Cladocerean and diatom stratigraphy of calcareous lake sediments from Kuusamo, NE Finland. Indications of Holocene lake-level changes

Cores of Holocene calcareous sediments (marls) from two small lakes (Putaanlampi and Ylimmäinen Kuivajärvi) in Kuusamo, NE Finland, were analysed for pollen, Cladocera, and diatoms. The cladoceran planktonic / littoral ratio shows consistent trends through the Holocene, with parallel changes at both sites, suggesting regional changes in water balance and lake levels. The diatoms were extensively dissolved, but the down-core occurrence of remains of several mesohalobous taxa absent from the younger sediments indicates that the lakes must have undergone stages of elevated eletrolyte concentration and, by inference, enhanced evaporation earlier in their history. The results are interpreted as indicating relatively dry conditions between 7000 and 5000 BP, with maximal dryness occurring around 6000 BP. The mid-Holocene dry interval is preceded and succeeded by periods of more humid conditions and higher lake levels. This pattern of Holocene humidity changes in NE Finland is closely comparable with the patterns previously suggested for Finnish Lapland and for south-central Sweden, but deviates from the pattern known from the southern parts of Fennoscandia