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doi:10.2204/iodp.pr.339.2012

Objectives

Drilling in the Gulf of Cádiz and off the West Iberian margin offers a unique opportunity to tackle key scientific goals enumerated in the IODP ISP related to

• Oceanic gateways and their global influence,

• Paleocirculation and climate,

• Rapid climate change,

• Sea level and related controls on sediment architecture, and

• Neotectonic activity and controls on continental margin sedimentation.

The extensive CDS that has been developing within the Gulf of Cádiz and extending around the West Iberian margin over the past 5 m.y. is a direct result of MOW (e.g., Madelain, 1970; Gonthier et al., 1984; Faugères et al., 1985; Nelson et al., 1993, 1999; Llave et al., 2001, 2006, 2007, 2010, submitted; Stow et al., 2002; Habgood et al., 2003; Hernández-Molina et al., 2003, 2006, submitted; Mulder et al., 2003, 2006; Hanquiez et al., 2007; Marchès et al., 2007). The high accumulation rates and expanded sedimentary records of drift deposits permit high-resolution examination of past environmental change (Llave et al., 2005; Voelker et al., 2006). The CDS deposits, therefore, hold the very best signal of MOW flow through the Strait of Gibraltar gateway and a clear record of its influence on the oceanography and climate of the North Atlantic Ocean and on NADW variability (Bigg and Wadley, 2001a, 2001b; Bigg et al., 2003). However, the region had not previously been drilled for scientific purposes, even though the Strait of Gibraltar gateway clearly has major implications for global climate and oceanography. The deeper target off western Portugal (APL-763) lies outside the direct influence of MOW but contains an expanded record of primarily hemipelagic sediment with which we can develop a Pleistocene marine archive of climate change.

We identified five broad scientific objectives for the proposed drilling program.

1. Understand the opening of the Strait of Gibraltar gateway and onset of MOW.

Tectonic adjustments along the suture line between the African and Eurasian plates could have lead to the opening of the Strait of Gibraltar gateway at the end of the Miocene at 5.3 Ma (Berggren and Hollister, 1974; Mulder and Parry, 1977; Maldonado et al., 1999; Blanc, 2002; García-Castellanos et al., 2009). Other authors have stressed that the cut into the threshold of Gibraltar was due to regressive erosion of a stream that was flowing toward the desiccated Mediterranean Basin, resulting in the opening of the strait (Blanc, 2002; Loget and Van Den Driessche, 2006). Reopening of the Strait of Gibraltar gateway marked the end of complete isolation of the Mediterranean Sea and the global effects of the Messinian salinity crisis (Ryan et al., 1973; Hsü et al., 1978; Comas et al., 1999; Duggen et al., 2003). Immediately following this initial opening, the gateway depth was most likely insufficient to allow very significant outflow of deep MOW into the Gulf of Cádiz. Therefore, the onset of deep MOW, the initiation of contourite drift sedimentation in the gulf, and the broader influence of warm saltwater influx in the North Atlantic lagged behind gateway opening. However, the actual timing of this event is uncertain.

Our first objective was to drill through the drift succession and into late Miocene sediments at several different sites and therefore date the basal age of drift sedimentation in the Gulf of Cádiz. We also aimed to evaluate the nature of change in the patterns of sedimentation and microfauna/microflora from the end of the Miocene through the early to middle Pliocene, from proximal regions closest to the Strait of Gibraltar gateway to distal regions on the West Iberian margin. This evaluation will allow us to determine any downstream variation in the onset of contourite deposition and of MOW bottom water signature.

2. Determine MOW paleocirculation and global climate significance.

The present-day flux of MOW through the Strait of Gibraltar gateway is nearly 2 Sv (i.e., 2 × 10⁶ m³/s), carrying warmer waters and more than 300,000 tons of excess salt into the North Atlantic every second. The consequent increase in density of NADW may stabilize, or in cases of decreased MOW flux, destabilize thermohaline circulation and therefore trigger climate change (Johnson, 1997; Rahmstorf, 1998; Bigg and Wadley, 2001a; Bigg et al., 2003). The importance of MOW in North Atlantic Ocean circulation and climate is now widely recognized. MOW millennial to long-term variability and its effects on thermohaline circulation is an active and prolific line of research at present. It has been inferred that the rate of deepwater formation reached its highest level in the Mediterranean during glacial stages, when contribution of NADW was at a minimum and Antarctic Bottom Water (AABW) at a maximum, but there are still many questions to resolve in this regard (e.g., Duplessy et al., 1988; Abrantes, 1988; Caralp, 1988, 1992; Sarnthein et al., 1994; Schönfeld, 1997, 2002; Cayre et al., 1999; Flower et al., 2000; Sierro et al., 2000, 2005; Shackleton et al., 2000; Schönfeld and Zahn, 2000; Cacho et al., 2000, 2001; Moreno et al., 2002; de Abreu et al., 2003; Schönfeld et al., 2003; Slater, 2003; Löwemark et al., 2004; Raymo et al., 2004; Voelker et al., 2006; Llave et al., 2006; Lebreiro et al., 2009; Lebreiro, 2010; Voelker and Lebreiro, 2010; among others).

