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doi:10.2204/iodp.proc.303306.101.2006

Summary and conclusions

The overall objectives of Expedition 303 were to establish late Neogene–Quaternary intercalibration of geomagnetic paleointensity, isotope stratigraphies, and regional environmental stratigraphies and, in so doing, develop a millennial-scale stratigraphic template. Such a template is required for understanding the relative phasing of atmospheric, cryospheric, and oceanic changes that are central to our understanding of the mechanisms of global climate change on orbital to millennial timescales.

The site locations for Expedition 303 are known from previous ODP/DSDP drilling or from conventional piston cores to (1) contain distinct records of millennial-scale environmental variability (in terms of ice sheet–ocean interactions, deep circulation changes, or sea-surface conditions) and (2) provide the requirements, including adequate sedimentation rates, for developing millennial-scale stratigraphies (through geomagnetic paleointensity, oxygen isotopes, and regional environmental patterns).

The drilling and core recovery phase of the expedition was a success because of the quality and nature of the cores recovered and the potential of the cores to meet the scientific objectives of the expedition. The common overall objective of Expeditions 303 and 306 (scheduled for March–April 2005) provided more flexibility to occupy sites as weather conditions allowed than is usual for an individual expedition. The October–November window in the North Atlantic ensured that this flexibility would be utilized.

It was decided in the early planning stages that the expedition would use the APC system only. Cores acquired using the XCB, particularly in the interval immediately below the limit of APC drilling, are usually disturbed by the drilling process and do not meet the standards of core quality required for high-resolution stratigraphic studies. Emphasis was placed on the recovery of complete, undisturbed composite sections utilizing multiple APC holes with drillover to increase the depth of APC recovery.

Sites U1302 and U1303

The overall objective at Sites U1302 and U1303 is to explore the record of LIS instability at this location close to Orphan Knoll (Fig. F1). Piston cores collected previously at or near Sites U1302 and U1303 (HU91-045-094P, MD99-2237, MD95-2024, and MD95-2025) show the presence of numerous detrital layers, some of which are rich in detrital carbonate (Hillaire-Marcel et al., 1994; Stoner et al., 1996, 2000; Hiscott et al., 2001). Isotopic data from planktonic foraminifers indicate that these detrital layers are associated with low-productivity meltwater pulses. The objective at Sites U1302 and U1303 is to document this manifestation of LIS instability further back in time to the base of the recovered section (~MIS 17). The mean sedimentation rates at Sites U1302 and U1303 are estimated to be ~13 cm/k.y. (Fig. F5), ensuring a high-resolution record.

At Site U1302, the first site to be drilled off Orphan Knoll (Fig. F1), we encountered a debris flow at ~105 mcd that was not recognized in seismic data (Toews and Piper, 2002). The top of the debris flow appears to be within the Brunhes Chron at ~700 ka, and the base of the section is estimated from nannofosssil stratigraphy to be at ~950 ka (Fig. F4). To avoid the debris flow, we traversed in dynamic positioning (DP) mode (with the base of the drill string lifted a few hundred meters above the seafloor) to Site U1303, located 5.68 km northwest of Site U1302. The debris flow was again encountered at Site U1303 at approximately the same depth, and drilling was again discontinued. The presence of a debris flow at shallow depths (~105 mcd) also precluded downhole logging operations as planned in the Expeditions 303 and 306 Scientific Prospectus, which was then deferred to the next planned site on Eirik Drift. Sites U1302 and U1303 can be easily correlated using a range of MST data, and it is clear that essentially the same section was recovered at the two sites. We generated a complete and continuous composite section by combining the five holes from Site U1302 and the two holes from Site U1303. Short segments of three cores from Site U1303 and the composite record from Site U1302 provide a continuous stratigraphic sequence to ~107 mcd.

• Sites U1302 and U1303 near Orphan Knoll have a detrital layer stratigraphy that is a proximal analog to the detrital stratigraphy of the central Atlantic and provides a detailed record of the instability of the Laurentide Ice Sheet since 700 ka.

