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doi:10.2204/​iodp.proc.302.102.2006

Data collected prior to Expedition 302

A chronologically arranged overview of several, but not all, geophysical data sets from the Lomonosov Ridge is presented here. These data sets were collected from ice islands (ARLIS II and LOREX), submarines (SCICEX), and icebreakers (all others). Locations of the seismic lines from these expeditions are shown in Figure F1.

ARLIS II 1961–1965

The second US Navy Arctic Research Laboratory Ice Station (ARLIS II) was deployed on an ice island in 1961 ~350 km northwest of Point Barrow, Alaska (Ostenso and Wold, 1977). During the ice island’s drift journey toward the Fram Strait, it reached its northernmost point on the Lomonosov Ridge flank, facing the Makarov Basin, in December 1963 (Fig. F1). The scientific program included bathymetric measurements, gravity observations, and continuous seismic reflection profiling. Reprocessing of the seismic reflection data (Weber and Sweeney, 1985) resulted in the interpretation of a 60 m thick section of conformable sediments draping the Lomonosov Ridge plateau along the ARLIS II’s first crossing of the ridge plateau from the Makarov Basin side. Further along the oblique drift track across the Ridge (Fig. F1), the thickness of the conformably draped and well-stratified sediments increased to >850 m on the Ridge's flat-topped crest (Weber and Sweeney, 1985).

LOREX 1979

North of the ARLIS II crossing of the Lomonosov Ridge, seismic reflection profiling was conducted from another drifting ice island in 1979, during the Canadian the Lomonosov Ridge Experiment (LOREX) (Weber, 1979) (Fig. F1). An air gun with a 0.164 L (10 inch³) chamber was deployed as the seismic source through a hole in the ice, in addition to a multichannel system for deeper penetration using explosive charges (Weber, 1979). LOREX also installed a 3 kHz subbottom profiler for higher resolution seismic acquisition (Weber, 1979). The seismic reflection data revealed the geological characteristics of the Lomonosov Ridge along the ice station’s crossing of a narrow section of the ridge near the North Pole (Fig. F1). The conclusion from these data was that the Ridge consisted of a series of tilted en-echelon fault blocks with their crests covered by thin (<75 m thick) drapes of unconsolidated sediments (Weber and Sweeney, 1985) (Fig. F2). Furthermore, substantial sediment erosion by currents was suggested in the area where the LOREX ice stations crossed the Lomonosov Ridge (Blasco et al., 1979).

Arctic Ocean 1991/ARK-VIII/3

After the completion of the LOREX project in 1979, it took 12 y before the next seismic reflection data were acquired over the Lomonosov Ridge. This occurred during the Arctic 1991 expedition (Fütterer, 1992; Jokat et al., 1992). The icebreakers Polarstern (Germany) and Oden (Sweden) conducted this expedition together. Seismic equipment was towed behind the Polarstern using a setup designed to function in Arctic sea ice. This configuration was based on previous Arctic seismic survey experiences that proved successful in this challenging environment (Grantz et al., 1986).

Towing equipment behind an icebreaker in the central Arctic pack ice, often approaching 10/10 ice coverage, is problematic. The icebreaking strategy is typically based on following existing leads—that is, by traveling along the periphery of larger ice floes. Because of this strategy, the ship’s track becomes highly irregular, requiring the streamer length to be shortened to a few hundred meters.

In situations where the ship is breaking ice, the freshly broken lead may close rapidly behind the icebreaker and trap the towed streamer. For this situation, a short streamer is also necessary. Moreover, meter-sized pieces of broken ice floes are continuously forced under the icebreaker’s hull. These pieces can impact, with great force, seismic equipment towed near the ship.

During the Arctic 1991 expedition, the seismic source consisted of two 3 L (~183 inch³) air guns that were suspended below a 1 ton weight to keep the air guns as close to the ship’s fantail as possible (Jokat et al., 1992). A 300 m long 12-channel streamer was deployed as a receiver. Seismic reflection profiles were successfully acquired over the Lomonosov Ridge using this setup in 7/10 to 8/10 ice conditions (Jokat et al., 1992). In addition to seismic reflection, sonobuoys were deployed during this expedition to map the velocity structure of the Lomonosov Ridge sediments and bedrock (Jokat et al., 1992). Two of the seismic reflection profiles, AWI-91090 and AWI-91091, crossed the Ridge crest completely at about 87°55′N and 87°40′N, respectively (Figs. F1, F3). These two profiles provided the first high-quality images of the <450 m thick, well-stratified, and apparently undisturbed drape of sediments (Jokat et al., 1992) on the Ridge crest. These two lines were subsequently used to identify proposed paleoceanographic drill sites.

During the Arctic 1991 expedition, subbottom profiling data were also acquired continuously using the Polarstern’s hull-mounted Parasound system, a parametric high-resolution subbottom profiler that operates in a frequency range between 2.5 and 5.5 kHz (Fütterer et al., 1992). The Parasound records along the AWI-91090 and AWI-91091 profiles showed that, even at higher resolution, the uppermost ~30–40 m of the Lomonosov Ridge stratigraphy appeared undisturbed (Fig. F4). The Polarstern was also equipped with an Atlas Hydrosweep multibeam bathymetric sonar that provided additional depth information of the Lomonosov Ridge crest along the 1991 expedition track (Fütterer et al., 1992). These data were incorporated into the International Bathymetric Chart of the Arctic Ocean (IBCAO) (Jakobsson et al., 2000), which provided the general bathymetry of the Lomonosov Ridge for Expedition 302.

