Radiogenic isotope record of Arctic Ocean circulation and weathering inputs of the past 15 million years



Lead (Pb), neodymium (Nd), and strontium (Sr) isotopic analyses were carried out on sediment leachates (reflecting the isotope composition of past seawater) and digests of the bulk residues (reflecting detrital continental inputs) of Integrated Ocean Drilling Program (IODP) Leg 302 and core PS2185 from the Lomonosov Ridge (Arctic Ocean). Our records are interpreted to reflect changes in continental erosion and oceanic circulation, driven predominantly by tectonic forcing on million-year timescales in the older (pre-2 Ma) part of the record and by climatic forcing of weathering and erosion of the Eurasian continental margin on thousand-year timescales in the younger (post-2 Ma) part. These data, covering the past ∼15 Ma, show that continental inputs to the central Arctic Ocean have been more closely linked to glacial and hydrological processes occurring on the Eurasian margin than on continental North America and Greenland. The constancy of the detrital input signatures supports the early existence of an Arctic sea ice cover, whereas the major initiation of Northern Hemisphere glaciation at 2.7 Ma appears to have had little impact on the weathering regime of the Eurasian continental margin.

1. Introduction

The importance of the Arctic Ocean and the circum-Arctic continental regions for modern global climate is obvious, in that this polar region is a sensitive responder as well as active driver of global and regional atmospheric and oceanic circulation patterns. The most evident examples are the influence of the Arctic Ocean on the Atlantic Meridional Overturning Circulation (AMOC) system and on the global insolation-albedo balance of solar energy input [e.g., Broecker and Peng, 1982; Aagaard et al., 1985; Gordon, 1986; Broecker, 1991; Ganachaud and Wunsch, 2000; Stocker, 2000; Peterson et al., 2006; Carlson et al., 2007; Mullarney et al., 2007]. Moreover, there is growing evidence that anthropogenic changes to the global climate system have started to affect the freshwater budgets within the Arctic Ocean region in ways that may readily impact the AMOC [Ganopolski et al., 1998; Peterson et al., 2006]. These most recent changes in the Arctic region are also important for the global energy budget, given that fluctuations of Arctic ice coverage have a significant impact on the albedo of the planet's surface, due to the large differences in reflectance between open waters and ice-covered areas [Holland and Bitz, 2003].

For these reasons, it has long been a goal of paleoclimatologists and paleoceanographers to understand the role of the Arctic Ocean in the past. Unfortunately, the availability of oceanic and terrestrial records of Arctic climate change had mostly been limited to the past half million years. For terrestrial records, this is a common problem resulting from the erosive environment. For marine records, sampling of longer records had been prevented by the difficulties of drilling in an ice-covered ocean. These limitations were overcome for the first time in 2004 by the Arctic Coring Expedition “ACEX” (IODP Leg 302). During this expedition a 428 m thick section of sediments was cored on the Lomonosov Ridge in the central Arctic Ocean: An unprecedented success that resulted in the recovery of the first continuous pre-Quaternary Arctic sediment sequences with a reliable stratigraphy reaching back into the Late Cretaceous [Moran et al., 2006; Backman et al., 2008].

Here, we present radiogenic lead (Pb) and strontium (Sr) isotopic data obtained from the leachable fraction of the bulk sediments, as well as Pb, neodymium (Nd) and Sr isotopic data from total digestions of the residual bulk sediments from the Neogene sections of the ACEX cores. To bridge the gap between the Neogene and the modern Arctic, we have also generated a higher-resolution record for the Late Quaternary from the neighboring piston core, PS2185, for which a well-developed age model of the past 200 ka is available [Spielhagen et al., 2004]. The Pb, Sr, and Nd isotope systems offer a unique insight into the Neogene paleoceanographic history of the Arctic Ocean, partly because these cores contain predominantly detrital sediments that are essentially barren of any biogenic material [Backman et al., 2006] and paleoceanographic information is difficult to obtain otherwise. Both cores record paleowater depths occupied by Arctic Intermediate Water (AIW) [Aagaard et al., 1985; Karcher and Oberhuber, 2002].