Site U1304

While en route to prospective sites on the Eirik Drift (Sites U1305–U1307), the unfavorable weather forecast for this area forced the ship to divert to Site U1304 at the southern edge of the Gardar Drift (Fig. F1). The sediments at Site U1304 comprise interbedded diatom and nannofossil oozes with clay and silty clay. The lithologies are generally interbedded on a centimeter or decimeter scale. Diatom assemblages are dominated by needle-shaped species of the Thalassiothrix/Lioloma complex. The site is located within the central Atlantic IRD belt and therefore provides a distal record (relative to that at Sites U1302 and U1303) of the ice sheet instability. Site U1304 provides a high-resolution, high-sedimentation-rate record at a water depth (3065 mbsl) sufficient to determine the millennial-scale changes in the influence of NADW at the site. The diatom-rich sedimentary section extends back into the uppermost Pliocene at 258 mcd. Mean sedimentation rates of 17.8 cm/k.y. are estimated for the last 0.78 m.y. and 12.2 cm/k.y. for the interval from 0.78 to 1.77 Ma. The diatom-rich stratigraphy implies that the site has been located at the subarctic convergence between the surface Labrador Current and the North Atlantic Current (see Bodén and Backman, 1996). The good preservation of benthic and planktonic foraminifers, the pristine magnetic properties, and the construction of a complete composite section from four holes indicate that the environmental record can be placed into a reliable and precise age model. With the clearing weather over Eirik Drift coinciding with completion of coring at Site U1304 and the scientific preference for obtaining logs at one of the Eirik Drift sites, logging was not conducted at Site U1304 in favor of Site U1305.

• Site U1304 provides a high-sedimentation-rate, high-resolution pelagic record at the southern edge of the Gardar Drift suitable for monitoring NADW and recording the detrital layer stratigraphy of the central Atlantic IRD belt since latest Pliocene time.

Sites U1305 and U1306

Three sites (Sites U1305, U1306, and U1307) were drilled on the Eirik Drift (Fig. F1). The first of these was the designated the “deepwater” site (Site U1305) in 3459 meters water depth at the western extremity of the Eirik Drift. The primary “shallow-water” site (Site U1306) in 2273 meters water depth is located 191 km northeast of Site U1305. The two sites were chosen by maximizing the thickness of the Quaternary sedimentary section in the MCS network obtained over the Eirik Drift during Cruise KN166-14 in summer 2002.

Conventional piston cores have shown that the sedimentation history on the Eirik Drift during the last glacial cycle is strongly affected by the WBUC, which sweeps along east Greenland and into the Labrador Sea (Hillaire-Marcel et al., 1994; Stoner et al., 1998). Based on two conventional piston cores (HU90-013-013P and HU90-013-012) from the Eirik Drift at similar water depths, the deepwater site (Site U1305) is expected to display relatively expanded interglacials and relatively condensed glacial intervals and the converse is true for the shallow-water site (Site U1306). The base of the section at both sites lies within the Olduvai Subchron at ~300 mcd, and the mean sedimentation rates are 17–18 cm/k.y. Sites U1305 and U1306 will provide complementary high-resolution records of the history of the WBUC, detrital layer stratigraphy signifying instability of the surrounding ice sheets, and the attributes for well-constrained age models using stable isotopes, biostratigraphy, and geomagnetic paleointensity.

• Sites U1305 and U1306 provide complementary records of Quaternary sedimentation on the Eirik Drift. The mean sedimentation rate (~17 cm/k.y.) is similar for both sites, but the patterns of sedimentation are expected to be different due to the contrasting water depths of the sites and the influence of the WBUC. The sites not only record the activity of the WBUC, and hence this component of NADW, but also monitor the detrital layer stratigraphy associated with instability of surrounding ice sheets, particularly the Greenland Ice Sheet.

