Magnetic Mineral Properties of Sediments and Their Relationship to Pore Fluid Chemistry, Lake Erie: A Preliminary Result

The legacy of development within the Lake Erie Basin has led to the inevitable input of a variety of contaminants including PCBs, heavy metals, and organochlorine pesticides into this ecosystem. Generally the sediment concentrations of these particular contaminants decrease from the western basin to the eastern basin, and from the southern area to the northern area of the central basin. Here we present evidence that the magnetic properties of sediments may provide mechanistic information about the early diagenesis of Fe in freshwater lakes. Specifically we hypothesize that there is a direct link between the build-up of porewater Fe2+ and the presence of magnetic minerals such as magnetite in these sediments. To test this hypothesis, we obtained four 30cm cores, spatially distributed from west to east and from north to south in Lake Erie. These cores were subsampled every half centimeter in the top 5 cm, and every centimeter below 5 cm and processed under N2 on board the R/V LIMNOS. The thermomagnetic behavior of the subsamples (J vs T) indicates the presence of magnetite in the lake sediments. Magnetic susceptibility (k) for cores 2 and 3 from central Lake Erie is, in general, higher than that for cores 1 and 4 from western and eastern Lake Erie. As susceptibility is commonly used as a proxy for magnetite content, this result indicates more magnetite in these cores, possibly the product of diagenetic iron reduction. Susceptibility of anhysteretic remanent magnetization (kARM) and the ratio of kARM/k for cores 2 and 3 are about 30-50% less than for cores 1 and 4, suggesting the grain size in the central Lake Erie is coarser than in near-shore areas. The magnetic properties through core 1 (western Lake Erie) are more variable than the other 3 cores, suggesting that depositional or post-depositional processes may have affected this core more strongly than in other areas. When the magnetic mineral data is combined with geochemical data we find similar trends in magnetic susceptibility (magnetite concentration) and the dissolved Fe2+ concentrations in the pore waters. Intriguingly there is little to no correlation with total solid-phase iron content in the sediments. This suggests that these magnetic mineral properties are not related to the total iron content of the sediment but instead may delineate zones of authigenic magnetite formation that is the result of microbial iron reduction.