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Eos, Vol. 86, No. 52, 27 December 2005

Volume 86 number 52 27 december 2005

EOS, TranSacTiOnS, amErican GEOphySical UniOn pages 549–556

than afew metersinlength. nearly all of the Icebreaker Expedition Collects Key Island expedition T-3cores collected in the 1960s and 1970s by the U.s. Geological survey forheatflowstudies were onlythree centime- Seafloor and Ice Data tersindiameter,and they contained very little of the upper 20–30 centimetersdue to the small corepipe’spropensity to not sample the PAGeS 549, 552 A New Pan-Arctic Stratigraphic Framework soft surface layers.Asexpected, suchnarrow cores also presented problems in providing Therecentlycompleted Healy- Trans- one coreobjective forHoTrAX wasthe adequate volumes of material formanypaleo- Arctic Expedition 2005 (HoTrAX’05) retrieved establishment of apan-Arctic Quaternary climatic proxymeasurements. 29 piston cores averaging nearly 12 metersin stratigraphy and thus the determination of some larger-diameter cores were later col- length from acomplete transect across the sedimentation rates in the central Arctic. lected in the central [e.g., central Arctic ocean (Figure1).These cores Both the stratigraphy and sedimentation Backman et al., 2004,and references provide acritically-needed sample cache for rates areneeded in order to determine avia- therein], butthey coveredonlylocalized both apan-Arctic stratigraphy and along- blepaleoclimate historyofthe a rea. areas and were still relatively short(maxi- awaited paleoclimate recordthatitishoped Earlier work in the Arctic washampered by mumlength of eight meters). HoTrAX has will greatlyimprove the understanding of how the lackofgood corematerial extending more changed this situation, and while thereare deepwater is exchanged between Arctic basins, howthe climate system in the Arctic worksoverlonger time intervals, and howthe Arctic system interacts with global systems. Thecoring wasdone from the U.s. Coast GuardCutter (UsCGC) Healy ,while oceano- graphic measurements were made from the swedish icebreaker Oden. In addition to coring and oceanography, HoTrAX mapped the seafloor with multi- beam bathymetryand collected chirpsonar profiles thatnot onlymapped the stratatoa sub-bottom depth of 50–100 meters ,but also provided detailed information on the geo- logic context of the coresites. Themultibeam bathymetryrevealed more extensive and deeper glaciogenic features on the Chukchi Borderland than previously mapped; showedpockmarkfields on the Mendeleyev ridge; and, forthe first time,dis- coveredlarge mudwaves (>500 meter wave- length, 5–30 meter amplitude) in the Alpha ridge area, indicating bottom currents in the deepArctic ocean. sediment erosion, includ- ing unconformities with older strata, was observedoverthe northwind ridge by chirp sonar and multibeam, and nearly uniform sediment drapes were profiled overmuchof the Mendeleyev and Alpha ridge system (Figure2).Aseparategeophysical compo- nent of HoTrAX collected 2200 kilometers of multichannel seismic data across some of the most inaccessible parts of the Arctic ocean.This data and pending results will help to resolvethe structureofthe central Fig. 1.Map of the HOTRAX ’05 transect showing the tracks of the U.S. Coast Guard Cutter Healy Arctic ridges and thus howthe early Arctic with a red line and the icebreaker oden in white.Coring sites are represented by yellow stars basins formed. (yellow numbers for the trans-Arctic leg) or red stars ( Healy’s first leg at the Alaskan margin), and dirty ice sampling stations are represented by green dots.The locations of Figures 2 and 3 are outlined in orange.The complete transect from Dutch Harbor,Alaska, to Tromsø, , is B y D. A. D ArBy ,M.JAkoBsson, AnD L.P oLyAk 8200 kilometers long. Eos, Vol. 86, No. 52, 27 December 2005

expanding the extent of grounded ice that once existed in this area (Figure2). Thecoring objective in this area wastodeter- mine the ageofthese grounded glacial masses and their origin,and thus severalcores were taken thatpenetratethe compacted diamicton (poorly-sorted sediments) beloworatthe mar- gins of ice-grounding features.In severalloca- tions along the Chukchi Borderland,at least two stratigraphicallydifferent diamictons were dis- covered, suggesting multiple ice-grounding events.An important result from the HoTrAX transect is thatthe seabed of the shallowest mapped areas of the Mendeleyev ridge (about 800 metersbelowsea level) did not contain anysigns of glacial erosion.This constrains the deepest glacial activity in the Arctic ocean as mapped so fartothe Lomonosovridge and Chukchi Borderland.

