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Influence of brine formation on Arctic Oceancirculation over the past 15 million years

BRIAN A. HALEY1*, MARTIN FRANK1, ROBERT F. SPIELHAGEN1,2 AND ANTON EISENHAUER1

1IFM-GEOMAR, Leibniz Institute for Marine Sciences, Wischhofstrasse 1-3, 24148 Kiel, Germany2Academy of Sciences, Humanities and Literature, 55131 Mainz, Germany*e-mail: bhaley@ifm-geomar.de

Published online: 2 December 2007; doi:10.1038/ngeo.2007.5

The early oceanographic history of the Arctic Ocean is important in regulating, and responding to, climatic changes. However,constraints on its oceanographic history preceding the Quaternary (the past 1.8 Myr) have become available only recently, because ofthe difficulties associated with obtaining continuous sediment records in such a hostile setting. Here, we use the neodymium isotopecompositions of two sediment cores recovered near the North Pole to reconstruct over the past ∼15 Myr the sources contributing toArctic Intermediate Water, a water mass found today at depths of 200 to 1,500 m. We interpret high neodymium ratios for the periodbetween 15 and 2 Myr ago, and for the glacial periods thereafter, as indicative of weathering input from the Siberian Putoranan basaltsinto the Arctic Ocean. Arctic Intermediate Water was then derived from brine formation in the Eurasian shelf regions, with only alimited contribution of intermediate water from the North Atlantic. In contrast, the modern circulation pattern, with relatively highcontributions of North Atlantic Intermediate Water and negligible input from brine formation, exhibits low neodymium isotoperatios and is typical for the interglacial periods of the past 2 Myr. We suggest that changes in climatic conditions and the tectonicsetting were responsible for switches between these two modes.

There is clear evidence that anthropogenic influences have startedto change the Arctic environment1. Constraining the extentand mechanisms of past environmental and oceanic changesin the Arctic region, one of the most sensitive responders toglobal climate change1–3, is crucial to better understand bothnatural and man-made variability. Until recently, however, ourunderstanding of Arctic oceanography before 1 Myr, includingthe intensification of Northern Hemisphere glaciation (INHG)at 2.7 Myr (refs 4–6), was very limited3,7,8. This was mainly dueto the technical difficulties that prevented drilling operations inan ice-covered ocean7. The central Arctic sediments recoveredin the summer of 2004 during the Arctic Coring Expedition(ACEX; Integrated Ocean Drilling Program, Leg 302) near theNorth Pole on the Lomonosov ridge (87◦5 N, 137◦ E; 1,250 mwater depth) now provide, for the first time, a continuousarchive from which Neogene changes of Arctic oceanographyand climate can be reconstructed3,7. The ACEX sediments showclear evidence of ice-rafted transport over the past 15 Myr,but otherwise the sedimentologically monotonous sequenceof essentially abiogenic, detrital sediments prevents furthermicrofossil-based paleoceanographic reconstructions3,7. We reporthere the Nd isotopic evolution of Arctic Intermediate Water (AIW)recovered from metal-oxide coatings on sediment particles9,10. TheNd isotopic composition of sea water reflects differences in theisotope composition of the rocks of the surrounding continentalland masses (that is, low 143Nd/144Nd reflects old continental crust,whereas high 143Nd/144Nd characterizes young mantle-derivedrocks), which are introduced into sea water through weatheringprocesses11. Given that the oceanic residence time of Nd is ofthe order of 600–2,000 years, Nd isotopes can be used as aquasi-conservative circulation tracer in the open ocean11.

Today, the AIW occupies water depths between ∼200 and1,500 m and is predominantly formed through cooling of thesaline surface waters of the North Atlantic Drift (NAD) inthe Greenland–Iceland–Norwegian (GIN) Sea12–14 (Fig. 1). Thiscirculation pattern results in a modern εNd (ref. 15) of −10.5 forAIW (P. S. Andersson et al., manuscript in preparation), which wereproduced from the leaches of coatings of various Arctic surfacesediments, thus confirming the reliability of the leaching methodto extract the past Nd isotope composition of sea water from thesesediments10 (see the Supplementary Information).

