HR: 17:45h
AN: B24A-08 INVITED     [Abstracts]
TI: How Much Magnetite Have Magnetic Bacteria Made? A Multiple Case Study of Swedish Varved Lake Sediments
AU: * Snowball, I
EM: ian.snowball@geol.lu.se
AF: Geobiosphere Science Centre, Lund University Solvegatan 12, Lund, 226 62 Sweden
AU: Zill‚n, L
EM: lovisa.zillen@geol.lu.se
AF: Geobiosphere Science Centre, Lund University Solvegatan 12, Lund, 226 62 Sweden
AU: Sandgren, P
EM: per.sandgren@geol.lu.se
AF: Geobiosphere Science Centre, Lund University Solvegatan 12, Lund, 226 62 Sweden
AB: Annually laminated (varved) lakes sediments are relatively common in Sweden because the distinct seasonality of climate induces winter ice cover and long periods of anoxia in the deeper parts of lake basins. The preserved annual cycle of sediment accumulation is characterized by a mineral rich layer, which is deposited during the spring snow melt, and an organic rich layer, which accumulates during the summer and winter. Mineral magnetic studies of several varved lake sediment sequences point to a common characteristic - high concentrations of a stable single domain magnetic mineral in the organic rich layers. In fact, in bulk sampled there is often a linear relationship between the amount of organic carbon and mass specific magnetic parameters that indicate the concentrations of ferrimagnetic minerals. The magnetic properties of catchment soils and sub-soils are quite different from the organic rich sediments and point to the production of magnetite in the lake. This magnetite is the carrier of an extremely stable natural remanent magnetization, which we have used to reconstruct the direction and intensity of the Earth's geomagnetic field over the past 9000 years, and it's concentrations in the sediment sequences also appear to be related to known climate changes during the Holocene. At this point of our studies, however, only one definite conclusion can be made: that the fine grained magnetite is produced within the lake, before or after sediment deposition. Where do we go from here? What methods can we use to test our hypothesis that magnetotactic bacteria are indeed responsible for the production of the single-domain magnetite? And how much secondary magnetite does a single lake contain? These are questions posed by this study. We attempt to answer the last one through using magnetic techniques and the known properties of magnetite. For the central parts of the lake basins we derive a conservative estimate in the range between 16 and 18 mg magnetite per square meter per year for the last 9000 years. This estimate implies that a cylinder of sediment with a radius of 50 m and thickness of 4-5 meters would contain approximately 1300 kg of secondary single domain magnetite.
DE: 0419 Biomineralization
DE: 0463 Microbe/mineral interactions
SC: Biogeosciences [B]
MN: Fall Meeting 2005