Next Section | Table of Contents
doi:10.2204/iodp.pr.302.2005
ABSTRACT
The first scientific drilling expedition to the central Arctic Ocean was completed in late summer 2004. Integrated Ocean Drilling Program Expedition 302, Arctic Coring Expedition (ACEX), recovered sediment cores deeper than 400 meters below seafloor (mbsf) in water depths of ~1300 m at the top of the world, only 250 km from the North Pole.
ACEX'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 continued 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) ensures that sediments atop the ridge are free of turbidites. The primary scientific objective of ACEX 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 facing ACEX was maintaining the drillship's location while drilling and coring 2–4 m thick sea ice that moved at speeds approaching half a knot. Sea-ice cover over the Lomonosov Ridge moves with 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 offshore results, based on analysis of core catcher sediments, demonstrate that biogenic carbonate only occurs in the Holocene–Pleistocene interval. The upper ~170 m represents a record of the past ~15 m.y. composed of sediment with ice-rafted sediment and occasional small pebbles, suggesting that ice-covered conditions extended at least this far back in time. Details of the ice cover, timing, and characteristics (e.g., perennial vs. seasonal) await further study. Earlier in the record, spanning a major portion of the Oligocene to late Eocene, an interruption in continuous sedimentation occurred. This may represent a hiatus encompassing a time interval of nondeposition or an erosional episode that removed sediment of this age from the ridge. The sediment record during the middle Eocene is of dark, organic-rich siliceous composition. Isolated pebbles, interpreted as ice-rafted dropstones, are present down to 239 mbsf, well into the middle Eocene section. An interval recovered around the lower/middle Eocene boundary contains an abundance of Azolla spp., suggesting that a fresh/low salinity surface water setting dominated the region during this time period. Although predictions based on geophysical data had placed the base of the sediment column at 50 Ma, drilling revealed that the latest Paleocene to earliest Eocene boundary interval, well known as the early Eocene Thermal Maximum (EETM), was recovered. During the EETM, the Arctic Ocean was subtropical with warm surface ocean temperatures. ACEX penetrated into the underlying sedimentary bedrock, revealing a shallow-water depositional environment of Late Cretaceous age.