In pronounced contrast to the modern situation, the ACEXrecord shows that AIW, with the exception of Late Quaternaryinterglacial periods, had a significantly more positive εNd valuethan today throughout the past 15 Myr (Figs 2 and 3). The largeand distinct εNd variability presented here documents that AIWformation was subject to major changes both on Myr (Fig. 2) andmillenial (Fig. 3) timescales. Below, we discuss the relationship ofthese changes to tectonic and climatic processes observed in theArctic region over time.

THE ‘NEOGENE’ RECORD (15 TO 2Myr)

The εNd of AIW became significantly (by ∼2 εNd units) morepositive between ∼15 Myr and ∼12 Myr (Fig. 2), followed by a10 Myr period (until ∼2 Myr), during which there is no indicationfor tectonic forcing of AIW circulation (that is, through changesof Arctic sea ways). Because the Pacific and Arctic Oceanswere isolated from each other before ∼5.5 Myr (ref. 16), theinterpretation of the ACEX Nd isotope record before this time isconstrained by the fact that there are only two sources with positiveenough εNd that can have influenced the isotopic composition of

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–150 

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Figure 1 Schematic map of the high northern latitude seas, ocean circulationand glacial ice sheet distributions. Light blue arrows indicate the main modernsurface currents12–14: the Gulf Stream, the NAD and the Norwegian Current (NC),which carry an εNd signal of −12.5 to the GIN Sea14 (P. S. Andersson et al.,manuscript in preparation). Crosses represent regions where modern intermediatewater forms, and modern intermediate depth circulation is shown by dark bluearrows12–14. AIW is, at present, formed in the GIN Seas, and dense brinecontributions from the Arctic shelves (dashed arrow) are negligible(P. S. Andersson et al., manuscript in preparation). AIW has a modern εNd of −10.5(bold value at the ACEX core location, indicated by the star; P. S. Andersson et al.,manuscript in preparation). (Nd isotope ratios are expressed as εNd values,corresponding to the measured 143Nd/144Nd normalized to the chondritic uniformreservoir (0.512638), multiplied by 10,000 (ref. 15)). In contrast, the dense waterinputs from the Labrador Sea are responsible for NADW having an εNd signature of−13.5 (bold value in North Atlantic)11,14,42. Shown in red are the potential supplyregions of Nd with a positive εNd signature in the Arctic: the ‘Icelandic basalts’ andthe Putorana basalts, as reflected in the distribution of sediments with >40%smectite content in the Kara Sea49 (the location of the Putorana basalts themselvesis indicated by the red ‘PB’). Generally during glacial stages, intermediate waterformation from North Atlantic surface-water sources shifted south of the GSR40, andbrine formation in the Arctic shelf regions was enhanced. The white regions indicateice distributions for particular times during the last glacial cycle: the purple outlineindicates the extension of the Fennoscandian ice sheet at 90 to 80 kyr and the greenoutline indicates the extension between 22 and 15 kyr, the Last Glacial Maximum47.The borders of the Laurentide ice sheet (dashed blue outline), only a crudeapproximation, are not relevant for this work.

AIW: the ‘Icelandic basalts’ (that is, the basaltic group associatedwith both Iceland and the Greenland–Scotland ridge, GSR)17,18 andthe Putorana flood basalts of Siberia19,20 (Fig. 1). It is suggested herethat the Putorana flood basalt source, through an indirect controlof GSR (Fig. 1) subsidence, offers the best explanation for the earlypart of the record.

Our preferred scenario is that the opening of the GSR ‘gateway’,which began well before 15 Myr (refs 21–23), allowed greaterpenetration of warm, saline North Atlantic surface waters intothe GIN seas, thus enhancing heat flow and ultimately moisturesupply to higher latitudes of northern Eurasia via the Westerlies

0 2 4 6 8

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Figure 2 Arctic Intermediate Water evolution from the Middle Miocene to thepresent. (i) NADW εNd data from Fe–Mn crusts in the North Atlantic compiled fromvarious publications11,32,33 (open circles and blue envelope). (ii) AIW εNd from ACEXsediment leaches with their 2σ external error, illustrating the evolution of AIW beforethe Quaternary (green circles). The stratigraphic framework of the sediments hasbeen presented previously3,7. Yellow circles represent core top samples, and thegreen box indicates the range of Late Quaternary data shown in detail in Fig. 3.Black diamonds are bulk sediment digestion data. (iii) Compiled global benthicforaminiferal δ18O data4 showing the climatically important final shifts(∼16–11Myr, and post ∼3Myr) of the transition from the ‘greenhouse’ to the‘icehouse’ world. (iv) Periods of enhanced Icelandic plume activity21–23.

