First Paleoceanographic drilling of Cenozoic sediments in the central Arctic Ocean

Sea-ice concentration in the central Arctic Ocean is typically 85–95% during summers and much of this ice consists of 2–5m thich multi-year ice. The Arctic Coring Expedition (ACEX, IODP Exp. 302) operated continuously in >90% ice cover while on site, which was dominated by multi-year ice. The primary goal of ACEX was to continuously recover a >400m thich sediment sequence draping the crest of the Lomonosov Ridge in the central Arctic Ocean between 87°N and 88°N. This would meet the fundamental paleoceanographic objective of ACEX, namely to determine the Cenozoic paleoenvironmental evolution in the central Arctic Ocean. ACEX was executed using three platforms: Vidar Viking (drilling platform), Oden (support icbreaker, operational HQ, research lab and science center), and Sovetskiy Soyuz (nuclear support icebreaker). Over a period of 22.5 days from August 15th to September 5th, the drill-ship lept its position over site in about 1200m water depth for 15.1 days (~97% of the time within ±50m) through manual steering of the ship. During that period, three scientists were helicoptered to/from the drill-ship twice/day. Three sites were drilled along a single reflection seismic profile (AWI-91090) showing a coherent seismostratigraphy. Ice drift determined the exact drilling location. A total of 495.47 m was cored, resulting in a recovery of 339.06 m in 110 XCB-cores and 8 APC-cores (68% recovery). The deepest hole was terminated at 427.9 mbsf. Preliminary analyses of about 4% of these sediments were made onboard the Oden during the offshore phase. The remaining sections were subsequently split and described by the onshore science party at Bremen University. Biogenic carbonate were encountered only in the Holocene and uppermost Pleistocene sections. The Pleistocene and Neogene sections show biogenic components. It follows that the central Arctic Ocean was not a sediment starved basin during these times. A 22 m thich (171-193 mbsf) palynologically barren interval separates the overlying middle Miocene from the underlying middle Eocene, which presumably preserves some of the intervening lower Neogene and upper Paleogene sections. Sand lenses occur frequently down to 198 mbsf (middle Miocene) and cm-sized pebbles occur more or less continuously down to 239 mbsf (middle Eocene). Both sand and pebbles are interpreted to represent ice-rafted debris. The average sedimentation rate in the middle Eocene to uppermost Paleocene section is about 1.6 cm/ka. A 107 m thick middle to upper lower Eocene interval hosts rich biosiliceous assemblages in which radiolarians by-and-large are missing, probably indicating reduces surface water salinities. The lower Eocene to uppermost Paleocene interval, below the biosilica interval, consists of dark clays/shales often showing sub-mm laminations and presence of pyrite nodules. The transition to the underlying sedimentary bedrock was not sampled because of recovery problems. The oldest sediment sampled consists of 1.39 m of shallow-marine dark layey mud of Campanian age.