Th or collective redistirbution or collective of this by of any article portion SPECIAL ISSUE FEATURE in published been has is article

Th e Coring Expedition (ACEX) Recovers Oceanography

A , Volume 4, a quarterly journal of Th 19, Number Cenozoic History or other means reposting, is only photocopy machine, permitted of the BY KATHRYN MORAN, JAN BACKMAN, AND THE IODP EXPEDITION 302 SCIENCE PARTY e Oceanography Society. Copyright 2006 by Th Society. e Oceanography

The Arctic Ocean is a small, nearly of total average area in all other oceans), the Lomonosov, were capped with thin-

landlocked ocean basin that is the shal- small basins (17 percent vs. 42 percent), ner sediment sequences that were ca. Th of approval the with lowest in the world (Figure 1). It has and large ridge areas (16 percent vs. 0.5–2 km thickness (Jackson and Oakey, maintained a polar location since form- 3 percent) (Jakobsson, 2002). 1990; Hall, 1979; Kristoffersen, 1990; ing in Early Cretaceous times (Grantz et During much of the Cenozoic, the Fütterer, 1992). Permission reserved. Society. All rights e Oceanography is gran al., 1990). The ocean’s two central deep shallow Arctic shelves and basins ac- In 1961, Heezen and Ewing (1961) all correspondence to: Society. Send [email protected] e Oceanography or Th basins (Amerasian and Eurasian) and cumulated massive amounts of sedi- recognized that the Mid-Atlantic Ridge its shelf seas occupy 2.6 percent of the ment from some of Earth’s largest rivers extended into the Arctic Ocean, this seg- global ocean area and less than 1 percent (Peterson et al., 2002). More than 6 km ment now known as the Gakkel Ridge. of the global ocean volume (Menard of sediment accumulated in the Amera- This realization led to the hypothesis and Smith, 1966; Jakobsson, 2002). The sian Basin since seafl oor spreading began that the was a con- mean water depth is ~ 1400 m, which opening this basin, ca. 120–130 million tinental fragment that broke from is ~ 2.5-km shallower than the global yeas ago (Ma) (Jackson and Oakey, 1990; the Eurasian continental margin dur- ocean mean depth (Jakobsson, 2002). Grantz et al., 1990), while the younger ing spreading along the Gakkel Ridge. The Arctic Ocean is further distinguished (~ 56 Ma) Eurasian Basin accumulated Aeromagnetic surveys supported this ted in teaching to and research. copy this for use Repu article from the other oceans by its large shelf sediment thicknesses of 2–3 km (Jack- assumption and suggested that Arctic areas (53 percent in Arctic vs. 13 percent son and Oakey, 1990). Ridges, such as seafl oor spreading initiated along the Gakkel Ridge during Chron24 near the MD 20849-1931, USA. 1931, Rockville, Box PO Society, e Oceanography Kathryn Moran ([email protected]) is Professor, Graduate School of Oceanography Paleocene-Eocene boundary (Wilson, and Department of Ocean Engineering, University of Rhode Island, Narragansett, RI, USA. 1963; Vogt et al., 1979). Reconstruction Jan Backman is Professor, Department of Geology and Geochemistry, Stockholm Univer- of the rift motion puts the Lomonosov sity, Stockholm, Sweden. Th e IODP Expedition 302 Science Party: David McInroy, Henk Ridge at the Barents/ margin in Brinkhuis, Steve Clemens, Th omas Cronin, Gerald Roy Dickens, Frédérique Eynaud, Jérôme the early Cenozoic. The Arctic ’91 expe- Gattacceca, Martin Jakobsson, Richard W. Jordan, Michael Kaminski, John King, Nalân Koc, dition, the fi rst non-nuclear icebreaker Nahysa C. Martinez, Jens Matthiessen, Th eodore C. Moore Jr., Matthew O’Regan, Jonaotaro effort to reach the North Pole (Fütterer, blication, systemmatic reproduction, Onodera, Heiko Pälike, Brice Rea, Domenico Rio, Tatsuhiko Sakamoto, David C. Smith, 1992), shot two seismic refl ection pro- Ruediger Stein, Kristen E.K. St. John, Itsuki Suto, Noritoshi Suzuki, Kozo Takahashi, Mahito fi les across the 40-km wide Lomonosov Watanabe, and Masanobu Yamamoto. Ridge. These profi les revealed a sediment