(consult ref. 24). This enabled extensive growth of northernEurasian ‘marine-based’ ice (either sea ice or floating ice shelves),beginning at ∼15 Myr. The most likely mechanism to explain theaddition of Putorana-sourced Nd to AIW (that is, at depth) isthus a substantial production of dense brines, formed throughsalt rejection during this sea-ice formation in the Eurasian shelfregions, in particular the Kara Sea region12,25 (P. S. Andersson et al.,manuscript in preparation; Fig. 1). Such brines are only producedin relatively small quantities by sea-ice formation in the modernArctic Ocean26. Brines are, however, extensively generated in thepolynyas of the modern Southern Ocean, which form throughkatabatic winds blowing seawards off the edge of the Antarctic icesheets and ice shelves27,28. In the modern Southern Ocean, brineformation processes are mainly responsible for the formation ofdense Antarctic Bottom Water27,28, demonstrating the potential ofthis mechanism to generate deep waters from saline surface watersin high latitudes. In the case of the Arctic Ocean, a prerequisitefor the enhancement of brine production at ∼15 Myr was theopening of the Fram Strait to the North Atlantic at about 17.5 Myr(ref. 29), which allowed the establishment of open-ocean salinities.An alternative explanation simply invoking enhanced inputs ofradiogenic Nd via rivers draining the flood basalts into the Arctic

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North Atlantic sources dominate AIW;Arctic brine formation limitedDecreasing North Atlantic sources of AIW;enhanced Arctic shelf brine formation

0 0.10 0.20 0.30Age (Myr)

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Figure 3 Late Quaternary evolution of AIW. AIW εNd obtained from sedimentleaches of core PS2185 (yellow circles). Core PS2185 (87◦31.9 ′ N; 144◦22.9 ′ E;1,051m water depth) was recovered at a location neighbouring the ACEX drill siteon the Lomonosov ridge and has a very detailed and reliable high-resolution agemodel for the latest Quaternary45. ACEX εNd data from the Late Quaternary (onlymeasured for some samples older than 200 kyr and not shown here; see text) showsimilar amplitudes of variation with respect to glacial–interglacial changes insediment properties7. The 2σ external error bars for the Nd data shown are thesame as in Fig. 2. Glacial–interglacial cycles are indicated by comparison with theglobally stacked benthic foraminiferal δ18O data4.

Basin is unlikely owing to the missing mechanism for subductionof a freshwater Nd isotope signature to AIW depths.

As a consequence of the above scenario, it is suggested that thepronounced increase in global benthic δ18O from the ‘mid-Mioceneclimatic optimum’ (15–17 Myr) to around 10 Myr (ref. 4; Fig. 2)may reflect, at least in part4,30,31, the initial growth of significantEurasian Shelf/Arctic Ocean ice. This is consistent with the recordof continuous ice-rafted debris deposition observed in the ACEXcore itself 3,7.

It has been shown that in the modern ocean, exchange ofNd between sea water and the Icelandic basalts can alter the Ndisotopic composition of bottom waters14,18. However, besides theconsiderations that modern deep water flows southward acrossthe GSR and the large distance of the GSR from the ArcticOcean, two other observations argue against an involvement ofthe Iceland basalts in the change of AIW εNd signature between15 and 12 Myr (Fig. 2): (1) the Nd isotope composition of theNorth Atlantic11,32,33 does not vary with fluctuations in the volcanicactivity of the Iceland basalts, as inferred from the history ofthe GSR plume activity21–23, and (2) deep-water exchange throughFram Strait was probably still restricted before 12 Myr (refs 23,34).Moreover, even with an open Fram Strait, such as today,GSR-influenced bottom waters are not transferred into the ArcticBasin (P. S. Andersson et al., manuscript in preparation). Thisleaves the Putorana basalts as the only likely source of radiogenicNd for AIW, as discussed above.

