化硅和有机质含量的X射线荧光扫描。所有核心代理数据将使用各种多变量和交叉光谱技术进行交叉关联。区域树木年轮气候记录将被用作验证水文变异性的独立手段。拟议的研究具有更大的社会影响，这些数据应该会引起水生态学家、自然资源管理者和关注淡水水系对气候变化的长期响应的气候学家的兴趣。长期干旱的时间和影响也将引起人类学家的兴趣，他们正在重建美洲原住民的迁徙，以应对更干旱的条件，包括北派尤特人、迈杜人、瓦肖人、约库特人和西莫诺人的祖先。此外，更好地将硅藻确立为气候变化指标的努力可能会增加用于解释区域气候变化及其对人类活动的相关影响的工具的数量。该项目将提供建立高质量年龄模型所需的年龄和扫描XRF分析，并为这套全新世岩心建立基本的沉积学框架。这个框架是所有正在进行的古生物学和地球化学工作的必要的第一步，这些工作正在由不同的研究生和教职员工与这些核心一起进行。它还将提供各种专门指标(硅藻、有机地球化学、花粉分析)所需的离散样品和硅藻工作所需的分析。目前，有5名研究生依靠这些核心进行论文和学位论文研究，并将从额外的年龄控制、地球化学数据和样本中受益匪浅。岩芯和数据将在明尼苏达州的国家湖泊核心储存库存档。

Work in the western US indicates that profound, long-term shifts in patterns of drought throughout the past 10-13,000 years (the Holocene) have had both historic and prehistoric impact on ecosystems and on humans. This grant will develop a long-term drought record from sediment cores in the northern Sierra Nevada, a climatically sensitive region. The project will focus on the alpine portion of the watershed where the effects of winter precipitation deficits are more directly measurable than in terminal lake basins. These effects include lowered lake levels during drought stages, warmer lake temperatures, variations in strength and depth of thermal stratification, and altered biotic structure. In particular, fossil diatom communities in lake sediments are expected to vary in response to drought, and may be used as a indicators of climate change and long-lasting droughts. An excellent diatom record potentially exists in sediments of Fallen Leaf Lake in the Lake Tahoe watershed for use as paleoclimate indicators, with expected multi-decadal to centennial resolution. Preliminary age models on cores taken in 2010 show great promise for developing a detailed chronology. The cores contain a continuous record extending back 13,300 yrs, with sedimentation rates averaging 1.4 mm/year. The team will test whether diatoms in sub-alpine lake sediment can serve as indicators of changes in winter precipitation, and whether changes in diatom, sedimentologic, and selected geochemical indicators can be related to drought conditions during the Medieval Climatic Anomaly and other Holocene long-term droughts. The lake is a good choice because it records large precipitation-driven lake level shifts, and the area's water balance and hydrologic response to drought is measurable. Preliminary data indicates shifts in the number of diatom species associated with a well documented multi-hundred year drought. The team will investigate diatom response to drought conditions by looking at organic geochemistry from discrete samples (% organic matter and C, N isotopes), compound specific geochemistry, and pollen analysis. In addition, the team will conduct X-ray fluorescence scans for biogenic silica and organic matter content at high resolution (0.2-0.5mm). All core proxy data will be cross-correlated using a variety of multivariate and cross spectral techniques. Regional tree-ring climatic records will be used as an independent means of validating hydrological variability.The proposed research has larger impacts to society, and this data should be of interest to aquatic ecologists, natural resource managers, and climate scientists concerned with the long-term response of fresh-water aquatic systems to climatic change. The timing and effects of prolonged drought will also be of interest to anthropologists reconstructing the migrations of Native Americans in response to drier conditions, including the ancestors of the Northern Paiute, the Maidu, the Washoe, the Yokuts, and the West Mono. Further, this effort to better establish diatoms as indicators of climate variation could add to the number of tools used to interpret regional climate change, and its associated impacts on human activities. This project will provide ages and scanning XRF analyses needed to build a quality age model, and establish the basic sedimentologic framework for this set of Holocene cores. This framework is a necessary first step for all of the ongoing paleontologic and geochemical work that is underway with these cores by various graduate students and faculty members. It will also provide the discrete samples needed for the various specialized indicators (diatoms, organic geochemistry, pollen analysis) and analyses necessary for the diatom work. At present, 5 graduate students rely on these cores for their thesis and dissertation research, and will benefit greatly by additional age control, geochemical data, and samples. Cores and data will be archived at the National Lacustrine Core Repository, Minnesota.