Backman, J., Moran, K., McInroy, D.B., Mayer, L.A., and the Expedition 302 Scientists Proceedings of the Integrated Ocean Drilling Program, Volume 302 Expedition 302 geophysics: integrating past data with new results1 Martin Jakobsson,2 Tom Flodén,2 and the Expedition 302 Scientists2 Chapter contents Introduction In preparation for IODP Expedition 302, Arctic Coring Expedition Introduction . 1 (ACEX), a site survey database comprising geophysical and geo- Data collected prior to Expedition 302. 2 logical data from the Lomonosov Ridge was compiled. The accu- Geophysical data collected during Expedition mulated database includes data collected from ice islands, ice- 302 . 4 breakers, and submarines from 1961 to 2001. In addition, seismic Summary . 6 reflection profiles were collected during Expedition 302 that com- References . 6 plement the existing seismic reflection data and facilitate integra- Figures . 8 tion between the acoustic stratigraphy and the Expedition 302 drill cores. An overview of these data is presented in this chapter. It is well recognized that collecting geophysical data in ice-cov- ered seas, in particular the Arctic Ocean, is a challenging en- deavor. This is because much of the Arctic Ocean is continuously covered with ice thicknesses that vary from 1 to 6 m. Over the continental shelves, sea ice can be absent during summer months, but it is present year-round in the central basins. This ice cover is the most dominant feature of the Arctic Ocean environ- ment. It circulates in the ocean basin in two main circulation pat- terns: the Transpolar Drift and the Beaufort Gyre (see the “Expe- dition 302 summary” chapter; Rudels et al., 1996). Expedition 302 sites are located within the less severe of these two ice circulation systems, the Transpolar Drift, which primarily moves sea ice from the shelves where it is formed (the Laptev and East Siberian Seas) across the basin and exits through the Fram Strait. During late summer, concentrations of Arctic sea ice can be <100% (10/10 ice cover), making it possible for icebreakers to op- erate. Average ice concentrations in the central Arctic Ocean dur- ing summer months can locally vary from partially open water (6/ 10) to completely ice covered (10/10). This sea-ice cover can move at speeds up to 0.5 kt. Early Arctic Ocean geophysical exploration was performed from ice-drift stations (Weber and Roots, 1990). However, the tracks from these drifting ice stations were controlled “by the whims of 1Jakobsson, M., Flodén, T., and the Expedition 302 nature” (Jackson et al., 1990), preventing detailed, systematic sur- Scientists, 2006. Expedition 302 geophysics: veys of predetermined target areas. These ice-drift stations were integrating past data with new results. In Backman, set up on stable icebergs that were trapped in sea ice and moved J., Moran, K., McInroy, D.B., Mayer, L.A., and the generally with the large drift patterns, but locally they were er- Expedition 302 Scientists, Proc. IODP, 302: ratic, so preselected locations could not be surveyed. In the late Edinburgh (Integrated Ocean Drilling Program Management International, Inc.). doi:10.2204/ 1980s, single icebreakers began to be used for oceanographic sur- iodp.proc.302.102.2006 vey work in the Arctic Ocean. Between 1991 and 2001, four scien- 2Expedition 302 Scientists’ addresses. tific icebreaker expeditions to the Lomonosov Ridge took place. Proc. IODP | Volume 302 doi:10.2204/iodp.proc.302.102.2006 M. Jakobsson et al. ACEX geophysics These cruises all experienced local sea-ice conditions mic reflection data (Weber and Sweeney, 1985) re- varying between 8/10 and 10/10. During these expe- sulted in the interpretation of a 60 m thick section of ditions, towed geophysical equipment was occasion- conformable sediments draping the Lomonosov ally damaged or lost, either because of a rapidly clos- Ridge plateau along the ARLIS II’s first crossing of the ing wake caused by local ice pressure or because ice ridge plateau from the Makarov Basin side. Further had cut the air gun array. along the oblique drift track across the Ridge (Fig. Conventionally powered icebreakers reached as far F1), the thickness of the conformably draped and as the North Pole for the first time during the 1991 well-stratified sediments increased to >850 m on the Expedition (Andersen and Carlsonn, 1992; Fütterer, Ridge’s flat-topped crest (Weber and Sweeney, 1985). 