Paleolimnology and paleohydrology

Although lake sediments and their contained fossils have been a source of paleo-environmental evidence since the 1940s (e.g. Wright 1966; Digerfeldt 1972), it was not until the late 1980s that the full potential of the paleolimnologic record began to be exploited. Paleolimnology is primarily concerned with the reconstruction of lake history from the sediments accumulating in the lake and from the fossils (e.g. diatoms, chironomids, cladocerans, ostracods) preserved in the sediments (Battarbee 2000; Smol 2002). Many advances have been made in paleolimno-logy in the past 25 years, including inorganic and organic sediment geochemistry, sediment paleomagnetism, stable isotope (13C, 15N, 18O, D) analyses of sediments and their contained fossils, and the use of transfer functions to derive quantitative reconstructions of past lake conditions (e.g. salinity, pH, lake-water temperature) (Smol 2002). Several of these advances were made in direct response to the need to develop a quantitative paleolimnology to address critical environmental issues associated with "acid-rain" research in north-west Europe and eastern North America.

In terms of Holocene paleoclimate research (Verschuren and Charman, this volume), major contributions have come from paleolimnologic studies in low-latitude areas in Africa, South America, and south-east Asia and in the Great Plains of North America. These studies, involving a wide range of paleoenviron-mental proxies such as diatoms, ostracods, stable isotopes, and geochemical ratios (e.g. Sr/Ca, Mg/Ca of ostracod carapaces), have provided unique evidence for major changes in lake-levels, salinity, and hydrology (Oldfield 2005). Many studies indicate a high degree of hydrologic variability, but a general tendency towards high lake-levels and wetter conditions during the early Holocene, followed by varying degrees of desiccation in the mid- and late Holocene (Oldfield 2005). Paleolimnologic evidence from northern and central Africa, for example, indicates extensive lakes and generally high lake-levels until about 6000 years ago (Gasse 2000), followed by irregular, often pulsed declines in lake-levels (Oldfield 2005).

Fine-resolution paleolimnologic studies also suggest high-frequency changes superimposed on the overall long-term changes at the orbital time-scales of COHMAP (Wright et al. 1993). For example, drought cycles detected by detailed paleolimnologic studies (e.g. Laird et al. 1998a,b) in the Great Plains of North America may be a complex climatic response to millennial-scale cooling events in the North Atlantic region and solar variation at several temporal scales (Fritz 2003).

Besides providing evidence of extremely low lake-levels, droughts, and drought-cycles (e.g. Digerfeldt 1988; Digerfeldt et al. 1992; Battarbee 2000; Fritz 2003), lake sediments can also provide records of other extreme events such as floods and debris flows (e.g. Nesje et al. 2001; Sletten et al. 2003; B0e et al. 2006). Detailed peat-stratigraphic studies involving fine-resolution analyses of plant macrofossils, testate amoebae, and quantitative reconstructions of bog moisture (e.g. Barber and Charman 2003; Booth and Jackson 2003; Booth et al. 2005 2006; Charman et al. 2006; Hughes et al. 2006; Verschuren and Charman, this volume) also provide valuable information about hydrologic changes, especially in the late Holocene.

The paradigm of Holocene climate history that has emerged, and is continuing to emerge, from detailed paleolimnologic and peat-stratigraphic studies is that there have been major changes in regional and local hydrology at a wide range of temporal scales, especially in low-latitude regions (Oldfield 2005). In addition, there appears to be considerable spatial variation in changes in Holocene hydro-logic patterns (Verschuren and Charman, this volume). There have also been a range of extreme events identified from lake sediments, as well as evidence for Holocene temperature changes in high-latitude areas (Oldfield 2005).

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