VOLUMETRIC CONSTRAINTS ON WATER MASS EXCHANGE

Mass balance calculations, assuming a simple two end-membermixing between brine water (εNd = +2; typical for the Putorana

basalt signature19,20) and North-Atlantic-sourced intermediatewater (εNd = −10; typical of the NADW record between 16 and8 Myr (refs 11,32,33); Fig. 2), imply the contribution of largeproportions (up to 30%) of brine-water-sourced AIW by ∼12 Myr.To achieve such mixing ratios, a severe concomitant restriction ofAtlantic NAD-sourced intermediate water input (with its negativeεNd signature) must have existed. The Nd isotope data clearlyindicate that intermediate water exchange between the NorthAtlantic and the Arctic was limited throughout the period from 12to 2 Myr (Fig. 2). Applying the end-member isotope compositionslisted above, a reduction in the ∼6 Sv modern flux of North-Atlantic-sourced intermediate water to the Arctic35 to 3 Sv, togetherwith a brine-sourced flux of 0.6 Sv would arrive at an εNd of−8 for AIW. There is evidence that past restrictions of NorthAtlantic water flow into the Arctic were even more severe than this50% estimate36,37. However, regardless of the exact values, brineformation fluxes of this order of magnitude (0.6 Sv) have beenobserved in modern interglacial oceans26–28,38,39, and demonstratethat the suggested magnitude of past-intensified brine formationon the Arctic Shelves together with a concomitantly restrictedNorth Atlantic inflow is realistic.

The most likely cause of such a reduction in North Atlanticwater inflow was a southward shift in the location of surface-water subduction in the North Atlantic to a position south ofthe GSR (that is, not in the GIN Seas). This situation is similarto that predicted for the glacial periods of the Quaternary40

(described below), the main difference being that, according to ourdata, this circulation pattern was a generally stable oceanographicconfiguration between 12 and 2 Myr. It is important to note that, incontrast to the Nd isotope signature of North Atlantic Deep Water(NADW) and its precursors in the North Atlantic11,32,33 (Fig. 2),the AIW εNd was remarkably stable between ∼4 and 2 Myr. Thiscontrast implies (1) that the INHG at 2.7 Myr (refs 4–6) was notdriven by changes in the Arctic Ocean, and (2) that the decrease inthe North Atlantic εNd record must reflect changes in contributionsfrom non-radiogenic Labrador Sea sources41,42. This is consistentwith the INHG being largely restricted to the Laurentide glaciersat this time, which increased the input of non-radiogenic Nd tothe Labrador Sea, but obviously did not have a strong influence onthe Nd isotope composition of waters reaching the GIN seas andthus AIW.

THE ‘QUATERNARY’ RECORD (2 Myr TO PRESENT)

The significant ‘excursion’ to εNd values as positive as −6 around2 Myr (Fig. 2) is ascribed to the growth of the first major NorthEurasian ice sheets, which grounded on large areas of the Arcticshelves, including the Kara Sea area, thus transferring increasedamounts of Nd with a positive isotope signature into the ArcticOcean. Analogous to the earlier Neogene part of the record,this ‘excursion’ is explained by enhanced brine-water productionresulting from increased sea-ice formation at the edges of theice sheets and concomitantly restricted North-Atlantic-sourcedintermediate-water input. However, the magnitude of the εNd

variation of this ‘event’, as well as the amplitude of variations duringthe subsequent glacial–interglacial cycles, demonstrates that thismode of circulation intensified after 2 Myr.

