Bacterial magnetite in lake sediments: late glacial to Holocene climate and sedimentary changes in northern Norway

Magnetic properties of sediments retrieved from a small lake in northern Norway spanning the last ∼14 500 cal. years have been analyzed in detail. The high-resolution core, with its ability to detect sub-centurial variability, records the rapid retreat of the Fennoscandian Ice Sheet (FIS) in northern Lyngen, and subsequently, the establishment of a stable Allerød to Younger Dryas watershed. From early Holocene and to the present magnetotactic bacteria (MB) producing stable single-domain (SSB) magnetite become the dominating magnetic carrier. The small, low relief catchment area, with its quartzite bedrock, scattered till and vegetated surface, generates a negligible contribution of detrital magnetic minerals. This allows for the opportunity to follow the presence and changes of the magnetosomes during the last 10 000 years. A strong positive statistical relationship (r2=0.83) between the anhysteretic remanent magnetisation susceptibility (χARM) and loss-on-ignition (LOI%) suggests an environmental ‘control’ over the bacteria magnitude [Quart. Int. 88 (2002) 13] that most likely is related to summer climate. A comparison between the magnetic mineralogy, as evident from the S-ratio (σIRM100mT/σSIRM3T), and the reconstructed summer temperatures from a nearby lake shows a close covariation between cooler summers and ‘harder’ S-ratios for a series of climatic events. Intervariability of LOI data and other proxies such as August sea-surface temperatures (SST) from the Nordic Sea, suggests land–ocean connections during converging phases of cooler climate, where the most prominent ‘in-phase’ periods are approximately (1) 6500–5000, (2) 3500–2400, and (3) 1700–1000 cal. years BP. The most significant change in the Holocene magnetic signal is initiated at ∼3700 cal. years BP, which we refer to here as ‘the Holocene Turnover’.