Background
The authors write that "lacustrine sediment magnetic assemblages respond sensitively to environmental changes," and that "characteristics of magnetic minerals, i.e. their concentration, mineralogy and grain size in sediments, can be studied by making mineral magnetic measurements, which yield large quantities of environmental data rapidly and non-destructively (e.g. Evans and Heller, 2003)."

What was done
Working with two sediment cores taken from Finland's Lake Lehmilampi (63°37'N, 29°06'E), Haltia-Hovi et al. constructed detailed chronological histories of several magnetic properties of the sediments, as well as a history their total organic carbon content.

What was learned
Based on their analyses, the four researchers discovered a "conspicuous occurrence of fine magnetic particles and high organic concentration" that was evident around 4,700-4,300 Cal. yrs BP, which time interval, in their words, "is broadly coincident with glacier contraction and treelines higher than present [italics added] in the Scandinavian mountains according to Denton and Karlen (1973) and Karlen and Kuylenstierna (1996)." And they report that from that time on towards the present, 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.

What it means
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.

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.

Evans, M.E. and Heller, F. 2003. Environmental Magnetism: Principles and Applications of Enviromagnetics. Academic Press, Boston, Massachusetts, USA, 299p.

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.