Laser vs. settling velocity differences in silt grainsize measurements: estimation of palaeocurrent vigour

The use of grain-size distribution of muds for the reconstruction of past deep ocean currents is becoming established and applied in the palaeoceanographic community. The methods are also applicable to shallow marine and tidal flat muds with similar inferences concerning the energy of wave and current sorting being drawn. Fine sediment grain-size distributions can be obtained using a variety of instruments based on fundamentally different theoretical principles. These machines may give varying, sometimes misleading, results giving divergent interpretations of flow speed history. The new evidence presented here, combined with earlier work, suggests that of the three most commonly used analytical methods for fine silts with clays, settling velocity (Sedigraph) should be the method and instrument of choice, with electrical resistance pulse counters (e.g. Coulter Counter) as a suitable alternative. The data also show that laser particle sizers should be avoided for palaeocurrent reconstructions because the measured size of platey minerals can be dominated by their large projected area. This causes them to be recorded as the same size as larger equant grains although they have much smaller settling velocity and were deposited in aggregates. This produces results with a weaker relationship to the dynamics of deposition. The central problem is thus that some coarse clay/fine silt is recorded as medium to coarse silt, the key size in the 'sortable silt' mean size method of inferring changes in flow speed. For coarse silts (e.g. loess) the laser gives more satisfactory results.