Palaeoenvironmental interpretations and reconstructions based on varved sediments began early in the history of varve-sediment research, and focused on using the interannual variations in the varve laminae thickness as a monitor of past environmental change (Anderson, 1961; Saarnisto, 1979; Renberg and Segerstrom, 1981; Perkins and Sims, 1983; Leonard, 1986; 0strem and Olsen, 1987; Cromack, 1991; Lotter, 1991; Deslodges, 1994). Variations in varve/laminae thickness have since been related to other environmental variables, such as temperature, precipitation, wind stress, coastal upwelling, and glacier activity/run-off. Palaeoenvironmental reconstructions have been derived from the coupling of varve chronologies with data from other evidence (pollen, diatoms, plankton, palaeomagnetic variations and geochemical changes).

The application of annually laminated (varved) sediments in palaeoenvironmental research has expanded with the growing need for millennia-long records of interannual to century-scale climate variability. In cases where the annual nature of the varves can be confirmed by radiometric and other methods, the laminae/varves provide annual resolution records that can be coupled with a wide range of palaeoenvironmental proxies to provide time series with accurate seasonal- to centennial-scale sample resolution. Proper interpretations require that the climate response time of the signal is taken into account.

Palaeoclimatic research has increasingly relied on the use of varved (annually laminated) lake and marine sediments to provide high-resolution chronologies for climate reconstructions.

Sites with laminated/varved sediments have complemented those with other sources of palaeoclimatic information, such as tree-rings, corals, ice cores and historical documents (Bradley and Jones, 1993). Where other annually dated records of palaeoenvironmental variability exist, varved sediments can provide useful complementary or longer time series. Although varves by definition are annual in nature, there will commonly be some significant counting error associated with varve-based chronologies. In addition, climate interpretations based on varved sediments are also subject to some controversy. Defining a clear, unambiguous palaeoenvironmental signal recorded in the sediments may be difficult. Of similar importance is the need to establish the climatic response time of the recorded signal.

Although varve-based palaeoenvironmental reconstructions are subject to some inherent uncertainties, they may ultimately provide some of the longest and most useful records of past annual to decadal-scale climatic variability. In contrast to most proxy time-series of annual/decadal climatic variability, varved sediments span millennia. Laminated/varved sediments therefore provide a key resource for unravelling the range of climatic variability, and for understanding how this past variability may be effected by climatic forcing unlike that of the present.

0 0

Post a comment