Sedimentary regimes in Arctic s Amerasian and Eurasian basins: Clues to differences in sedimentation rates

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1 Accepted Manuscript Sedimentary regimes in Arctic s Amerasian and Eurasian basins: Clues to differences in sedimentation rates Emma Sellen, Martin Jakobsson, Jan Backman PII: S (07)00183-X DOI: doi: /j.gloplacha Reference: GLOBAL 1314 To appear in: Global and Planetary Change Received date: 5 June 2007 Revised date: 10 October 2007 Accepted date: 11 October 2007 Please cite this article as: Sellen, Emma, Jakobsson, Martin, Backman, Jan, Sedimentary regimes in Arctic s Amerasian and Eurasian basins: Clues to differences in sedimentation rates, Global and Planetary Change (2007), doi: /j.gloplacha This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

2 Sedimentary regimes in Arctic's Amerasian and Eurasian basins: Clues to differences in sedimentation rates Emma Sellén 1 (corresponding author, emma.sellen@geo.su.se), Martin Jakobsson 1, and Jan Backman 1. Department of Geology and Geochemistry, Stockholm University, Stockholm, Sweden Abstract A standard lithostratigraphic model based on cores retrieved from the ice island T-3 was developed by Clark et al. (1980) for the Amerasian Basin of the Arctic Ocean. We have investigated whether or not it is possible to apply this lithostratigraphy to cores from the Lomonosov Ridge, which can be correlated to Eurasian Basin cores, for the purpose of correlating the Amerasian and Eurasian stratigraphies. Published averaged sedimentary proxies from a selected set of T-3 cores are used to correlate with the identical published proxies for the included Lomonosov Ridge cores. The standard lithostratigraphic classification could not be applied to the Lomonosov Ridge cores, which is interpreted to result from differences in sedimentary regimes in the Amerasian and Eurasian Basins. These differences also apply to the barrier between the two basins, the Lomonosov Ridge. The general sedimentation rates are three to four times lower in the Amerasian Basin than in the Eurasian Basin if the first down-core paleomagnetic inclination change is used to correlate between the two basins whereas correlation based on sediment coarse fraction suggests only two times lower rates in the Amerasian Basin. Keywords: Arctic Ocean; stratigraphy; sedimentary regimes; stratigraphic correlation; sedimentation rates 1

3 1. Introduction Numerous short sediment cores taken from the Amerasian Basin of the Arctic Ocean were originally described by Clark et al. (1980). These cores were retrieved from the ice island T-3, which functioned as a platform for sediment coring from 1963 to 1973 (Clark et al., 1980). They established a lithostratigraphic classification synthesized from 67 sediment cores (below referred to as Clark's "type core"), in which 13 lithological units were identified and labeled A to M. This classification subsequently has been widely applied on cores raised from the Alpha Ridge, the Northwind Ridge and the Mendeleev Ridge (e.g., Minicucci and Clark, 1983; Morris et al., 1985; Mudie and Blasco, 1985; Scott et al., 1989; Clark et al., 1990; Poore et al., 1993; Ishman et al., 1996; Jokat, 1999; Phillips and Grantz, 2001), and used for correlation of sediment cores over large distances in the Arctic Ocean. An age model was developed for these sediment cores that relied on the identification of the Brunhes-Matuyama paleomagnetic reversal boundary, dated to 781 ka (Lourens et al., 2004), and the assumption of a linear sedimentation rate (Clark et al., 1980). Several studies have shown, however, that many measured changes in Pleistocene magnetic polarity directions in Arctic Ocean sediments represent excursions of shorter durations within the Brunhes Chron (Løvlie et al., 1986; Bleil and Gard, 1989; Jakobsson et al., 2001; Nowaczyk et al., 2001; Nowaczyk et al., 2003; Spielhagen et al., 2004). This paper aims to investigate whether or not the lithostratigraphic units introduced by Clark et al. (1980) are possible to identify in cores retrieved from the Lomonosov Ridge during the LOREX 79 expedition (Blasco et al., 1979), the expedition ARK-VIII/3 (Fütterer, 1992), and the Arctic Ocean 96 expedition (Backman et al., 1997). The main 2

4 objective for correlating stratigraphies between the Amerasian Basin and the Lomonosov Ridge is to establish a consistent chronostratigraphic framework for Arctic Ocean sediment cores and to re-evaluate interpretations of many previous studies that has applied the age model of Clark et al. (1980). 2. Background: Investigated Cores and Their Stratigraphy 2.1 Lomonosov Ridge Sediments deposited on the Lomonosov Ridge range in thickness from a few tens of meters to more than 1000 m (Jackson and Oakey, 1990; Jokat, 2005). Sea ice rafted debris originating from the Kara Sea and the Laptev Sea areas, transported with the Transpolar Drift, is presently the main sedimentary source for the Lomonosov Ridge together with a smaller amount of iceberg rafted debris (Spielhagen et al., 1997; Wahsner et al., 1999; Eicken et al., 2000). Sea ice in the Transpolar Drift generally survives for up to three years before it leaves the Arctic Ocean via the Fram Strait (Stokes et al., 2005). Morris et al. (1985) investigated three gravity cores that are used in this present study. Core B-24 was recovered from the crest of the Lomonosov Ridge at 1600 m depth, B-17 at 2360 m was taken on the flank towards a local depression in the Lomonosov Ridge morphology, the so-called "intra-basin", and B-8 at the foot of the ridge in the Makarov Basin at 3956 m depth (Figures 1 and 2; Table 1). Core PS from the crest of the Lomonosov Ridge was described by Fütterer et al. (1992) and Spielhagen et al. (1997). Box core from an adjacent site on the 3

