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doi:10.2204/iodp.pr.302.2005
INTRODUCTION
Geological Setting
Ever since Bruce Heezen and Maurice Ewing recognized, in their 1961 paper, that the mid-ocean rift system extended from the North Atlantic into the Arctic Ocean, it has been assumed that the Lomonosov Ridge (Fig. F1) was originally a continental fragment broken off of the Eurasian continental margin. Aeromagnetic surveys of the Eurasian Basin have since mapped a remarkably clear pattern of magnetic lineations which can be interpreted in terms of seafloor spreading along the Gakkel Ridge since Chron C24 at ~53 Ma (Wilson, 1963; Vogt et al., 1979; Kristoffersen, 1990). If we compensate for that motion of the seafloor, the Lomonosov Ridge is brought into juxtaposition with the Barents/Kara Sea margin in early Cenozoic reconstructions. Zircon-bearing bedrock samples from the Lomonosov Ridge at 88.9°N yield a latest Permian (~250 Ma) age (Grantz et al., 2001). The only known source for zircons aged ca. 250 Ma in the circum-Arctic is in the post-tectonic syenites of northern Taymyr Peninsula and nearby islands in the Kara Sea, lending support to the tectonic model in which the ridge is interpreted to be a continental sliver that separated from the Eurasian plate.
As the Lomonosov Ridge moved away from the Eurasian plate and subsided, sedimentation on top of this continental sliver began and continued to the present, providing a >400 m thick stratigraphic sequence. The elevation of the ridge above the surrounding abyssal plains (~3 km) indicates that sediments on top of the ridge have been isolated from turbidites and originate from hemipelagic, biogenic, eolian, and/or ice-rafted input.
Two key seismic profiles (AWI-91090 and AWI-91091) were acquired across the Lomonosov Ridge in about 8/10 sea-ice cover in 1991 (Jokat et al., 1992). At 88°N in ~1 km of water, the ridge is 80 km wide with a 410 m thick section of acoustically stratified sediments that cap the ridge above an unconformity (Fig. F2). Below this unconformity, sediments are present in down-faulted asymmetric half-grabens.
Several dozen short cores (<10 m) of Pleistocene and Holocene age exist from the central parts of the Lomonosov Ridge, indicating average sedimentation rates of ~10 m/m.y. (e.g., Gard, 1993; Jakobsson et al., 2000, 2001; Backman et al., 2004).
Prior to the Arctic Coring Expedition (ACEX), little information was available about pre-Pleistocene paleoenvironments in the central Arctic Ocean. Temperate marine conditions existed during the Late Cretaceous (Campanian–Maastrichtian) based on evidence provided by silicoflagellates and diatoms from three short T-3 and CESAR cores, all retrieved from the Alpha Ridge in the Amerasian Basin (Clark et al., 1980; Bukry, 1981; Thiede et al., 1990). One 3.64 m long core (F1-422) containing a 1.65 m section with middle Eocene diatoms and silicoflagellates provided the sole evidence for early Cenozoic marine conditions in the Arctic (Bukry, 1984; Ling, 1985).
Scientific Objectives
The history of Arctic paleoceanography is so poorly known that we can look at the recovery of any material as a true exploration that will, by definition, increase our knowledge and understanding of this critical region. The key objective of ACEX was to recover a continuous ~450 m thick sediment sequence including the upper part of the underlying acoustic basement (bedrock) from the crest of the Lomonosov Ridge. The overall, primary scientific goal was to determine the paleoenvironmental development in the central Arctic Ocean during post-Paleocene times and to decipher its role in the global climate evolution. A secondary scientific goal was to acquire information about the early tectonic evolution of the Eurasian Basin.
Specific paleoceanographic objectives are to:
- Determine the history of ice rafting and sea ice;
- Study local versus regional ice sheet development;
- Determine the density structure of Arctic Ocean surface waters, the nature of North Atlantic, and the conveyor and onset of Northern Hemisphere glaciation;
- Determine the timing and consequences of the opening of the Bering Strait;
- Study the land-sea links and the response of the Arctic to Pliocene warm events;
- Investigate the development of the Fram Strait and deepwater exchange between the Arctic Ocean and Greenland/Iceland/Norwegian Sea; and
- Determine the history of biogenic sedimentation.
The tectonic objectives are focused on ridge evolution. Specific tectonic objectives for drilling on the Lomonosov Ridge are to:
- Investigate the nature and origin of the Lomonosov Ridge by sampling the oldest rocks below the regional unconformity in order to establish the pre-Cenozoic environmental setting of the ridge and
- Study the history of rifting and the timing of tectonic events that affected the ridge.
Strategy
The biggest challenge facing ACEX was maintaining the drillship's location during drilling and coring in moving, heavy sea ice. Sea-ice cover over the Lomonosov Ridge moves with the Transpolar Drift and is affected by local responses to wind, tides, and currents. Until now, the high Arctic Ocean Basin, known as "mare incognitum" to the scientific community, had never been deeply cored before because of these challenging sea-ice conditions.
Plans for this first-ever event were carefully crafted over several years and included a fleet of three icebreaker-class ships: a drilling vessel, the Vidar Viking, which remained at a fixed location and suspended over 1600 m of drill pipe through the water column and into the underlying sediments; a Russian nuclear icebreaker, Sovetskiy Soyuz; and the diesel-electric icebreaker Oden. The Sovetskiy Soyuz and Oden protected the Vidar Viking by breaking "upstream" floes into small bergy bits to allow the Vidar Viking to stay positioned to drill and recover the sediment cores.
This strategy was successful. Planners had predicted that the fleet could maintain the drillship's station for up to 2 full days, yet the stationkeeping ability was stretched to more than 9 consecutive days. The three ships coordinated their efforts through a central Fleet Manager, at times on a minute-to-minute basis. The fleet kept the Vidar Viking on location in 9/10, multiyear ice for up to 9 days—a landmark feat that has empowered scientists to continue to explore this least known of our oceans through scientific ocean drilling for many years to come.
The Sovetskiy Soyuz conducted the first attack on oncoming heavy floes, whereas Oden was the last defense in protecting the drilling operation against the oncoming ice (Fig. F3). During these defensive operations, the officers on the Vidar Viking kept station by manually driving the powerful thrusters with the bow continuously maneuvered to head into the direction of the oncoming ice. The ice management defense strategies were continuously updated with information from a full-time ice and weather forecast team onboard the Oden and Sovetskiy Soyuz.
Coring operations were conducted by Seacore, Ltd., using a specially built drill rig for the Vidar Viking. Coring tools were provided by the British Geological Survey. Cores were collected on the Vidar Viking from five boreholes drilled to a maximum depth of 428 meters below seafloor (mbsf). A ~160 m record of wireline geophysical logs was collected in one borehole.
The cores, collected in plastic liners, were sealed for postexpedition analyses onshore at the Integrated Ocean Drilling Program repository in Bremen, Germany. Before they were stored, cores were analyzed for physical properties using Geotek's nondestructive multisensor track. Selected intervals were sampled to extract pore water and microbiology samples. During the expedition, core catcher samples were routinely transferred to the Oden twice a day for analyses that included micropaleontology, stratigraphy, petrophysics, chemistry, and sedimentology.