Radiogenic isotopes have been applied for various purposes in marine research (see Frank [2002] or Banner [2004] for recent reviews). The isotope composition of detrital particles allows the fingerprinting of sediment provenances and, by analyzing the clay fraction, the reconstruction of paleocurrent pathways [e.g., Grousset et al., 1988; Revel et al., 1996; Fagel et al., 1999, 2004; Hemming, 2004]. The radiogenic isotope compositions of the dissolved Sr and Pb are preserved in suitable authigenic (seawater-derived) sediment components, such as carbonates or FeMn oxide precipitates, and can be used to reconstruct isotopic changes of seawater itself. The information derived from these seawater records differs depending on the isotopic system analyzed. The global oceans' isotopic uniformity of elements with long oceanic residence times (τ), such as Sr (∼2 Ma) [Palmer and Edmond, 1989] requires that variation on long timescales results from multimillion-year tectonic process or large-scale global climatic changes, such as Himalayan uplift or Cenozoic global cooling [e.g., Edmond, 1992; Raymo and Ruddiman, 1992; Derry and France-Lanord, 1996; Zachos et al., 2001]. Moreover, the global uniformity of the marine Sr isotopes has enabled the establishment of reliable stratigraphies of marine sediment records over the entire Phanerozoic [e.g., Veizer et al., 1999; Peucker-Ehrenbrink and Ravizza, 2000]. In contrast, the concentrations and radiogenic isotope ratios of elements with oceanic residence times on the order of, or shorter than the mixing time of the ocean (Pb has τ = 50–200 years) [Schaule and Patterson, 1981] are not efficiently homogenized and thus show significant differences between water masses. In the case of Pb, high particle reactivity mainly renders its isotope composition useful as a tracer for local weathering inputs to the oceans (see summary of Frank [2002]).

The radiogenic isotope records of the ACEX sediments presented here contribute to reconstructing the Arctic terrestrial and marine evolution over the past ∼15 Ma. It needs to be stressed that while the data presented are robust, the interpretations we offer are, to a large extent, unconstrained by independent evidence of Arctic climatic/oceanographic change. This is particularly true for the older part of the record, and reflects the fact that the ACEX data represent the first continuous paleoceanographic record available from the Arctic Ocean for the past 15 Ma. Indeed, the only other pre-Quaternary radiogenic isotope seawater data were provided by Winter et al. [1997], in a study on diagenetic Fe-Mn micronodules in short sediment cores from the Canadian Basin of the Arctic Ocean [also Haley et al., 2007]. Winter and colleague's valuable records are included in the discussion of our new data, but it is pointed out that the stratigraphies of these Canadian Basin sediments, which are generally difficult to constrain, were established under the paradigm that the Arctic Ocean has been a sediment-starved ocean basin. This paradigm has, in the meantime, been revised for several Arctic regions [Backman et al., 2004] and remains the subject of current research efforts. Therefore the ages of the sediments used by Winter et al. [1997] may be much younger than estimated.

2. Materials and Methods

The ACEX samples analyzed were provided by IODP, and samples from the Late Quaternary sediment core PS2185 were made available from the core repository of IFM-GEOMAR (core locations shown in Figure 1). Details of the stratigraphy and composition of the sediments, as well as the previous paleoceanographic proxy interpretations, of core PS2185 have been published previously [Spielhagen et al., 1997, 2004]. The location, sedimentary setting and all other pertinent details of the ACEX cores have either been described previously [Backman et al., 2006; Moran et al., 2006] or are contained in other contributions to this volume [e.g., Backman et al., 2008]. They are therefore not reiterated here. We have adopted the ACEX age model presented by Backman et al. [2008], the Neogene part of which is an update of the initially published age model [Moran et al., 2006]. Our study focuses on the record of past ∼15 Ma, for which a nearly continuous record of sedimentation on the Lomonosov Ridge is available [Frank et al., 2008]. Overall, these sediments are essentially devoid of biogenic components and do not show any signs of past anoxia. The almost complete lack of biogenic material, which prevents the application of standard paleoceanographic analyses (e.g., oxygen or carbon isotope analyses of foraminifera), is beneficial from the point of view of our study; The low levels of organic carbon input to the sediments have retarded diagenetic progression [Froelich et al., 1979], thus stabilizing the FeMn oxide coatings from which we extract our dissolved seawater isotope record (as described below).

Figure 1.

Arctic Ocean and North Atlantic region. The site locations of IODP Leg 302 and PS2185 are both on the Lomonosov Ridge, which separates the Canadian and Eurasian basins of the Arctic Ocean. Cross is the North Pole. The thick black line shows the limits of the shelf regions, and blue arrows show modern surface ocean currents. Highly generalized isotopic geochemical provenance regions are indicated by different colors; the gray area of northern Russia reflects the lack of isotopic information from this area. These colors are the same used in Figure 3. The two important ocean gateways are the Greenland-Scotland Ridge (GSR) between the North Atlantic and Greenland-Iceland-Norwegian seas (GIN or Nordic seas), and the Fram Strait between the GIN seas and the Arctic Ocean.

