Measurement of the sortable silt current speed proxy using the Sedigraph 5100 and Coulter Multisizer IIe: precision and accuracy

The weight percentage and mean size of the 10-63 mu m terrigenous silt frac tion (termed 'sortable silt') of fine-grained marine sediments have recentl y been argued to be proxies for near-bottom palaeocurrent intensity. This p aper details the accuracy and precision of the Sedigraph 5100 and the Coult er Multisizer IIe in determining the weight percentage and mean size of the sortable silt at abundances characteristic of deep-ocean sediments. Three sortable silt standards and a fine silt/clay diluent were generated from na tural deep-sea sediment of the Iceland Basin. These were used to produce si lty clays with standard additions of sortable silt ranging from 1% to 20% o f the total by weight. Accuracy was measured relative to the size of the pu re spike measured by each instrument. At sortable silt abundance >5%, the S edigraph and the Coulter Multisizer estimate both weight percentage and mea n grain size precisely and accurately, while below 5%, instrumental noise d egrades their performance. On the basis of the Sedigraph results, a series of percentage errors is defined, which may be applied to sortable silt mean size and weight percentage estimates used in palaeoceanographic near-botto m current reconstruction. It is suggested that, ideally, palaeocurrent reco nstructions should be based on Sedigraph determinations of the 0-63 mu m gr ain size distribution, yielding both mean and abundance, or on Coulter Mult isizer determinations of the sortable silt grain size mean. The Coulter Mul tisizer may prove especially useful in regions of very depleted (<5%) sorta ble silt component.