A new dimension to sediment magnetism: Charting the spatial variability of magnetic properties across lake basins

摘要

We have investigated the variability of the magnetic properties of surface sediments across eight Minnesota lake basins. The measured magnetic properties are controlled by the competing fluxes of allochthonous and autochthonous magnetic particles, and differ according to location in the basin. Shoreline sediments are dominated by detrital magnetic particles, whereas littoral and profundal sediments are characterized by a combination of bacterial magnetosomes and detrital particles. The position of the oxic-anoxic interface, which may occur in the water or within the sediment column, controls the depth at which living magnetotactic bacteria occur, and determines the degree of preservation of their magnetosome chains in the surface sediment. The preservation potential of undisturbed chains is higher for bacterial magnetite formed at the top of the sediment column in the littoral area than for magnetosomes originating in the water column in the profundal area. Bacterial magnetite in the profundal facies will contain a higher proportion of chains collapsed during settlement through the water column to the lake bottom. This process increases the fraction of interacting magnetosomes, which in turn artificially lowers the ARM ratio (X_(ARM)/IRM), which ceases to be a reliable grain size indicator in the profundal environment. Our results indicate that a holistic approach to interpreting limnologically-derived paleoecological data should be employed. Specifically, a thorough understanding of evolving and interrelated factors such as basin morphology and limnologic conditions is crucial for a more confident interpretation of the sedimentary record in terms of environmental conditions at the time of sediment deposition.