Comparative varve counting and magnetic properties of the 8400-yr sequenceof an annually laminated sediment in Lake Valkiajarvi, Central Finland

Two varve counts made nearly 20 yrs apart and by different authors in the s mall and well sheltered meromictic lake of Valkiajarvi were compared with a view to establishing how similar, or otherwise, these two independent varv e chronologies might be. The results were significant, the difference betwe en the two varve counts being less than 2% for most of the sediment length, even though the average varve thickness was only 0.3 mm. The continuous an d essentially uniform varve record - so far the longest in Finland - covers 8400 yrs and could be applied for accurate dating of palaeoenvironmental i ndicators in the sediment. As an example, we present a varve-dated pollen d iagram for Lake Valkiajarvi. In addition, some magnetic parameters (susceptibility, ARM, SIRM) were meas ured on the sediment sequence to support the stratigraphic division and cor relation of the cores, and to outline the development of the basin since th e last deglaciation. Magnetic variables reflected mainly the variation in m inerogenic material in the sediment, and were therefore related to changes in the catchment. The magnetic parameters also showed a drastic change some 6000 yrs ago, the reason for which, unfortunately, is still not fully unde rstood.