Structure and Variability of Mediterranean Outflow Water Flow Recorded in Contourite Layers in the Gulf of Cadiz and west of Portugal

Abstract:

Water exchange between the Atlantic Ocean and the Mediterranean Sea has important effects on world hydrography, and the exchange varies on both shorter and longer time scales in response to climate, sea-level and tectonics. Drift deposits cored during IODP Exp. 339 provide new information about the Mediterranean Outflow Water (MOW) history. Drift sediments are primarily muddy but they often include layers with coarse silt and sand-sized material which are termed silty or sandy contourites. Several coarser layers are being studied in detail using high-resolution grain size and CAT scanning techniques, along with analysis of the sand fraction and thin section, X-ray and XRF techniques to characterize the beds and the events which created them. Many coarser beds are mixtures of finer sediment, often (but not always) similar in grain size to the sediment deposited before and/or after the bed, and coarser sediment which often (but not always) overlaps in grain size with the finer sediment in the contourite layer. The coarser sediments may have accumulated during numerous shorter episodes of higher-speed flow while the finer sediments may have accumulated at times when flow speeds were reduced. Bioturbation has usually mixed the two or more grain size populations to form the observed layers, and grain-size grading patterns as well as the size and amount of coarser material may relate to the nature of the flow events which formed them. Hiatuses at bed contacts and within beds suggest that flows were erosional at times. At most sites, two or three drill holes 10 to 100 m apart were collected, and sediments in adjacent drill holes could be correlated based on sediment properties. While the nature and sequence of contourite layers in adjacent holes are often quite similar, correlated beds can have somewhat different thickness, grain sizes and layering patterns. This lateral variability may be due to the presence of larger bed forms or other larger-scale depositional or erosional process. While these silty and sandy contourites apparently provide important information about the both the short-term and long-term history of MOW events, we need to distinguish variations related to current flow history with those related to bed forms or other local sedimentary processes to fully characterize the nature and effects of these deep currents.