The only other available Arctic sea-water Nd isotope datafor the past 5 Myr, recovered discontinuously from Fe–Mnmicronodules of various water depths in the Canadian Basin, alsoindicate a major decrease in Nd isotope composition around 2 Myr(ref. 8), supporting the contention that the oceanographic stabilityin the Arctic Basin of the preceding 10 Myr was disrupted at thistime. Unfortunately, both the uncertainties in the age models of thesediments from which these data were obtained43 and the different

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water depths that they reflect8 limit the use of these data for directcomparison to the data presented here. Indeed, accurately datingArctic samples is still a major challenge, and whereas the NeogeneACEX chronology used here is considered robust44, the QuaternaryACEX stratigraphy is not as well defined. For this reason, the latestQuaternary record (the past 400 kyr; Fig. 3) was obtained fromsediment samples of a neighbouring core on the Lomonosov ridge(PS2185), for which a robust age model is available45.

The ‘Quaternary’ Lomonosov ridge data presented hereindicate that at ∼2 Myr a threshold of Arctic glaciation andoceanography was reached, resulting in a Nd isotope variabilitywith a large (4 εNd-unit) amplitude on millennial timescales(Figs 2 and 3). These variations reflect switches between an‘interglacial mode’ of AIW circulation, characterized by the modernconfiguration (Fig. 1), and a ‘glacial mode’. During the ‘glacialmode’, AIW was controlled by enhanced input of shelf waters frombrine sources, in particular the Kara Sea, and a coeval stronglyrestricted input from North Atlantic sources. This configuration isessentially identical to the circulation described previously for the‘Neogene’ record. That is, in this ‘glacial mode,’ Atlantic-sourcedintermediate water formed further south in the North Atlantic40,and thus had limited influence on the Arctic Basin owing to thebarrier of the GSR, a restriction that was enhanced during theglacial Quaternary drops in sea level. This restricted Atlantic waterinflow also caused the Quaternary Arctic Ocean to be less wellventilated during glacials, consistent with observations of cyclic Mnenrichments in Arctic sediments46.

An important feature of the changes in Late Quaternary AIWformation is evident in the Nd isotope record during MarineIsotope Stage 3, between 59 and 24 kyr: AIW formation seems tohave changed to the ‘interglacial mode’ ∼30 kyr before the LastGlacial Maximum (15–22 kyr; Fig. 3). This unexpectedly early shiftcan be explained by the fact that the Kara Sea shelf region wasnot covered by an ice sheet after 50 kyr (Fig. 1; ref. 47), thusinhibiting brine formation, and the consequent input of Nd witha positive εNd signature to intermediate depths, in this area. Thestriking correspondence in timing of the change in AIW signatureand the independently derived terrestrial glaciation record47 lendsstrong support for the proposed mechanism of interaction betweenvariable dense brine production on the Arctic shelves and changesin North-Atlantic-sourced intermediate-water inputs to explainAIW variability.

The Nd isotope data and the scenario presented here providethe first framework for a Neogene oceanographic and climatichistory of the Arctic. Although the history of the Arctic Ocean isfar from resolved through this one record, the large Nd isotopevariability observed confirms the high sensitivity of the ArcticOcean to climate forcing mechanisms on all timescales.

METHODS

The sea-water-derived Nd in the metal-oxide coatings of the sediment wasextracted by leaching, following the protocols in refs 9,10, but modified inthe following ways: (1) a buffered acid leach step was omitted as the Arcticsediments are virtually devoid of a carbonate component; (2) the leachsolution to extract the oxide phase (a dilute reducing/complexing solutionof hydroxylamine.HCl and acetic acid, buffered to pH = 4) was diluted10-fold compared with that of ref. 10 to avoid any contamination causedby leaching of clays. The leachate solution was separated from the sediment,evaporated in double-distilled concentrated HNO3 and then run through astandard ion-exchange procedure to separate and purify the Nd for analysis(AG 50W-X12 resin for cation separation; di-(2-ethylhexyl)phosphate resinfor rare-earth element separation). The procedural blank was negligible. TheNd isotopic composition was measured at the mass spectrometry facility ofIFM-GEOMAR using a Thermo Triton thermal ionization mass spectrometer,correcting for instrumental fractionation using 146Nd/144Nd = 0.7219 (ref. 15).