We sampled the ACEX sediments with the goal of obtaining a coarse ∼1 Ma resolution “Neogene” record (defining here the “Neogene” section as the sediments above the 18.2 Ma hiatus [Moran et al., 2006; Backman et al., 2008] and below the “Quaternary” section younger than ∼2 Ma). The stratigraphy for most of the Neogene section is based on Be isotopes [Frank et al., 2008] and a small number of biostratigraphic constraints, mainly dinoflagellates [Backman et al., 2008]. In order to constrain the variations during the most recent glacial-interglacial changes, we also carried out ka resolution measurements on sediments from piston core PS2185 (87°31.9′N, 144°22.9′E; 1051 m water depth), for which a detailed age model based on multiple dating techniques is available [Spielhagen et al., 2004].

All samples were freeze-dried and ∼0.5 g (dry weight) was chemically leached in order to extract the seawater-derived Pb and Sr preserved in the authigenic ferromanganese sediment coatings. The leaching procedure generally followed the protocols provided by Bayon et al. [2002] and Gutjahr et al. [2007] as follows. The sediments were first rinsed with MilliQ water three times. For each of these rinses, and for each subsequent step described, the sediment and solution were gently shaken for 60 min and then centrifuged, and the supernatant was decanted. A buffered-acetic acid step to remove carbonates after the initial MilliQ rinses was omitted, because the Arctic sediments do not contain significant amounts of carbonate [Backman et al., 2006]. Furthermore, we did not use a MgCl2 leaching step [Gutjahr et al., 2007] because we found no significant difference in the Pb or Sr isotopic compositions of the final leach with or without this step. Therefore, after rinsing the sediments with MilliQ water, we immediately carried out the leaching step to extract the radiogenic isotopes contained in the ferromanganese coating. The leach solution consisted of 5 mM hydroxylamine HCl and 1.5% acetic acid, buffered to pH = ∼3.5 to 4 with NaOH (i.e., a tenfold dilution of the leach solution used by Gutjahr et al. [2007]). We used this diluted solution in order to avoid the unintentional leaching of the abundant clays in these essentially 100% detrital Arctic sediment samples. Because of their relatively high concentrations of Sr and Pb, often with distinct isotopic signatures, contamination from clays can quite readily bias the signal of the authigenic metal oxides. However, while minimizing clay contamination, the dilute leach solution used resulted in readily measurable amounts of extracted Pb and Sr (typically 20 to 500 ng Pb; 100 to 3000 ng Sr). After collecting the leachate, the sediment samples were rinsed three times with MilliQ water and stored. The leachate was evaporated to dryness, then taken up in 1 mL concentration HNO3 and evaporated to dryness again three times in order to destroy the matrix of the leach solution. After this, 70% of the sample was used for chromatographic separation and purification of Sr (and Nd) following combined standard procedures [Horwitz et al., 1992; Cohen et al., 1988], and the remaining leachate fraction was used for chromatographic extraction and purification of Pb following Galer and O'Nions [1989].

Twenty sediment samples were subjected to a further leaching step with a tenfold more concentrated leach solution (i.e., equivalent to the one used by Gutjahr et al. [2007]); rerinsed and dried; weighed; and completely dissolved in sealed Teflon vials using a mixture of 1 mL conc. HNO3/5 mL concentration HF kept at 180°C for 3 d. After cooling, the samples were sonicated for >60 min. and, having verified complete dissolution, were evaporated to dryness and redissolved in 5 mL conc. HCl. Once evaporated to dryness again, the samples were refluxed for an hour in 1 mL 6 M HCl at 110°C followed by the same chromatographic separation and purification procedures for Nd, Sr and Pb as described above. Total procedure blanks were <2 ng Pb, ∼3 ng Sr and negligible for Nd. The blanks did not bias the results, and core top sediment analyses clearly support the conclusion that our leaches accurately reflect contemporaneous seawater (Table 1 and Figure 2 and section 3).

Figure 2.

Pb, Nd, and Sr isotopic data from various Arctic core top samples. The leachate data are plotted versus water depth and compared to the isotopic signatures of both modern seawater and sediments from the Arctic (sample locations and water depths are given in Table 1). The correspondence of the weak leach data to modern, measured Nd and Sr isotopic values for seawater supports the claim that the weak leach of metal oxide coatings faithfully reflects contemporaneous seawater, with little contamination from the detrital fraction. “Modern” seawater Pb isotopes reflect a mixture of natural (Holocene) Pb and recent anthropogenic inputs. The core top Pb isotopes of the weak leaches all fall between these end-members (as predicted by Gobeil et al. [2001] in the Arctic Ocean), again supporting the claim that signal extracted from the metal oxides is that of contemporaneous seawater.

