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We have been studying the effects of diagenesis on the paleomagnetic signal of lacustrine sediments by examining tephra layers found in unaltered and altered states in the same diagenetic environment. In this paper, we report rock magnetic data from seven such layers in six diagenetic environments ranging from saline-alkaline to mildly alkaline. The effects of diagenesis can be complicated and varied but certain patterns are evident. These patterns indicate that both the physical state of the magnetic grains and the geochemistry of the porewaters are important in determining the effects of diagenesis. In some cases, diagenesis simply reduces the intensity of the original magnetization, but in others it produces a new magnetization that completely overwhelms the original one. Measurements of various rock magnetic parameters have allowed us to relate the changes in intensity to changes in the particle-size distribution of the magnetic carriers. In particular, decreases in intensity appear to be associated with selective dissolution of the fine-grained magnetic carriers or with a general reduction in the quantity of magnetic carriers of all grain sizes; increases in intensity seem to result from a reduction in the numbers of coarse-grained magnetic carriers. Plots of the demagnetization behavior of natural and saturation isothermal remanent magnetizations may be useful in discriminating between unaltered and altered material, especially when used in conjunction with other rock magnetic information.

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