162 Oceanography Vol. 19, No. 4, Dec. 2006 Figure 1. Physiographic map of the Arctic Ocean (IBCAO, 2003), showing the location of the drill core sites (red dot) on the Lomonosov Ridge. Colors represent water depth where dark blue is ~ 4000 m and the Lomonosov Ridge (light blue) varies from 800 m to 1300 m.

sequence over 400-m thick on the ridge freshwater supply from the Nordic Seas, to the Integrated Ocean Drilling Pro- crest (Holland et al., 2001), indicating and a deepening of Arctic Intermediate gram’s Arctic Coring Expedition (IODP the presence of a unique archive of the Waters (Holland et al., 2001; Curry and ACEX) conducted in August 2004, the past 56 million years of sedimentation Mauritzen, 2005). As such, extracting a paleoceanographic record of the central and paleoclimate history in the central Cenozoic record that describes the pres- Arctic extended only to the mid-Pleis- Arctic Ocean. ence or absence of ice in the Arctic, its tocene (~ 200–500 thousand years ago), The signifi cance of this paleoenvi- associated impact on Earth’s albedo, and where analyses were based on short pis- ronmental record is rooted in the Arc- the temporal variations of surface and ton cores, rarely longer than 10 meters tic Ocean’s infl uence on global climate, deep-ocean temperature and salinity is (Backman et al., 2004). Pre-Pleistocene specifi cally in terms of sea ice and the of fi rst-order importance. material had rarely been recovered, and formation of cold, dense, bottom wa- when it was, only piecemeal. The pa- ters that drive global thermohaline cir- THE ARCTIC CORING leoenvironmental record compiled from culation (Broecker, 1997; Holland et EXPEDITION ACEX these short cores, although invaluable for al., 2001). Recent studies have demon- Our knowledge of the Arctic Ocean ba- recent reconstructions of central Arctic strated the Arctic Ocean’s critical role sin, limited by the logistical diffi culties glacial events, neither allowed the sci- in freshening the upper ~1.5 km of the of working in harsh, ice-covered regions, entifi c community to address divergent northern North Atlantic due to an in- is commensurate with our knowledge of hypotheses, nor address a series of long- crease in the export of sea ice, increased the other ocean basins 50 years ago. Prior standing questions concerning the earlier

Oceanography Vol. 19, No. 4, Dec. 2006 163 Vidar Viking by circling upstream in the fl owing sea ice, breaking the fl oes into smaller pieces that would not dislodge the drilling vessel from within a 75-m radius from a fi xed position. Despite thick and pervasive ice cover, the fl eet and ice-management teams suc- cessfully enabled the drilling team to recover cores from three sites. Ice condi- tions became unmanageable only twice, Figure 2. Aerial photograph of the three icebreakers during drilling forcing the fl eet to retrieve the pipe and Sovetskiy Soyuz operations. Th e (top) breaks the large, unbroken move away until conditions improved. fl o e s ; Oden (middle) breaks these pieces into smaller ice to allow the Vidar Viking (lower) to maintain station over the drill site. From the technological standpoint, ACEX was the fi rst expedition to drill in the central Arctic Ocean where heavy sea ice prevails year-round. The approach used proved successful and is applicable Cenozoic evolution of the Arctic Ocean 200 people, including scientists, technical for future scientifi c and exploration (Houghton et al., 2001). Closing this staff, icebreaker experts, ice management drilling in this challenging environment. knowledge gap required a new techno- experts, ship’s crew, and educators. logical approach. At the drill sites, temperatures hov- A 56 MA SEDIMENT RECORD To attack these important scientifi c ered near 0°C and occasionally dropped ACEX drilled and cored three sites on questions, the Ocean Drilling Program to -12°C. Ice fl oes 1 to 3 meters thick the Lomonosov Ridge within 20 km of (ODP) and the IODP developed a fun- blanketed more than 90 percent of the each other (Figure 1). The sites were damentally new approach to Arctic ocean surface, and ice ridges, several positioned along a site survey seismic Ocean studies, using multiple vessels to meters high, were encountered where refl ection profi le collected in 1991. This drill in deep water of the central Arctic fl oes converged. The ice drifted at speeds profi le was interpreted to represent a Ocean near the North Pole. ACEX used of up to 0.3 knots and changed direc- continuous Cenozoic sedimentary re- two large icebreakers to enable a third, tion over short time periods, sometimes cord atop rifted continental crust (Jokat outfi tted as a drillship, to maintain po- within an hour. et al., 1992). Although the sites are lo- sition over a site in heavy, moving sea The Swedish diesel-electric icebreaker cated up to ~ 15-km apart along the ice (greater than 90 percent sea surface Vidar Viking was converted into a drill- seismic line, continuity of the seismic cover) for extended periods (Figure 2). ship for this expedition by adding a geo- refl ectors among the sites and physical This approach overcame the diffi culty technical drilling system that was capable property data allow sediment cores re- of maintaining position over a drill site of suspending greater than 2000 m of covered from each site to be correlated to in waters that are blanketed in moving drill pipe through the water column and each other; they are interpreted together ice fl oes. In August 2004, the three ice- into the underlying sediments and by as a single depth time series spanning the breakers met at the ice edge, northwest creating a hole in the hull (moonpool) Cenozoic (Moran et al., 2006). of Franz Josef Land, and headed north, capable of accommodating the drilling Glimpses of pre-Miocene climatic as a convoy, to begin ACEX, IODP Expe- system (see Evans et al., this issue). The conditions in the central Arctic were dition 302. This expedition successfully two other icebreakers, a Russian nuclear meager prior to ACEX because they recovered core in water depths between vessel, Sovetskiy Soyuz, and the Swedish were based on only four short cores 1100 and 1300 m. ACEX involved over diesel-electric vessel, Oden, protected the (< 10-m long) from the western Arctic

