Pliocene-Pleistocene ice rafting history and cyclicity

Transcription

1 PALEOCEANOGRAPHY, VOL. 15, NO. 6, PAGES DECEMBER 2000 Pliocene-Pleistocene ice rafting history and cyclicity in the Nordic Seas during the last 3.5 Myr Eystein Jansen, l, 2 Torben Fronval,3, 4 Frank Rack,5, 6 and James E. T. Channell 7 Abstract. A continuous 3.5 Myr IRD record was produced from Ocean Drilling Program (ODP) Site 907. A timescale based on magnetic polarity chrons, oxygen isotope stratigraphy (for the last 1Myr) and orbital tuning was developed. The record documents a stepwise inception of large-scale glacial cycles in the Nordic Seas region, the first being a marked expansion of the Greenland ice sheet at 3.3 Ma. A second step occurred at 2.74 Ma by an expansion of large scale ice sheets in the Northern Hemisphere. Ice sheet variability around the Nordic Seas was tightly coupled to global ice volume over the past 3.3 Myr. Between 3 and 1 Ma, most of the variance of the IRD signal is in the 41 kyr band, whereas the last 1 Myr is characterized by stronger 100 kyr variance. The Gamma Ray Porosity Evaluator (GRAPE) density record is closely linked with IRD variations and document sub orbital variability resembling the late Quaternary Heinrich ond cycles. 1. Introduction Glaciers have existed in the areas surrounding the Nordic Seas since at least the late Miocene. This has been documented by records of ice-rafted debris (IRD) which also document the major steps in the intensification of glaciation in the high-latitude areas [Jansen and Sjoholm, 1991; Wolf and Thiede, 1991; Larsen et al., 1994; Wolf-Welling et al., 1996; Fronval and Jansen, 1996a]. Major ice sheets repeatedly grew and disintegrated in the area through the late Neogene and Quaternary, but so far, no continuous record has been produced to gauge the variability of glaciers and ice sheets on the neighboring continents. Consequently, major insight into the relationships with the global evolution of glaciation and the response of the regional ice masses to forcing factor such as orbital forcing has been scarce. It was therefore a key objective of Ocean Drilling Program (ODP) Leg 162 to core continuous sediment sections in the area to enable such studies. Here we report the first long-term, continuous record of ice rafting in Nordic Seas from the spliced sediment sequenc established fi om the three holes drilled at ODP Site 907 on the Iceland Plateau, north of Iceland. This is, to our knowledge, the first time such a continuous record has been developed with a resolution which resolves Milankovitch cyclicity. 1 Department of Geology, University of Bergen, Bergen, Norway. 2Also at Nansen Environmental and Remote Sensing Center (NERSC), Bergen, Norway. 3Department of Geology, University of Copenhagen, Copenhagen, Denmark. 4Now at: Ma rsk Oil and Gas, Copenhagen, Denmark. 5Department of Geodesy and Geomatics Engineering, University of New Brunswick, Fredericton, New Brunswick, Canada. 6Now at: Joint Oceanographic Institutions, Inc. Washington, D.C. 7Department of Geology, University of Florida, Gainesville. Copyright 2000 by the American Geophysical Union Paper number 1999PA /00/1999PA Site 907 is located on the flat part of the Iceland plataeu at 69ø14.99'N, 12ø41.89'W (Figure 1) in a water depth of 1800 m There is no evidence for sedimentary structures indicative of grain flows and contourites. It should thus be possible to derive a reliable open-ocean record of IRD from this site. The site was cored during Leg 151 (Hole 907A) and Leg 162 (Holes 907B and 907C). The Iceland Plateau is located in the southwestern part of the Nordic Seas (Figure 1). It is defined by the 1800-m contour and gently increases in depth eastward away fi'om the spreading axis. The southeastern part of the plateau drops off into the Norway basin; to the south the plateau shallows toward the Iceland shelf. In the central part of the plateau, sediments from the last -1 Myr consist of bioturbated clayey silts and silty muds with abundant to rare planktonic and benthic foraminifers and several distinct volcanic glass layers. Ice-rafted material is also common, and glacial-interglacial cycles can easily be recognized in the sediments. Clayey silts and silty clays with high amounts of IRD but virtually no biogenic carbonate and only rare volcanic glass characterize the period fi'om-3 to -1 Ma. Sediments older than -3 Ma consist of silty clays with high abundances of biogenic silica and volcanic glass and small quantities of ice-rafted material [Jansen et al., 1996; Myhre et al., 1995]. High-resolution records from the Iceland plateau should enable us to study the evolution of climate at high northern latitudes in detail: both the long-term trends and the climatic variability. We here focus on the time period fi'om 3.5 Ma to the present, which covers important climatic events, such as the initiation of large-scale glaciation in the Northern Hemisphere and the middle Pleistocene climate shift, and address how the sensitivity of the climate system at high northern latitudes (primarily the ice sheets) responded to orbital forcing during this important transitional period. The record of IRD is believed to represent the flux of melting icebergs originating from glaciers large enough to calve in the ocean. Ice masses on Greenland, around the Arctic Ocean, in the Svalbard/Barents Sea area, in Scandinavia, and on Iceland are potential areas where these icebergs may originate that subsequently deposit their IRD in the Nordic Seas. The basic determining factor for IRD deposition is the availability of calving icebergs that can release their sediment contents upon melting in the ocean. 709

