Strength variability of the Mediterranean Outflow Water during late Quaternary: Preliminary results from IODP Site U1588

The Mediterranean Sea is thought to play a role in changing past ocean circulation and North Atlantic climate, through the outflow of warm, saline intermediate waters (Mediterranean Outflow Water; MOW) into the North Atlantic. Previous studies mostly focused on the Gulf of Cádiz, immediately after the MOW existing the Mediterranean, but how the MOW varied along the northward transport is still unclear. Fine grain-size parameters have been widely employed to infer paleo-flow speeds of near-bottom currents in the deep sea, in particular the terrigenous non-cohesive “sortable silt” (denoted as SS) controlled by selective deposition. Here we present terrigenous sediment grain size results on IODP Site U1588 (37°57.61′N, 9°30.99′W, 1339 m water depth), which was retrieved from the Iberian Margin during the IODP Expedition 397. Our aim is to reconstruct strength variations in the lower branch of the MOW over the past ~250,000 years. After removing organic matter (leached with 10% H2O2 at 85 ℃) and marine carbonates (leached with 0.5 M HCl), the terrigenous detrital component of about 100 samples were measured on a Malvern Mastersizer 3000 instrument. Our grain-size results show a bimodal distribution, with a small peak near 1 μm and the main mode between 5–8 μm. The correlation between the percentage and mean of the sortable silt fraction (10–63 μm) is significant (R2=0.43, P<0.01), permitting the use of SS-mean as a reliable indicator of the deep-sea current strength. The calculated SS-mean is from ~14.2 to 18.2 μm, corresponding to the flow speed of ~3.3 to 10.2 cm/s. Based on the shipboard age model, our results show a persistent low-latitude forcing of MOW flow speed over the past 250,000 years, with strong precessional and glacial cycles.