The standard deviation of 27 runs of a Nd solution (SPEX), diluted to the sameconcentrations as the samples, resulted in the stated 2σ external reproducibility(0.50 εNd units), although the internal error given by replicate runs of theJNdi-1 standard48 was better (0.3 εNd units; n = 39). A suite of Arctic core topsamples replicated the modern Arctic Deep Water εNd signature within error(see the Supplementary Information), demonstrating that a pure sea-watersignal was extracted with the applied protocol. To further assure that no Ndwas leached from the clays, several Nd and Sr isotopic analyses of completebulk sediment dissolutions were made, after the sediments had undergoneleaching, for comparison with the leached Sr and Nd isotope compositions10.In all cases, the potential clay-derived Nd contribution was less than our statedmeasurement error (see the Supplementary Information), corroborating theextraction of a pure sea-water signal.

Received 29 June 2007; accepted 24 July 2007; published 2 December 2007.

References1. Peterson, B. J. et al. Trajectory shifts in the Arctic and Subarctic freshwater cycle. Science 313,

1061–1066 (2006).2. Gerdes, R. & Koberle, C. Comparison of Arctic sea ice thickness variability in IPCC Climate of the

20th Century and in ocean-sea ice hindcasts. J. Geophys. Res. 112, C04S13 (2007).3. Moran, K. et al. The Cenozoic palaeoenvironement of the Arctic Ocean. Nature 441, 601–605 (2006).4. Zachos, J., Pagani, M., Sloan, L., Thomas, E. & Billups, K. Trends, rhythms, and aberrations in global

climate 65 Ma to present. Science 292, 686–693 (2001).5. Raymo, M. E., Lisiecki, L. E. & Nisancioglu, K. H. Plio-Pleistocene ice volume, Antarctic climate, and

the global δ18O record. Science 313, 492–495 (2006).6. Ravelo, A. C., Andreasen, D. H., Lyle, M., Lyle, A. O. & Wara, M. W. Regional climate shifts caused by

gradual global cooling in the Pliocene epoch. Nature 429, 263–267 (2004).7. Backman. J., Moran. K., McInroy. D. B., Mayer. L. A. & the Expedition 302 Scientists. Proc. IODP Exp.

Rep. 302 (Integrated Ocean Drilling Program Management International, College Station,Texas, 2006).

8. Winter, B. L., Johnson, C. M. & Clark, D. L. Strontium, neodymium, and lead isotope variations ofauthigenic and silicate sediment components from the Late Cenozoic Arctic Ocean: Implications forsediment provenance and the source of trace metals in seawater. Geochim. Cosmochim. Acta 61,4181–4200 (1997).

9. Piotrowski, A. M., Goldstein, S. L., Hemming, S. R. & Fairbanks, R. G. Temporal relationships ofcarbon cycling and ocean circulation at glacial boundaries. Science 307, 1933–1938 (2005).

10. Gutjahr, M. et al. Reliable extraction of a deepwater trace metal isotope signal from Fe–Mnoxyhydroxide coatings of marine sediments. Chem. Geol. 242, 351–370 (2007).

11. Frank, M. Radiogenic isotopes: Tracers of past ocean circulation and erosional input. Rev. Geophys.40, 1001 (2002).

12. Aagaard, K., Swift, J. H. & Carmack, E. C. Thermohaline circulation in the Arctic Mediterranean Seas.J. Geophys. Res. 90, 4833–4846 (1985).

13. Karcher, M. J. & Oberhuber, J. M. Pathways and modification of the upper and intermediate waters ofthe Arctic Ocean. J. Geophys. Res. 107, 3049 (2002).

14. Lacan, F. & Jeandel, C. Denmark Strait water circulation traced by heterogeneity in neodymiumisotopic compositions. Deep-Sea Res. I 51, 71–82 (2004).

15. Jacobsen, S. B. & Wasserburg, G. J. Sm-Nd isotopic evolution of chondrites. Earth Planet. Sci. Lett.50, 139–155 (1980).

16. Gladenkov, A. Y., Oleinik, A. E., Marincovich, L. & Barinov, K. B. A refined age for the earliestopening of Bering Strait. Palaeogeogr. Plaeoclimatol. Palaeoecol. 183, 321–328 (2002).

