University of Cambridge, United Kingdom

Integrated Ocean Drilling Program (IODP) Expedition 399 combines IODP Proposal 644-Full2 and ancillary proposal letter (APL)-763 and focuses on the broader significance of Mediterranean Outflow Water (MOW) on North Atlantic circulation and climate. The expedition will address important questions highlighted in the IODP Initial Science Plan related to paleocirculation and climate, the influence of oceanic gateways, and sea level and neotectonic control on sediment architecture along continental margins. In order to answer these questions, we propose targeted drilling of a late Neogene continental margin sequence in the Gulf of Cadiz and off West Iberia. The high rates of accumulation associated with contourite depositional system (CDS) deposits in this region provide an expanded sedimentary record that permits detailed examination of paleocirculation patterns linked to past environmental change. Expedition 339 offers a unique opportunity to understand the global link between paleoceanographic, climatic, and sea level changes from Messinian to recent time. The Gulf of Cadiz and off-West Iberia CDS form an extensive compound sedimentary body, which has been developing along the midslope over the past 5 m.y. under the direct influence of MOW. It therefore holds an unmistakable signal of MOW through the Gibraltar Gateway, reopened following tectonic adjustments at the end of the Messinian Salinity Crisis, and hence a clear record of Mediterranean Sea and MOW influence on the North Atlantic Ocean. The importance of the Gulf of Cadiz is clearly reflected in the large number of regional studies and multinational interest shown over the past 40 y. An extensive array of high-quality data exists for the region, and a detailed seismic stratigraphic framework has recently been proposed. But, despite such extensive surveying, the region has not yet been drilled for scientific purposes, even though the Gibraltar Gateway clearly has major implications for global climate and oceanography. We have identified the following five broad scientific objectives, which require seven drill sites through the Pliocene to Quaternary sedimentary record: 1. Understand the opening of the Gibraltar Gateway and onset of MOW, 2. Determine MOW paleoceanography and global climate significance, 3. Establish a marine reference section of Pleistocene climate change, 4. Identify sea level changes and sediment architecture of the Cadiz CDS and Iberian margin, and 5. Ascertain synsedimentary neotectonic control on architecture and evolution of the CDS. To achieve these major scientific objectives, it is essential to integrate the results of the proposed drill sites with a dense network of existing high-resolution seismic reflection profiles. Interpretation of this seismic network is already well established, although the inferred ages require drilling confirmation. Seven primary sites (and two alternates sites) have been selected that will allow us to identify and calibrate the third- and fourth-order depositional units and associated widespread erosive discontinuities across the CDS. This is of great significance, both regionally and globally, for 1. Monitoring the long-term variability of MOW and its global climatic significance; 2. Constraining the main paleoceanographic events from late Miocene to recent time, including high-resolution focus on late Pleistocene and Holocene rapid climate events; 3. Evaluating the influence of opening of the Gibraltar Gateway on North Atlantic oceanography and climate and monitoring the effects of sea level change on MOW flux; 4. Understanding the architecture of a complex CDS and the nature of its unit stacking pattern related to allogenic and autogenic controls; and 5. Investigating the dramatic large-scale asymmetric cycles of seismic character evident on high-resolution records, thereby elucidating their link with Quaternary-Pliocene climate/sea level and paleoceanographic changes. In addition, drilling proposed Site SHACK-04 on the western Iberian margin is expected to recover a continuous sediment record with high sedimentation rates that preserves a signal of millennial-scale climate variability throughout the Pleistocene. This record will constitute a marine reference section of Pleistocene climate variability that can be correlated confidently to polar ice core and European terrestrial archives. The site will be used to document suborbital climate variability over numerous Pleistocene glacial-interglacial cycles; determine interhemispheric phasing (lead/lag) of the climate system; and correlate terrestrial, marine, and ice core records.