Do geochemical estimates of sediment focusing pass the sediment test in the equatorial Pacific?



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The paleoceanographic recording fidelity of pelagic sediments is limited by chemical diagenesis and physical mixing (bioturbation and horizontal sediment transport). Diagenesis and bioturbation are relatively well-studied, but the effects of physical sedimentation have been largely ignored. Modeling U series isotopes (e.g., 230Th) can potentially quantify horizontal sediment movement, but model horizontal sediment focusing often equals or exceeds the vertical particle rain. We find no evidence of this level of sediment focusing in the equatorial Pacific from geophysical data or sediment core comparisons. The overestimate of sediment focusing by 230Th is probably caused by poor model assumptions: that sediment does not fractionate (does not sort according to size during transport) and that 230Th cannot leak from slowly accumulating sediments. Both assumptions are weak. U series methods do hold promise to quantify sediment focusing if properly calibrated. With calibration the trade-offs between seeking high sedimentation rates for better time resolution and the blurring by horizontal sediment focusing can be better assessed. Copyright 2005 by the American Geophysical Union.


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