17. Mertz, D. F., Devey, C. W., Todt, W., Stoffers, P. & Hofmann, A. W. Sr–Nd–Pb isotope evidenceagainst plume-asthenosphere mixing north of Iceland. Earth Planet. Sci. Lett. 107, 243–255 (1991).

18. Lacan, F. & Jeandel, C. Neodymium isotopic composition and rare earth element concentrations inthe deep and intermediate Nordic Seas: Constraints on the Iceland Scotland Overflow Watersignature. Geochem. Geophys. Geosyst. 5, Q11006 (2004).

19. Sharma, M., Basu, A. R. & Nesterenko, G. V. Temporal Sr-,Nd- and Pb-isotopic variations in theSiberian flood basalts: Implications for the plume-source characteristics. Earth Planet. Sci. Lett. 113,365–381 (1992).

20. Basu, A. R., Hannigan, R. E. & Jacobsen, S. B. Melting of the Siberian mantle plume. Geophys. Res.Lett. 25, 2209–2212 (1998).

21. Poore, H. R., Samwourth, R., White, N. J., Jones, S. M. & McCave, I. N. Neogene overflow of NorthernComponent Water at the Greenland–Scotland Ridge. Geochem. Geophys. Geosyst. 7, Q06010 (2006).

22. Wright, J. D. in Tectonic Boundary Conditions for Climate Reconstructions (eds Crowley, T. J. &Burke, K.) (Oxford Univ. Press, New York, 1998).

23. Wright, J. D. & Miller, K. G. Control of North Atlantic Deep Water circulation by theGreenland–Scotland Ridge. Paleoceanography 11, 157–170 (1996).

24. Driscoll, N. W. & Haug, G. H. A short-circuit in thermohaline circulation: A cause for northernhemisphere glaciation? Science 282, 436–438 (1998).

25. Tutken, T., Eisenhauer, A., Wiegand, B. & Hansen, B. T. Glacial-interglacial cycles in Sr and Ndisotopic composition of Arctic margin sediments triggered by the Svalbard/Barents Sea ice sheet. Mar.Geol. 182, 351–372 (2002).

26. Shapiro, G. I., Huthnance, J. M. & Ivanov, V. V. Dense water cascading off the continental shelf. J.Geophys. Res. 108, 3390 (2003).

27. Gill, A. E. Circulation and bottom water production in the Weddell Sea. Deep-Sea Res. 20,111–140 (1973).

28. Marsland, S. J., Bindoff, N. L., Williams, G. D. & Budd, W. F. Modeling water mass formation in theMertz Glacier Polynya and Adelie Depression, East Antarctica. J. Geophys. Res. 109, C11003 (2004).

29. Jakobsson, M. et al. The early Miocene onset of a ventilated circulation regime in the Arctic Ocean.Nature 447, 986–990 (2007).

30. Shackleton, N. J. & Kennett, J. P. in Init. Rep. DSDP 29 (eds Shackleton, N. J. & Kennett, J. P.) 743–755(US Govt. Printing Off., Washington, DC, 1975).

31. Miller, K. G., Fairbanks, R. G. & Mountain, G. S. Tertiary oxygen isotope synthesis, sea level history,and continental margin erosion. Paleoceanography 2, 1–19 (1987).

32. Burton, K. W., Lee, D.-C., Christensen, J. N., Halliday, A. N. & Hein, J. R. Actual timing ofneodymium isotopic variations recorded by Fe–Mn crusts in the western North Atlantic. Earth PlanetSci. Lett. 171, 149–156 (1999).

nature geoscience VOL 1 JANUARY 2008 www.nature.com/naturegeoscience 71

© 2008 Nature Publishing Group

ARTICLES

33. O’Nions, R. K., Frank, M., von Blanckenburg, F. & Ling, H.-F. Secular variation of Nd and Pbisotopes in ferromanganese crusts from the Atlantic, Indian and Pacific Oceans. Earth Planet Sci. Lett.155, 15–28 (1998).

34. Engen, Ø. Evolution of High Arctic Ocean Basins and Continental Margins. Thesis, Univ. Oslo,Norway, 184 pp (2005).

35. Schauer, U., Fahrbach, E., Osterhus, S. & Rohardt, G. Arctic warming through the Fram Strait:Oceanic heat transport from 3 years of measurements. J. Geophys. Res. 109, C06026 (2004).