5 Lomonosov Ridge was used to supplement the missing top 30 cm in core (Fütterer, 1992). Core 96/12-1pc was acquired from the ridge crest for which a detailed late Pleistocene stratigraphy has been developed using sediment physical properties including color, paleomagnetism, biostratigraphy, chemostratigraphy, and optically stimulated luminescence dating (Jakobsson et al., 2000a; Jakobsson et al., 2001; Jakobsson et al., 2003a). The Lomonosov Ridge cores 96/12-1pc and PS have previously been correlated with cores from the Eurasian Basin by Backman et al., (2004) and Spielhagen et al. (2004). Their correlations suggest that these Lomonosov Ridge cores represent the Eurasian Basin sedimentation regime. 2.2 Alpha and Mendeleev Ridges The Alpha and Mendeleev Ridges include several seamounts and valleys with depths ranging from 740 to 2000 m (Jakobsson et al., 2003b). Between 500 m and 1000 m of pelagic sediments is draped over the crest of the Alpha Ridge (Jokat, 2003) and between 300 m and 3500 m sediment covers the Mendeleev Ridge (Lebedeva-Ivanova et al., 2006). At present, the clockwise Beaufort Gyre is the dominating force dictating the drift of the pack ice as well as the icebergs in the Amerasian Basin (Phillips and Grantz, 2001). Sea ice transported in the Beaufort Gyre can survive there for several decades before being carried out of the Arctic Ocean (Stokes et al., 2005). The sediment incorporated in the sea ice and icebergs were originally entrained predominantly on the 4

6 shelf region of the Canadian Arctic Archipelago, but some also originates from the Chukchi Sea and the Laptev Sea shelves (Darby, 2003). During the Pleistocene, the continental Laurentide ice sheet delivered large amounts of iceberg-rafted debris from the Canadian Arctic Islands to the Amerasian Basin (Phillips and Grantz, 2001). Peak abundances of coarser grain sizes are related to increased iceberg discharge and sea ice rafting (Polyak et al., 2004). The weight percentage of coarse fraction (>63μm) has decreased during the Holocene, indicating that sea ice is the primary transport agent of material rather than icebergs (Darby et al., 2002; Polyak et al., 2004). A total of 580 sediment cores and samples were recovered from the Amerasian Basin from T-3, between 1600 m and 3600 m depth (Clark et al., 1980). The T-3 cores ranged in length from about 30 cm to 550 cm and were distributed over more or less the entire Alpha Ridge. A few Mendeleev Ridge and Nautilus Basin cores were also incorporated in Clark's synthesized type core (Figure 1) Litho- and chronostratigraphy In general, Clark s type core is described as being composed of silty and arenaceous lutites with a few coarse-grained, carbonate-rich, pinkish white layers. The lithological units of one of Clark's key cores, FL439 from the Alpha Ridge (Clark and Hanson, 1983), is shown in Figure 3, together with its proposed correlation to B-8 (Morris et al., 1985). Clark et al. (1980) used lutite as a term to describe a clayey silt or silty clay (Ericson et al., 1964). More specifically, the sedimentary parameters used for recognition of the individual units A to M were percentages of quartz-feldspar, total amount of detrital grains, coarse fraction larger than 63 µm, content of Fe-Mn particles, 5

7 and abundance of foraminiferal tests. Furthermore, visual identification of the units based on sediment texture, degree of bioturbation, color and content of erratics, were also included. The average weight percentage of sand-sized material was considered to be the most important sedimentary characteristic when identifying and correlating the lithostratigraphic units in several hundred cores distributed over several hundred thousand square kilometers (Clark et al., 1980). The chronostratigraphy of units A to M was established by classifying magnetic signatures in the cores, assuming linear sedimentation rates within identified magnetic chrons (Clark et al., 1980). The first reversed polarity signature, which consistently appeared within lithologic unit K (approximately cm) in all measured cores, was interpreted to represent the Brunhes-Matuyama boundary (Figures 3 and 4). A recent age estimate of this geomagnetic reversal boundary is 781 ka (Lourens et al., 2004). The Brunhes-Matuyama magnetic reversal was originally placed in T-3 core FL224 from the Nautilus Basin off the Chukchi Cap at about 1 m core depth (Steuerwald et al., 1968; Clark, 1970). In the longer cores used by Clark et al. (1980), a second polarity direction change was observed in unit D (on average between 227 cm and 261 cm). This change was interpreted to represent the Matuyama-Gauss boundary (2.581 Ma; Lourens et al., 2004). In the 1960's through early 1980's, no other independent age determinations were used for any of the T-3 cores and, thus, the assumed ages of the proposed magnetostratigraphies resulted in sedimentation rates that consistently were on the order of mm/1000 years (Backman et al., 2004). In support for the concept of a sediment starved Arctic Ocean, Clark et al. (1980) referred to observed abundances of ferromanganese particles in sediments as well as extensive ferromanganese coatings on 6

8 larger pebbles, low amounts of foraminifera and ice rafted erratics, and heavily bioturbated sedimentary units. The lithostratigraphy for the Lomonosov Ridge developed by Morris et al. (1985) was subdivided into units Alpha to Mu using sediment color and texture, degree of bioturbation, mineralogy, occurrence of planktic foraminifera, contents of bulk carbonate, coarse fraction, and the abundance of ferromanganese micro-nodules (Figure 3). This stratigraphy was synthesized from cores B-8, B-17 and B-24. However, there is no stratigraphy or sedimentological data published for B-17 and this core is therefore not included in Figure 4. Core B-24 is dominated by sandy, silty clay which is yellowish brown or olive brown in color. Magnetic reversals were not found in any of the three cores, and an age model was established by adopting a constant sedimentation rate of 1.5 mm/1000 years using Lin kova s (1965) results from the Marvin Spur, approximately 200 km away from B-24 (Figure 1). The sediments in core PS generally consist of olive brown and brown mud with some silty and sandy layers (Fütterer, 1992). There is a clear cyclicity expressed by prominent, mottled brown mud units (Figure 4). In the original chronostratigraphy developed by Spielhagen et al. the Brunhes-Matuyama boundary was placed at 342 cm downcore. The Matuyama-Gauss and Gauss-Gilbert (3.596 Ma, Lourens et al., 2004) boundaries were placed at 450 cm and 631 cm, respectively, consistent with the low sedimentation rate scenario proposed by Clark et al. (1980). Sedimentation rates in PS thus ranged from about 1.7 to 4.4 mm/1000 years. However, this age model was significantly updated by Spielhagen et al. (2004) after correlation to core 96/12-1pc and based on the co-occurrence of the calcareous nannofossil taxa G. muellerae and E. 7

