Which is the better proxy for paleo-current strength: Sortable-silt mean size (SS) or sortable-silt mean grain diameter (d SS)? A case study from the Nordic Seas
2015
Tegzes, Andrea D. | Jansen, Eystein | Telford, Richard J.
The coarseness of the 10-63 m terrigenous silt (i.e., sortable-silt) fraction tends to vary independently of sediment supply in current-sorted muds in the world's oceans, with coarser sediments representing relatively greater near-bottom flow speeds. Traditionally, the coarseness of this size fraction is described using an index called sortable-silt mean size ( SS), which is an arithmetic average calculated from the differential volume or mass distribution of grains within the 10-63 m terrigenous silt fraction, where the relative weights of the individual size bins become increasingly disproportionate, with respect to the actual number of grains within those size bins, toward the coarse end of the size range. This not only increases the absolute value of the apparent mean size within the 10-63 m terrigenous silt fraction, but it may also affect the apparent pattern of relative changes in the coarseness of the sortable-silt fraction along the core. In addition, it makes SS more prone to biases due to, for example, analytical errors. Here we present a detailed analysis of grain-size distributions over three selected Holocene time intervals from two complementary sediment cores (JM97-948/2A and MD95-2011), extracted from the center of a high-accumulation area along the flow path of the main branch of the Atlantic Inflow into the Nordic Seas and show that differential-number-based statistics, which likely better describes variations in the actual coarseness of the sortable-silt fraction, may provide a more robust alternative to (S) over bar(S) over bar
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