doi:10.2204/iodp.proc.339.103.2013
Site U13851
Expedition 339 Scientists2
Background and objectives
Future advances in our understanding of the Earth’s climate system will rely on our ability to link high-resolution sedimentary archives from the oceans, ice cores, and terrestrial sequences and to interpret these records in the context of novel Earth system modeling approaches. Few places exist in the world where sufficiently detailed and unambiguous marine-ice-terrestrial linkages are possible. One challenge for the Integrated Ocean Drilling Program (IODP), and the broader drilling community in general, is to identify and recover marine, ice, and terrestrial sequences from appropriate locations and, with adequate temporal resolution, to study processes of the integrated climate system. Site U1385 on the West Iberian margin (Figs. F1, F2) represents such a location and provides a rare opportunity for recovering key sequences needed to link marine-ice-terrestrial archives.
The impetus for drilling this site comes from Nick Shackleton’s pioneering work on Core MD95-2042 from the Iberian margin, which has played a pivotal role in our understanding of millennial-scale climate variability over the last glacial cycle. Shackleton et al. (2000) demonstrated that the oxygen isotopic record of the planktonic foraminifer Globigerina bulloides in Core MD95-2042 could be correlated precisely to δ18O variation (i.e., temperature) in the Greenland Ice Core during the last glacial (Fig. F3). Transitions marking the onset and end of interstadials were abrupt, resembling a “square-wave” form, which suggests that the Polar Front migrated rapidly, leading to sudden changes in North Atlantic surface waters. By comparison, the δ18O benthic curve resembled the temperature record from Antarctica, both in shape (the “triangular” form of Antarctic Isotope Maxima) and phasing relative to Greenland and North Atlantic surface temperature records. This Antarctic affinity largely reflects changes in local deepwater δ18Odw, which may be due to changes in deepwater sourcing and/or source signature (Skinner et al., 2007). The strength of the Portuguese margin sediment record is the ability to correlate millennial-scale climate events from the marine environment to polar ice cores in both hemispheres. Moreover, the narrow continental shelf and proximity of the Tagus River result in rapid delivery of terrestrial material to the deep-sea environment off Portugal, thereby providing a record of vegetation changes and permitting correlation of marine and ice-core records to European terrestrial sequences (e.g., Shackleton et al., 2000, 2004; Sánchez Goñi et al., 1999, 2002; Tzedakis et al., 2004, 2009; Roucoux et al., 2006; Margari et al., 2010). Few other, if any, places exist in the world ocean where such detailed and unambiguous marine-ice-terrestrial linkages are possible. For this reason, the Iberian margin has become a focal point for studies of climate variability over the last several glacial cycles. Extending this remarkable sediment archive further back in time is the primary goal of Site U1385.
Numerous CALYPSO piston cores have been retrieved and studied from the Iberian margin, and piecemeal sections extend as far back as marine isotope Stage (MIS) 15. From experience on cruises of the Marion Dufresne (MD03 and MD08), deeper penetration is limited using conventional piston coring systems because of the highly cohesive nannofossil oozes of MIS 9, 11, and 13. Thus, the drilling capability of the R/V JOIDES Resolution was needed for obtaining these continuous sections and extending the Iberian margin record to study the evolution of millennial-scale climate variability across successive glacial cycles and its role in glacial–interglacial climate transitions.
Site U1385 (37°34.285′N, 10°7.562′W) coincides with the position of Core MD01-2444, which has been studied extensively as part of the Pole-Ocean-Pole project (e.g., Fig. F1) (Skinner and Shackleton, 2006; Vautravers and Shackleton, 2006; Martrat et al., 2007; Margari et al., 2010). The water depth (2578 meters below sea level [mbsl]) places the site under the influence of Northeast Atlantic Deep Water today; however, it was influenced to a much greater degree by southern sourced waters during glacial periods (Curry and Oppo, 2005).
The overall objective of Site U1385 was to recover a late Pleistocene sediment record that will greatly improve the precision with which marine sediment records of climate change can be correlated to and compared with ice-core and terrestrial records. Specific objectives to be accomplished by postcruise research include
- Documenting the nature of millennial-scale climate variability for older glacial cycles of the Quaternary, including changes in surface and deepwater circulation during the “100 k.y. world,” Mid-Pleistocene Transition (MPT), and “41 k.y. world”;
- Deriving a marine sediment proxy record for the Greenland Ice Core beyond the oldest ice (~125 ka) to examine the amplitude and pacing of Dansgaard/Oeschger-type variability during previous ice ages;
- Determining interhemispheric phase relationships (leads/lags) by comparing the timing of proxy variables that monitor surface (Greenland) and deepwater (Antarctic) components of the climate system, thereby overcoming problems of age determination on millennial and submillennial timescales;
- Studying how changes in orbital forcing and glacial boundary conditions affect the character of suborbital-scale climate variability and, in turn, how millennial-scale variability interacts with orbital geometry to produce the observed glacial–interglacial patterns of climate change;
- Reconstructing the climate transitions into and out of glacial periods at high temporal resolution;
- Reconstructing the history of changing local dominance of northern-sourced versus southern-sourced deep water on orbital and suborbital timescales during the Quaternary;
- Investigating climate variability during past interglacial periods in comparison to the Holocene;
- Linking terrestrial, marine, and ice-core records by analyzing pollen and terrestrial biomarkers that are delivered to the deep-sea environment by rivers; and
- Developing an orbitally tuned timescale and contributing to the development of a global stratigraphy having sufficient resolution to study abrupt climatic events and their phase relationships.