2 710 JANSEN ET AL.: PLIO-PLEISTOCENE IRD HISTORY 75 ø 36øW 24 ø 12 ø 0 ø 12øE 75 ø 70 ø 70 ø 65 ø 65 ø 60 ø 60 ø 55 ø 55 ø 50ON I I 50ON 36øW 24ø 12 ø 0 ø 12øE Figure 1. Map of the northern North Atlantic and the Nordic Seas with location of ODP/DSDP sites referred to in the text. Availability is closely coupled with the supply rate (i.e. local ice extent), although sea surface temperature and current directions also to some degree influence the supply of icebergs and melting rates. On the basis of the close correlation of IRD flux and the onshore glaciation record which has been documented for the last glacial-interglacial cycle [Baumann et al., 1995; Fronval and Jansen, 1996b; Fronval and Jansen, 1997] we use the IRD records as a firstorder monitor of the extent of of glaciation on the surrounding continents. Iceberg drift reconstructions from the use of IRD provenance indicators [Bischofet al. 1990] demonstrate that Greenland was the major IRD source area for the Iceland plateau area during the last glacial period. This connection does not necessarily apply to the entire Pliocene- Pleistocene, but it is reasonable to believe that the IRD record at Site 907 to a large extent reflects fluctuations of the Greenland ice sheet. Site 907 is relatively close to Greenland, and the subpolar gyre system would tend to force icebergs southeastward from the coast of east Greenland (Figure 1). Hence Greenland is a likely origin for icebergs crossing over the site. Owing to gyre type of circulation of the Nordic Seas icebergs may have circumnavigated the Nordic Seas from Scandinavia and ended up in the Iceland Sea. The almost complete melting of the northern Eurasian ice sheets in many interglacials would make this a likely source area for IRD in the Iceland Sea at least during major deglaciations. In sum, we believe our record from Site 907 reflects a combined signal mainly derived from Greenland with an important component from the Scandinavia/Barents Sea region and possibly from Iceland. Earlier studies of IRD in the late Quaternary documented that IRD flux is tightly coupled to the waxing and waning of ice sheets on land, with highest flux of IRD occurring at glacial maxima and the deglaciations following these [Fronval et al., 1995; Baumann et al., 1995; Fronval and Jansen, 1997]. We interpret the IRD record on the assumption that this relationship is also valid for the past 3-4 Myr, and that maxim and minima in the number of of IRD per gram of dry sediment record reflects glacial maxima and minima. 2. Methods 2.1. Analytical Methods We sampled the cores at 10-cm intervals except where prohibited by routine whole-round sampling. Samples were wet sieved on 125- and 63-1 m screens and dried. To determine the amount of ice-rafted material in the sediments, counts of ice-rafted grains were performed on the >125 fraction. Approximately 500 IRD grains were counted per

3 .--. sample. IRD is defined as all minerogenic grains (excluding volcanic grains and pyrite) and is expressed as number per gram dry sediment. Multisensor track (MST)data may also document IRDrelated parameters [Rack, 1996]. Saturated bulk density (gamma ray attenuation porosity evaluator(grape)) and magnetic susceptibility were measured shipboard by the MST system soon after the core sections had equilibrated to room temperature [see Myhre et al., 1995; Jansen et al., 1996]. The GRAPE density and magnetic susceptibility was measured each 2 cm on the MST, giving a sampling interval of 1 kyr. Although measured at the same spacing, the width of the response function of the susceptibility loop produces a smoothing which reduces the resolution of the record relative to that of the GRAPE. During Leg 162, the GRAPE system gave densities which were systematically lower compared to measurements done on discrete samples in the shipboard laboratory and GRAPE bulk density measurements carried out on Leg 151 on cores from Hole 907 A [Jansen et al., 1996]. To overcome this problem and provide a consistent 8OOO ,2 2,0 -- 1,8 1,6 1,4 1, _ ' m.. - >' ,-, 150 = 100 ß 50 JANSEN ET AL.: PLIO-PLEISTOCENE IRD HISTORY 711 GRAPE data set, we corrected the GRAPE data from Holes 907B and 907C by applying the following regression: corrected GRAPE = x (original GRAPE) , R 2 = 0.93 (1) In the GRAPE and magnetic susceptibility records, data from volcanic ash layers have been omitted, as high concentrations of volcanic glass have a strong impact on these parameters. Deviant values, due to occurrence of large dropstones, have also been omitted from the GRAPE and magnetic susceptibility records. The oxygen isotope measurements were performed on a Finnigan MAT 251 mass spectrometer at Bergen University, fitted with an on line Finnigan automatic carbonate reaction system ("Kiel device"). Results are reported with respecto the Pee Dee Belemnite (PDB) standard through calibration against the National Institute of Standards and Technology (NIST) 19 standard. The reproducibility of the system is +0.07%o for 8 ]8 O based on replicate measurements of an internal carbonate standard. The benthie isotope records were produced from analyses of the benthie foraminifers Cibicides wuellerstorfi, Cassidulina teretis, and Oridorsalis umbonatus. A number of studies have shown that carbonate shells of C. wuellerstorfi and O. umbonatus are precipitated in oxygen isotopic disequilibrium with the ambient