Table 1. Arctic Ocean Piston Core Top Leachate and Bulk Digestion Data
SampleaWeight,b gLatitude, °NLongitude, °EWater Depth, m87Sr/86SrcɛNdd206Pb/204Pbe206Pb/204Pbf206Pb/204Pbg
  • a

    Piston core samples “PS” are core top samples.

  • b

    Dry weight of sample.

  • c

    The ratio 87Sr/86Sr corrected to NBS987 = 0.710245; 2σ error = 0.000031 (n = 77) (see section 2 for details).

  • d

    The ratio 143Nd/144Nd corrected to JNdi-1 = 0.512115; normalized to CHUR = 0.512638; 2σ error = 0.5 (n = 34) (see section 2 for details); ɛNd of modern AIW is −10.5 [Porcelli et al., 2006; P. S. Andersson et al., The balance of Nd isotopes in Arctic Ocean Water, submitted to Geochimica Cosmochima Acta, 2008].

  • e

    The ratio 206Pb/204Pb corrected to NBS981 = 16.9405; 2σ error = 0.03 (n = 96) (see section 2 for details).

  • f

    The ratio 207Pb/204Pb corrected to NBS981 = 15.4963; 2σ error = 0.03 (n = 96) (see section 2 for details).

  • g

    The ratio 208Pb/204Pb corrected to NBS981 = 36.7219; 2σ error = 0.09 (n = 96) (see section 2 for details).

  • h

    Leachate data.

  • i

    Bulk sediment dissolution after weak leach (see section 2 for details).

PS59/280-1 bulk digestioni1.0190042750.720089−9.518.62915.62138.614

Measurements of the isotopic compositions of Sr and Pb (the latter following the Tl doping procedure of Belshaw et al. [1998]) were carried out using an Axiom multicollector inductively–coupled plasma–mass spectrometer (MC-ICP-MS). The Nd isotope compositions were measured on a Triton Thermal Ionization Mass Spectrometer (TIMS), both instruments operated at IFM-GEOMAR. We report the 87Sr/86Sr results with 2σ external reproducibility of ±31 ppm, from 51 runs of a SPEX® Sr solution. The 2σ external reproducibility of 206Pb/204Pb, at ±0.03 (n = 96 of a SPEX® Pb solution), is somewhat higher than normally reported for Pb isotope data, but reflects the fact that we ran all our samples and standards at the low concentrations (20 ppb Pb) equivalent to our most dilute leachate samples. Nd isotope measurements, run on the TIMS, are reported with a 2σ external reproducibility of ±0.5 ɛNd units (from 44 runs of a SPEX Nd solution, although the 2σ internal precision, based on 54 analyses of the JNdi-1 standard solution used for instrument bias correction, is better at ±0.3 ɛNd units). The reported Nd error for the bulk digest data (±0.5 ɛNd units) is artificially higher than necessary, due to use of a measurement method optimized for samples with much less Nd. However, as will be shown later, the bulk digest Nd data is relatively invariant, and the interpretations associated with these data are not affected by the slightly higher errors. In the case for Sr and Pb, the variability of the samples measured was much higher than the reported reproducibilities (by a factor of 15 for Sr and 20 for Pb in the leachates).

Mass bias corrections were made using 88Sr/86Sr = 8.375209, 146Nd/144Nd = 0.7219 and for Pb, a 205Tl/203Tl = 2.3875 of added Tl (NIST SRM 997). All reported isotope ratios were normalized to the following accepted values: NBS SRM 987: 87Sr/86Sr = 0.710245; NBS SRM 981: 206Pb/204Pb = 16.9405, 207Pb/204Pb = 15.4963, 208Pb/204Pb = 36.7219 [Galer, 1999]; JNdi-1: 143Nd/144Nd = 0.512115 [Tanaka et al., 2000]. Nd isotope values are reported in standard epsilon unit notation, normalized to CHUR = 0.512638 [Jacobsen and Wasserburg, 1980].

3. Results

The efficacy of our leaching protocol to extract the isotopic signature of past seawater from Fe-Mn coatings, while avoiding clay contamination, was assessed by analyses of a set of core top samples from various locations in the Arctic Ocean (locations of the cores are listed in Table 1). These leachates should reproduce the isotope composition of modern seawater. The Sr isotope composition of seawater is globally homogenous, and its present-day 87Sr/86Sr of 0.70916 is well constrained [Henderson et al., 1994; McArthur et al., 2001]. As shown in Figure 2, despite different sample locations and different compositions of the local sediments, the core top leachate Sr isotope compositions reproduce the modern seawater values within analytical uncertainties (Table 1). This is the case despite the large difference between the Sr isotopic compositions of seawater and the sediments themselves (from the “bulk digests” reported here, and other modern sediment analyses shown in the Figure 2), which corroborates that clay contamination of the leachates is negligible [Gutjahr et al., 2007].

