PRELIMINARY SCIENTIFIC ASSESSMENT

IODP Expedition 306 was based on two separate proposals titled (A) "Ice sheet–ocean atmosphere interactions on millennial timescales during the late Neogene–Quaternary using a paleointensity-assisted chronology for the North Atlantic" and (B) "Installation of a CORK near Hole 642E to document and monitor bottom water temperature variations through time." Following Expedition 303, Expedition 306 was the second cruise of the North Atlantic paleoceanography study that aimed to generate a late Neogene–Quaternary chronostratigraphic template for North Atlantic climate proxies and allowed their correlation at a sub-Milankovitch scale and their export to other parts of the globe by using a PAC.

Based on the Expeditions 303 and 306 Scientific Prospectus and the results of Expedition 303, Sites IRD3A and IRD4A and two sites on Eirik Drift were selected to become the primary Expedition 306 sites. Continuous weather observations performed throughout Expedition 306 showed, however, that coring operations on Eirik Drift were not possible at any time because of extremely bad weather conditions in the Labrador Sea. Thus, the alternate Site GAR1B was elevated to a primary site instead. In total, we lost 10 days because of severe weather conditions in the first part of the expedition. Therefore, only three sites (Sites U1312 [IRD4A], U1313 [IRD3A], and U1314 [GAR1B]) could be drilled instead of the planned four sites. However, Expedition 306 was completed successfully and the objectives outlined in the 303/306 Scientific Prospectus and Addendum were fulfilled:

• For the North Atlantic paleoceanography study, we recovered complete sedimentary sections at three sites. The excellent sections from Sites U1313 and U1314 provide the requirements, including adequate sedimentation rates, to study millennial-scale environmental variability in terms of ice sheet-ocean interactions, deep circulation changes, or sea-surface conditions. Site U1313 provides a unique and complete Pliocene–Pleistocene sediment section with remarkably constant sedimentation rates. This site will allow for an optimal reconstruction of the phasing of temperature records and their relationship to ice sheet instability and changes in deepwater circulation throughout the last 5 m.y. High sedimentation rates of 13–14 cm/k.y. will allow a high-resolution study of paleoenvironmental change during the late Messinian. At Site U1312, unfortunately, only two holes could be drilled because of severe weather conditions. Although the recovery in these holes was very good, a complete splice could only be obtained between 68 and 159 mcd. The upper 68 mcd of Hole U1312A was affected by coring disturbance. Below 159 mcd, both holes are of high quality but splicing the two records was not possible because of a very uniform sediment composition. Furthermore, at this site, sedimentation rates are quite low, especially in the late Miocene, the major target at Site U1312 (<1–2 cm/k.y.). Thus, high-resolution studies could not be performed at this site. The almost-complete sedimentary sequence from Site U1312 representing the last ~11 m.y., however, will allow the study of short- and long-term climate variability and ocean-atmosphere interactions under very different boundary conditions, as outlined in the previous sections.


• Our second Expedition 306 main target was the CORK program. Here, a borehole observatory with a 150 m thermistor string was successfully deployed in a cased and CORKed borehole close to Site 642. The data recovered in the coming years will provide for the first time a directly measured record of BWT over approximately the last 100 y from a paleoceanographically very important key area of the North Atlantic.

Because two of the 306 sites (U1312 and U1313) are redrillings of Sites 607 and 608, expectations of the type and age of sediments existed prior to the 306 drilling. At the third Site U1314, information about the sedimentary type and sedimentation rates/age had been obtained for the upper 30 m from a Marion Dufresne piston core. In this context, the material recovered met expectations.

Besides the coring and CORK activities, the downhole logging at Sites U1313 and in Hole 645E was extremely useful and another highlight of the expedition. An excellent correlation between downhole logging and shipboard MST logging was possible. This allows, for example, calculation of "real" sedimentation rate values. In Hole 645E, FMS logging yielded very good results and allows correlation to existing core data and filling of coring gaps (60% of the formation!). Together with the other downhole logging data, detailed information about permeability, fluid flows, and temperature gradients of the area will be available.

Looking back at the process of the expedition and the evolution of the weather conditions, we have to mention that early spring (March) was not the right season to perform drilling operations in the North Atlantic. During the second part of the expedition, we had optimum weather conditions for conducting the CORK program very successfully. The CORK working area, however, was very far away from our main working area of the North Atlantic Paleoceanography program (i.e., >5 days of transit). Here, it would have been useful to have the approval for an alternative coring program close to the CORK area in advance. If weather conditions would have been too severe to conduct the CORK program, we had no real alternative program at that time.

Some of the drill sites proposed in the original proposal (572-Full3) and the Expeditions 303 and 306 Addendum could not be drilled during Expeditions 303 and 306, respectively: Sites IRM2A and IRM3A (revisiting the area of ODP Sites 918 and 919) (Larsen, Saunders, Clift, et al., 1994) were already skipped prior to the expeditions because of weather windows, and the high-priority LAB8 sites could not be drilled during Expedition 306 because of severe weather conditions during the campaign. These sites should be considered in future drilling expeditions in the North Atlantic to complete the "Ice sheet-ocean-atmosphere interactions on millennial timescales during the late Neogene–Quaternary" objectives.

The IRM3A and IRM2A sites—located in the Irminger Basin on the path of the East Greenland Current and deep waters resulting from Denmark Strait overflow, the major component of NADW—are sensitive to millennial-scale instabilities of coastal ice sheets of the East Greenland/Icelandic area and are influenced by instabilities of the Laurentide Ice Sheet. Drilling results from some of the LAB8 transect sites could extend the climate record back into the Miocene and answer questions about the sedimentary architecture of sediment drifts, the role of the Western Boundary Under-Current (WBUC) in NADW formation, and provide a unique record of Greenland Ice Sheet instability.

In a future drilling campaign related to the "Ice sheet-ocean-atmosphere interactions on millennial timescales during the late Neogene–Quaternary," it might also be useful to consider drill sites strongly influenced by IRD input from the British-Fennoscandian Ice Sheet. Here, potential candidates could be DSDP Site 548, drilled southwest of Ireland across the continent/ocean boundary on the sediment-starved Goban Spur (De Graciansky, Poag, et al., 1985), and ODP Site 644 located on the Vøring Plateau (Eldholm, Thiede, Taylor, et al., 1987). A comparison between records from Sites U1305–U1307 (western "end-member" proximal to the Laurentide/Greenland Ice Sheet), Sites IRM3A and IRM2A ("end-member" of coastal ice sheets of the East Greenland/Icelandic area), Sites U1304 and U1308 (central part of the IRD belt), and new sites at the location of DSDP Site 548 (eastern "end-member" proximal to the British Ice Sheet) and ODP Site 644 ("end-member" proximal to the Fennoscandian Ice Sheet) (see Fronval et al., 1997) will allow a high-resolution (millennial to submillennial) reconstruction of the (contemporaneous and/or nontemporaneous) history of the Laurentide, Greenland, and British-Fennoscandian ice sheets' instability and their relationship to surface water and deepwater characteristics.