seawater [Duplessy and Shackleton, 1985; Graham et al., 1981]. The oxygen isotopic offsets appear constant with a mean value of %o for C. wuellerstorfi and -0.4%o for O. umbonatus. On the basis of this evidence we have adjusted the measured 80 values of C. wuellerstorfi and O. umbonatus by adding 0.64 and 0.4%o, respectively. C. teretis is considered to calcify8 close to equilibrium and no correction was applied to the fi O values [Jansen et al., 1990; D.A.R. Poole et al., Stable isotope fractionation in recent benthie foraminifera from the Barents and Kara Seas, submitted to Paleoceanography, 2000]. The planktonic isotope records were produced by analyses of the planktonic foraminifer Neogloboquadrina pachyderma (sinistral). Owing to extensive carbonate dissolution in sediments older than 1 Ma it was only possible to produce isotopic records for the last 1 Myr. Older sediments are essentially barren of biogenic carbonate Construction of Composite Section One advanced piston corer (APC) hole (Hole 907A) was drilled during ODP Leg 151. Two additional holes (Holes 907B and 907C) were drilled during Leg 162 in order to get complete recovery from the site. A composite section was created on Leg 162 from these three holes on the basis of GRAPE, magnetic susceptibility, and natural gamma radiation records, confirming the continuity of recovery [Jansen et al., 1996]. In general, the methods used at Site 907 were similar to those used to construct composite depth sections during ODP Leg 138 [Hagelberg et al., 1992] and ODP Leg 154 [Curry et al., 1995]. Records of the sediment physical properties were moved along a depth scale core by I:. core as correlations between holes were made. Although core 0,,,,,,,,1,,,, ' '' ' ' [' ' ' ' distortion within a given core in some cases was significant, the core depths were only adjusted by a single constant, and Depth (mcd) no differential stretching or squeezing was performed within Figure 2. IRD per gram of dry sediment, gamma ray porosity evaluator the cores. Two spliced records were assembled by the Leg (GRAPE) density, and magnetic susceptibility records versuspliced 162 shipboard scientific party. One makes use of Hole 907A depth (meters composite depth (MCD)). The stratigraphic position of whenever possible (907ABC splice), whereas the other is polarity chron boundaries used in the age model are shown. based primarily on Holes 907B and 907C (907BC splice)

4 712 JANSEN ET AL.: PLIO-PLEISTOCENE IRD HISTORY 0 ODP Site 9O7 lo 15 I'"'l'"'l'"'l'"'l'"'l'"'l 5,5 5 4,5 43,5 3 2, ,5 5 4,5 4 3, b 180-planktonic b 180-benthic IRD/g dry SED (7 point smooth) Figure 3. Planktonic and benthie foraminiferal/518 0 records and IRD data (seven point smoothed) from the upper 21 m (mcd) at Site 907. Segments with low stratigraphic resolution of the isotope records due to very low abundances of foraminifera tests are indicated by dotted lines. The stratigraphic positions of oxygen isotope stages 1 to 29 are indicated. [Jansen et al., 1996]. In this study we used the 907ABC splice with revisions as explained below. Owing to problems in the shipboard splice in a few instances with apparent "double coring" (overlap between cores) a new improved version of the composite depth section and an improved 907ABC splice was created postcruise. This effort was prompted by discrepancies between the composite depths of magnetic polarity reversals among holes, particularly within the Matuyama and Gauss chrons. By creating this new composite depth scale we eliminated these problems. Meters below sea floor/meters composite depth (mbsf/mcd) offsets for the new composite section and tiepoints for the new 907ABC splice are given by Channell et al. [1999a]. All data are plotted against this spliced depth scale on Figure 2 and reported as mcd. 2.3 Age Conversion The time control for the Site 907 record was developed in three phases: (1)palcomagnetic reversal chronology, (2) a combination of the palcomagneti chronology with oxygen isotope stratigraphy for the last 1 Myr, (3) tuning the IRD record to orbital parameters. A detailed magnetic polarity record ([Charmell et al., 1999a]) provides the basic age constraints. This interpretation of the magnetic polarity stratigraphy within the last 3.5 kyr differs only slightly from those of the Leg 151 and Leg 162 initial reports [Myhre et al., 1995; Jansen et al., 1996]. Ages for palcomagnetic polarity chrons are assigned from Cande and Kent [1995]. The positions of the palcomagnetic polarity intervals with respect to the records are indicated on Figure 2. Second, oxygen isotope stages 1-30 were recognized primarily in the planktonic foraminiferal and partly in the benthic foraminiferal /5180 records (Figure 3). The timescale of Shackleton et a/.[1990] was employed for these. The lower part of the isotope record is less reliable than the upper. We therefore used isotope stage boundary ages only back to Stage 19 at the Brunhes/Matuyama boundary. Below this level we constructed ages by linear interpolation between polarity chron boundaries, assuming constant sedimentation rates between age tie-points. The age model of Site 907 shows that sedimentation rates were relatively constant (around 20 m/myr). The sampling rate gives an average stratigraphic resolution of the IRD record of between 4000 and 5000 kyr Tuned Timescale For the period 3.5 to 1 Ma the only time constraints come from the palcomagnetic record, and all samples were dated b y linear interpolation between polarity chron boundaries. In order to improve on this situation and produce a highresolution timescale also for the interval older than 1 Ma we tuned the IRD record to the orbital parameters. Since there is Supporting IRD data for Figures 2 and 6 are available a noticeable 41 kyr component in the untuned IRD record electronically at World Data Center-A for Paleoclimatologx., NOAA/NGDC, 325 Broadw_ay, Boulder CO (e-marl: (Figures 4a and 4b), we did this by simply tuning the 41 kyr paleo mail.ngdc.noaa.gov; URL: component of the IRD record below isotope stage 19 to the

