Another exceptional character of the Arctic Ocean is that it is almost entirely covered by a lid of floating sea-ice, extending at least 1,000 km in all directions from the North Pole. The sea-ice coverage is over 97% during winters and 85-95% during summers. First-year ice is generally not thicker than 2 m. Sea-ice that survives one or several summer melt cycles is referred to as multi-year ice, is generally 3-5 m thick and covers 60% of the ice-pack area. Virtually all ice-floes are criss-crossed by even thicker pressure ridges, the scars of colliding floes. Arctic's sea-ice is ultimately melted in the Nordic Seas and the North Atlantic after having been transported there by currents via the narrow Fram Strait, separating Svalbard from Greenland. Drift velocities is commonly on the order of a few tenths of a knot within the Arctic Basin, although large variations have been observed with respect to both speed and direction.

This sea-ice infested environment has effectively prevented scientists from performing systematic investigations of the basins and their geological histories. Even the simplest of questions, such as the time-dependent distribution of sea-ice, has remained unanswered because of the logisitical difficulties involved with the permanent sea-ice. The importance of this small ocean basin lies in its geographic position, around the North Pole, and its lid of floating sea-ice. Because of its reflectivity, sea-ice influences Earth's solar radiation balance and thus the global climate. The sea-ice also exerts an influence on both the distribution of fresh-water and the global thermohaline circulation through production of ventilated waters to the deep oceans, which makes the Arctic to one of the "lungs" of the World Ocean.

Moreover, a major element in the evolution of Cenozoic paleoclimates has been the transformation from warm Eocene oceans with low latitudinal and bathymetric thermal gradients into the more recent modes of circulation characterized by strong thermal gradients, cold deep oceans and cold high latitude surface waters. About 92% of all water in today's oceans are colder than ~10°C. In the Eocene, 50 million years ago, all water in the oceans was warmer than 10°C. Bottom temperatures in the early Eocene, the time of maximum Cenozoic warmth, were on the order of 12°C, helping prevent large-scale continental ice sheets to develop.

The transition to today's world, Antarctica and Greenland covered by continental ice-caps, and a seasonally variable but persistent sea-ice cover in the Arctic, is linked the change in climate that increased both latitudinal and bathymetric temperature gradients. It follows that throughout the course of the Cenozoic, the climate on Earth has changed from one extreme (early Paleogene greenhouse lacking ice) to another (Neogene icehouse with bipolar glaciation). It has long been recognized that our lack of knowledge about the role the Arctic played in the maintenance and development of this climates is a major gap in our ability to understand and model global environmental change. The history of Arctic paleoceanography is so poorly known that we still may look at the recovery of any part of Arctic's climate archive, as preserved in its sediments, as a true exploration that will increase our knowledge and understanding of this critical region. It is against this scientific background that the Arctic Coring Expedition (ACEX) was developed.

The scientific inspiration that subsequently led to ACEX surfaced aboard F/S Polarstern in 1991 when two reflection seismic profiles (AWI-91090 and AWI-91091) were acquired by Wilfried Jokat and Yngve Kristoffersen across the Lomonosov Ridge between 87° and 88°N. These profiles show a neatly draped sediment sequence being over 400 m thick, at a modest water depth of ca 1200 m, which were considered to represent an undisturbed and continuous record of lower Eocene to Recent sediments. Continuous sampling and recovery of this sequence were thought to provide access to a unique archive of climatic and paleoceanographic information, enabling us to unravel the long-term Cenozoic paleoenvironmental and paleoclimate history of the central Arctic Ocean.

An unsuccessful attempt to recover this 400 m thick sediment sequence on the crest of the Lomonosov Ridge near 88°N was made in 1996 in a single-ship operation.Two years later this lesson resulted in an ODP proposal that envisioned a three-ship operation composed of a drilling platform and two supprt ice-breakers: the Oden and one nuclear powered ice-breaker. The group of scientists behind the development of this proposal was enlarged to about a dozen persons after it being well received by the ODP. The proposal survived the transition from ODP to IODP and a plethora of planning efforts.

Three ice-breakers, the converted ice-breaker Vidar Viking, the Oden and the nuclear powered Sovetskiy Soyuz, met at enembayment of the ice edge near 83°N and 42°E, for a joint transit to the first drill site at 87°55’N and 138°32’E, arriving there on August 14, 2004. An intensive period followed, involving drilling beginning on 15 August, moving a few nautical miles to new drilling site locations along the approved reflection seismic line, and logging, until September 5th, when Hole M4000C and ACEX drilling was terminated.

We anticipated to recover muds with ice-rafted sand in the upper part of the sediment column, changing progressively towards mudstone in the bottom part of the sediment drape, perhaps with some biosilica in the Eocence, judging from a T3 ice-island gravity core from the Alpha Ridge, the sole evidence of Paleogene biosiliceous sedimentation prior to ACEX. Age information was a priority during the expedition. But prior to ACEX, we had no knowledge about which micropaleontological groups would be preserved in the Cenozoic sediments on the Lomonosov Ridge.

Thus, an unusally large group of nine biostratigraphers was invited as shipboard scientists, representing expertise of calcareous nannofossils, diatoms, foraminifers, ostracodes, palynology, radiolarians, and silicoflagellates, in order to establish an age model. Age control represents the crucial first step towards understanding the paleoenvironmental development in the central Arctic Ocean.

Post Cruise Core Opening
In November 2004, the entire ACEX science party, half of which was shipboard scientists, met at the University of Bremen to split, describe and sample the recovered cores.

We are still debating the age model. Yet, ice-rafted sand-lenses occur throughout the uppermost 198 m of the sediment column, having an early Miocene age of about 18 Ma. A surprise was the presence of a major hiatus, not evident in the pancake-layer stratigraphy as observed in the critical reflection seismic profiles, at 198.13 mbsf. The immediately underlying sediments are of middle Eocene age, on the order of 44 Ma. Dropstones occur fairly consistently throughout the Neogene sediments and into the underlying middle Eocene unit. The deepest (=oldest) dropstone, a 1 cm piece of gneiss, occurs at 239.34 mbsf, resting in sediments of about 46 Ma. At least seasonal sea-ice must have existed at that time, perhaps like in the present Baltic Sea, used for recreational swimming during summers and partly frozen over during winters. Considering the consistent presence of sand lenses and the cyclic lithological expression throughout the Neogene sediments, they may very well represent continuous presence of sea-ice. The cause of the early Miocene through middle Eocene hiatus on the Lomonosov Ridge is still unresolved. A cause linked to the largee-scale circulation history of the Arctic Ocean driven by tectonics appears as a reasonable path to follow.

Core sections from representative units/subunits.