SORTABLE SILT IS A WIDELY USED PROXY FOR BOTTOM CURRENT STRENGTH. WE EXAMINE THE IMPACT OF DIFFERENT PREPARATION AND INTERPRETATION PROCEDURES IN THE ASSESSMENT OF BOTTOM CURRENT STRENGTH AND VARIABILITY

Bottom currents are a vital, but poorly studied part of ocean circulation. Determining the flow strength and direction of bottom currents and paleo bottom currents in marine environments has been explored for decades e.g. Heezen and Hollister (1971), Ledbetter (1981), and the papers written as part of the High Energy Benthic Boundary Layer Experiment (HEBBLE). More recently McCave and his students (e.g. McCave et al., 1995, McCave and Hall, 2006, McCave et al., 2017) have proposed the use of the mean size of sortable silt (10-63 um) as a measure of paleocurrent strength. Whether or not sortable silt (SS) mean can be used as a current velocity indicator when part of the sediment is ice-rafted (IRD) has been open to discussion. Jonkers et al., 2015, suggested that the addition of ice-rafted debris (IRD) can increase the SS mean and hence invalidate the interpretation of stronger bottom currents. Luna, 2018 found that when weight % IRD exceeded ~5%, changes in the SS mean paralleled the changes in weight % IRD. McCave and Andrews, 2019, argue that both the SS% and SS mean need to be well correlated if bottom current strength can be reliably determined. We will present SS%, SS mean and weight % IRD for Pliocene sediments form ODP Site 697 in the Jane Basin.

We will also present grain-size data from five IODP samples from Site 1564 (SubAntarctic Front) where the fine fraction has been prepared for analysis using different processing methods in order to determine how susceptible the sortable silt mean and percentage are to different preparation procedures. Each sample has been split into four equal sizes in order to determine if sortable silt is affected by the drying and rewetting of the fine fraction, as well as crushing and sonification during disaggregation. This will allow us to explore and compare the different preparation processes and determine whether or not, and to what extent, initial procedures influence the size and weight % distribution of sortable silt. This crucial preparatory step is not commonly discussed in the methods section of papers focusing on grain size analysis and may need to be taken into consideration.