Testing of the Sortable Silt Hypothesis: Preliminary Experimental Results

Accumulations of fine sediments along continental shelf and deep-sea bathymetric contours, known as contourites, form a sedimentary record dependent on bottom-current behavior, which is related to past oceanographic conditions such as ocean-basin-scale circulation. It has been hypothesized that the coarse silt (10-63 μm) grain size range within a deposit can be used as a proxy for paleocurrent velocity. This range is chosen for its non-cohesive behavior while still being mobile under the 10-40 cm/s deep-water current velocities. This is commonly referred to as the Sortable Silt hypothesis and specifically uses the mean within the coarse silt range as well as the abundance (e.g., percentage by mass) within this range of a given sample. Slow deposition rates on contourites (2-10 cm/kyr) make it difficult to test this hypothesis in the field for a wide range of sediment compositions and current velocities. Only a few laboratory studies have sought to test the hypothesis over a limited range of conditions with results that were either inconclusive or conflicting. The purpose of this study is to further test the Sortable Silt hypothesis in the laboratory using a non-circulatory flume. Unlike traditional laboratory flumes, sediment and water enter one end of the flume and are not recirculated. A non-circulatory flume maintains constant concentration with time at the sediment source and also allows for examination of downstream changes in grain size. A variety of velocities, sediment concentrations, and silt and clay mixtures were tested resulting in the deposition of silt and clay. Samples of the deposited material were then analyzed for grain-size distribution using x-ray-monitored settling particle size analyzer (SediGraph), which is the preferred instrument for paleoceanographic applications. The results of these ongoing experiments will help to further improve understanding of fine sediment transport and the effects that cohesive sediment may have on the Sortable Silt hypothesis.