Fig. 1. The Arctic Ocean drilling carried out on the Lomonosov Ridge by IODP Leg 302 ACEX.

Fig. 2. D/V Vidar Viking of Norway with nuclear I/B Sovietsky Soyuz of Russia in far distance.

The Arctic coring expedition (ACEX) of Leg 302 of the Integrated Ocean Drilling Program (IODP) was carried out during August-September 2004 for 39 days, in which two members of the Kyushu University participated. It was highly successful in obtaining 428m long cores of extremely important sub-seafloor sediment samples from the region (〜88°N) near the North Pole for the first time. The expedition team, sailed on three vessels of different tasks assigned respectively, was lead by Professor Jan Bachman of the Stockholm University of Sweden and Dr. Kate Moran of the University of Rhode Island of the United States of America. The shipboard scientific team consisted of 17 highly motivated, ambitious, and talented scientists. including two graduate students from eight different nations from many parts of the world. Among them, our representatives from Kyushu University were Mr. Jonaotaro Onodera and Professor Kozo Takahashi, Department of Earth & Planetary Sciences, Graduate School of Sciences, who took responsibility as shipboard paleontologists. We embarked on icebraker R/V Oden, belonging to the Royal Academy of Sweden, and departed from the port of Tromso located at 70°N, Norway on the 7th of August, 2004. The cruise in the mid night sun environment was very special to most of us living in the light/dark environment together with the experience in ice braking. After sailing in sea-ice free waters, R/V Oden met with two counterpart vessels near the ice edge at about 82°N: nuclear ice braker I/B Sovietsky Soyuz (〜23,000 tons) of Russia; and drilling vessel D/V Vidar Viking of Norway.

It was indeed a significant achievement in filling the gap of human knowledge in terms of obtaining core samples from a time span of almost continuously blank geological information. This is because that conventional so-called piston coring had been usually limited to about 10m in depth below the sea-floor and thus covered the sediments belonging to the time span from the present up to about 100,000 to 300,000 years or less. This has been largely due to our inability to bring in a drilling vessel into a special ice laden environment like the Arctic Ocean thus far. However, the IODP made the impossible possible and the ACEX team came back with flying colors with a load of sediment samples that no one had ever seen before! Thanks to the ice management team on board R/V Oden and I/B Svietsky Soyuz, which consisted of ten able people including captains of other ships, and physical oceanographers from Canada, Russia, Sweden, Finland, England, and so on. Without their assistance, the ACEX would have not been able to drill a core. Such a sea-floor drilling experience in the ice laden environment had never been achieved in the relatively deep water of depths greater than 1200m on the Lomonosov Ridge (〜88°N) near the North Pole. The ice brakers kept crushing the Arctic sea-ice of thickness varying from 2m to 7m into fairly small size. This kept D/V Vidar Viking continuously protected from the great horizontal pressure of large sheets of sea-ice and made the drilling vessel stationary at the spot exactly above a drilling hole. Practically, the ship kept propelling at the speed of about 0.2 knots in the midst of the mashed potatos-like sea-ice condition to keep her at the drill spot due to flow of water and sea-ice (Figs. 2-3). The drilling operation was sometimes barely accomplished in extremely severely cold air temperatures such as -12℃ when drilling fluids began freezing, even though it was summer and with the midnight sun (Fig. 4).
The reconstruction of the geography and the environmental conditions of the Arctic Ocean are invaluable for understanding the history of the Earth, including climate change. Today we humans live in the short interval of the so-called "the interglacial period," which sits in the midst of the socalled the glacial ages. The planet Earth experienced with gradual cooling towards the glacial age since the warm period of the Cretaceous Period in the Mesozoic Era, about 75 million years ago (Ma). In this cooling transition we saw an emergence of continental ice sheets (Greenland, Fenoscandia, & Laurentide) at about 2.65 Ma in the Northern Hemisphere. We do not understand the mechanism responsible for the build-up of the Northern Hemisphere glaciation while we understand reasonably well the southern Hemisphere counterpart (〜36 Ma of the Antarctic ice sheet due to the opening of the Drake Passage〜40 Ma). The ACEX sediment samples (Figs. 5-6) may be able to provide us with a clue in this regard. Only the sediment samples can tell the truth. The initial formation of sea-ice in the Arctic Ocean appears to be at least 17 Ma and possibly as old as 44 Ma, based on the ship board observations of ice rafted debris of pebbles that can be transported only by sea-ice or icebergs but not by water or winds. Today, the anthropogenic global warming calls for attention and the possibility of the quantitative decrease of the Arctic sea-ice has been debated. For the possible and likely future event of which we lose the Arctic sea-ice entirely, we must learn about the ice free Arctic Ocean environment from past examples (the Paleocene-Eocene thermal maximum: Fig. 7) with the invaluable ACEX core samples just obtained.
The ACEX clarified the detailed environmental changes in the Arctic region during the past 56 Myrs due to the successful drilling of 428m below the sea-floor, including 400m of sediments accumulated in the Arctic and the remaining underlain 28m of the basement materials deposited at the edge of the Siberian continent up to 〜80Ma prior to its migration to the current location. However, one thing it did not uncover was the puzzle of the Pacific-Atlantic Ocean connection in terms of timing and mechanisms. It is clear that much of the past 56 Myrs of the paleo-Arctic Ocean was with shallow brackish and biologically productive waters and considerably semi-enclosed conditions. The present day analogy is the Black Sea with anoxic subsurface and deeper waters. Based on what we saw in the record of the Arctic sediments from the ACEX, we must fill the last gap in the blank spot in our knowledge map: the Bering and Okhotsk Seas. Particularly the former is located in the pathway of the Pacific and Atlantic Oceans via the Arctic Ocean. The currently proposed IODP drilling of the Bering and Okhotsk Seas by the author and other international scientists, if materialized, will no doubt provide understanding of the circulation which could have served a significant role in transport of mass and heat between the major basins of the past world. In all, the understanding of the subarctic marginal seas together with the Arctic environmental changes will give us significant progress in the uncovering of the global environmental and climate change history.

Fig. 3. Satellite view: sea-ice braking by two ice-brakers and positioning for drilling by D/V Vidar Viking (courtesy of IODP Leg 302 ACEX sea-ice management group).	Fig. 4. Commuting by helicopters in the midnight sun from R/V Oden to D/V Vidar Viking.	Fig. 5. The microscopy work in the paleontology laboratory on board R/V Oden belonging to the Royal Swedish Academy of Science; most of the scientists stayed on this vessel.

Fig. 6. Automated non destructive MST measurements of cores (e.g., density, magnetic susceptibility) on board D/V Vidar Viking.	Fig. 7. Silicoflagellates, diatoms, and ebridians lived in the paleo-Arctic Ocean at about 44 Ma. The environmental conditions are determined as shallow, warm, productive, brackish, and semi-closed by these abundant and well preserved diversified siliceous microfossils.