Transport mechanism and paleoclimatic significance of terrigenous silt deposited in varved sediments of an African rift lake

  • Johnson T
  • McCave I
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Grain-size measurements of fine-grained sediments based on laser diffraction may contain spurious information due to the over-estimation of the size and proportion of platy particles. Consequently, some regard the use of laser diffraction particle sizing in palaeoceanography inappropriate. Here, it is shown experimentally that such concerns are not warranted. Laser diffraction particle sizing is known to be fast, precise and allows for detailed particle sizing over a broad size range; it is therefore potentially a very powerful technique if the complications associated with it can be overcome. As most sediments are mixtures of different components transported by different mechanisms, inferences of past environmental parameters require decomposition of the grain-size record in question. Useful decomposition can only be performed if changes in the contribution of the components are registered predictably by the measuring device. This study reports on mixing experiments which show that the Fritsch A22 laser diffraction particle sizer does indeed register small changes in the contribution of the mixing components in a predictable way. Mixing proportions estimated from the measurements do, however, differ from the initial mixing proportions, but these can be converted with only small errors. Application of the conversion equations to a North Atlantic grain-size record that showed consistent slowdown of deep-ocean circulation in response to millennial scale ice-rafting events during the last glacial does not quantitatively affect the original inferences. Laser diffraction particle sizing of fine-grained sediments therefore yields reproducible and useful data for palaeoceanographic reconstructions.

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