Arctic Ocean 1996/ARK-XII/1

The Oden and Polarstern teamed up again during the Arctic Ocean 1996 expedition. The geophysical data consisted of seismic reflection profiling, seismic refraction experiments (Kristoffersen et al., 1997), and high-resolution subbottom profiling using a chirp sonar (Backman et al., 1997). It was the first time that chirp sonar data were collected from the central Arctic Ocean. When compared with the Arctic 1991 expedition, the geophysical surveying was concentrated in areas of the Lomonosov Ridge located closer to the Siberian continental margin (Fig. F1). Seismic reflection profiling was also conducted successfully along a transect crossing the Ridge near the North Pole (Fig. F1). In total, more than 700 km of seismic reflection data were collected. An air gun array consisting of four sleeve guns with a total volume of 5.5 L (~336 inch³) mounted in a steel cage and depressed by a 1 ton weight was initially used. This array was lost because of impact with ice (Kristoffersen et al., 1997). A lighter depressor foil was subsequently constructed for two spare 3 L (~183 inch³) Prakla Seismos air guns. The lighter depressor worked well with less risk to the seismic source. A 200 m 16-channel streamer was towed as the receiver, using an offset of 150 m behind the icebreaker.

Chirp sonar profiles provided important information about shallow sediment erosion (Fig. F5). Previously unmapped areas of the Ridge crest located between about 85°24′N and 87°17′N were substantially eroded, indicating that the ridge crest had been affected by ice grounding down to about 1 km water depth (Jakobsson, 1999; Jakobsson et al., 2001; Polyak et al., 2001). A pronounced unconformity was mapped to show that >50 m of the sediment column was estimated to be missing in portions of the eroded areas (Jakobsson, 1999). The chirp sonar data also demonstrated that at water depths >1 km, the seismic stratigraphy consisted of well-stratified and apparently undisturbed sediments.

Another outcome of the Arctic Ocean 1996 expedition was a largely revised bathymetric portrayal of the Lomonosov Ridge between 85°20′N, 135°E and 87°40′N, 155°E (Jakobsson, 1999). These data were critical for supplementing the Arctic Ocean bathymetric database used to develop the new IBCAO map (Jakobsson et al., 2000).

ARK-XIV/1a

The major goal of expedition ARK-XIV/1a in 1998 was to sample and acquire geophysical data from the Alpha Ridge in the Amerasian Arctic Ocean (Jokat, 1998). Because of the extreme ice conditions that prevail in the Alpha Ridge region, two ships were used: the Polarstern as the scientific platform and the Russian nuclear icebreaker Arktika as the support icebreaker. Despite working in extremely hard ice conditions with up to 6 m thick sea ice, a total of 320 km of multichannel seismic reflection data was collected from the Alpha Ridge along three profiles, showing a sediment thickness ranging from 500 to 1200 m (Jokat, 2003).

During the return route toward the Laptev Sea, a track was chosen along the strike of the Lomonosov Ridge in more favorable ice conditions. Several profiles were successfully acquired along and crossing the Ridge crest (Jokat, 1998) (Figs. F1, F6). These profiles show a more variable topography of the Ridge south of 85°N compared to the 1991 and 1996 survey areas north of 85°N. The 1998 data also show that the sediment thickness atop the ridge gradually increases toward the Laptev Sea margin. One primary and three alternate Expedition 302 drilling sites were proposed along the 1998 seismic reflection profiles. These alternate sites were selected primarily as backups for ice contingency purposes.

SCICEX 1999

During the SCICEX 1999 cruise, the US nuclear submarine USS Hawkbill mapped the Lomonosov Ridge without being restricted by Arctic pack ice. A much larger area could be mapped because this ice constraint was eliminated (Edwards and Coakley, 2003) (Fig. F1). The SCAMP system, mounted on the USS Hawkbill, continuously recorded chirp sonar profiles, sidescan data, and swath bathymetric data along a regular survey track over the Lomonosov Ridge. The track was designed to investigate the nature of the erosion of the Ridge crest and acquire additional site survey data near the two primary seismic reflection profiles, AWI-91090 and AWI-91091. The SCAMP chirp sonar data confirmed that erosion was limited to water depths shallower than 1000 m on the ridge crest (Polyak et al., 2001) and that the area near the two seismic reflection profiles showed an apparently undisturbed sediment stratigraphy (Edwards and Coakley, 2003).

Arctic Ocean 2001

As of 2001, the Expedition 302 site survey database contained geophysical data that indicated undisturbed sediment sequences at several locations on the Lomonosov Ridge. However, there were no seismic reflection cross-lines on the Ridge, and the Ocean Drilling Program Site Survey Panel requested that these be collected. For this reason, a site survey component was added to a Swedish expedition in 2001 (Kristoffersen et al., 2001). Two generator-injector guns with 8.5 L (~519 inch³) combined capacity and an 8-channel streamer with an active length of 200 m were deployed as the seismic reflection system (Kristofferson et al., 2001). Under tough ice conditions, 100 km of seismic reflection data were collected with the purpose of crossing the previously acquired AWI-91091 profile (Fig. F1). The difficult ice conditions reduced the number of survey days from five to three, and 400 m of hydrophone cable was also damaged.

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