New Evidence for Bottom Currents in the Central Arctic

Themechanisms of deepwater exchange between Arctic basins, suchasthe nature and location of deepcurrents, arelong- Fig. 2.Examples of Healy’sKnudsen chirpsonar (center frequency3.5 kHz) records showing running enigmas thatseveral researchers (a) glacial erosion and probable glacially deposited diamictons on Northwind Ridge and have tried to resolve[e.g., Jones et al.,1995]. (b) pockmark collapse structures,perhaps caused by gas escape from possible methane build- To studythese mechanisms, the cores from ups in the sediment on Mendeleyev Ridge as well as uniform sediment drapes.Data was pro- Lomonosovridge were taken in adepres- cessed using Sioseis software. sion along the ridge wheredeepwater exchange between the Amundsen and still manyareas thatneed to be cored, there neverbeen encountered by previous cores Makarov basins is thought to occur.Evi- is sufficient quality corematerial to meet from this area. In addition, severalhigh-resolu- dence of bottom currents on the ridge and most of the Pleistocene stratigraphic and tion cores from the shelf and slope north of expanded sediment stratigraphy in this paleoclimate objectivesdriving HoTrAX. Alaska with over12metersofprobableHolo- depression or gapwill be the foc us of future Thelong cores obtained by HoTrAX cene sediments were collected. investigations to determine amoredetailed along the Mendeleyev and Alpha ridges Pleistocene paleoclimate recordfor this area showexcellent preliminarycorrelations Evidence for Glacial Erosion than previouslypossible. from onboardlithological description and Thebottom mapping indicates thatthe multi-sensor corelogs suchasthose of mag- Theexpedition started on the northwind deepest passage in Lomonosovridge, netic susceptibility (Ms), whichpeak when ridge and immediatelydiscoveredglacial ero- located on the Makarov Basin side of the concentrations of fine-grained magnetic sion markings on the seafloor to depths of intra-basin, is shallowerthan shown on the minerals arepresent and thus allowmatch- nearly 1000 meters, whichismorethan 200 current International Bathymetric Chartof ing of suchdeposits among different cores metersdeeper than previouslymapped in this the Arctic ocean and on the russian bathy- (Figure3). Furthermore, some unique strati- area [ Polyak et al., 2001]. some of theseglacial metric mappublished in 1999 by the rus- graphic features, suchasdistinctive changes features were mapped forthe first time,greatly sian Head Department of navigation and in lithologyand prominent layers of coarse detritus deposited by , provide a basis forcorrelation with previouslyrecov- ered sediment records [e.g., Jakobsson et al., 2000], including the 2004 drilling on the Lomonosovridge [ ShipboardScientific Party, 2005].This correlation indicates apro- nounced increase in sedimentation rates from the Alpha ridge to the southernMen- deleyev ridge closer to the areas whereice retreats during summer. In the longer cores, up to 80 distinct cycles, thought to be glacial-interglacial fluctuations, were described based on color changes and texture. similar butfewer cycles have been described previouslyincores from various areas of the Arctic ocean [e.g., Jakobsson et al., 2000]. However, the processes behind these clearly visible sediment cycles arenot fully understood.TheHoTrAX cores mayhelp in resolving this enigma, whichisakey to the Fig. 3.Magnetic susceptibility logged on board with the multi-sensor core logger and a pho- Arctic ocean’spaleoenvironmental history. tograph of the core HLY0503-14JPC showing the prominent cycles that were observed in all some of the deeper units, below8-10 metersin retrieved cores from the central Arctic Ocean.The logged cores are from the Alpha and Men- the HoTrAX cores from the Alpha ridge,have deleyev ridges, and a preliminary correlation is exemplified by selected tie lines. Eos, Vol. 86, No. 52, 27 December 2005 oceanography.Evidence of bottom currents, conditions around the southernMendeleyev Formoreinformation about HoTrAX, visit including disturbed sediment stratification ridge area, conditions quicklychanged over the Website: http://www.odu.edu/sci/ocean- and extensive ripple fields, wasfound in a Alpha ridge and Makarov Basin. surprisingly ography/hotrax/index.htm broader area than expected here. large open leads in the ice were encountered Another important region of uncertain bot- in