Our second objective was to date the principal unconformities and minor discontinuities identified on seismic records, thereby calibrating the seismic stratigraphic framework. We can therefore assess their link to paleocirculation variation and events and evaluate their global correlation and significance. Detailed reconstruction of paleoceanographic conditions over centennial, millennial, and longer timescales at high resolution will disclose their evolution through time and help elucidate the background driving forces.

3. Establish a marine reference section of Pleistocene climate (rapid climate change).

Few marine sediment cores have played such a pivotal role in paleoclimate research as those from the Portuguese margin Shackleton sites, so-called to honor Nick Shackleton’s seminal work in highlighting the global importance of these sections (Shackleton et al., 2000). These cores preserve a high-fidelity record of millennial-scale climate variability for the last glacial cycle that can be correlated precisely to polar ice cores in both hemispheres (Fig. F7). Moreover, the narrow continental shelf off Portugal results in rapid delivery of terrestrial material (e.g., pollen) to the deep-sea environment, thereby permitting correlation of marine and ice-core records to European terrestrial sequences. IODP drilling at Site U1385 (proposed Site Shack-04 and location of piston Core MD01-2444) on the Iberian margin during Expedition 339 provided a rare opportunity to recover a marine reference section of Pleistocene climate variability that can be correlated confidently to polar ice cores and terrestrial archives.

Our third objective (and the principal objective of Site U1385 [APL-763]) was to recover a Pleistocene sediment archive offshore Portugal that will greatly improve the precision with which marine sediment records of climate change are correlated to and compared with ice-core and terrestrial records. By yielding multiproxy records that can be placed on an integrated stratigraphy, drilling of proposed Site Shack-04 resulted in major advances in our ability to reconstruct millennial-scale climate variability during the Pleistocene and understand its underlying causes.

4. Identify external controls on sediment architecture of the Gulf of Cádiz CDS and West Iberian margin.

The effects of sea level change on Pliocene–Quaternary sedimentary architecture in the Gulf of Cádiz and along the West Iberian margin have been considerably amplified by the direct influence of the Strait of Gibraltar gateway cross-sectional area (see above). Detailed analysis of the sedimentary architecture of CDS drift deposits is essential to distinguishing between climate and sea level change and to the further development of the global sequence stratigraphic model. In particular, the global role of bottom currents and evolution of alongslope depositional systems has not been adequately considered as a major component of the generalized sequence stratigraphic framework for continental margins. Furthermore, the cyclicity of MOW flux and resultant deposits has a relationship to eustatic influences on the Gibraltar sill that needs to be defined as a control on circulation of the entire North Atlantic.

Our fourth objective was to establish the nature of sedimentation and the timing of associated hiatuses by first-order drilling, dating, facies analysis, and correlation between all drill sites. This will further allow us to determine the stacking pattern and evolution of the Pliocene–Quaternary drift deposits and evaluate contourite cyclicity at seismic to sediment scales. Combining facies analysis and sedimentation rates with detailed compositional studies allows a more complete understanding of sediment supply and remobilization within the bottom-current system and quantification of a sedimentary budget and flux for the CDS in the gulf.

5. Ascertain synsedimentary neotectonic control on architecture and evolution of the CDS.

Charting the chronology of neotectonic activity that has had significant effect on development and architecture of the CDS is essential to a more complete understanding of the margin system in this region. Margin evolution anywhere is controlled by complex interaction of many different forcing variables, most importantly sea level and climate, sediment supply, and tectonics. The Gulf of Cádiz provides an excellent example of how recent tectonic activity has markedly affected submarine topography (Medialdea et al., 2004; Hernández-Molina et al., 2006; Fernández-Puga et al., 2007; Llave et al., 2007; García et al., 2009; Terrinha et al., 2009; Zitellini et al., 2009) and, consequently, controlled changes in the distribution of branches of MOW and their influence in the North Atlantic (Llave et al., 2007). The initiation and early history of MOW with regard to timing and tectonic control of the Strait of Gibraltar gateway is the focus of Objective 1 (above).

Our fifth objective was to chart the chronology of neotectonic activity that has had significant effect on development and architecture of the CDS and to identify the principal morphological changes that have resulted from this tectonic activity. We will thus be able to evaluate the direct influence of recent diapiric activity on the evolution of the CDS, particularly with regard to the erosion of channels and moats through the softer cores of diapir strings. There is also an excellent opportunity to determine the rate of diapiric movement and its change through time.

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