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doi:10.2204/iodp.sp.302.2004


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 was a continental fragment originally split off from 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, 1990a). If we compensate for that motion of the seafloor, the Lomonosov Ridge is indeed 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 c 250 Ma old zircons 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 indeed has continued to the present, providing what may be a continuous stratigraphic sequence (Figures F2 - F3). 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 are likely of purely pelagic origin, chiefly biogenic, aeolian or ice-rafted.

Deep-penetration reflection seismic profiles were acquired from the Lomonosov Ridge on icebreaker-based expeditions in 1991, 1996, and 1998 (Jokat et al., 1992; 1998; 1999; Kristoffersen, 1997a). The first deep-penetration seismic cross-lines from the Lomonosov Ridge were collected in 2001 (Yngve Kristoffersen, personal communication, and ODP Data Bank). The first high-resolution chirp profiles were collected in 1996 (Jakobsson, 1999). In 1999, the SCICEX program collected abundant high-resolution seismic chirp data, swath bathymetry and sidescan sonar backscatter data from a USN nuclear submarine (Pyle et al., 1997), contributing many new exciting results (Polyak et al., 2001; Edwards et al., 2001) including a much improved bathymetric chart of the Arctic Ocean (Jakobsson et al., 2000a; see also www.ngdc.noaa.gov/mgg/bathymetry/arctic/arctic.html).

High-resolution chirp sub-bottom profiler (Figure F4) and interferometric swath bathymetry and backscatter data collected over the Lomonosov Ridge from the USS Hawkbill in the Spring of 1999 have been provided to the site-survey data center at LDEO. The chirp data in Figure F4 show the continuous drape of mantle-bedded pelagic sediments that are the primary target for this drilling program.

Two of the key seismic profiles (AWI-91090 and AWI-91091) were acquired across the Lomonosov Ridge in about 8/10 ice cover during the 1991 expedition (Jokat et al., 1992). At 88°N in 1 km of water, the ridge is 80 km wide with a 450 m thick section of acoustically stratified sediments that cap the ridge above an unconformity (Figure F3). Below this unconformity, sediments are present in down-faulted asymmetric half-grabens. Seismic velocities from refraction experiments are typical for deep-sea sediments above the unconformity (1.5-2.2 km/s) and are >4 km/s below.

Several dozens of short cores (<10 m) of Pleistocene and Holocene age exist from the central parts of the Lomonosov Ridge, indicating average sedimentation rates of ~7-10 m/m.y. (e.g., Gard, 1993; Jakobsson et al., 2000b; 2001). By assuming that the tectonic model of the onset of Cenozoic marine sedimentation on the ridge is approximately correct in terms of timing (50 Ma: Jokat et al., 1992) and considering the total thickness of the section (450 m) above the unconformity, a rate of 7-10 m/m.y. is consistent with the average sedimentation rate of the entire section: 9 m/m.y.

Little information is 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 mid- or late Eocene silicoflagellates also has been retrieved from the Alpha Ridge, providing the sole evidence for early Cenozoic marine conditions in the Arctic (Bukry, 1984). Thus, existing core material, at best, represents a few percent of the Cenozoic paleoceanographic history of the Arctic Ocean.

It is concluded that the 450 m-thick sediment sequence draping the crest of the Lomonosov Ridge between 87°N and 88°N (Figures F2 - F3) contains a unique archive of climatic and paleoceanographic information, which is the key to unravelling the long-term (50 Ma) Cenozoic environmental history of the central Arctic Ocean.

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