ABSTRACT
Annually laminated or varved sediments are deposited and preserved under certain environmental conditions in lakes and in the ocean. Clastic varves are formed in proglacial and periglacial lakes in arctic and alpine regions, whereas organic varves dominate in mesotrophic to eutrophic lakes in midlatitudes and in the tropics. Evaporitic varves are deposited in saline lakes and along shallow coasts in arid regions. However, many transitional forms exist between these three varve types. In the marine environment, organic and clastic varves can be formed in oxygen minimum zones and in areas with high sedimentation rates. After detailed investigation of fabric and composition of varved sediments using thin sections and scanning electron microscopy as well as identification of the processes that cause cyclic deposition, varve counts can be transformed into ‘absolute’ or ‘floating’ chronologies. Counting errors can be quantified by multiple counts of two or more investigators on several cores from the same site, while systematic chronological errors can only be determined by comparison with chronologies from other records or with other independent dating methods. Future developments of varve studies are directed towards image analyses of scanned thin sections, radiographs and polished sediment surfaces, all of which provide fast, repeatable, non-destructive, objective and high-resolution measurements making varve chronologies even more reliable.