36. Jansen, E., Bleil, U., Henrich, R., Kringstad, L. & Slettmark, B. Paleoenvironmental changes in theNorwegian Sea and the Northeast Atlantic during the last 2.8 m.y.: Deep Sea Drilling Project/OceanDrilling Program sites 610, 642, 643 and 644. Paleoceanography 3, 563–581 (1988).

37. Henrich, R. & Baumann, K.-H. Evolution of the Norwegian Current and the Scandinavian Ice Sheetsduring the past 2.6 m.y.: Evidence from ODP Leg 104 biogenic carbonate and terrigenous records.Paleocean. Paleoclim. Paleoecol. 108, 75–94 (1994).

38. Bjork, G. A one-dimensional time-dependent model for the vertical stratification of the upper ArcticOcean. J. Phys. Oceanogr. 19, 52–67 (1989).

39. Cavalieri, D. J. & Martin, S. The contribution of Alaskan, Siberian, and Canadian coastal polynyas tothe cold halocline layer of the Arctic Ocean. J. Geophys. Res. 99, 18343–18362 (1994).

40. Ganopolski, A., Rahmstorf, S., Petoukhov, V. & Claussen, M. Simulation of modern and glacialclimates with a coupled global model of intermediate complexity. Nature 391, 351–356 (1998).

41. Farmer, G. L., Barber, D. & Andrews, J. Provenance of Late Quaternary ice-proximal sediments in theNorth Atlantic: Nd, Sr and Pb isotopic evidence. Earth Planet. Sci. Lett. 209, 227–243 (2003).

42. Lacan, F. & Jeandel, C. Acquisition of the neodymium isotopic composition of the North AtlanticDeep Water. Geochem. Geophys. Geosyst. 6, Q12008 (2005).

43. Backman, J., Jakobsson, M., Løvlie, R., Polyak, L. & Febo, L. A. Is the central Arctic Ocean a sedimentstarved basin? Quat. Sci. Rev. 23, 1435–1454 (2004).

44. Frank, M. et al. Beryllium isotopes in central Arctic Ocean sediments over the past 12.3 million years:Stratigraphic and paleoclimatic implications. Paleoceanography (2007, in the press).

45. Spielhagen, R. F. et al. Arctic Ocean deep-sea record of northern Eurasian ice sheet history. Quat. Sci.Rev. 23, 1455–1483 (2004).

46. Jakobsson, M. et al. Manganese and color cycles in Arctic Ocean sediments constrain Pleistocenechronology. Geology 28, 23–26 (2000).

47. Svendsen, J. I. et al. Late quaternary ice sheet history of northern Eurasia. Quat. Sci. Rev. 23,1229–1271 (2004).

48. Tanaka, T. et al. JNdi-1: A neodymium isotopic reference in consistency with LaJolla neodymium.Chem. Geol. 168, 279–281 (2000).

49. Wahsner, M. et al. Clay-mineral distribution in surface sediment of the Eurasian Arctic Ocean andcontinental margin as indicator for source areas and transport pathways—A synthesis. Boreas 28,215–233 (1999).

AcknowledgementsThe ACEX sediments were acquired through joint efforts of the Integrated Ocean Drilling Program(IODP), the European Consortium for Ocean Research Drilling (ECORD) and the Swedish PolarResearch Secretariat. We thank IODP Leg 302 members, in particular K. Moran and J. Backman. Wealso thank J. Heinze and A. Kolevica for support in the laboratory, J. Fietzke and F. Hauff for their helpin running the mass spectrometers and D. Bauch and J. Zachos for helpful discussions.Correspondence and requests for materials should be addressed to B.A.H.Supplementary Information accompanies this paper on www.nature.com/naturegeoscience.

Author contributionsSamples given to M.F. (ACEX core) and taken by R.F.S. (PS2185) were analysed by B.A.H. at the massspectrometry facility of IFM-GEOMAR run by A.E. All authors contributed equally to the discussionand interpretation of the results.

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