9 huxleyi. The revised chronostratigraphy (Jakobsson et al., 2000a) where the Brunhes- Matuyama boundary is replaced with the Biwa II excursion (~220 ka, Lund et al., 2006), implies a sedimentation rate of 1.6 cm/1000 years, which is about an order of magnitude larger than the previous estimate. The sediment composition of core 96/12-1pc resembles that of the relatively neighboring core PS2185-6, with a distinct difference in a gray silty layer that is present in 96/12-1pc between about 130 and 180 cm (Figure 4). The latter core is characterized by lithologic cycles that have been correlated to variations in manganese concentration, considered to be a proxy indicator for glacial-interglacial variability (Jakobsson et al., 2000a). The manganese/color cycles in core 96/12-1pc were referred to a low latitude oxygen isotope curve (Bassinot et al., 1994). The resulting correlation was used to construct an age model for core 96/12-1pc (Jakobsson et al., 2000a). Occurrences of Gephyrocapsa spp. and E. huxleyi between 222 cm and 195 cm in 96/12-1 pc was used as a control point. This interval was considered to represent MIS 5 and the age model thus showed a sedimentation rates in the upper 2.2 m of the core of nearly 2 cm/1000 years, in stark contrast to the mm-scale rates proposed by Clark et al. (1980) for cores in the Amerasian Basin (Jakobsson et al., 2001). Absolute dating by Optically Stimulated Luminescence (OSL) subsequently confirmed the age model established for core 96/12-1pc (Jakobsson et al., 2003a). 3. Lithostratigraphic re-evaluation approaches The lithostratigraphies from Clark et al. (1980), Morris et al. (1985), and Fütterer (1992), were redrawn (Strater version 1.4) and compared for similarities. The original 8

10 lithostratigraphic descriptions for core 96/12-1pc were available, redrawn and used correspondingly. Grain size and microfossil abundance data averaged over broad intervals from selected sets of the T-3 cores have been published by Clark et al. (1980). These data are used here for comparisons with the corresponding data sets published for the Lomonosov Ridge cores (Morris et al., 1985; Spielhagen et al., 1997; Jakobsson et al., 2001). 3.1 Coarse Fraction The coarse fraction (CF) is here defined as grain sizes >63 µm. Clark et al. (1980) analyzed selected cores for the CF and the mean of several T-3 cores were calculated for the lower, middle and upper parts of each lithostratigraphic unit (A to M). The CF of the LOREX B-cores was analyzed at 1-cm intervals and measured as weight percent (Morris and Clark, 1986). Results were presented as a mean value for each individual lithostratigraphic unit (Morris et al., 1985). In order to use the grain size records from Clark et al. (1980) and Morris et al. (1985), their published graphs were digitized. The CF data from core represents a 5 cm sample interval (Stein et al., 1994; Spielhagen et al., 1997). The CF data of core 96/12-1pc were determined at a sample resolution of about 2 cm. These latter grain size data sets were provided in digital form for this study (Jakobsson et al., 2001; Spielhagen et al., 2004). 3.2 Foraminiferal abundance Planktic foraminifers occur discontinuously in the upper parts in most central Arctic Ocean piston/gravity cores (Clark et al., 1980; Aksu, 1988; Scott et al., 1989; 9

11 Ishman et al., 1996; Spielhagen et al., 1997; Jakobsson et al., 2001). Abundances of foraminifers per gram sediment were used, in addition to CF data, for core-to-core correlations. Clark et al. (1980) analyzed thin sections for each lithologic unit (A to M) in 3 to 5 different cores and averaged the result. These averaged results were given as percentage of foraminiferal tests in the >63 µm size fraction. The abundance of planktic foraminifers per gram of coarse sediment (>63µm) was determined by Morris et al. (1985) as averages for each of their lithostratigraphic unit. The foraminiferal abundances published by Clark et al. (1980) and Morris et al. (1985) were digitized in order to be included here. Spielhagen et al. (1997) counted the planktic foraminferal concentrations in a representative split ( 500 tests) of the µm fraction, whereas Jakobsson et al. (2001) reported the foraminiferal abundance in the >150 µm size fraction. 3.3 Paleomagnetic polarity reversals The T-3 cores were split up into 15-cm-long segments upon recovery and the paleomagnetic polarity was determined from 2 cm samples taken at 5 cm intervals in each segment (Steuerwald et al., 1968; Clark, 1969). Unfortunately, only the interpreted zones of paleomagnetic polarity directions are available because the measurements were never published for the T-3 cores, nor were any intensity, declination or inclination values calculated (Darby, 1989). For core B-8, Morris et al. (1985) analyzed samples at 10-cm intervals. Core PS was sampled at 4-5 cm intervals using vertically orientated paleomagnetic cube samples (about 7 cm 3 ); the core was also sampled using an U-channel (28 mm x 28 mm, Frederichs, 1995). The upper ~4.2 m of core 96/12-1pc 10