5 JANSEN ET AL.: PLIO-PLEISTOCENE IRD HISTORY k 294k :: I CI ;, ;'98 i'.. i i, ' ' I ' ' ' ' I ' ' ' ' I ' ' ' ' I ' ' ' ' I ' ' ' ' 0,01 0,02 0,03 0,04 0,05 0,06 frequency (cycles/kyr) I I I I I I I I I I I I I I I I I I I I I I I [ I I I I I I I I 0,5 1 1,5 2 2,5 3 3,5 Age (Ma) 8000m c) ooo i o 0,5 1 1,5 2 2,5 3 3,5 Age (Ma) 4,0c d) :' - 3,5-_. 3,0 2,5. 1,5' 1,0' 0,5'? Depth (mcd) Jansen et al. Fig. 4 Figure 4. (a) Power spectra (Blackman-Tukey method) of IRD per gram of dry sediment record from ODP Site 907. Dashed line shows spectrum from untuned age model. Solid line shows spectrum from tuned age model (see text). (b) IRD pe rem of dry sediment record on the untuned age model filtered with a band pass filter centered at 41 kyr.. (c) IRD per grem of dry sediment vs age. Dashed line represents untuned age model, solid line tuned age model. (d) Age versus depth plot for untuned age model (dashed line) and tuned age model (solid line). Fix points (isotope stages and polarity chron boundaries) are shown by crosses. BW, bandwidth; CI, Confidence interval

6 714 JANSEN ET AL.' PLIO-PLEISTOCENE IRD HISTORY tilt parameter, using the Berger and Loutre [1990] solution. The IRD record was filtered with a band pass filter centered at 41 kyr. The resulting filtered output is shown in Figure 4b. This was tuned to the orbital target by assuming no phase lag between maximum IRD and maximum tilt. Comparisons of IRD records with orbital parameters for the last two glacials cycles indicate a minimal time lag, that is maximum IRD coincides with tilt maxima within a few thousand years [Fronval and Jansen, 1997, E. Jansen, unpublished data, 1999]. This simple assumption is kept as a premise for the tuning process throughout the length of the record. The tuning was done interactively using the Analyseries software package [Paillard et al., 1996]. The tuning was obvious in most parts of the record and only small adjustments were necessary. In a few instances, however, low signal in the 41 kyr component of IRD made the procedure slightly ambiguous. To minimize possible errors, we kept all polarity chron boundaries as fixed points and did not allow any adjustment exceeding half a tilt cycle (20 kyr). The difference between the two timescales is depicted in Figure 4c and the resulting age versus depth plot is shown together with the untuned one in Figure 4d. The resultant frequency spectrum is shown together with the spectrum of the untuned time series in Figure 4a. For the remainder of this work, all data sets are based on this tuned timescale. 3. Results and Discussion 3.1. G!aciation History Previous studies documented small amounts of IRD back to approximately 11 Ma in Nordic Seas sediment sections [Fronval and Jansen, 1996a; Wolf-Welling et al., 1996]. The IRD record for the period Ma from Site 907 on the tuned timescale is compared with the IRD-record from ODP Sites 644/642 from the Voring plateau on the eastern side of the Norwegian Sea (Figure 5). The age model for the Voring plateau record [Jansen and Sjoholm, 1991; Fronval and Jansen, 1996a] is based on the paleomagnetic record of Bleil [1989], recalculated on the Cande and Kent [1995] age scale. It is not until about 3 Ma that major peaks of IRD appear in the record (Figure 5). While the inception of glaciation therefore probably occurred close to the Middle/Late Miocene boundary, major glacial intervals are not apparent until at just before 3 Ma. The first evidence of this intensification is recorded at site 907 at 3.3 Ma and at Ma, whereas we observe the first major IRD peaks at 2.74 Ma on the Voting Plateau offshore Northern Europe (Figure 5). This difference in timing probably reflects an earlier response of the Greenland Ice sheet during this intensification phase compared with the inception and expansion of ice sheets in northern Europe. A similar time lag is also evident with respect to the Laurentide ice sheet, as IRD records from O DP Sites 607, 609, and 610 in the North Atlantic do not show major IRD peaks with probable Laurentide origin until at about Ma, during isotope stages G6-100 [Kleiven, 1995, 2000]. At site 610 (location on Figure 1) small IRD peaks at about 3 Ma are also observed [Kleiven, 2000], however, the first large peaks with several orders of magnitude higher IRD abundances do not occur until isotope stage G6 at 2.74 Ma. In addition, to provide continuous open ocean sediment sections enabling detailed studies of lrd history, ODP Leg 162 also drilled the glacial fans of the continental margins off east Greenland and Svalbard. These sites enable a direct tie to the history of the ice sheets of the hinterland. The results from / b) ' ' [ I ' ' I I [ I I I i i i i i! i 2,2 2,4 2,6 2,8 3 3,2 Age (Ma) [ I ' I 3,4 3,6 Figure 5. Comparison of (a) IRD/g dry sediment record from ODP Site 644/642 [Jansen and Sjoholm, 1991] with age model recalculated to the Cande and Kent [ 1995] timescale and (b) IRD per gram record from ODP Site 907 on the tuned age model (see text).

