Item

Abstract

Holocene palaeomagnetic secular variation (PSV) in inclination and declination recorded in the sediment remanent magnetization of two small lakes, Lake Lehmilampi (63°37'N, 29°06'E) and Lake Kortejärvi (63°37'N, 28°56'E) in eastern Finland is presented. As an outcome of systematic coring, eight cores, 300–753 cm in length, were investigated. All samples (Lehmilampi n= 1320, Kortejärvi n= 943) were subjected to palaeo- and mineral magnetic analyses. The directions of the characteristic remanent magnetization (ChRM) were obtained from progressive alternating field demagnetization of the natural remanent magnetization (NRM) followed by principle component analysis. The younger sections of the sediment columns in the studied lakes are annually laminated, providing detailed chronologies for dating PSV features back to 5100 cal. BP. The underlying older sections of the cores were dated by palaeomagnetic pattern matching in respect to varve-dated Lake Nautajärvi PSV data (Ojala & Saarinen 2002), thus yielding a composite age model covering nearly the whole Holocene epoch. Average sedimentation rates ranging from ∼0.68 to 0.74 mm yr−1 enabled recording of changes in the geomagnetic field at decadal resolution. The carriers of remanence are dominantly magnetite of stable single-domain to pseudo-single-domain grain size, accompanied by magnetic minerals of harder coercivity. The sediments from both lakes exhibit strong and stable single-component magnetizations nearly throughout the whole cores. The sediment magnetization lock-in delay is estimated to range between 80 and 100 yr. PSV data were transformed into time-series and subsequently stacked to comprise North Karelian stack, and Fisher statistics were used to calculate mean directions together with the 95 per cent confidence level (α95). A comparison of declination and inclination features of the North Karelian stack with previously published data expresses remarkable similarity, therefore confirming the similar source behind the changes in the NRM directional records. The high quality of the PSV data extracted from Lehmilampi and Kortejärvi, in terms of dating as well as amplitude reconstruction, have a high potential to improve existing and future geomagnetic field models.