Description
Based on analyses of a number of different magnetic measurements of ferromagnetic minerals contained in two sediment cores that were extracted from Finland's Lake Lehmilampi (63°37'N, 29°06'E), Haltia-Hovi et al. discovered that a "conspicuous occurrence of fine magnetic particles and high organic concentration" occurred around 4,700-4,300 Cal. yrs BP -- which they say "is broadly coincident with glacier contraction and treelines higher than present in the Scandinavian mountains according to Denton and Karlen (1973) and Karlen and Kuylenstierna (1996)" -- after which they report there was a "decreasing trend of magnetic concentration, except for the slight localized enhancement in the upper part of the sediment column at ~1,100-900 Cal. yrs BP," where the year zero BP = AD 1950.

Changes of these types in prior studies have been attributed to magnetotactic bacteria (e.g. Magnetospirillum spp.), which Haltia-Hovi et al. describe as "aquatic organisms that produce internal, small magnetite or greigite grains" that are used "to navigate along the geomagnetic field lines in search of micro or anaerobic conditions in the lake bottom," as described by Blakemore (1982) and Bazylinski and Williams (2007). And they further state that the studies of Snowball (1994), Kim et al. (2005) and Paasche et al. (2004) "showed magnetic concentration enhancement, pointing to greater metabolic activity of these aquatic organisms in the presence of abundant organic matter," which is also what Haltia-Hovi et al. found in their study, where they report that the "concentration of organic matter in the sediment is highest, together with fine magnetic grain sizes, in the time period 1,100-900 Cal. years BP," which time interval they say "is associated with warmer temperatures during the Medieval Climate Anomaly according to the varve parameters of Lake Lehmilampi," citing the precise core-dating by varve-counting work of Haltia-Hovi et al. (2007). And all of these observations, taken together, strongly suggest that the peak warmth of the Medieval Warm Period (about AD 850-1050) was very likely somewhat greater than that of the Current Warm Period.

Other References
Bazylinski, D.A. and Williams, T.J. 2007. Ecophysiology of magnetotactic bacteria. In: Schuler, D. (Ed.) Magnetoreception and Magnetosomes in Bacteria. Springer, Berlin, Germany, pp. 37-75.

Blakemore, R.P. 1982. Magnetotactic bacteria. Annual Review of Microbiology 36: 217-238.

Denton, G.H. and Karlen, W. 1973. Holocene climatic variations -- their pattern and possible cause. Quaternary Research 3: 155-205.

Haltia-Hovi, E., Saarinen, T. and Kukkonen, M. 2007. A 2000-year record of solar forcing on varved lake sediment in eastern Finland. Quaternary Science Reviews 26: 678-689.

Karlen, W. and Kuylenstierna, J. 1996. On solar forcing of Holocene climate: evidence from Scandinavia. The Holocene 6: 359-365.

Kim, B., Kodama, K. and Moeller, R. 2005. Bacterial magnetite produced in water column dominates lake sediment mineral magnetism: Lake Ely, USA. Geophysical Journal International 163: 26-37.

Paasche, O., Lovlie, R., Dahl, S.O., Bakke, J. and Nesje, E. 2004. Bacterial magnetite in lake sediments: late glacial to Holocene climate and sedimentary changes in northern Norway. Earth and Planetary Science Letters 223: 319-333.

Snowball, I. 1994. Bacterial magnetite and the magnetic properties of sediments in a Swedish lake. Earth and Planetary Science Letters 126: 129-142.