7 JANSEN ET AL.' PLIO-PLEISTOCENE IRD HISTORY a) ' O=, i i i i i! i i l i i i i i 3 2,5 ' I b) ooo 0 i i i 1 1,2 846 Benthie O-18 i I i i i i i i i i, 1,4 1,6 1, ,5 3 3,5 4 4,5 5 5,5 6 6, Benthie O-18 ' 2,2 ' ' ' 214 ' ' ' 216 ' ' ' 28 ' ' '.-- 2,5 3 3,5 4 4,5 :5 5,5 6 6, Benthie O ,5 3 3,5 4 4,5 5 5, ,2 3,4 3,6 3,8 4 Age (Ma) Figure 6. IRD per grem of dry sediment record from ODP site 907 compared with benthie foraminiferal oxygen isotope record from Eastern Equatorial Pacific Site 846 [Mix et al., 1995; Shackleton et al., 1995]. Numbering refers to some of the main isotope stages. (a) 0-1 Ma, (b) 1-2 Ma, (c) 2-3 Ma, and (d) 3-4 Ma. Note that IRD-record only extends to 3.5 Ma. 6,5 these sites corroborate the IRD history of site 907 by pointing to much earlier large-scale glacial activity in Greenland than in the European Arctic. At Site 987 off Scoresbysund on the east coast of Greenland (Figure 1) two major phases of debris flow deposition, indicative of an active ice sheet grounded at the paleo shelf break were discovered. These occur in the earliest part of the Gilbert chron and in the early part of the Matuyama chron ( Ma) [Channell et al., 1999b; Solhelm et al., 1998]. The younger debris flow unit probably corresponds to isotope stages and represents the first fan build up in the north European Arctic. The expansion of a Svalbard/Barents Sea ice sheet to the shelf break was much later than in Greenland. It occurred in the lower part of the Matuyama

8 716 JANSEN ET AL.: PLIO-PLEISTOCENE IRD HISTORY chron, probably synchronously with the second debris flow phase off Greenland, i.e., also at isotope stages [Channell et al., 1999b; Solheim et al., 1998]. A comparison of the Site 907 IRD record with the benthic foraminiferal oxygen isotope record from ODP Site 846 (Eastern Equatorial Pacific) which represents the global ice volume record, reveals many interesting correllations (Figure 6). The first main IRD peak coincides with the first major glacial after 4 Ma, isotope stage M2 at 3.3 Ma, documenting a strong participation of Northern Hemisphere glaciation in the first major global ice volume increase of the last 4 Myr. IRD events related to isotope stages KM2 and G20 are also present. From isotope stage G6 the background IRD level rises in 907 and the IRD peaks become more systematic, in tandem with the more pronounced high-amplitude beat of the oxygen isotope record. This evidence together with evidence from ODP Sites 644 (Figure 5) and 610 [Kleiven, 1995, 2000] which reveals that major IRD peaks, probably originating from northern Europe and northern America, also appear in Stage G6, indicates repeated widespread glaciations over Northern Hemisphere landmasses from this time. The largest IRD peaks seem to correlate with the glacial stages G6, G4, 104, 100, 98 and 96 (Figure 6), which all exhibit heavy benthic oxygen isotope values. Detailed synthetic carbonate records from ODP Sites 980, 981, and 983 in the North Atlantic, display carbonate minima during the same periods as well as a minimum at 3.3 Ma at the time of the first IRD peak at Site 907 [Ortiz et al., 1999]. For the last 2.5 Myr, there is good correlation between the local proxy (IRD in 907) and the global ice volume record (benthic foraminiferal O isotopes). Periods with low amplitude oxygen isotope change have less pronounced IRD signals, and the clear 41 kyr cyclicity in the Early Pleistocene Ma oxygen isotope record is accompanied by a very similar signal in the IRD record. Although the IRD signal is weak in the Ma interval, as is the amplitude of the oxygen isotope record, there is continuous presence of IRD in the record. In contrast with many of the preceeding and following interglacials where there is no or very little IRD, the baseline level of IRD is higher in the Ma interval and never gets to zero, pointing to a situation with incomplete deglaciations during this interval. The transition to longer periods with dominant 100 kyr cyclicity after the mid-pleistocene climate shift at 0.9 Ma is also evident in the IRD record (Figure 6a). The IRD record from the Nordic Seas appears to be a faithful recorder of the global climate signal and provides strong evidence that Northern Hemisphere glaciation was a major element in the global ice volume variations throughouthe past 3.5 Myr. On the tuned timescale, IRD peaks tend to appear at the end of glacials as defined by the heavy oxygen isotope values. As it is likely that the highest IRD input would occur at deglaciations after ice maxima, this apparent phasing may be indicative of a coherent age model. Clearly, there are intervals when this phasing breaks down, possibly due to local departures from the global ice volume record or to inaccuracies in the timescale Orbital Cyclicity To evaluate the evolution of the frequency distribution of the IRD signal, we produced evolutionary frequency spectra by moving a 500 kyr window along the time series with 20% overlap. For each 500 kyr period, we performed a spectral analysis. (Figure 7). The spectra show the emergence of 41 kyr cycles at the main intensification of glaciation at at this 100k 41k 21k Ma " : Ma,, 2.4_ Ma i?? :,il... iiii',iii',! 1_ Ma -1.7 Ma Ma -0.9 Ma ::i;i -0.5 Ma... '1',,,, I' '"'"'""' I... I ' ' "i'""' I''' ' o o,o o,oz 0,03 0,04 o,os 0,06 Frequency (cycles/kyr) Figure 7. Evolutionary spectra for IRD per grem of dry sediment at ODP Site 907. Each curve represents the power spectrum for a 500 kyr interval, as shown to the fight of the curve. location after 2.9 Ma. Before this time, there are no clear signs of orbital frequencies in the IRD record. Some variance in the precessional frequency is seen in the Ma window, otherwise the variance at this frequency is rather low throughouthe investigated period. The sample resolution close to 5 kyrs should be sufficiento capture precessional variance, and the absence of such variability could either be due to imperfections with the timescale or a real lack of precessional influence. With the exception of the Ma interval, which has a low amplitude IRD signal with little power at any frequency, the 41 kyr signal is maintained through to Ma. The tuning of the timescale to the tilt factor will naturally emphasize this frequency, but it is clear also from the untuned record (see Figure 4b) and from just a visual examination of the data on the untuned timescale that this is a robust feature of the IRD record. Variance around 100 kyr appears to emerge during the mid-pleistocene transition, dominating the frequency spectra for the past 0.9 Myr, thereby indicating that the local ice sheets of the region clearly change their variability toward 100 kyr cyclicity as has previously been shown both for North Atlantic sea