Site U1307

Site U1307 was not in the initial plan for Expedition 303, but was occupied when a storm moving northeastward across the North Atlantic blocked our passage to our intended next site (Site U1308). Site U1307 was placed at a location on the Eirik Drift (Fig. F1) where the Quaternary sedimentary section appears thin relative to Site U1306, providing APC access to the underlying Pliocene section. Two holes were drilled at Site U1307, reaching a maximum depth of 162 mcd in the uppermost Gilbert Chron (~3.6 Ma). The mean sedimentation rate for the recovered section was 4.9 cm/k.y. Interval sedimentation rates between polarity reversals ranged from 2.7 to 7.6 cm/k.y. Poor weather and excessive ship heave curtailed drilling at this site, and the two holes were insufficient to generate a complete composite section. The site did, however, establish the feasibility of recovering the Pliocene sedimentary section on the Eirik Drift using the APC. The site extends the environmental record back to ~3.6 Ma and will provide invaluable age control throughout the MCS network established on the Eirik Drift by the KN166-14 cruise in 2002.

• Site U1307 provides a record of sedimentation on the Eirik Drift since ~3.6 Ma. The tight age control on the sedimentary record will be used to provide age control for seismic reflectors that can be traced through the Cruise KN166-14 seismic network, thereby contributing to the understanding of the sedimentary architecture of the Eirik Drift.

Site U1308

The final site of Expedition 303 was Site U1308, a reoccupation of DSDP Site 609. Shipboard and shore-based analytical techniques have changed considerably in the 22 y since this site was originally drilled (in 1983). For example, the shipboard facilities for the construction of composite sections were not introduced until 8 y later (1991).

Site 609 has been the focus of some of the most important developments in paleoclimate research in the last 15 y. Layers of IRD containing detrital carbonate (Heinrich events) were recognized at this site in the early stages of their correlation to the Greenland ice core record (e.g., Bond et al., 1993). The 1500 y cycle in petrologic characteristics such as hematite-stained grains and Icelandic glass has also been recognized at this site (Bond et al., 1999). Most of the recent work on Site 609 sediments has been conducted on the last glacial cycle due in part to uncertainties in the continuity of the section at greater depth. The objective of the reoccupation of Site 609 was to recover a demonstrably complete sedimentary section that could be used to establish the isotopic characteristics of NADW, monitor the detrital layer stratigraphy of the central Atlantic IRD belt, and place this record into a well-constrained chronostratigraphy. Several factors affected core quality at Site U1308. Sea swells reaching 6 m during drilling in Hole U1308A affected this hole as well as other holes at this site. In addition, sticky clay and other debris caught around the bit and in the BHA were believed to be the cause of loss of core from the base of core liners and crushed liners in intervals from all holes. To obtain a complete, undisturbed stratigraphic record at Site U1308 required six holes consuming all the remaining operational time. The maximum penetration at Site U1308 was 341 mbsf to the upper Miocene at ~6 Ma (Fig. F4). However, the complete composite section is limited to the uppermost 247 mcd, extending well within the Gauss Chron at ~3.1 Ma, with the mean sedimentation rate since that time being ~8.3 cm/k.y. (Fig. F5).

• Site U1308 is a reoccupation of a classic site (Site 609) in the central Atlantic that has driven many of the most important advances in paleoceanography during the last 10–15 y. A demonstrably complete section was recovered back to 3.1 Ma, with almost continuous recovery back to ~3.5 Ma. The site will provide a record of central Atlantic detrital layer stratigraphy, as well as a means of monitoring NADW, within a well-constrained chronostratigraphy.

The drilling and recovery phase of Expediton 303 has been an unqualified success due to the dedication of IODP staff, the Transocean employees, and members of the science party. The weather also played an important role in permitting the recovery of high-quality cores at all sites. A total of 4656 m of high-quality core was recovered from sites with mean sedimentation rates in the 5–18 cm/k.y. range. The sites were chosen to recover Pliocene and Quaternary records of millennial-scale environmental variability in terms of ice sheet–ocean interactions, deep circulation changes, or sea-surface conditions. The sites provide the requirements, including adequate sedimentation rates, for developing millennial-scale stratigraphies (through geomagnetic paleointensity, oxygen isotopes, and regional environmental patterns). We expect research on these cores in the coming years to break new ground in the fields of paleoclimatology, paleoceanography, and paleomagnetism.

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