164 Oceanography Vol. 19, No. 4, Dec. 2006 Ocean basin on the Alpha-Mendeleyev and Northern Hemisphere glaciations is boundary (Coxall et al., 2005) and syn- Ridge. These cores consisted of black reduced or even eliminated. Recent sup- chronous changes in calcium carbonate biosiliceous material, suggesting poorly port for the synchroneity of these events compensation depth (Tripati et al., 2005). ventilated bottom waters at ~ 70 Ma comes from detailed oxygen isotope re- Among our results, the best age con- (million years ago) and ~ 35 Ma, and an cords that document a sharp increase in trol spans the middle Miocene (Fig- estimate for Cretaceous sea surface tem- δ18O values across the Eocene-Oligocene ure 3). Paleoenvironmental analyses peratures of 15°C (Jenkyns et al., 2004). In the youngest part of the ACEX re- cord, the Plio-Pleistocene, a change to higher sedimentation rates at 1.9 Ma occurs at a time when Northern Hemi- sphere glaciation intensifi ed and global cooling accelerated. Immediately prior to this switch to higher rates, over the time interval ~ 3 Ma to 2.5 Ma, sediment physical properties show a large infl ux of coarser-grained sediment that is re- fl ected in the consolidation index, acous- tic velocity, and bulk density (Figure 3), potentially refl ecting increased sea ice and icebergs roughly coincident with the large expansion of continental ice sheets (Shackleton et al., 1984). ACEX initial results show that ice- rafted debris (IRD), in the form of fi ne to coarse sand and dropstones, was pres- ent as early as 45 Ma (Figure 3). Further analyses of the IRD will help to discern sea-ice versus iceberg delivery and peren- nial versus seasonal sea-ice regimes. For example, if the source of sea-ice IRD is from a location that is far from the Lo- monosov Ridge where multiple ice years are required to transport it, then we can discriminate seasonal versus perennial sea ice (Darby, 2003). These occurrences of IRD in ACEX cores suggest that sea ice Figure 3. Synthesis of the ACEX coring results showing the age model. Litho- and/or icebergs were present in the cen- logic units are identifi ed as separate colors. Th e “greenhouse” world of the Eocene is distinctly diff erent in sediment characteristics from the “icehouse” tral Arctic earlier than the late Tibetan world of the Miocene. A long hiatus (located in the upper orange shaded Plateau tectonic uplift event, as far back zone of the age model) separates sharp changes in the sediment properties as the Eocene. With an earlier inception above and below (bulk density, organic carbon content, occurrence of pyrite). Th e bulk density shows a normal increase in depth during both the green- of Northern Hemisphere glaciation, the house and icehouse times; the yellow solid lines are consolidation trends for lag between the onset of major Southern each unit and show a reverse density trend within the transition zone.

