A Cenozoic History of the Arctic Ocean
Total Page：16
File Type：pdf, Size：1020Kb
Th or collective redistirbution of any portion article of any by of this or collective redistirbution SPECIAL ISSUE FEATURE articleis has been published in Th e Arctic Coring Expedition (ACEX) Recovers Oceanography A 19, Number journal of Th 4, a quarterly , Volume Cenozoic History permitted photocopy machine, only is reposting, means or other of the Arctic Ocean BY KATHRYN MORAN, JAN BACKMAN, AND THE IODP EXPEDITION 302 SCIENCE PARTY e Oceanography Society. 2006 by Th Copyright 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. with the approval of Th 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). gran is e Oceanography All rights Society. reserved. Permission al., 1990). The ocean’s two central deep shallow Arctic shelves and basins ac- In 1961, Heezen and Ewing (1961) or Th e Oceanography [email protected] Send Society. to: all correspondence 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 Lomonosov Ridge 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 to copy this article Repu for use copy this and research. to ted in teaching 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 e Oceanography Society, PO Box 1931, Rockville, MD 20849-1931, USA. 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/Kara Sea 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, reproduction, systemmatic blication, 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).