Proceedings of the Integrated Ocean Drilling Program 302: Arctic Coring Expedition (ACEX)

The first scientific drilling expedition to the central Arctic Ocean, Integrated Ocean Drilling Program Expedition 302, Arctic Coring Expedition (ACEX), recovered sediment cores to 428 m below seafloor (mbsf) in water depths of ~1300 m, 250 km from the North Pole. Expedition 302’s destination was the Lomonosov Ridge, hypothesized to be a sliver of continental crust that broke away from the Eurasian plate at ~56 Ma. As the ridge moved northward and subsided, marine sedimentation occurred and continues to the present, resulting in what was anticipated from seismic data to be a continuous paleoceanographic record. The elevation of the ridge above the surrounding abyssal plains (~3 km) ensured that sediments atop the ridge were free of turbidites. The primary scientific objective of Expedition 302 was to continuously recover this sediment record and to sample the underlying sedimentary bedrock by drilling and coring from a stationary drillship. The biggest challenge during Expedition 302 was maintaining the drillship’s location while drilling and coring in 2–4 m thick sea ice that moved at speeds approaching 0.5 kt. Sea-ice cover over the Lomonosov Ridge moves with one of the two major Arctic sea-ice circulation systems, the Transpolar Drift, and responds locally to wind, tides, and currents. Until now, the high Arctic Ocean Basin, known as “mare incognitum” within the scientific community, had never before been deeply cored because of these challenging sea-ice conditions. Initial results reveal that biogenic carbonate is present only in the Holocene–Pleistocene interval. The upper 198 m represents a relatively high sedimentation rate record of the past 18 m.y.  composed of sediment with ice-rafted debris and dropstones, suggesting that ice-covered conditions extended at least this far back in time. A hiatus at 193.13 mbsf spans 25 m.y. from the early Miocene to the middle Eocene (~18–43 Ma). The sediment record during the middle Eocene is of dark, organic-rich biosiliceous composition. Isolated pebbles, interpreted as ice-rafted dropstones, are present to 239 mbsf, well into the middle Eocene interval. Around the lower/middle Eocene boundary an abundance of Azolla spp. occurs, suggesting that a fresh and/or low-salinity surface water setting dominated the region during this time period. Drilling recovered the uppermost Paleocene to lowermost Eocene boundary interval, well known as the Paleocene/Eocene Thermal Maximum (PETM). During the PETM, the temperature of the Arctic Ocean surface waters exceeded 20°C. Drilling during Expedition 302 also penetrated into the underlying sedimentary bedrock, revealing a shallow-water depositional environment of Late Cretaceous age.