12 from the Arctic Ocean 96 expedition was sampled continuously with thin-walled cubic plastic boxes (6.2 cm 3 ), while the lower 3 m was sampled using two 1.5-m-long U- channels (2 x 2 cm, Jakobsson et al., 2001). The summary polarity columns published for the cores above were used in the core-to-core correlation. 4. Results Cores B-8, B-24, PS2185-6, , 96/12-1pc and the synthesized data from Clark et al. (1980), were compared to each other using paleomagnetic inclination changes, coarse fraction (>63µm), foraminifera abundance, color, and lithostratigraphy (Figure 4). Morris et al. (1985) correlated the lithostratigraphy of B-8 from the Makarov Basin to Clark s type core (Figure 3). However, the three LOREX B-cores were taken in, or in close proximity to, the intra-basin located on the central Lomonosov Ridge (Figure 2). This is a conduit for Amerasian Eurasian Basin water mass exchange and an area of extensive current activity (Björk et al., 2007), which thus influences the sedimentary depositional patterns. Core B-8 is likely affected by turbidite deposits originating from the flank of the Lomonosov Ridge, considering its position at the base of the ridge (Figure 2) (Stein and Korolev, 1994). A current that flows inside and along the intrabasin walls (Björk et al., 2007) presumably affected the sediment record in B-17 extensively. The extent to which B-24 has been affected is more difficult to assess. Despite its location on a ridge high, it is close to the highly dynamic area of the intrabasin (Björk et al., 2007). Therefore, any correlation between these cores and cores taken from the Lomonosov Ridge or the Alpha Ridge appears uncertain. Cores PS and 96/12-1pc have previously been correlated using paleomagnetism, sediment physical 11

13 properties, and CF data (Jakobsson et al., 2001; Spielhagen et al., 2004) and they are recovered from similar depositional environments (Figure 2). 4.1 Paleomagnetic Polarity Direction Changes The first conspicuous change in geomagnetic polarity direction (Figure 4) in cores PS (at about 343 cm) and 96/12-1pc (at about 270 cm) has been interpreted to represent the Biwa II excursion (220 ka, Jakobsson et al., 2001; Spielhagen et al., 2004). Assuming that this excursion was recorded regionally in Arctic Ocean sediments, an obvious potential correlation horizon, and hence isochron line, is available by linking the first down-core geomagnetic polarity direction change in the Lomonosov Ridge cores to the first recognized corresponding change in the Alpha Ridge cores, which typically occurs in the upper 60 cm to 93 cm in these latter cores (Clark et al., 1980). This correlation implies that sedimentation rates in Amerasian Basin cores roughly were 3 to 4 times lower compared to the rates on the Lomonosov Ridge. No further correlations, using the paleomagnetic inclination pattern could be made. 4.2 Lithostratigraphy The lithological units as described by Fütterer (1992) and Jakobsson et al. (2001) correlate fairly well between core PS and 96/12-1pc (Figure 4). While the upper units differ somewhat in grain size and color, the lower parts of the cores, below 163 cm in PS and below 190 cm in 96/12-1pc, show alternating layers of dark brown and light brown clay with similar downcore coarse fraction distributions and foraminiferal contents. The lithological units established by Clark et al. (1980) have not been 12

14 recognized in cores PS and 96/12-1pc. The proposed lithostratigraphic correlation by Morris et al. (1985) between B-8, from the Makarov basin at the foot of the Lomonosov Ridge and Clark s type core (Figure 3), was primarily based on the recognition of a pinkish white sandy layer, located at 70 cm in B-8 and at the base of unit M in Clark's type core. This pinkish white layer has not been observed in PS or 96/12-1pc, nor was it possible to recognize any of Clark s other pinkish white or white coarser layers, judging from the descriptions and color core photos of PS and 96/12-1pc. 4.3 Sediment Color An exact color correlation is not possible due to the lack of core photos of the T-3 cores. The variation in sediment color was, in all studies, assigned through visual examination. There are some distinct changes, however, in the described sediment colors that appear to be traceable among the cores discussed. All cores vary in color, mainly from yellowish to olive to grayish brown. The top layer is generally described as having a dark brown color (Figure 4). There is a section of alternating darker brown and lighter brown clays recognized in both PS (below 163 cm) and 96/12-1pc (below 190 cm). A correlation based on color from these cores to B-24, and the colors described by Clark et al. (1980), cannot be established. 4.4 Coarse Fraction 13

15 The CF data of B-8, PS and 96/12-1pc, as well as the synthesized coarse fraction for the Alpha Ridge cores (Clark et al., 1980; Morris et al., 1985; Jakobsson et al., 2001; Spielhagen et al., 2004) were used in the next core correlation step (Figure 4). There are some similarities between PS and 96/12-1pc on the one hand and the grain size curves for B-8 and B-24 on the other. A prominent CF peak slightly above the first paleomagnetic polarity reversal is present in both PS and 96/12-1pc, between g5 and g6 (Figure 4) and may correlate to the lower CF peak in B-8. Following the correlation by Morris et al. (1985) this would correspond to the lowermost high plateau (>30 %) CF values for B-24. In general, there is virtually no resemblance between the CF data generated from B-24 and PS2185-6/96/12-1pc, and it follows that CF-based correlations to the LOREX B-cores must be considered uncertain. The cause to this difference is probably due to differences in their depositional settings (Figure 2). We have attempted to link PS and 96/12-1pc to the synthesized CF for Clark's type core, despite the fact that the latter data consist of only a few averaged values for each lithostratigraphic unit. A connection may be possible by correlating the g5-g6 peak with Clark's synthesized CF interval between 93 cm and 153 cm (Figure 4), which may appear reasonable. This correlation implies (a) that sedimentation rates on the Lomonosov Ridge crest is about two times higher compared to Clark's synthesized type core and (b) that Clark's proposed geomagnetic polarity pattern differs from those derived from the Lomonosov Ridge. Due to the apparent lower resolution of the Alpha Ridge data (only a few averaged values were published for each lithostratigraphic unit) the details observed in the Lomonosov Ridge cores cannot be distinguished in the Clark s synthesized data. 14

