Parts A and B Summary

DISCUSSION AND CONCLUSIONS

Following Expedition 303, Expedition 306 was the second cruise of the North Atlantic climate study that aims to generate a late Neogene–Quaternary chronostratigraphic template for North Atlantic climate proxies that can be correlated at a sub-Milankovitch scale and exported to other parts of the globe by using a PAC. In addition, Expedition 306 also included the installation of a CORK near Site 642 (Vøring Plateau, Norwegian margin) to investigate the feasibility of reconstructing bottom water temperature histories at the decade to centennial timescale by making high-precision temperature-depth measurements.

Based on the Expeditions 303 and 306 Scientific Prospectus and the results of Expedition 303, proposed Sites IRD3A and IRD4A and two sites on Eirik Drift were originally selected to become the primary Expedition 306 sites. Extremely bad weather conditions in the Labrador Sea, however, did not allow coring operations on Eirik Drift at any time. Thus, we concentrated our work related to the North Atlantic paleoceanography study in the area directly south of the central North Atlantic IRD belt and on the southern Gardar Drift. These locations are known either from Leg 94 or from conventional piston coring to have the potential for paleomagnetic and isotopic age control and sedimentation rates high enough for high-resolution reconstructions of sea-surface and bottom water characteristics and ice sheet instabilities during late Neogene to Pleistocene times.

The main objective at Site U1312, a reoccupation of Site 608, was to obtain continuous records of surface and deepwater characteristics and their interactions with ice sheet instabilities during Neogene–Quaternary times. In this context, an important target at this site was the recovery of a complete undisturbed upper Miocene section by means of APC coring. The Holocene to upper Miocene sedimentary succession at Site U1312 consists of varying mixtures of biogenic and detrital components, primarily nannofossils, foraminifers, and clay minerals. At ~3.5 Ma, the progressive but oscillatory deterioration of the northern hemisphere climate, which gradually led to the onset of major continental ice sheets at ~2.7 Ma, is reflected in the increase in detrital sediment input, followed by Late Pliocene–Pleistocene climate-controlled short-term variability in detrital input. Average sedimentation rates were low during the late Miocene and in the latest Pliocene and Pleistocene (1–2 cm/k.y.) and higher in the Early Pliocene (3–8 cm/k.y.).

Site U1313 is a reoccupation of Site 607. Site 607 has been very important for generating a Late Pliocene to Pleistocene stable isotope stratigraphy that can be interpreted in terms of ice sheet variability and changes in NADW circulation. At the site of Core VM 30-97, located close to Site 607, Heinrich events are marked by the distinctive detrital carbonate signature, and planktonic-foraminifer-derived SSTs warmed markedly during the Heinrich events and during the Last Glacial Maximum, in distinct contrast to the climate records from the subpolar North Atlantic.

At Site U1313, four holes with a maximum penetration to 308.6 mbsf were drilled. The Holocene to uppermost Miocene sedimentary succession at Site U1313 consists primarily of nannofossil ooze with varying amounts of foraminifers and clay- to gravel-sized terrigenous components. The detrital components become much more important and variable in the Upper Pliocene–Pleistocene interval of the sequence, probably reflecting northern hemisphere ice sheet instability. Bio- and magnetostratigraphy indicate nearly constant sedimentation rates of ~4.1–4.5 cm/k.y. throughout the Pliocene–Pleistocene time interval, whereas in the late Messinian, sedimentation rates were ~13–14 cm/k.y. Correlation between the holes was excellent in the upper 168.5 mcd because of pronounced variations in nearly all physical properties measured. In particular, L* from color reflectance measurements mimic variations in the global benthic oxygen isotope stack (e.g., Lisiecki and Raymo, 2005), and a preliminary age model was constructed by matching sharp L* variations with glacial and interglacial terminations. Alkenone-derived SSTs show variability from ~1319the Late Pliocene and the latest Miocene, respectively. The consistency of downhole logging data with both core data and age models will allow mapping of the spliced core record to actual depth, resulting in more accurate sedimentation rate calculations as well as more detailed age-depth models. Of special note is the dramatically consistent linear correlation of downhole natural gamma radiation (upper 225 mbsf) with the recent Lisiecki and Raymo (2005) benthic oxygen isotope record during the last 5.4 m.y.

Site U1314 was drilled on the southern Gardar Drift, close enough to the IRD belt to record Heinrich-type detrital layers that monitor ice sheet instability, at a water depth of 2800 m, allowing a high-resolution monitoring of NADW and its short-term (sub)millennial variability.

The Upper Pliocene to Holocene sedimentary sequence at Site U1314 consists of an alternation of predominantly nannofossil oozes enriched in biogenic and terrigenous components and terrigenous silty clay with varying proportions of calcareous and siliceous organisms. This alternation is also reflected in the carbonate content varying between ~5 and 70 wt%. Sand- and gravel-sized sediment, common at Site U1314 from 0 to 240 mbsf, provides direct evidence of ice rafting and documents the influence of Pliocene–Pleistocene glaciations on this region. Site U1314 yields abundant moderately well to well-preserved assemblages of calcareous and siliceous microfossils throughout the section and an excellent paleomagnetic record of the Brunhes, Matuyama, and the upper part of the Gauss Chrons. Even several short geomagnetic reversals are present in the paleomagnetic record. Sedimentation rates based on microfossil datums and paleomagnetic reversals indicate decreasing rates from ~11–11.5 cm/k.y. during the Late Pliocene to ~7.0–7.5 cm/k.y. during the Pleistocene. Stratigraphic correlation was straightforward at Site U1314 because most of the sediment physical properties show prominent short-wavelength amplitude variations. The resulting mcd scale is well resolved, and the spliced section is complete down to 281 mcd.

At Site U1315, a borehole observatory consisting of a CORK to seal the borehole from the overlying ocean and a thermistor string/data logger unit to document BWT variations and monitor its subbottom diffusion over a 5 y period, was successfully installed in a new hole ~180 m deep, close to Site 642. To assess current background thermal conditions in the region, logging down to almost 600 mbsf was performed in Hole 642E using the TAP tool in combination with the triple combo and the FMS-sonic tool strings. The upper 10 m of the borehole has a very steep thermal gradient (~2500km. At a depth of ~500 mbsf, a strong positive temperature excursion to ~42indicate inflow.

From the multidisciplinary (i.e., sedimentological, micropaleontological, and geochemical) studies to be performed on these cores (together with cores from Expedition 303) in the coming years, new milestones in the understanding of mechanisms and causes of abrupt climate change as one of the major challenges in global climate change research today are expected to be reached.

The success of our expedition was substantially supported by the excellent cooperation between the IODP staff, the Transocean employees, and the Shipboard Science Party, and the strong efforts of all of them.