Paleoenvironmental Significance of Varved Lake Sediments in Fennoscandia and their Contribution to PAGES Related Programs

achieved include the studies of the Greenland ice-cores and the construction of regional dendro-climatological time-series from tree-ring data. Most freshwater lake-sediments in Fennoscandia were deposited during the late Pleistocene and the Holocene. Dating of these sedimentary sequences has, therefore, relied heavily upon the radiocarbon dating method. However, the existence of annually laminated (varved) lake sediments in Finland and Sweden has been known for several decades. A considerable amount of work has been expended on methodological studies, with respect to undisturbed sediment core recovery as well as subsequent analytical techniques. The most common type of varve structure found in Fennoscandia is: (i) a light coloured mineral layer deposited during the spring snowmelt flood; (ii) a brown summer layer consisting predominantly of relatively coarse grained autochthonous organic matter, and (iii) a dark brown (often black) layer of fine grained organic matter that settles out during the winter, when the lakes are ice covered. Occasionally, thin micro-layers of mineral material can be deposited as a consequence of autumn storms (Figure 1). However, other varve types are known, such as those that contain distinct layers composed of diatom frustules or calcium carbonate. Common to all varves is the absence of post-depositional disturbance (primarily in the form of bioturbation). In many cases culturally induced eutrophication during the last few hundred years initiated anoxia and the subsequent preservation of varves. The individual layers that constitute a single varve thus provide physical, chemical and biological proxies of environmental change at a seasonal resolution. Sites located in the near vicinity of research centers and field stations have often been studied with respect to tracing increasing human influence in the form of agriculture and air pollution. Other studies have investigated the relationships between recent meteorological observations and micro-fossil records, for example pollen, spores and diatom frustules. All these studies have used the inherent calendar Paleoenvironmental Significance of Varved Lake Sediments in Fennoscandia and their Contribution to PAGES Related Programs