16 4.5 Foraminiferal abundance The abundance of foraminiferal tests from B-24, 96/12-1pc, PS2185-6, and the average values from the Alpha Ridge cores, were not straightforward to correlate (Figure 4; Clark et al., 1980; Morris et al., 1985; Jakobsson et al., 2001; Spielhagen et al., 2004). The uppermost parts are by and large relatively rich in planktic foraminifers in the Lomonosov Ridge cores and as well in Clark's type core. In the case that a hiatus is present somewhere in the upper cm of B-24 and considering the short distance between this core and the two other Lomonosov Ridge cores, one may speculate that the lower peak abundances of foraminiferal tests in B-24 may correlate with f2-f5 in 96/12-1pc/PS (Figure 4). However, due to its position by the intra-basin it is probably unsuitable to use for correlation. Clark's type core shows continuous presence of foraminiferal tests in the upper meter, underlain by a short barren interval, which rests on a short "peak" interval (between 153 cm and 185; Figure 4). This "peak" in the type core may be correlated to the three distinct peaks of foraminiferal abundances (f3 to f5) in PS and 96/12-1pc as the sediment below all these peaks are virtually barren. This correlation assumes that foraminifera disappeared from both the Alpha Ridge and the Lomonosov Ridge at the same time. However, the synthesized data available for the Alpha Ridge cores is not detailed enough to distinguish the three individual peaks in this interval. The above correlation would yield a sedimentation rate that is only approximately 1.3 times lower on the Alpha Ridge. 15

17 5. Discussion and conclusions Three different correlations (Figure 4), based on paleomagnetic inclination pattern, CF and foraminifera concentrations has been suggested between Lomonosov Ridge cores (PS and 96/12-1pc) and the stratigraphy established by Clark et al. (1980). The standard lithological units A to M and colors were of no help since they cannot be recognized in the Lomonosov Ridge cores. The most obvious correlation is probably the first paleomagnetic inclination reversal/excursion since it is consistently recognized in cores from both the Alpha and Mendeleev Ridges and the Lomonosov Ridge. However, both the CF and the foraminifera abundance correlation conflicts with the inferred paleomagnetic correlation. Although a correlation between core B-8 and Clark s type core has previously been made, it was not possible to recognize this Alpha Ridge lithostratigraphy in either PS or 96/12-1pc from the Lomonosov Ridge. The correlation between B-8 and the Alpha Ridge lithostratigraphy might not be valid because of the location of B-8 in the Makarov Basin at 3956 m depth and due to the extensive current activity in the area and the possibility of turbidite deposits. Paleomagnetic measurements appears to have potential to be a good correlation tool for sediment cores in the Arctic Ocean. The first reversal was identified in all cores, except for the shorter cores B-8 and B-24, which did not reach deep enough into the sediment column. The excursions found in PS and 96/12-1pc are often quite thin and, due to a less detailed record and possibly lower sedimentation rates for the Alpha Ridge cores, it is possible that they are not documented in these sediment or alternatively 16

18 have merged together. More detailed paleomagnetic measurements on longer cores are necessary in order to verify this assumption. The second proxy, the described lithostratigraphies, did not result in a correlation which could indicate that the lithostratigraphic units (A to M) are not possible to use outside of the Amerasian Basin. Within the Laurentide Ice Sheet there were several active ice streams that released a large amount of icebergs into the Amerasian Basin. The ice streams alternates between short periods of rapid ice discharge and a phase of stagnation. This would result in episodically released IRD that should be possible to observe in the sedimentary records (Stokes et al., 2005). The pinkish-white and white carbonate coarse layers found in the Alpha Ridge cores (Clark et al., 1980) could be evidence of this. We do not, however, see these carbonate layers in cores from the Lomonosov Ridge and the Eurasian Basin (Fütterer, 1992; Jakobsson et al., 2001), which could indicate that the sediment is mainly of Eurasian origin. Correlating sediment cores using only color descriptions can be highly uncertain as sediment color change when exposed to oxygen or other laboratory conditions. The assignment of color based on visual description must also be considered to contain a certain degree of subjectivity. The characteristic alternating dark and light colored layers described in both PS and 96/12-1pc could not be identified in core B-24 and B-8 or in the lithostratigraphy for the Alpha Ridge cores. Therefore, a correlation based on cyclicity in colors across from the Lomonosov Ridge to the Alpha Ridge was not achieved. These individual layers might not be possible to distinguish in the Alpha Ridge cores as the lower sedimentation rates contribute to merging layers together. However, 17

19 new HOTRAX material show a prominent cyclical stratigraphy with alternating dark brown layers as has been found in the Lomonosov Ridge cores (Darby et al., 2005). This may indicate that the T-3 cores do not give an account of all aspects of the depositional history for the cored sediments in the Amerasian Basin. The CF correlation provided one link between the Lomonosov Ridge cores and the Alpha Ridge cores (g5-g6), which implies a sedimentation rate that is about two times higher on the Lomonosov Ridge. The data available from the T-3 and LOREX B-cores are poorly resolved and the correlation needs to be improved by making more CF analyses on the longer cores retrieved from both ridges during the HOTRAX 05 expedition (Darby et al., 2005). The use of foraminifera abundances as a correlation proxy did not result in a good connection between the Alpha Ridge and the Lomonosov Ridge. The correlation was in part difficult due to different units used and the sparse averaged data for the Alpha Ridge. A shift from agglutinated foraminifera to calcareous faunas has been observed in several Arctic sediment cores and this could be a useful biostratigraphic marker for correlation in the Arctic Ocean (Evans and Kaminski, 1998; Backman et al., 2004; Polyak et al., 2004). This marker could not be used in this study because it was not used by Clark et al. (1980), when constructing the lithostratigraphic units for the Alpha Ridge cores. Carbonate dissolution of corrosive waters could be a reason for the presence of only agglutinated foraminifera, as well as lower sedimentation rates (Jakobsson et al., 2001). However, it might be problematic to use as a marker because the shift did not seem to occur at the same time in the whole Arctic Ocean (Backman et al., 2004). 18

