Aim of the project

The Arctic Ocean and its marginal seas play a fundamental role in the global ocean/climate system. The dense, cold, bottom waters of most of the world’s oceans, which originate in the Nordic seas, strongly influence global thermohaline circulation, driving world climate. The permanent Arctic sea ice cover has a tremendous influence on the Earth’s albedo and the distribution of fresh water. The sea ice cover varies both seasonally and over longer time periods and thus has a direct influence on global heat distribution and climate. While understanding the history of the Arctic Ocean is critical for climate, ocean circulation or tectonic models that would be truly global, the logistical difficulties associated with the work in this remote and harsh region have prevented us from gathering the critical data needed to document the role of this key region in the development and maintenance of the global climate system.

The primary goal of IODP Expedition 302 (7 August-13 September 2004), ACEX, was the continuous recovery of a >400 m thick sediment sequence draping the crest of the Lomonosov Ridge in the central Arctic Ocean between 87°N and 88°N. This would meet the fundamental paleoceanographic objectives of ACEX, namely to determine the Cenozoic paleoenvironmental evolution in the central Arctic Ocean and to decipher the Arctic’s role in the global development from the Paleogene greenhouse (warm climates lacking polar ice-caps) to the Neogene icehouse (cold climates with polar icecaps).

Another major goal was to sample the transition across the regional unconformity to establish the pre-Cenozoic environmental setting of the ridge.

These sediment sequences represent a unique archive of the past 55 million years of paleo-environmental evolution in the central Arctic Ocean, whereas the transition into the acoustic basement and its uppermost parts represent a similarly unique archive of the early tectonic evolution of the Eurasian Basin.

Prior to ACEX, knowledge about the paleo-environmental history of the central Arctic Ocean was limited to short box-, gravity- and piston-cores (generally

The ACEX program was developed from ODP/IODP Proposal 533 by Jan Backman, Kate Moran, Bernard Coakley, Margo Edwards, Rene Forsberg, Ruth Jackson, Wilfried  Jokat, Yngve Kristoffersen, Larry Mayer, Martin Jakobsson, Evgeny Musatov and Nikita Bogdanov. Wilfried Jokat and Yngve Kristoffersen collected all airgun seismic survey data; Bernard Coakley, Margo Edwards and Martin Jakobsson provided high-resolution chirp sonar, sidescan sonar and bathymetry data. The geophysical site survey data for ACEX were collected through a series of international scientific programs organized and financed by the following operators: 1991 (SPRS and AWI), 1996 (SPRS and AWI), 1998 (AWI), 1999 (USNSF- SCICEX), and 2001 (SPRS).

Offshore phase

ACEX began when ODP Proposal 533 was initially submitted in March 1998. This proposal was ranked #1 by ODP in 2000 and 2001, and subsequently also ranked #1 by IODP in 2002. Because of the high ranking JOIDES established an Arctic Detailed Planning Group (ADPG) in 2000 that by and large laid the foundation for the future ACEX planning efforts. In the ADPG, presented and accepted in August 2001 (JOIDES Journal 2001. Vol. 27, 16-27), the highest ranked and preferred platform option consisted of three vessels: ” … a drilling platform, with two supporting icebreakers, a 75 000 HP Russian nuelear icebreaker … and one hunter icebreaker (HIB, there are several to choose from including the Oden … ”. The Swedish Polar Research Secretariat committed Oden to ACEX for a duration of 30 days as a Swedish contribution to IODP and ECORD.

The offshore phase of ACEX was executed using the following three platforms: Vidar Viking (drilling platform), Oden (project operational HQ, fleet and ice management center, IT and communications center, research lab and science center), and Sovetskiy Soyuz (supporting icebreaker). The route to and from the Lomonosov Ridge is presented in figure 1. Vidar Viking began to keep station over the first coring site at 1 100 on 15 August and ended station keeping over the last site at 1 400 on 5 September. Over this period of 21.1 days, Vidar Viking managed to keep its position with phenomenal precision, -98% of the time within ±50 m, in >90% ice cover (largely multi-year ice) for 15.1 days through manual driving of the ship, a truly remarkable achievement by Captain Jørgen E. Haave and his mates. This was made possible through the excellent service provided from weather prognosis, wind forecasting, and ice drift predictions using ice drift buoys helicoptered out and placed on approaching ice floes. The three ships are illustrated in photograph 1.

The deepest hole was terminated at 427.9 metres below seafloor. A total of 495.47 m was cored, resulting in a recovery of 339.06 m in 8 APC-cores, 110 XCB-cores and one Wash-core (68.4% recovery). Preliminary analyses of about 4% of these sediments were made onboard Oden during the offshore phase of ACEX.

Preliminary results

The sites are located only a few nautical miles apart along a single seismic line (AWI-91090), showing an identical and coherent Cenozoic seismostratigraphy. Preliminary results from shipboard investigations of care-catchers describe a thick Middle Miocene through Pleistocene sequence that shows large amplitude, cyclic variability in the density, magnetic susceptibility and acoustic velocity of the sediments. Biogenic carbonate in the form of foraminifers, ostracodes and calcareous nannofossils were encountered only in the Holocene and the uppermost Pleistocene section. The current age control in the Pleistocene and Neogene sections hence relies on dinoflagellate cyst biostratigraphy, indicating a Pleistocene sedimentation rate on the order of 2 cm/ka. Late and Middle Miocene sediments were deposited at rates varying between about 1 and 2 cm/ka. A 24 m thick interval separates the overlying Middle Miocene from the underlying Middle Eocene and presumably preserves some of the early Neogene and late Paleogene sections. This interval shows changes in physical properties and, from visual inspection through the core liners, pronounced colour variability.

Dinoflagellate cysts, diatoms, ebridians and silicoflagellates are common to abundant in the Middle Eocene section, which bottoms in a spectacular layer showing massive occurrences of glochidia and massulae (megaspores) of the freshwater hydropterid fern Azolla at the Early/Middle Eocene boundary, suggesting strongly reduced surface water salinity or perhaps even a brief episode of fresh water conditions at the surface. It is yet not known if the Azolla spores represent an indigenous signal, indicating fresh-to-nearly-fresh surface waters, or if they have been transported into a marine Arctic basin from a neighbouring freshwater system. However the sporadic and rare occurrences of radiolarians suggest that Arctic’s surface water salinities were indeed reduced throughout the Eocene interval containing biosilica. Biosilica is not present prior to the late Early Eocene.

The dinoflagellate species Apectodinium augustum occurs abundantly at Around 380 m in pyrite-rich mudstones, indicating that the Initial Eocene Thermal Maximum interval has been recovered, during which the Arctic Ocean experienced surface temperatures on the order of 20°C. Benthic foraminifers suggest that the Early Eocene through latest Paleocene sediments were deposited in a shelf environment. Mudstone of Late Paleocene age rests unconformably on Campanian marine sands, sandstone and mudstone.