the Lomonosovridge area onlytobefol- Acknowledgments tom current activity between basins is the lowedbyasudden returntosevereconditions. ‘Cooperation Gap’ area wherethe Men- Ice thicknesses occasionallygreater than three We thank captains Dan oliverand Tomas deleyev and Alpha ridges join and where meterswereencountered as the twoships Årnell and the crewsofthe UsCGC Healy and deepwater exchange between the Canada neared the ; similar conditions pre- the Oden formaking the science operations and Markarov basins should occur.HoTrAX vailed as the expedition movedsouth toward possible.Also,weare indebted to the entire mapped and cored this gap(lowareainthe in mid-september. HoTrAX science party forgenerating the pre- ridge). Likethe Lomonosovridge gap, this Continuous ice thickness profiles were col- liminaryresults reflected in this paper.The U.s. area also showedevidence of bottom cur- lected from an EM-31 electromagnetic induc- national science Foundation’soffice of Polar rents, seen by thepresence of manganese tion sensor hung overthe side of the near Programs, the U.s. Coast Guard, the swedish coatings on dropstones due to lowsedimen- the bowaswell as from 30 stations across the Polar research secretariat, and the swedish tation rates expected from current move- entireArctic forspot measurements of ice science Council (Vr) funded HoTrAX. ments.Thus, in addition to anew paleocli- physical properties. severalpatches of mate record, the multibeam bathymetric data extremelyconcentrated sediment were References and sediment cores from Lomonosovridge encountered and sampled, especiallyinthe and the ‘Cooperation Gap’ area contain simi- trans-polar drift, the major ice drift pattern Backman, J.,M.Jakobsson, r. Løvlie, and L. Polyak (2004), Is the central Arctic ocean a sediment lar evidence of bottom currents, and together from the Laptevsea area to the strait starved basin?, Quat. Sci. Rev.,23, 1435–1454. with hydrographic data mayprovide anew across the central Arctic ocean. Jakobsson, M., r. Løvlie,H.Al-Hanbali, E.Arnold, pictureofbottom water movement in the J. Backman, and M. Mörth (2000), Manganese color central Arctic ocean. cycles in Arctic ocean sediments constrain Pleis- Plans for tocene chronology, Geology,28, 23–26. and Beyond Jones, E. P. ,B.rudels, and L. G.Anderson (1995), Sea Ice Observations Deep waters of the Arctic ocean: origins and cir- Work has alreadybegun to establish the culation, Deep Sea Res., Part I,42, 737–760. This expedition wasonlythe second stratigraphy and chronologyofHoTrAX Polyak, L., M. H. Edwards, M. Jakobsson, and B. J. Coakley (2001), Existence of Arctic ice shelves crossing of the central Arctic basin to be cores with avariety of measurements, during the Pleistocene inferred from deep-sea done the hardway,bybreaking ice (the first including paleomagnetic fluctuations, radio- glaciogenic bedforms, Nature,410, 453–457. involved the UsCGC Polar Sea andthe Cana- carbon and beryllium-10 dating, optical stim- shipboard scientific Party (2005),Arctic Coring dian icebreaker LouisSt. Laurent in 1994). ulation luminescence,and biostratigraphy. Expedition (ACEX): Paleoceanographic and tec- tonic evolution of the central Arctic ocean,Prelim. Becauseofits ambitious science agenda, the Thegoal is to establish acorrelated late- Rep. 302, 46 pp., Integrated ocean Drill. Program, twoicebreakers(Healy and Oden)did not Cenozoic stratigraphic recordfromthis tran- Washington, D. C. (Available at http://www.ecord. reachthe north Pole until 12 september sect across the entireArctic ocean.This org/exp/acex/302Pr.pdf) 2005, later in the year than anyprevious non- recordwould provide the chronological nuclear surface vessels. framework forawide variety of paleoclimate Author Information Although 2005 wasundoubtedlythe most proxies in the Arctic ocean.This work will ice-free summer in recorded historywithin the be timed forthe International Polar year in Dennis A. Darby, old Dominion University,nor- Amerasian Arctic ocean, HoTrAX encoun- 2007, and this initiative will be used to folk,Va., Martin Jakobsson, stockholm University, tered manypatches of very thick ice in the launchacomprehensive investigation of the sweden; Leonid Polyak, ByrdPolar research Cen- central basin and Eurasian Arctic.While this Arctic ocean paleoceanography and its ter,ohio state University,Columbus. cruise started in early August with easy ice links with the global paleoclimate.