20 The result of our correlation shows that we may indeed have different sedimentation regimes in the Amerasian Basin and the Eurasian Basin. This is reasonable since the Alpha Ridge is mainly affected by the Beaufort Gyre and mostly receives material derived from the North American continent, whereas the Lomonosov Ridge is mostly affected by the Transpolar Drift that brings material form the Siberian shelves (Wahsner et al., 1999; Darby, 2003). Fluctuations of the path of the Transpolar Drift as a response to the Arctic Oscillations could also have had an effect on these sedimentation differences as it is sometimes moved much closer to North America (Darby and Bischof, 2004). Sediment material originating from the Laurentide Ice Sheet and the Innuitian Ice Sheet has mainly been deposited in the Amerasian Basin whereas the Eurasian Basin has had more sediment input from the Eurasian Ice Sheet (Darby et al., 2002). From this it follows that it might not be possible to establish a common lithostratigraphy throughout the entire Arctic Ocean; the standard lithostratigraphy developed by Clark et al. (1980) only applies to the central part of the Amerasian Basin. The possibility of higher sedimentation rates in the Eurasian Basin is plausible as vast amounts of sediments are discharged onto the Eurasian shelves from some of the world s largest rivers (for example the Yenisey, Lena and Ob, together with Pechora, Kolyma, and Severnaya Dvina) that drain much of the Eurasian Arctic landmass (Peterson et al., 2002). In order to provide a more reliable stratigraphic correlation it is necessary to compare more data from more detailed analyses than the synthesized data that was used for the Alpha Ridge cores. The results from the HOTRAX 05 expedition have generated a great amount of new material that can hopefully bring more light to this problem (Darby et al., 2005). 19

21 6. Acknowledgements Financial support was received from Stockholm University, the Swedish Research Council (VR) and the Swedish Royal Academy of Sciences through a grant financed by the Knut and Alice Wallenberg Foundation. Leonid Polyak is thanked for constructive comments on the manuscript. This is a contribution from the Stockholm University Climate Research Centre (SUCLIM). 7. References Aksu, A.E., Upper Cenozoic history of the Labrador Sea, Baffin Bay, and the Arctic Ocean, a paleoclimatic and paleoceanographic summary. Paleoceanography, 5, Backman, J., Knies, J., Knudsen, J.-O., Kristoffersen, Y., Lif, A., Musatov, E. and Stein, R., Geological coring and high resolution chirp sonar profiling. In: E. Grönlund (Editor), Polarforskningssekretariatets årsbok 1995/96. Polarforskningssekretariatet, Halmstad, pp Backman, J., Jakobsson, M., Løvlie, R., Polyak, L. and Febo, L.A., Is the central Arctic a sediment starved basin? Quaternary Science Reviews, 23, Bassinot, F.C., Labeyrie, L.D., Vincent, E., Quidelleur, X., Shackleton, N.J. and Lancelot, Y., The astronomical theory of climate and the age of the Brunhes-Matuyama magnetic reversal. Earth and Planetary Science Letters, 126 (1-3), Björk, G., Jakobsson, M., Rudels, B., Swift, J.H., Anderson, L., Darby, D.A., Backman, J., Coakley, B., Winsor, P., Polyak, L. and Edwards, M., Bathymetry and deep-water exchange across the Lomonosov Ridge at N. Deep-Sea Research I, 54, Blasco, S.M., Bornhold, B.D. and Lewis, C.F.M., Preliminary results of surficial geology and geomorphology studies of the Lomonosov Ridge, Central Arctic Basin. Geological Survey of Canada-Paper, 79-1C, Bleil, U. and Gard, G., Chronology and correlation of Quaternary magnetostratigraphy and nannofossil biostratigraphy in Norwegian-Greenland Sea sediments. Geologische Rundschau, 78 (3), Clark, D.L., Paleoecology and sedimentation in part of the Arctic Basin. Arctic, 22,

22 Clark, D.L., Magnetic Reversals and Sedimentation Rates in the Arctic Ocean. Geological Society of America Bulletin, 81, Clark, D.L., Whitman, R.R., Morgan, K.A. and Mackay, S.D., Stratigraphy and glacial-marine sediments of the Amerasian Basin, central Arctic Ocean. Geological Society of America Special Paper, 181, Clark, D.L., Chern, L.A., Hogler, J.A., Mennicke, C.M. and Atkins, E.D., Late Neogene climate evolution of the Central Arctic Ocean. Marine Geology, 93, Darby, D.A., Sediment composition and sedimentary processes in the Arctic Ocean. In: Y. Herman (Editor), The Arctic Seas. Van Nostrand Reinhold Company, New York, pp Darby, D.A., Bischof, J.F., Spielhagen, R.F., Marshall, S.A. and Herman, S.W., Arctic ice export events and their potential impact on global climate during the late Pleistocene. Paleoceanography, 17 (2), /2001PA Darby, D.A., Sources of sediment found in sea ice from western Arctic Ocean, new insights into processes of entrainment and drift patterns. Journal of Geophysical Research, 108. Darby, D. and Bischof, J., A Holocene record of changing Arctic Ocean ice drift analogous to the effects of the Arctic Oscillation Paleoceanography, 19 (PA1027), doi: /2003pa Darby, D., Jakobsson, M., Polyak, L. and party, H.s., Icebreaker expedition collects key Arctic seafloor and ice data. EOS Transactions, American Geophysical Union, 86 (52), 549, 552. Eicken, H., Kolatschek, J., Freitag, J., Lindemann, F. and Kassens, H., A key source area and constraints on entrainment for basin-scale sediment transport by Arctic Ocean. Geophysical Research Letters, 27, Ericson, D.B., Ewing, M. and Wollin, G., Sediment cores from the Arctic and subarctic seas. Science, 144, Evans, J.R. and Kaminski, M.A., Pliocene and Pleistocene chronostratigraphy and paleoenvironment of the central Arctic Ocean, using deep water agglutinated foraminifera. Micropaleontology, 44 (2), Frederichs, T., Regional and temporal variations of rock magnetic parameters in Arctic marine sediments. Berichte zur Polarforschung, 164, (in german). Fütterer, D., Arctic '91: The Expedition ARK-VIII/3 of RV "Polarstern" in Berichte zur Polarforschung, 107, 271. Gard, G. and Crux, J.A., Reworked Jurassic-Neogene calcareous fossils in the central Arctic. Marine Geology, 119, Ishman, S.E., Polyak, L.V. and Poore, R.Z., Expanded record of Quaternary oceanographic change: Amerasian Arctic Ocean. Geology, 24, Jackson, H.R. and Oakey, G.N., Sedimentary thickness map of the Arctic Ocean. In: A. Grantz (Editor), The Geology of North America; The Arctic Ocean Region, vol. L. The Geolological Society of America, Boulder. Jakobsson, M., Løvlie, R., Al-Hanbali, H., Arnold, E., Backman, J. and Mörth, M., 2000a. Manganese and color cycles in Arctic Ocean sediments constrain Pleistocene chronology. Geology, 28,