Unfortunately, anthropogenic contamination prevents the determination of the natural “present-day” (i.e., Holocene) Pb isotopic composition of seawater [e.g., Schaule and Patterson, 1981; Shen and Boyle, 1988; Gobeil et al., 2001]. However, this very contamination can be used to evaluate if a true seawater Pb signal was successfully extracted. Specifically, the core top leachate Pb isotopic data appear to reflect a mixture of modern, anthropogenically influenced seawater, and modeled natural seawater of preindustrial time (Figure 2) [Gobeil et al., 2001]. The core top Pb isotopic data can thus be simply described as a mixture between the modeled unaltered and anthropogenically altered modern seawater Pb, which supports our conclusion that a pristine contemporaneous seawater Pb isotope signal was extracted by application of our weak leaching technique. Finally, we rule out the possibility that the Pb isotopic variations reflect redistribution of sediments carrying preformed metal oxide coatings because the Nd isotopic records of these same metal oxides clearly reflect the Nd isotopic composition of modern AIW at the Lomonosov Ridge (Table 1 and Figure 2).

Our down core radiogenic isotope records (Table 2 for both the ACEX and PS2185 samples) will be discussed in sections 3.13.3: The Nd and Sr isotopic composition of the total dissolutions (tracing sediment provenance); the Sr isotopic composition of the leachates (possibly reflecting exchange of the Arctic Ocean with the open ocean); and the Pb isotopic compositions of both the leachates and the total dissolutions (as a proxy of weathering processes on the continents).

Table 2. Arctic Ocean Down Core Leachate and Bulk Digest Radiogenic Isotope Data
SampleaCore SectionDepth,b m, cmAge, MaWeight,c gLeachateTotal Dissolution
  • a

    IODP Leg 302 (“ACEX”) core locations: Hole 2:87.55°N; 139.22°E; 1211m depth; Hole 4: 87°52N: 136°11°E: 1289m depth; PS2185 core location: 87.32°N; 144.23°E; 1051m depth.

  • b

    IODP Leg 302 core depths given in m (mcd); PS2185 core depths given in cm (corrected after Spielhagen et al. [2004]).

  • c

    Dry weight of sample.

  • d

    The ratio 87Sr/86Sr corrected to NBS987 = 0.710245; 2σ error = 0.000031 (n = 77) (see section 2 for details).

  • e

    The ratio 206Pb/204Pb corrected to NBS981 = 16.9405; 2σ error = 0.03 (n = 96) (see section 2 for details).

  • f

    The ratio 207Pb/204Pb corrected to NBS981 = 15.4963; 2σ error = 0.03 (n = 96) (see section 2 for details).

  • g

    The ratio 208Pb/204Pb corrected to NBS981 = 36.7219; 2σ error = 0.09 (n = 96) (see section 2 for details).

  • h

    The ratio 143Nd/144Nd corrected to JNdi-1 = 0.512115; normalized to CHUR = 0.512638; 2σ error = 0.5 (n = 34) (see section 2 for details).

