Environmental influence on relative palaeointensity estimates from Holocene varved lake sediments in Finland

Relative palaeointensity records were reconstructed from two sediment cores obtained from two small lakes, Lake Lehmilampi and Lake Kortejarvi, in eastern Finland. The sediments are annually laminated, and they cover the last 5100 and 3700 years in Lake Lehmilampi and Lake Kortejarvi, respectively. Mineral magnetic investigations indicate uniform magnetic properties with magnetic mineralogy dominated by single and pseudo single domain magnetite and only small variations in the concentration of magnetic minerals, which makes them interesting targets for reconstructing relative palaeointensity. Three estimates of relative palaeointensity were obtained by normalising the natural remanent magnetisation after 30 mT alternating field (AF) demagnetisation (NRM30mT) by (1) low-field magnetic susceptibility, (2) anhysteretic remanent magnetisation after 30 mT AF demagnetisation and (3) saturation isothermal remanent magnetisation imprinted at 1000 mT (SIRM1000mT). Comparison of the palaeointensity estimates with relative mean X-ray density reflecting variations in sediment composition, suggested the normalised records were carrying an environmental imprint, which contaminated especially the centennial features. This interpretation was confirmed by coherence analyses, which also indicated that NRM30mT/SIRM1000mT from Lake Lehmilampi was the most efficiently normalised. Despite the inevitable environmental imprint, relative palaeointensity record NRM30mT/SIRM1000mT from Lake Lehmilampi shows similar millennial features in comparison with relative palaeointensity reconstructions from other lakes in the area, archaeomagnetic records and CALS7K.2 geomagnetic field model. (C) 2011 Elsevier BM. All rights reserved.