DIATOM EVIDENCE OF HYDROCLIMATE VARIABILITY IN THE MIDWESTERN UNITED STATES SINCE 2150 CAL YR BP

The Midwest region of the United States is an important agricultural center and home to more than 70 million people. The region is vulnerable to both droughts and flooding, which are likely to become more common in the future with a changing climate. Understanding the patterns and drivers of hydroclimate variability in the region in the recent past can inform forecasting and preparation for future hydroclimate variability. Lake sediments provide an opportunity to reconstruct past hydroclimate variability at high temporal resolution using a variety of physical, geochemical, and biological sedimentary proxies. Here we present a 206-cm-long diatom record from Clear Lake, IA, (43.13° N, 93.37° W) that spans the last 2150 cal yr BP. Diatom assemblages were analyzed throughout the core in continuous intervals at approximately decadal resolution. Diatom assemblages overall were dominated (36–86%) by small, tychoplanktonic fragilarioid taxa, e.g. Staurasirella pinnata, Staurosira construens, Pseudostaurosira brevistriata. The dominance of small tychoplanktonic taxa is likely related to Clear Lake’s relatively shallow maximum depth (modern z_max = 5.9 m). Planktonic Stephanodiscus minutulus and Asterionella formosa are found at low abundances throughout the core, which suggests the lake rarely stratified and was not substantially deeper than the modern setting through the late Holocene. The top 36 cm are characterized by an increase in meroplanktonic Aulacoseira ambigua and A. granulata, which co-dominate the assemblages (15-46%). Three distinct peaks in Aulacoseira abundance are centered at 72, 55, and 26 cm. Aulacoseira is indicative of abundant nutrients, particularly silica, and usually associated with water column mixing to supply nutrients. However, the increased relative abundance of meroplanktonic taxa for some intervals may indicate higher lake levels have occurred, periodically, in the last millennium.