Paleolimnological records of climatic change in North America

Lacustrine fossil records provide long time series of data on limnological and climatic conditions; these data are useful for establishing natural patterns of climate variability and for generating testable hypotheses about atmospheric circulation and climate-ecosystem linkages. Shoreline features can indicate past lake-level fluctuations that may reflect changes in moisture balance, but often these records are discontinuous and are evidence of only extreme conditions. The organisms, geochemistry, and sedimentology of lake sediments may provide a more continuous sequence of direct and indirect lake-climate interactions in the past. The most clearly interpretable paleolirnnological records of climatic change are those that use several lines of evidence to corroborate a climatic hypothesis and are from sites near an ecotone or in regions of extreme climate. In all cases, hydrologic setting mediates a lake's response to climate and must be considered in interpreting sedimentary sequences. Discussion of the impact of recent and future climatic changes necessitates a baseline against which the magnitude of observed or predicted changes can be measured. Commonly we look to recent history as a reference point for comparison; however, the landscape of recent decades, or in some cases even centuries, has been altered greatly by human activities. Thus, to determine whether recent or future climatic change is unique or unusual, we need long natural (preanthropogenic) time series against which observed or predicted climatic states and variability can be compared. Paleolimnological archives are a tool for looking at environmental patterns prior to anthropogenic impact and show a range of climatic states far greater than those documented by written records. Former strand lines of pluvial lakes Bonneville and Lahontan in the western U.S., for example, testify to moisture extremes unsurpassed in recent centuries or even millennia. In addition, one can us: the paleolimnological record as an independent climatic proxy against which patterns and rates of terrestrial response to climatic change can be compared. Here, I present a broad overview of paleoclimatic reconstruction in North America from lacustrine records, with a focus on the late Quaternary. The review is limited to climatic inference from paleolirnnological records, a term I use in a restrictive sense to refer to records of past lake states (physical, biological, and geochemical), as opposed to records of past stream hydrology (e.g. Ely et al. 1993; Knox 1993) or terrestrial conditions. I summarize paleolimnological tools available for reconstruction of past climate, the capabilities and limitations of these tools, Acknowledgments I thank Dan Engstrom for many fruitful discussions in recent years on lakes and climate and H. E. Wright, Jr., for conlments and some major conceptual issues that arise in studying paleolimnological records. Paleolimnological records Climatic influences on lakes are extremely complex and include a diversity of direct and indirect effects. Atmospheric temperature and wind directly affect water temperature and stratification, moisture availability (precipitation minus evaporation, PE) is a primary control on lake level, and temperature and light directly affect organismal growth rates. In turn, stratification patterns and lake levels exert a major influence on lake-water chemistry, which affects growth and reproduction of organisms. In addition, both simple and complex impacts of climate on terrestrial ecosystems and on surface and subsurface hydrology affect a lake's hydrochemistry and thus the structure and function of aquatic communities. Because of the complexity of these influences on the geochemical precipitates and biological fossils in lake sediments, a unique climatic solution from the paleolimnological record may be difficult to obtain. However, this complexity can be addressed through careful site selection and analysis of multiple sedimentary proxies, enabling paleolimnological records to be robust and clearly interpretable archives of past climate and climatic change. Lake-level change: Direct linkage of lakes andclimateRecords of changing lake levels are probably the clearest evidence for climatic change. In the western U.S., former shoreline features (strandlines, stromatolites, littoral sediments) document rises and falls in the levels of large lakes that covered vast areas of the Great Basin during the late Pleistocene (Benson et al. 1990). Sensitivity analysis of coupled climatic and hydrologic models can constrain the climatic conditions that drove the tremendous lake-level variations (Hostetler and Benson 1990) and on the manuscript. point to potential lake'-climate feedback (~ostetler 'et al. This work was supported by grants from NSF (ATM 901994). Former deltas and other sedimentological features 05875-02) and the NOAA Paleoclimatology Program (NA36GP0302). of influent streams also can be used to infer fluctuations