Century-scale paleoclimatic reconstruction from Moon Lake, a closed-basin lake in the northern Great Plains

Estimates of past lake-water salinity from fossil diatom assemblages were used to infer past climatic conditions at Moon Lake, a climatically sensitive site in the northern Great Plains. A good correspondence between diatom-inferred salinity and historical records of mean annual precipitation minus evapotranspiration (P ET) strongly suggests that the sedimentary record from Moon Lake can be used to reconstruct past climatic conditions. Century-scale analysis of the Holocene diatom record indicates four major hydrological periods: an early Holocene transition from an open freshwater system to a closed saline system by 7300 B.P., which corresponds with a transition from spruce forest to deciduous parkland to prairie and indicates a major shift from wet to dry climate; a mid-Holocene period of high salinity from 7300 to 4700 B.P., indicating low effective moisture (P ET); a transitional period of high salinity from 4700 to 2200 B.P., characterized by poor diatom preservation; and a late Holocene period of variable lower salinity during the past 2,200 yr, indicating fluctuations in effective moisture. Potential changes in water availability in important to infer past changes in general circulation patterns in agricultural areas are a major concern, especially in North America (Harrison and Metcalfe 1985; Harrison drought-prone regions, such as the northern Great Plains, 1989), Europe (Guiot et al. 1993), and Africa (Street and where even minor climatic change might increase the Grove 1976). Within topographically closed lakes, flucfrequency and severity of drought. To measure or predict tuations in the water budget affect both lake level and possible anthropogenic influences on the climate system, lake chemistry. Because diatoms are sensitive to lake we must first understand natural climatic variability. Proxy chemistry, fossil diatom assemblages preserved in lake records of past climatic conditions can provide the long sediments record past hydrochemical conditions. In this time series needed to establish these natural patterns. study, we use quantitative salinity-inference techniques Lakes are intricately linked to the hydrological system based on modern diatom distribution in relation to waterand can be sensitive recorders of climatic change. Patterns chemistry gradients in the northern Great Plains (Fritz ofcontinental-scale lake-level fluctuations have been used 1990; Fritz et al. 199 1, 1993) to reconstruct changes in effective moisture (precipitation minus evapotranspiration, P ET) for a climatically sensitive site, Moon Lake, 1 Present address: PEARL, Dept of Biology, Queen's UniNorth Dakota. Century-scale climatic fluctuations at this versity, Kingston, Ontario K7L 3N6. site are then compared with other paleoclimatic records Acknowledgments from the northern Great Plains to assess broad-scale reWe thank D. R. Engstrom for providing the 2LOPb dating. H. gional trends vs. E. Wright, Jr., D. R. Engstrom, J. Xia, G. A. King, M. Santelmann, and G. L. Jacobson assisted with fieldwork. We also Study area thank B. Cumming for providing assistance with statistical analysis of the data and critical reviews throughout the manuscript The northern Great Plains is a vast region of prairie process. The work is in partial fulfillment of the requirements and steppe in central North America characterized by mward a Ph.D. degree at the University of M~n~eso t a by K. level to rolling topography (Fig. 1). Prior to agricultural R. L. This research was funded in part by NSF grants (ATM 90settlement, the natural vegetation was composed of three 05875-02, ATM 94438 and BSR 8649 6), the NOAA major plant communities: eastern tall-grass prairie, cenPaleoclimatology Program (NA36GP0302) and a fellowship to tral mixed-grass prairie, and western short-grass plains K. Laird under a NSF research training grant in paleorecords (steppe) (Carpenter 1940). Climate is extreme and varito the University of Minnesota (BIR 9014277). able, often with winter temperatures as low as -29OC and Limnological Research Center Contribution 473. summer temperatures as high as 38°C and varying pre-