PS2185-6 18.40915.58038.480     
PS2185-6 18.59915.57938.650     
IODP Leg 3024/1/2/80-822.320.160.510.70892718.26315.63138.2410.714820−11.218.32215.54638.371
IODP Leg 3024/1/2/80-822.320.160.990.70888018.65415.64738.8650.715982−10.718.32215.53838.418
IODP Leg 3024/1/2/80-822.320.160.500.708963        
IODP Leg 3024/1/2/80-822.320.161.000.70892218.60015.56838.600     
PS2185-6 261.500.140.51         
PS2185-6 277.500.160.50 18.65915.57338.506     
PS2185-6 307.000.190.50 18.47015.60538.423     
PS2185-6 321.000.190.50 18.57915.53938.514     
PS2185-6 347.000.230.500.70919518.63515.60738.687     
PS2185-6 359.000.240.500.70919918.65515.59338.683     
PS2185-6 367.000.270.500.70919818.64715.54938.562     
PS2185-6 370.000.280.510.70923818.64515.59438.6860.716907−11.418.64615.56738.718
PS2185-6 373.000.300.500.70921718.57615.57238.114     
IODP Leg 3024/2/1/130-1325.980.410.510.70912318.70515.58938.6400.716695−10.118.66115.57138.621
IODP Leg 3024/2/1/140-1426.080.420.510.70923118.63615.57438.627     
IODP Leg 3024/2/2/1-36.190.430.500.70922118.62915.55838.608     
PS2185-6 379.000.310.500.709272        
IODP Leg 3024/2/2/11-136.290.430.500.70922518.62615.56938.641     
IODP Leg 3024/2/2/21-236.390.440.510.70925518.61215.55638.6220.717959−11.418.63515.55938.669
IODP Leg 3024/2/2/31-336.490.450.500.70922318.65115.58238.639     
IODP Leg 3024/2/2/41-436.590.450.500.70913518.73315.61238.759     
IODP Leg 3024/2/2/51-536.690.460.510.70913318.73215.62938.8180.718218−10.618.63915.53738.555
IODP Leg 3024/2/2/61-636.790.470.510.70913518.69815.60538.716     
IODP Leg 3024/2/2/71-736.890.480.50 18.54515.58038.575     
IODP Leg 3024/2/2/80-826.980.480.510.70922318.59315.57238.6260.717201−10.518.59315.55438.636
IODP Leg 3024/2/2/81-836.990.480.500.70919518.54015.55338.507     
IODP Leg 3024/2/2/91-937.090.490.510.709096        
IODP Leg 3024/2/2/101-1037.190.500.520.70909818.68515.59638.7020.716992−10.518.60515.57038.637
IODP Leg 3024/2/2/111-1137.290.500.520.70909718.62115.55938.537     
IODP Leg 3024/2/2/121-1237.390.510.520.70917918.58215.58938.648     
IODP Leg 3024/2/2/131-1337.490.520.51 18.56215.53738.463     
IODP Leg 3024/2/2/141-1437.590.520.500.70913118.58315.58238.580     
IODP Leg 3023/2/2/68-709.980.690.500.70919718.46615.56038.453     
IODP Leg 3024/3/2/143-14512.520.860.510.70909218.63915.58438.674     
IODP Leg 3024/4/1/110-11215.301.060.50         
IODP Leg 3022/5/1/0-221.141.460.500.70909918.42415.57738.368     
IODP Leg 3022/5/1/100-10222.141.530.500.70910818.16715.52337.925     
IODP Leg 3022/5/0-222.651.560.500.70914418.51515.54638.475     
IODP Leg 3022/5/2/50-5223.151.600.550.70910618.45415.56838.4190.715775−10.318.51515.51838.437
IODP Leg 3022/6/1/94-9627.081.870.500.70911518.46415.51338.3580.714966−10.018.51115.56338.531
IODP Leg 3022/7/2/24-2632.882.270.500.70910118.23815.57238.219     
IODP Leg 3022/7/2/52-5433.162.290.540.70911518.50815.52038.454     
IODP Leg 3022/10/2/70-7246.783.230.500.70910118.50415.55938.5180.715660−12.118.40815.51338.457
IODP Leg 3022/11/3/70-7252.163.600.510.70899818.61415.62038.7760.717255−11.718.51315.56138.678
IODP Leg 3022/13/1/50-5258.514.040.510.70900818.54115.56238.572     
IODP Leg 3022/14/1/80-8260.814.190.520.70908318.45415.61238.528     
IODP Leg 3022/16/2/70-7271.614.940.500.70904118.34715.65038.429     
IODP Leg 3022/19/1/25-2781.465.620.520.70901018.38515.56138.344     
IODP Leg 3022/20/2/70-7288.426.100.50 18.43315.50738.360     
IODP Leg 3022/21/2/70-7293.426.440.500.70900218.43015.54038.395     
IODP Leg 3022/23/2/70-73102.047.040.510.70908318.50215.56238.512     
IODP Leg 3022/23/2/70-72102.047.040.51 18.45815.55438.468     
IODP Leg 3022/25/1/78-80111.497.690.500.70890018.46015.60538.532     
IODP Leg 3022/25/1/78-80110.997.691.020.70887118.46015.52838.4290.716227−9.718.51215.50938.399
IODP Leg 3022/25/1/78-80110.997.690.490.708887        
IODP Leg 3022/25/1/78-80110.997.691.020.70887118.49115.54538.450     
IODP Leg 3022/26/2/56-58117.278.090.500.70907618.46815.53338.4500.715322−9.9   
IODP Leg 3022/27/2/70-72122.418.440.51 18.48015.55838.469     
IODP Leg 3022/29/2/70-72130.418.990.500.70912418.47715.55438.4520.716087−10.318.46115.53838.476
IODP Leg 3022/29/3/70-73131.919.100.510.70911918.51915.57538.591     
IODP Leg 3022/30/2/78-80135.5010.460.500.70898418.52115.56538.558     
IODP Leg 3022/32/1/74-76140.4511.560.510.70907518.47715.58738.601     
IODP Leg 3022/34/2/78-80151.2912.310.500.70913718.56015.58938.610     
IODP Leg 3022/25/2/78-80155.0612.570.520.70884718.39915.56738.3930.717040−11.018.52115.55538.579
IODP Leg 3022/37/2/88-90164.9013.250.500.709005        
IODP Leg 3022/38/2/20-22167.1013.400.51 18.55315.58838.624     
IODP Leg 3022/38/2/80-82167.7013.440.500.70887618.51015.56838.552     
IODP Leg 3022/40/2/80-82175.3913.970.500.70897518.48815.57238.5960.715561−10.6   
IODP Leg 3022/42/2/80-82184.8014.620.510.70901418.46215.56138.581     
IODP Leg 3022/43/2/70-72189.6914.960.500.70889418.45915.54838.513     

