Introduction to special section on Cenozoic Paleoceanography of the Central Arctic Ocean

[1] In late summer 2004, the Integrated Ocean Drilling Program (IODP) conducted one of the most transformational missions in the almost 40 year history of scientific ocean drilling: the Arctic Coring Expedition (ACEX) [Backman et al., 2006; Moran et al., 2006a]. This technically challenging expedition [Moran et al., 2006b] to the Lomonosov Ridge near 88 N recovered the first long-term Cenozoic sediment record from the Arctic Ocean–extending previous records from 1.5 Ma to an unprecedented 56 Ma. Glimpses of the breadth of this transformation were seen during the ACEX cruise when the massulae from freshwater ferns (Azolla) were found in 49 Ma old sediments [Brinkhuis et al., 2006] and the presence of Apectodinium augustum confirmed that the Paleocene-Eocene thermal maximum (PETM) (55 Ma) was unexpectedly recovered [Sluijs et al., 2006]. [2] Soon after the expedition, when the cores were opened and analyzed, ice-rafted debris was found in surprisingly old (46 Ma) sediments [Moran et al., 2006a]. These middle Eocene sediments are characterized also by their high organic carbon content [Stein et al., 2006]. The exciting early results attracted other investigators that expanded the scientific investigating team to more than 40 people. Analyses were extended to include studies that revealed surprisingly high Arctic Ocean surface water temperatures [Sluijs et al., 2006] and an amplified hydrological system [Pagani et al., 2006] during the PETM. Initial analyses revealed an extensive hiatus encompassing about 26 million years that occurred below a short interval showing starkly alternating black and white layers that is now dubbed the ‘‘zebra’’ interval; thus the time interval from the late early Miocene (18 Ma) to the middle middle Eocene (44 Ma) is missing. Although the hiatus is a lost window in time for the Arctic paleoclimate record, it spawned other studies that integrated the regional tectonic history with ACEX results revealing a major oceanographic reorganization at 17.5 Ma–ventilation of the Arctic Ocean to the North Atlantic through the Fram Strait [Jakobsson et al., 2007]. In terms of ventilation and water mass history, Neogene neodymium isotope data reveal that Arctic Intermediate Waters originate from brine formation on Eurasian shelf areas between 15 and 2 Ma, and during glacial periods throughout Pleistocene times [Haley et al., 2008a]. They conclude that this mode of Arctic circulation differs markedly from today’s situation, showing relatively high contributions of North Atlantic Intermediate Water and minor input from brine formation. [3] In this special section, the transformation continues. Results from the large ACEX scientific ‘‘family’’ include: a robust age model; detailed analyses of the middle Eocene that document a unique brackish water environment; sea ice and iceberg history reconstructions as well as the provenance of ice-rafted debris from the Eocene to present; evolution of depositional environments; times of isolation and connection to the global ocean; geochemical analyses of the organic carbon-rich sediments; and unique applications of high-resolution proxies and cyclostratigraphy. Highlights from this special section are presented here.