9 JANSEN ET AL.' PLIO-PLEISTOCENE IRD HISTORY 717 0,0-50,0-100,0 150,0-200,0-250,0-300,0-350,0 - b) i 2,2-2 1,8 1, , i i 1,25 1,3 1,35 1,4 1,45 1,5 Age (Ma) ' I ' ' ' I 1,55 1,6 Figure 8. (a) Magnetic susceptibility, (b) GRAPE density, and (c) IRD per grem records from the interval 1.6 to l.2 Ma at ODP Site 907. All records are plotted on tuned timescale (see text). Vertical dashed lines denote positions of IRD peaks. surface temperature records [Ruddiman et al., 1989] and for the global ice volume signal [Imbrie et al., 1984; Shackleton et al., 1990; Raytoo, 1992] MST Data and Suborbital Variability The IRD record has a resolution that does not resolve suborbital-scale variability. However, the higher resolution (1 kyr) of the multisensor track (MST) data should provide evidence of such variability, if it exists. To investigate this, we plotted GRAPE density data and magnetic susceptibility data from the MST alongside IRD (Figures 2 and 8). As can be seen on Figures 2 and 8, there are many resemblences between the MST data and the IRD record, in particular between GRAPE and IRD. The significant increase in GRAPE density from 2.9 Ma closely corresponds with the onset of repetitive IRD peaks. Also, the preceeding IRD peaks are accompanied by density increases. The subdued variability in the Ma period is reflected in the GRAPE and magnetic susceptibility data, as is the shift toward longer cycles after 1 Ma. The magnetic susceptibility record has some similarities with the IRD but appears to respond later to the glacial intensification than IRD and density. Studies have indicated that GRAPE records from the Nordic Seas reflect grain size distributions, which are linked to the transport of terrigenousedimento the deep sea [Rack et al., 1996]. From Figures 2 and 8 it is evident that a very good correlation exists between the IRD and GRAPE curves at Site 907, with high IRD corresponding to high GRAPE values. In the open ocean the higher GRAPE densities during glacial periods are due to a poorer grain size sorting and therefore to lower porosity [Moros et al., 1997] of sediments with a large ice-rafted component. We may thus expect the GRAPE record to reflect the ice-rafting signal. The details of the GRAPE data shown in Figure 8 document this general correlation. Fine-scale presumed IRD peaks are superimposed on the general 41 kyr cyclicity. This pattern is clearest in the Ma interval, but is also clear between 1.3 and 1.23 Ma and elsewhere. This indicates that IRD pulses operate on suborbital timescales. The spacing appears to be of the order of 5-10 kyrs, i.e., roughly corresponding to the general cyclicity of late Quaternary Heinrich events. Similar suborbital IRD variations in the same period, dominated by 41 kyr cyclicity, were also observed in isotope stage 40 at Ma by Raytoo et al. [1998] in another Leg 162 site, Site 983, south of Iceland (Figure 1). This points to the existence of millennial-scale ice sheet variability, probably in the form of collapses and surges, also before the emergence of 100 kyr cycles, i.e., in a period when ice sheet maxima were smaller and when ice sheet variability was less than in the late Quaternary. Since the origin of the IRD at Site 907 is in the Nordic Seas region, this testifies

10 718 JANSEN ET AL.: PLIO-PLEISTOCENE IRD HISTORY that the suborbital variability was not a reflection of iceberg supply from the Laurentide ice sheet. Ice sheet destabilizations on suborbital timescales are probably inherent in the ice sheet dynamics generally, and do not require ice sheets of the size of the late Quaternary ice sheets to be triggered. The existence of similar events in isotope stage 3 [Bond et al., 1993; Frorival et al., 1995] and isotope stage 5 [McManus et al., 1994; Frorival and Jansen, 1997], and during isotope stage 10 [Oppo et al., 1998] also indicate this relationship between ice sheet size and destabilisations, as these are all stages with intermediate ice volume maxima. The connections between magnetic susceptibility and sediment texture and mineralogy within the Nordic Seas are not well understood [Rack et al., 1996]. A general correlation between low magnetic susceptibility and stadial periods is found in last glacial interval in the Nordic Seas [Fronval et al., 1995; Rasmussen et al., 1996] and the Reykjanes Ridge [Moros et al., 1997]. In general, we find a similar correspondance between glacial periods (as indicated by high IRD content) and low magnetic susceptibility (Figure 8). Investigations fi'om south of Iceland indicate that this pattern in the magnetic susceptibility signal is a reflection of the titanomagnetite content of the sediment, which most probably reflects the intensity of the bottom water currents [Moros et al., 1997]. This suggests that the magnetic susceptibility variations at Site 907 are not directly linked to IRD variability but somehow relates to bottom water circulation differences between warm and cold periods. Low-susceptibility IRD from the crystalline continental rocks dilute the susceptibility signal during glacials, and enhanced winnowing of high-suscpetibility fines from the volcanic ridges during interglacials probably explains the situation. We emphasize that this is different fi'om the situation in the central North Atlantic where magnetic susceptibility, at least within the last glacial period, is directly linked to supply of IRD [Grousset et al., 1993] General Discussion The strong correspondence