3.1. Nd and Sr Isotopic Signature of Bulk Sediments

The Nd and Sr isotopic compositions of the bulk digests (i.e., total sediment) cluster around a mean value of −10.8 ɛNd (±0.8) and 0.71652 in 87Sr/86Sr (±0.00109). In Nd-Sr isotope space (Figure 3), these data fall within the range of previously measured bulk sediments from the Canadian Basin of the Arctic Ocean [Winter et al., 1997], and tightly cluster compared to the range of possible source rocks (where the isotopic fields in Figure 3 illustrate all possible sediment provenance regions, color-coded the same as in Figure 1). In detail, the ACEX samples plot directly above the field of “Eurasian shelf” sediments, as estimated from a mixture of Barents, Kara, and Laptev seas shelf sediments [Tütken et al., 2002]. Moreover, with only a few exceptions, all bulk sediment dissolution data plot within the field of modern central Arctic sea ice sediments, known to be derived from the Kara and Laptev Sea shelf regions [Tütken et al., 2002].

Figure 3.

Sr and Nd isotopes and sediment provenance. The yellow points are the bulk digest data from the entire Neogene and Quaternary Arctic record. These cluster in similar isotopic space to the bulk sediment data for the Canadian Basin, delimited by the orange rectangle [Winter et al., 1997]. The unfilled regions represent dust data [Biscaye et al., 1997]. The inset shows a comparison of the data reported here to that of Tütken et al. [2002]. Here, the rectangles represent data from sea ice sediment, and the filled ovoid regions are shelf surface sediments. The blue unfilled region is modern central Arctic sea ice sediment [Tütken et al., 2002]. Data sources are Putorana basalts from Sharma et al. [1992]; NW Europe from Revel et al. [1996]; North Atlantic groups I (“Iceland/East Greenland shelves”), II (“Norwegian Sea/Gulf of St. Lawrence”), and III (“Baffin Bay/Hudson Strait/Labrador Sea”) from Farmer et al. [2003] (their group designations in quotes); Greenland, Canada (including Superior Province) from Goldstein and Jacobsen [1988] and McColluch and Wasserburg [1978]. Also included are the regions summarized by Winter et al. [1997].

3.2. Strontium Isotope Signature of Arctic Ocean Bottom Water

During application of sediment leaching protocols, small amounts of Sr from the clays may be released, potentially contaminating the seawater Sr isotopic signature of the authigenic metal oxides [e.g., Gutjahr et al., 2007]. The weak leaching protocols used here clearly generated an authigenic Sr-isotopic record of AIW (Figure 4) that corresponds well to the established global seawater Sr isotopic record [Hodell et al., 1991; Edmond, 1992; Veizer et al., 1999; McArthur et al., 2001]. This is striking in view of the large isotopic difference between the leachate 87Sr/86Sr signature and the bulk sediment 87Sr/86Sr signature (Figure 4).

Figure 4.

Down core Sr isotopic data. (a) Neogene record of weak leach and bulk sediment dissolution from IODP Leg 302. The overall close correspondence of the weak leach data to contemporaneous seawater [McArthur et al., 2001], compared to the “isotopic pull” potential from clay contamination, suggests that the authigenic metal oxide signal is robust down core (see text). (b) Same weak leach data on a magnified scale, making clearer the greater deviations of the leach signals from the seawater curve prior to 8 Ma. The red data points possibly indicate times when the Arctic Ocean 87Sr/86Sr deviated from the Global Ocean Sr isotopic value. (c) Quaternary leachate Sr isotopic data, which should all be the same given the long oceanic residence time of Sr (see text). The maximum estimate for divergence resulting from clay contamination is relatively small (defined by the red datum) but enough to equally well account for the deviations in the early record (Figure 4b). There are negative deviations in the Sr isotopic record that we attribute to diminutive detrital carbonate contamination.