23 Jakobsson, M., Cherkis, N., Woodward, J., Macnab, R. and Coakley, B., 2000b. New grid of arctic bathymetry aids scientists and mapmakers. EOS Transactions, American Geophysical Union, 81 (9), 89, 93, and 96. Jakobsson, M., Løvlie, R., Arnold, E.M., Backman, J., Polyak, L., Knudsen, J.-O. and Musatov, E., Pleistocene stratigraphy and paleoenvironmental variation from Lomonosov Ridge sediments, central Arctic Ocean. Global and Planetary Change, 31, Jakobsson, M., Backman, J., Murray, A. and Løvlie, R., 2003a. Optically stimulated luminescence dating supports central Arctic Ocean cm-scale sedimentation rates. Geochemistry, Geophysics, Geosystems, 4, Jakobsson, M., Grantz, A., Kristoffersen, Y. and Macnab, R., 2003b. Physiographic provinces of the Arctic Ocean. Geological Society of America Bulletin, 115 (12), Jokat, W., Expedition gives fresh view of Central Arctic geology. EOS Transactions, American Geophysical Union 80, 465, Jokat, W., Seismic investigations along the western sector of Alpha Ridge, Central Arctic Ocean. Geophysical Journal International, 152, Jokat, W., The sedimentary structure of the Lomonosov Ridge between 88 N and 80 N. Geophysical Journal International, 163 (2), Lebedeva-Ivanova, N.N., Zamansky, Y.Y., Langinen, A.E. and Sorokin, M.Y., Seismic profiling across the Mendeleev Ridge at 82 N: evidence of continental crust. Geophysical Journal International, 165 (2), Lourens, L.J., Hilgen, F.J., Laskar, J., Shackleton, N.J. and Wilson, D., The Neogene period. In: F.M. Gradstein and J. Ogg (Editors), A Geological Time Scale Cambridge University Press, Cambridge, pp Løvlie, R., Markussen, B., Sejrup, H.P. and Thiede, J., Magnetostratigraphy in three Arctic Ocean sediment cores; arguments for geomagnetic excursions within oxygen-isotope stage 2-3. Physics of the Earth and Planetary Interiors, 43 (2), Lund, S., Stoner, J.S., Channell, J.E.T. and Acton, G., A summary of Brunhes paleomagnetic field variability recorded in Ocean Drilling Program cores. Physics of the Earth and Planetary Interiors, 156, Minicucci, D.A. and Clark, D.L., A Late Cenozoic stratigraphy for glacial-marine sediments of the eastern Alpha Cordillera, central Arctic Ocean. In: B.F. Molnia (Editor), Glacial-marine sedimentation. Plenum Press, New York, pp Morris, T.H., Clark, D.L. and Blasco, S.M., Sediments of the Lomonosov Ridge and Makarov Basin: a Pleistocene stratigraphy for the North Pole. Geological Society of America Bulletin, 96, Morris, T.H. and Clark, D.L., Pleistocene calcite lysocline and paleocurrents of the central Arctic Ocean and their paleoclimatic significance. Paleoceanography, 1 (2), Mudie, P.J. and Blasco, S.M., Lithostratigraphy of the CESAR cores. In: H.R. Jackson, P.J. Mudie and B. S.M. (Editors), Initial Geological Report on CESARthe Canadian Expedition to Study the Alpha Ridge, Arcitc Ocean. Geological Survey of Canada, Ottawa, pp