between the global ice volume record and the IRD record fi'om Site 907 implies that glaciations in the Nordic Seas region are fully integrated with the global ice volume signal at least for the last 3.5 Myr. Although it is not possible to distinguish Northern and Southern Hemisphere contributions to the global ice volume signal, the data indicate that Northern Hemisphere climates were into the ice age mode from at least 3.3 Ma. Yet the history of active glaciers extends further back in time. Smaller-magnitude IRD peaks at the Miocene/Pliocene boundary and at the middle/late Miocene boundary [Frorival and Jansen, 1996a; Wolf-Welling et al., 1996] occur at times of marked global climatic change, when glacial sediments were deposited offshore Greenland and in Iceland [Larsen et al., 1994; Solhelm et al., 1998; Geirsdottir and Eiriksson, 1994], indicating that ice masses have existed at least during the past 12 Myr. The stepwise enhancement of major glacial cycles, documented by the IRD peaks at 3.3 and around 3 Ma, most likely reflects expansions of the Greenland ice sheet since sediments offshore Norway and Svalbard do not record major IRD inputs in the form of large IRD peaks until Ma. Evidence from Iceland [Geirsdottir and Eiriksson, 1994] and Alaska and Arctic Canada also document glaciations at---3 Ma [Matthews and Ovenden, 1990; Brigham-Grette and Carter, 1992], but in Arctic America, these were intersected by temperate phases much warmer than the present tundra situation. The main establishrnent of Northern Hemisphere glaciation apparently occurred in isotope stage G6 at Ma. At this time major IRD peaks appear from all of the Northern Hemisphere ice sheets, on both sides of the Nordic Seas (Figure 5), and in sites at various latitudes of the North Atlantic origin [Kleiven, 1995, 2000]. The same age for the first major IRD peak is also reported fi'om the North Pacific [Maslin et al., 1995]. At 2.7 Ma, icebergs emanated from all the main ice sheet loci, followed by a series of glaciations which probably were even larger, as indicated by the increasing O isotope values for the glacials after 2.7 Ma (Figure 6). This progression of increasingly severe glacials is most likely the reason the continental record of western Europe does not show evidence of true glacial climates until after the Gauss/Matuyama boundary at 2.6 Ma [Zagwijn, 1992]. The influence of ice sheets on North Atlantic-Nordic Seas ocean circulation increased after 2.7 Ma, both by downwind and albedo feedbacks, which cooled the surface waters [Kleiven, 2000], and by increasing the variability of deep water ventilation [Raymo and Ruddiman, 1992]. Before 2.7 Ma, the variability of oceanic circulation is not so easily explained as a result of ice sheet influence and amplifictaion. Yet the stepwise change into widespread continental glaciation in the Pliocene probably exerted major influence on oceanic circulation variability. An underlying cooling trend probably existed which increased the sensitivity of the high-latitude regions to the formation of ice sheets of larger magnitude than the more restricted ones that formed during the late Miocene and earlier parts of the Pliocene. The progression seen in our record in which the area most prone to glaciation (Greenland) apparently first developed large ice sheets is a strong indication that there was a gradual cooling process in effect, at least from before 3.3 Ma. [Maslin et al., 1995] suggested that the combination of such a cooling trend with the particular strength of orbital forcing at this time could have started the system of repeated glaciations. Model experiments with the Louvain de la Neuve two-dimensional climate model supports this. These experiments concluded that when exposed to a CO2-1owering, the orbital configuration led to establishment of large Northern Hemisphere ice sheets [Li ot al., 1998]. If this is correct, questions still remain as to (1) why the system continued in the glacial mode until today, even when orbital forcing changed, and (2) what caused the underlying trend. Many have suggested this to be diminishing atmospheric CO2-1evels, alone or in combination with tectonically forced processesuch as mountain building and uplift of plateaux in Tibet/Himalaya and North America [Ruddiman and Kutzbach, 1989; Raytoo and Ruddiman, 1992; Maslin et al., 1995] or effects of the closure of the Panamanian gateway [e.g. Haug and Tiedemann, 1998]. One problem with these explanations lies with the timing, as much of the change apparently took place quite a long time before 3.3 Ma. The interval from to Ma is characterized by a number of large IRD peaks indicating severe glaciations/deglaciations. The North Atlantic benthie 5 3C records show that glacials from this time on tend to be accompanied by reduced production of North Atlantic Deep Water (NADW)with interglacials showing relatively high NADW flux [Raymo et al., 1992], i.e., the same pattern as in the late Quaternary. There is, however, a strong dissimilarity between the North Atlantic and the Nordic Seas records. In the Nordic Seas, strong carbonate dissolution and sediments barren of carbonate characterize the whole interval Ma, except