1992). Geophysical results from this expedition col- lected two important reflection profiles, AWI-91090 LOREX 1979 and AWI-91091, that crossed the Lomonosov Ridge North of the ARLIS II crossing of the Lomonosov between 87° and 88°N. These profiles imaged a ~450 Ridge, seismic reflection profiling was conducted m thick, well-stratified and apparently undisturbed from another drifting ice island in 1979, during the drape of sediments overlying a prominent acoustic Canadian the Lomonosov Ridge Experiment unconformity (Jokat et al., 1992) that spawned the (LOREX) (Weber, 1979) (Fig. F1). An air gun with a idea to conduct a paleoceanographic drilling expedi- 0.164 L (10 inch3) chamber was deployed as the seis- tion to this Ridge. mic source through a hole in the ice, in addition to a The use of US Navy nuclear submarines for geophys- multichannel system for deeper penetration using ical mapping was implemented through the Science explosive charges (Weber, 1979). LOREX also in- Ice Exercise program (SCICEX) (Newton, 2000). The stalled a 3 kHz subbottom profiler for higher resolu- development of the Seafloor Characterization and tion seismic acquisition (Weber, 1979). The seismic Mapping Pods (SCAMP), which hold a Chirp subbot- reflection data revealed the geological characteristics tom profiler, swath bathymetric profiler, and side of the Lomonosov Ridge along the ice station’s cross- scan sonar, was an essential part of the SCICEX pro- ing of a narrow section of the ridge near the North gram (Chayes et al., 1996). In 1999, the Lomonosov Pole (Fig. F1). The conclusion from these data was Ridge geophysical database was augmented with that the Ridge consisted of a series of tilted en-eche- acoustic data acquired during the SCICEX program lon fault blocks with their crests covered by thin using the SCAMP system mounted on the US nuclear (<75 m thick) drapes of unconsolidated sediments submarine USS Hawkbill (Edwards and Coakley, (Weber and Sweeney, 1985) (Fig. F2). Furthermore, 2003). substantial sediment erosion by currents was sug- gested in the area where the LOREX ice stations crossed the Lomonosov Ridge (Blasco et al., 1979). Data collected prior to Expedition 302 Arctic Ocean 1991/ARK-VIII/3 After the completion of the LOREX project in 1979, A chronologically arranged overview of several, but it took 12 y before the next seismic reflection data not all, geophysical data sets from the Lomonosov were acquired over the Lomonosov Ridge. This oc- Ridge is presented here. These data sets were col- curred during the Arctic 1991 expedition (Fütterer, lected from ice islands (ARLIS II and LOREX), subma- 1992; Jokat et al., 1992). The icebreakers Polarstern rines (SCICEX), and icebreakers (all others). Loca- (Germany) and Oden (Sweden) conducted this expe- tions of the seismic lines from these expeditions are dition together. Seismic equipment was towed be- shown in Figure F1. hind the Polarstern using a setup designed to func- tion in Arctic sea ice. This configuration was based ARLIS II 1961–1965 on previous Arctic seismic survey experiences that The second US Navy Arctic Research Laboratory Ice proved successful in this challenging environment Station (ARLIS II) was deployed on an ice island in (Grantz et al., 1986). 1961 ~350 km northwest of Point Barrow, Alaska Towing equipment behind an icebreaker in the cen- (Ostenso and Wold, 1977). During the ice island’s tral Arctic pack ice, often approaching 10/10 ice cov- drift journey toward the Fram Strait, it reached its erage, is problematic. The icebreaking strategy is typ- northernmost point on the Lomonosov Ridge flank, ically based on following existing leads—that is, by facing the Makarov Basin, in December 1963 (Fig. traveling along the periphery of larger ice floes. Be- F1). The scientific program included bathymetric cause of this strategy, the ship’s track becomes highly measurements, gravity observations, and continuous irregular, requiring the streamer length to be short- seismic reflection profiling. Reprocessing of the seis- ened to a few hundred meters. Proc. IODP | Volume 302 2 M. Jakobsson et al. ACEX geophysics In situations where the ship is breaking ice, the that chirp sonar data were collected from the central freshly broken lead may close rapidly behind the ice- Arctic Ocean. When compared with the Arctic 1991 breaker and trap the towed streamer. For this situa- expedition, the geophysical surveying was concen- tion, a short streamer is also necessary. Moreover, trated in areas of the Lomonosov Ridge located meter-sized pieces of broken ice floes are continu- closer to the Siberian continental margin (Fig. F1). ously forced under the icebreaker’s hull. These pieces Seismic reflection profiling was also conducted suc- can impact, with great force, seismic equipment cessfully along a transect crossing the Ridge near the towed near the ship. North Pole (Fig. F1). In total, more than 700 km of During the Arctic 1991 expedition, the seismic seismic reflection data were collected. An air gun ar- source consisted of two 3 L (~183 inch3) air guns that ray consisting of four sleeve guns with a total vol- were suspended below a 1 ton weight to keep the air ume of 5.5 L (~336 inch3) mounted in a steel cage guns as close to the ship’s fantail as possible (Jokat et and depressed by a 1 ton weight was initially used. al., 1992). A 300 m long 12-channel streamer was de- This array was lost because of impact with ice (Krist- ployed as a receiver.