24 Nowaczyk, N.R., Frederichs, T.W., Kassens, H., Nørgaard-Pedersen, N., Spielhagen, R.F., Stein, R. and Weiel, D., Sedimentation rates in the Makarov Basin, central Arctic Ocean: a paleomagnetic and rock magnetic approach. Paleoceanography, 16, Nowaczyk, N.R., Antonow, M., Knies, J. and Spielhagen, R.F., Further rock magnetic and chronostratigraphic results on reversal excursions during the last 50 ka as derived from northern high latitudes and discrepancies in precise AMS 14 C dating. Geophysical Journal International, 155, Peterson, B.J., Holmes, R.M., McClelland, J.W., Vörösmarty, C.J., Lammers, R.B., Shiklomanov, A.I., Shiklomanov, I.A. and Rhamstorf, S., Increasing river discharge to the Arctic Ocean. Science, 298 (5601), Phillips, L.R. and Grantz, A., Regional variations in provenance and abundance of ice-rafted clasts in Arctic Ocean sediments: implications for the configuration of Late Quaternary oceanic and atmospheric circulation in the arctic. Marine Geology, 172, Polyak, L., Curry, W.B., Darby, D.A., Bischof, J. and Cronin, T.M., Contrasting glacial/interglacial regimes in the western Arctic Ocean as exemplified by a sedimentary record from the Mendeelev Ridge. Palaeogeography, Palaeoclimatology, Palaeoecology, 203, Poore, R.Z., Philips, R.L. and Rieck, H.J., Paleoclimate record for Northwind Ridge, Western Arctic Ocean. Paleoceanography, 8, Scott, D.B., Mudie, P.J., Baki, V., MacKinnon, K.D. and Cole, F.E., Biostratigraphy and late Cenozoic paleoceanography of the Arctic Ocean: Foraminiferal, lithostratigraphic, and isotopic evidence. Geological Society of America Bulletin, 101, Spielhagen, R.F., Bonani, G., Eisenhauer, A., Frank, M., Frederichs, T., Kassens, H., Kubik, P.W., Mangini, A., Nørgaard-Pedersen, N., Nowaczyk, N.R., Schäper, S., Stein, R., Thiede, J., Tiedemann, R. and Wahsner, M., Arctic Ocean evidence for Late Quaternary initiation of northern Eurasian ice sheets. Geology, 25, Spielhagen, R.F., Baumannc, K.H., Erlenkeuser, H., Nowaczyke, N.R., Nørgaard- Pedersen, N., Vogt, C. and Weiel, D., Arctic Ocean deep-sea record of northern Eurasian ice sheet history. Quaternary Science Reviews, 23, Stein, R. and Korolev, S., Shelf-to-basin sediment transport in the eastern Arctic Ocean. In: H. Kassens, H.-W. Hubberten, S.M. Pryamikov and R. Stein (Editors), Russian-German Cooperation in the Siberian Shelf Seas: Geo-System Laptev Sea. Reports on Polar Research 144, Alfred Wegener Institute, Bremerhaven, pp Stein, R., Schubert, C., Vogt, C. and Fütterer, D., Stable isotope stratigraphy, sedimentation rates, and salinity changes in the latest Pleistocene to Holocene eastern central Arctic Ocean. Marine Geology, 119, Steuerwald, B.A., Clark, D.L. and Andrew, J.A., Magnetic stratigraphy and faunal pattern in Arctic Ocean sediments. Earth and Planetary Science Letters, 5, Stokes, C.R., Clark, C.D., Darby, D. and Hodgson, D.A., Late Pleistocene ice export events into the Arctic Ocean from the M'Clure Strait Ice Stream, Canadian Arctic Archipelago. Global and Planetary Change, 49,

25 Wahsner, M., Müller, C., Stein, R., Ivanov, G., Levitan, M., Shelekhova, E. and Tarasov, G., Clay-mineral distribution in surface sediments of the Eurasian Arctic Ocean and continental margin as indicator for source areas and transport pathways-a synthesis. Boreas, 28, Captions Figure 1. Schematic map of the Arctic Ocean showing locations of the studied cores (yellow stars) from the Lomonosov Ridge and two key T-3 cores (Minicucci et al., 1983; Morris et al., 1985; Spielhagen et al., 1997; Jakobsson et al., 2001) and the cores used by Clark et al. (1980) to develop the standard Amerasian Basin lithostratigraphy (pink dots). The individual cores are listed in Table 1. The bathymetry is based on the IBCAO DTM (Jakobsson et al., 2000a). The major ocean currents-the Beaufort Gyre (BG) and the Trans-Polar Drift (TD) are represented by green lines and the blue lines show the sea ice drift patterns in the Arctic Ocean (Gard and Crux, 1994; Darby, 2003). The drift track for ice-island T3 is shown as an orange line (Hunkins and Tiemann, 1977). Figure 2. Bathymetric map and cross-section (IBCAO, Jakobsson et al., 2000a) with positions for cores on the Lomonosov Ridge (Morris and Clark, 1986; Backman et al., 1997; Spielhagen et al., 1997) and intra-basin currents after Björk et al. (2007). Orange line represents the cross-section; red line represents currents originating in the Makarov Basin; yellow line represents currents originating in the Amundsen Basin Figure 3. Correlation of core FL-439 (from the Alpha Ridge with standard lithostratigraphic units by Clark et al. (1980) with cores B-8 and B-24 (from the 24

26 Lomonosov Ridge) adapted from Morris et al. (1985), Minicucci et al. (1983), and Morris et al. (1986). Ages of the paleomagnetic chrons are inferred from Lourens et al., (2004). Figure 4. Previously published correlations between the Lomonosov Ridge cores PS and 96/12-1pc (brown lines, Spielhagen et al., 1997; Jakobsson et al., 2001; Spielhagen et al., 2004) and between B-24, B-8 (both from the Lomonosov Ridge) and the synthesized lithology for the Alpha Ridge (orange lines, Clark et al., 1980; Morris et al., 1985) based on lithology, color, CF (wt%>63μm), foraminifera concentrations and paleomagnetic inclination pattern. Correlation between cores , 96/12-1pc and the lithology for the Alpha Ridge based on paleomagnetic inclination pattern (red), CF (blue) and foraminifera concentrations (green) is also presented. (pw = pinkish white coarse layer) 25

27 Table 1 Core Sites on the Lomonosov Ridge Core Latitude Longitude Wate Depth (m) Length (cm) Expedition B ' ' LOREX 79 B ' ' 2360 LOREX 79 B ' ' LOREX ' ' ARK-VIII/3, /12-1pc ' ' Arctic Ocean 96 FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T

28 FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T FL ' ' T

29 28

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