THE ROLE OF MAGNETOSOMES IN THE SEDIMENTS OF LAKE ELY, PA: THEIR PRODUCTION AND DESTRUCTION

Lake Ely, a small, post-glacial lake located in northeastern Pennsylvania, is unique in having a strong sedimentary magnetization dominated by the magnetosomes produced by magnetotactic bacteria. Measurements of Fe, S, and O₂ at different depths in the lake's water column, as well as the magnetization of material filtered from the water indicate that magnetotactic bacteria live in the water column at the oxic-anoxic interface that occurs at depths of about 15 m. Magnetic measurements and TEM observations of lake sediments and material caught in a sediment trap show that magnetosomes are the major constituent of the magnetic minerals in the lake sediments. Variations in the saturation isothermal remanent magnetization (SIRM) are observed to correlate with the historic rainfall record over the past 60 years suggesting the potential of the magnetosomes providing a useful paleoclimate record. However, in the pre-settlement sediments the magnetization is observed to decrease in intensity by an order of magnitude at depths between 30 and 70 cm in the sediment column. The ratio of anhysteretic remanent magnetization to SIRM, a measure of magnetic particle grain-size, is observed to drop significantly at depths between 60 and 70 cm, indicating a coarsening of magnetic grain size. These data indicate that sulfur reduction diagenesis is dissolving the magnetite magnetosomes in the lake sediments. Thus, Lake Ely provides not only a laboratory for understanding the environmental controls on the production of magnetosomes, but a detailed record of the factors causing their dissolution due to reduction diagenesis. The down-core variation of susceptibility (χ) and the ratio SIRM/χ show “spikey” behavior in the zone of magnetization decrease indicating that the magnetic products of reduction diagenesis appear to be heterogeneously distributed in the sediments. Detailed forward modeling of isothermal remanent magnetization (IRM) acquisition curves indicates that as magnetosomes dissolve, detrital magnetite and hematite particles become relatively more important, magnetically, in the sediments. Modeling of mineral magnetic measurements will allow a detailed magnetic record of the dissolution of magnetosomes and the secondary magnetic minerals produced by reduction diagenesis in freshwater lake sediments.