Item

Abstract

Standard mineral magnetic measurements, including magnetic susceptibility, ARM and IRM, were carried out on two Holocene sediment cores from Lake Lehmilampi (Finland), which span the last ~9,700 years. Paleoenvironmental changes were inferred from stratigraphic variations in the sediment mineral magnetic properties. Total organic carbon (TOC) measurements supplement the bulk mineral magnetic data. Development of Lake Lehmilampi is closely connected with Lake Pielinen, from which Lake Lehmilampi was gradually isolated ~5,066 Cal. years BP. Since the isolation of Lake Lehmilampi, its sediment has been annually laminated, with clastic-organic varves in the deep basin. Magnetic mineralogy is dominated by ferrimagnetic minerals, interpreted as magnetite throughout the sediment column, with a variable, but minor contribution of antiferromagnetic minerals. Sediment magnetic properties indicate a series of changes in sediment composition during the Holocene, which correlate well with sediment lithology and TOC. Magnetic properties differ in pre- and post-isolation sediments with respect to concentration of fine-grained magnetite, as expressed by χARM and χARM/SIRM, showing variation in response to organic matter content in post-isolation varved sediments. This may reflect authigenic bacterial production of magnetite in the weakly anoxic environment. Preliminary data suggest that magnetosome production in Lake Lehmilampi is controlled by lake productivity, and ultimately by climate.