Age Determination in Lake Deposits

Abstract

It is almost impossible to overemphasize the importance of good chronological control to paleolimnology. Age control allows us to determine rates of processes and fluxes of materials, and to test hypotheses of linkage between archives and hypothesized external controls of those archives. Geologists differentiate between relative age versus absolute dating methods. Relative age determinations are based on the concepts of superposition (older sediments are on the bottom, in the absence of tectonic disturbance) and lithological correlation. In contrast, absolute dating methods are done without necessary reference to other analyses or locations, to produce an age determination (i.e., 100,000 yr before present). Some methods, such as paleomagnetics, amino acid racemization, and biostratigraphy, lie in a gray area between these two, providing absolute dates or age ranges in certain circumstances and relative age constraints in others. In this book, I will refer to the general study of both relative and absolute age determination as geochronology, and use the term geochronometry to refer to absolute dating. Lithological correlation involves matching similar lithologies between outcrop or core localities, allowing a network of age relationships to be established between various sites. This can be done at any scale, from within a lake to intercontinental, although lithostratigraphical correlations based on core or outcrop observations are most commonly useful only at a local, intrabasinal level. Correlation within basins is often achieved using reflection seismic stratigraphy. Depositional or unconformity surfaces can normally be recognized on seismic lines that extend over the scale of individual sub-basins to entire lakes (Nelson et al., 1994; Lezzar et al., 1996; Van Rensbergen et al., 1998). When dated cores are obtained or outcrops studied along these seismic lines, a correlation network can be established, with probable ages attached to specific seismic horizons. Intrabasinal correlation can also be done by correlating distinctive patterns of change in features such as magnetic intensity, patterns of stable isotopic change in sediments, or biostratigraphical markers, that may be consistent across a lake basin. Sometimes, relative correlations can be made between lakes.