Late Pliocene History of Mediterranean Outflow Water in the Gulf of Cadiz (IODP Expedition 339, Site U1389)
Abstract:The opening and closure of ocean gateways play an important role amongst climate forcing mechanisms: surface and deep-water circulation are altered, and hence global heat transport. An important component of North Atlantic circulation patterns is the warm and saline Mediterranean Outflow Water (MOW) that enters the North Atlantic via the Gibraltar Strait. Its onset and early history after the opening of the Gibraltar Strait at 5.3 Ma are poorly constrained and its impact on oceanography and climate in the Pliocene are largely unexplored. Current research efforts in the scope of projects funded by the Austrian Science Fund (FWF-P25831-N29) and the Max Kade Foundation aim to improve our knowledge about this early phase of MOW. Research focuses on Sites U1387C (upper Miocene-lower Pliocene; ~5.8-3.8 Myrs) and U1389E (middle-upper Pliocene; ~3.7-2.8) drilled during IODP Expedition 339 in the northern Gulf of Cádiz close to the source area of MOW.
At IODP Site U1389E, a 280m-thick sequence of upper Pliocene contourites is studied to reconstruct MOW behavior and potential paleoceanographic and paleoclimatic feedback mechanisms during the Mid-Pliocene Thermal Optimum. Currently, a reliable age model is developed for the studied interval based on bio-, chemo-, and cyclostratigraphy. Several biostratigraphic datums have been identified from analyses of calcareous nannoplankton and planktic foraminifera which indicate a depositional age between 3.65 Ma and 2.87 Ma. In some intervals of the record a further refinement of the age model at the millenial scale is complicated by poor recovery. However, preliminary results from high-resolution geochemical analyses of the bulk sediment (TOC, CaCO3 and S) and shells of the planktic foraminifer Globigerinoides ruber (δ18O, δ13C) reveal distinct cyclic patterns in several well recovered intervals. Based on an average sedimentation rate of ~25cm/ka, these cycles most likely reflect precessional forcing. Current paleoceanographic research on the late Pliocene history of MOW focuses on these well-recovered intervals and rely on the combination of δ18O and Mg/Ca records of benthic foraminifera and Zr/Al records collected by XRF-scanning of the core material.