Earth and Atmospheric Sciences, Department of


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A DISSERTATION Presented to the Faculty of The Graduate College at the University of Nebraska In Partial Fulfillment of Requirements For the Degree of Doctor of Philosophy, Major: Geosciences (Geology). Under the Supervision of Professor Sherilyn C. Fritz.
Lincoln, Nebraska: May, 2009
Copyright (c) 2009 Jens Schmieder.


A high-resolution multi-proxy analysis was used to reconstruct moisture balance fluctuations spanning the last 6500 years and to investigate the origins of interdunal lakes in the central Sand Hills of Nebraska. Detailed paleoecological data from Beaver Lake, plus analyses from two nearby lakes, were integrated to reconstruct regional climate history. Six episodes of multi-centennial droughts occurred, with a dramatic change in overall mean state between 4000 and 3800 yr BP. An extensive low lake stand between 6500-5750 yr BP was likely predominantly climatically driven rather than geomorphically induced.

Paleohydrological reconstructions of five lakes in the Nebraska Sand Hills were compared in order to 1) determine whether droughts of the past 4000 yrs were spatially and temporally coherent across the region, 2) distinguish local variation in climate from regional patterns, 3) compare the paleolimnological results with the existing dune records, and 4) assess the frequency of variation among the sites. Results indicate frequent alterations between high and low lake-levels during the past 4000 yr. Extended droughts were more common prior to 2000 yr BP, while the last two millennia were hydrologically more complex, and climate shifts alternated on shorter timescales. This record refines the existing Holocene drought history of the Nebraska Sand Hills and aids in our understanding of how drought episodes can vary in terms of magnitude, spatial extent, and temporal scale.

Diatom diversity trends were analyzed for a suite of Nebraska lakes to evaluate the sensitivity of diatom communities to various environmental gradients. Diatom diversity of natural lakes showed the strongest correlation with total nitrogen, turbidity, and conductivity, whereas in reservoir lakes and sand pits, diversity was predominantly driven by phosphorus, nitrate-nitrogen species, turbidity, and pH. Differences in diversity trends are primarily a manifestation of nutrient and light availability, which in turn, is dependent on the physical and geographic properties that characterize each lake system.