Oceanography Vol. 19, No. 4, Dec. 2006 165 indicate major changes in the Arctic site, although distal from the coastline, of this interval show that major changes basin environment since that time. Ear- suggest a relatively fresh Eocene basin, occurred to the hydrology of the region lier, during the Eocene, the Lomonosov strongly infl uenced by high produc- during the PETM warming (Pagani et al., Ridge was shallow and supported an up- tivity and the mixing and recycling in review). Within and below the PETM per water column dominated by fresh- of nutrients. interval, agglutinated benthic foramini- water conditions near the surface and At the base of the middle Eocene, a fers occur, typifying a shallow marine deeper conditions that preserved or- massive occurrence of the freshwater depositional environment during the ganic matter. Sometime after the middle hydropterid fern Azolla suggests greatly latest Paleocene through early Eocene Eocene, a major change in the Arctic reduced surface-water salinity or per- (Figure 3) (Moran et al., 2006). environment occurred, which is repre- haps even fresh surface-water conditions sented in our sediment record as a long at this time (49.5 Ma) (Brinkhaus et al., SUMMARY hiatus. Sedimentation, at a slow rate, was 2006). A concomitant occurrence of ma- The ACEX successfully recovered the fi rst probably re-established by the Miocene rine ebridian and diatom assemblages major paleoceanographic record from (Figure 3), but, with the exception of the points to a warm and highly stratifi ed the central Arctic using IODP’s new one interval, was devoid of benthic or shallow upper water column occurring Mission-Specifi c Platform operational planktonic remains until ~ 14 Ma when at the end of the early Eocene (Figure 3). approach. The results are exciting and sedimentation rates increased. During the earliest Eocene, the (sub-) reveal a constricted Arctic Ocean basin The Paleogene ACEX sedimentary re- tropical dinofl agellate species Apecto- during the early Cenozoic, dominated cord differs signifi cantly from the overly- dinium augustum dominated the fos- by shallow water in the early Eocene and ing sediments in terms of physical prop- sil record in the pyrite-rich mudstone warm surface water conditions during erties and an increase in organic carbon cores of Unit 3. A global increase in A. the PETM. A change from low to high (Figure 3) (Moran et al., 2006). Below augustum occurred during the Paleocene organic carbon sediment occurs later at the hiatus, in the middle Eocene, dino- Eocene Thermal Maximum (PETM) ~ 49.5 Ma during a time of fresh, warm fl agellate cysts, diatoms, ebridians, and (Crouch et al., 2001), the largest known surface water. The initiation of Northern silicofl agellates are common to abun- climatic warming of the Cenozoic. Re- Hemisphere cooling began much earlier dant and typify an ice-free, brackish to cords from mid-latitude oxygen isotopes than previously estimated and may have sometimes fresh, productive, and warm (δ18O) of biogenic carbonate suggest been synchronous with the onset of Ant- environment. Collectively, these assem- a sea-surface warming of 4–5°C. This arctic glaciation. ACEX also confi rmed blages represent a highly productive ne- warming perturbed the hydrologic cycle, the previously hypothesized continental ritic (shallow water) environment. Black resulting in a surface-water salinity in- origin of the Lomonosov Ridge by recov- fi rm clay in this interval is characterized crease at low latitudes (Zachos et al., ering core material deeper than the Ce- by high organic content, fi ne lamina- 2003) that was probably matched by a nozoic sedimentary sequence across the tions, and pyrite similar to other black salinity decrease at high latitudes. ACEX regional seismic unconformity. shale deposits, suggesting shallow-water analyses of the A. augustum using TEX86 deposition (Figure 3) (Wignall, 1994). A (Sluijs et al., 2006) show that even at ex- ACKNOWLEDGEMENTS shallow-water setting is consistent with treme high latitudes in the Arctic Ocean, We thank the IODP ECORD (European the physiography of the Arctic Ocean sea-surface temperatures were on the Consortium of Ocean Research Drilling) basin where, for example, over 40 per- order of 20°C. The geographic location Science Operator (ESO), led by the Brit- cent of the modern seafl oor is shallower of the ACEX sediment record was close ish Geological Survey; the Swedish Polar than 200 m. This percentage was prob- to its current position during the time Research Secretariat who provided ship- ably greater in Eocene times when higher of the PETM, which suggests that this time for Oden as well as the ice and fl eet sea level fl ooded much of what is now temperature probably represents a sea- management; icebreaker Captains Årnell, dry land. Thus, results from the ACEX sonal summer high. Additional studies Backman, Davidjan, Haave, Shirley,

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