British Geological Survey, United Kingdom

During a follow-up expedition, we will conduct the first multidimensional, cross-hole experiments attempted in the oceanic crust, including linked hydrologic, microbiological, seismic, and tracer components. After completion of drillship operations, we will initiate multiyear tests using the new network of subseafloor observatories, allowing us to examine a much larger volume of the crustal aquifer system than has been tested previously. By monitoring, sampling, and testing within multiple depth intervals, we can evaluate the extent to which oceanic crust is connected vertically and horizontally; the influence of these connections on fluid, solute, heat, and microbiological processes; and the importance of scaling on hydrologic properties. This work is helping us to understand the nature of permeable pathways, the depth extent of circulation, the importance of permeability anisotropy, and the significance of hydrogeologic barriers in the crust. ACEX's destination was the Lomonosov Ridge, hypothesized to be a sliver of continental crust that broke away from the Eurasian plate at approximately 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 ( approximately 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 approximately 170 m represents a record of the past approximately 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.