11 JANSEN ET AL.: PLIO-PLEISTOCENE IRD HISTORY 719 in a few intervals Ma [Baumann and Huber, 1999]. The glacial cycles are not reflected by the typical late Quaternary shifts between carbonate-rich, warm interglacials and glacials with lower carbonate content due to less carbonate production and dilution by terrigenous input. Instead, the sediments appear to be continuously influenced by IRD, although to a variable degree, as the Site 907 record documents. Carbonate is almost constantly absent all over the Nordic Seas, except in the shallow areas [Fronval and Jansen, 1996a; Wolf- Welling et al., 1996; Baumann and Huber, 1999] between 3.5 and 1.1 Ma, and the small remaining quantities are strongly influenced by dissolution [Henrich, 1989]. In contrast, the last 1 Myr were characterized by high carbonate content in interglacials and dissolution only during major deglaciations [Henrich, 1989]. Since high carbonate content is indicative of relatively warm surface waters and low carbonate dissolution (good ventilation), the carbonate-barren intervals of the period prior to 1 Ma indicate that there was less inflow of warm Atlantic water and a reduced ventilation of deep and intermediate waters compared with late Quaternary interglacials and glacials. Long periods during which the IRD levels never go to zero (Figure 4) are also an argument that the northward heat flux was diminished. Two aspects make this situation an enigma: First in the North Atlantic, there is the consistent pattern of interglacials with high NADW production during the entire last 3 Myr [Raymo et al., 1997]. To produce this pattern, there must have been areas of strong overturning in the high-latitude North Atlantic. Overturning could have occurred south of where it happens today, i.e., south of the Greenland-Scotland Ridge, or in a manner with convection only to sill depths, as discussed below. The other problem is the discovery of forested areas on the northern tip of Greenland, dated to Ma [Funder et al., 1985; S. Funder, personal communication, 1995]. If dated correctly, this evidence of climates warmer than now would indicate periods of strong northward heat transport and/or absence of sea ice along northern Greenland. Neither Site 907 nor the sites on the V ring plateau (644/642) include interglacial sediments which document such a warm phase. One possibility is that there were indeed periods of warm water influx into the Nordic Seas, but a hydrographic situation with slightly less salty water only allowed convection to sill depths (---lkm). This could create overflows to feed NADW, but may have left the deeper parts below about lkm water depth poorly ventilated so that carbonate produced at the surface was dissolved at depth. Ongoing work to study the chemistry of overflow waters immediately downstream from the sill may provide needed documentation to support or disprove this idea. Recently, Mcintyre et al. [ 1999] documented from O D P sites downstream of the overflows that some of the interval between 2.0 and 1.4 Ma was characterized by very negative 5 3C, i.e. nutrient-rich and poorly ventilated overflo water. Similar but less direct evidence comes from carbonate records from the same area [Ortiz et al., 1999]. While this situation may explain the extended carbonate dissolution phase, it is hard to conceive that less salinity influx could lead to warmer Arctic conditions, as temperature and salinity normally is correlated in this region. The lack of complete deglaciations, as documented by the continuous IRD flux at both Sites 907 and 644, would also indicate that the interglacial phases of the interval prior to 1 Ma in general were cooler with less northward oceanic heat flux than in late Quaternary interglacials. According to the IRD data, the last 1.8 Myr are characterized by repeated severe glaciations on the continent surrounding the Nordic Seas. The first expansion of ice sheets out of mainland southern Norway into the North Sea basin is believed to have happene during this phase at Ma [Sejrup et al., 1991]. Within the last Myr the records document a shift toward glaciations of longer duration (lower-frequency fluctuations) as well as an evolution toward more pronounced interglacial periods (longer intervals without IRD). The IRD record fi'om Site 907 does not, however, exhibit higher-amplitude fluctuations and higher absolute values as should be expecteduring a period of large late Pleistocene continental ice sheets. This could be due to dilution by foraminifers or could reflect that the absolute size of the Greenland ice sheet did not change significantly during glacials in the last 1.8 Myr. On the eastern side of the basin, at ODP site 644, there is a clear indication of increased IRD flux at the mid-pleistocene climate shift [Jansen and $joholm, 1991], possibly indicating that the northern European ice sheets were more influenced by this change than the Greenland ice sheet. Regardless of this, we believe the marked shift toward variance around 100 kyr in the past 0.9 Myr (Figure 7) is a strong indication that all ice sheets in the region changed their mode of variance at the time of the mid-pleistocene climate shift, fi'om 41 kyr dominance to variance at lower frequencies. It is remarkable how closely the IRD record mimics the global climate record both in terms of amplitude and in the way it documents local ice sheet response to the orbital forcing function and how this changed with time. 4. Conclusions A continuous IRD record covering the last 3.5 Myr documents that the inception of large-scale glacial cycles in the Northern Hemisphere occurred stepwise, with a marked expansion of the Greenland ice sheet at 3.3 Ma as the first step. A second step occurred in isotope stage G6 at 2.74 Ma when all main ice sheet loci in the Northern Hemisphere started to feed icebergs to the surrounding ocean. Since 3.3 Ma, the ice sheets surrounding the Nordic Seas have been tightly coupled to the global ice volume variations, as shown by the close resemblance between the IRD record and the benthic oxygen isotope record. Between 3 and 1 Ma most of the variance of the IRD signal is in the 41 kyr band, whereas the last 1 Myr is characterized by stronger 100 kyr variance. GRAPE density, and to some extent, magnetic susceptibility are closely linked with IRD variations. The GRAPE record documents a number of oscillations which probably reflect IRD variations on suborbital timescales (5-10 kyr cycles) similar to the late Quaternary Heinrich/Bond cycles. Acknowledgments. We thank the captain, crews, and scientific staffs of ODP legs 151 and 162 for their hard work to ensure the successes of the legs. ODP and the Bremen core repository staff are thanked for samples and sampling assistance. We also thank Rune S raas and Odd Hansen for mass spectrometry operation and Gerd Solbakken and Stig Monsen for sample preparation. This study was supported by the Norwegian Research Council.

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