It is possible, through isotopic mass balance considerations, to estimate a maximum clay contamination of ∼2% from the Quaternary leachates [Gutjahr et al., 2007]. That is, the 35 leachate samples of the Late Quaternary, when seawater 87Sr/86Sr was essentially constant (due to the >1 Ma residence time of Sr), show <2% clay contamination (Table 2 and Figure 4). Given that the detrital fraction of Arctic sediments of the entire past 15 million years is largely of similar bulk composition [Backman et al., 2006], we do not expect that this 2% limit will underestimate clay contamination for the older sediments. Indeed, there is a close correspondence of the leachates with contemporaneous seawater after ∼8 Ma (Figure 4). Prior to 8 Ma, some 87Sr/86Sr values are slightly higher than the supposed maximum 2% clay contamination. It is also apparent that some Quaternary sediment leachates representing glacial periods have 87Sr/86Sr values lower than seawater (Figure 4), which likely reflects a minor contribution of Sr originating from trace amounts of Mesozoic or older detrital carbonates. The deviations from seawater observed in these leach samples are only relatively small (∼250 ppm), suggesting that detrital carbonate contamination was also small.

3.3. Lead Isotope Signatures in Bulk Sediment and Leaches

The Pb isotope compositions of the bulk sediments and the leaches for both the ACEX and PS2185 cores are generally similar (Table 2), as shown in 206,207,208Pb/204Pb space in Figure 5. These isotopic ratios center around ∼18.5 (206Pb/204Pb), ∼15.6 (207Pb/204Pb) and ∼38.6 (208Pb/204Pb; Figure 5). These Lomonosov Ridge sediment data overlap with the low end of Pb isotope values of bulk sediments and Fe-Mn micronodules from the Canadian Basin (Figure 5) [Winter et al., 1997] and with the high end of the Pb isotopic spaces delimiting the Putorana basalts [Sharma et al., 1992], which will be described in more detail below.

Figure 5.

Arctic weak leach and bulk sediment dissolution data in 206Pb/204Pb-207Pb/204Pb-208Pb/204Pb space. Yellow symbols are IODP Leg 302 weak leaches; green symbols are PS2185 weak leaches; the gray region reflects all the bulk sediment dissolution data measured here. All these Lomonosov Ridge data fall in similar Pb isotopic space, supporting the claims that the bottom water Pb isotopic value is derived from exchange with settling clays and that the source of these clay has been broadly similar over the past 15 Ma (see text). These fields overlap with those of Canadian Basin samples [Winter et al., 1997] and Putorana basalts [Sharma et al., 1992] but not with the fields defined by North Atlantic samples. This implies that the North American sources of Pb, as seen in the North Atlantic, are only of minor influence in the Canadian Basin of the Arctic (see text).

The Lomonosov Ridge sediments are also distinctly less radiogenic than North Atlantic deep water Pb isotope records [Burton et al., 1997; O'Nions et al., 1998; Abouchami et al., 1999; Reynolds et al., 1999; Foster and Vance, 2006] (Figure 5). In general, our Arctic Pb isotope data are consistent with the Nd and Sr isotope data from the same bulk sediment dissolutions, in that these Pb isotopes also imply that there has been a largely invariant “Eurasian sediment” signature at the Lomonosov Ridge over the past 15 Ma. Furthermore, because the Pb isotope composition of the Canadian Basin samples lies between the Lomonosov Ridge fields and the North Atlantic fields (in 206,207,208Pb/204Pb space), it is reasonable to suggest that the Canadian Basin recorded a component of Canadian/Greenland-sourced Pb that is also characteristic of the North Atlantic records [Winter et al., 1997], but distinct from the Lomonosov Ridge.

We chose to graphically display only the 206Pb/204Pb data (omitting 207Pb/204Pb and 208Pb/204Pb) versus age in Figure 6. As indicated above, over the past 15 Ma AIW 206Pb/204Pb varied little at the Lomonosov Ridge, with only slight positive deviations, from a median value of 18.47, centered around 13 Ma and 3.5 Ma (Figure 6). There is only one divergence from this median toward less radiogenic values between 6 and 4 Ma. The constancy of the seawater (leachate) data is mirrored in the bulk sediment digest data, which also scatter closely around the same median line (Figures 5 and 6).

Figure 6.

Pb isotopic evolution of Arctic Ocean Intermediate water. (left) “Quaternary” data from PS2185 (green circles). (right) “Neogene” IODP Leg 302 data. The blue circles are data from North Atlantic crusts; and the blue line in Figure 6 (left) is benthic δ18O data to indicate the global climatic state (i.e., glacial, interglacial) [Lisiecki and Raymo, 2005]. The Canadian Basin data of Winter et al. [1997] are also shown in Figure 6 (right), with the caveat of a possibly erroneous age model (see text). The median “Neogene” and “Quaternary” values are also shown, and deviations from these values are indicated by green (positive deviations) and blue (negative deviations) shading. The main features of the Neogene data are the minima centered around 5 Ma and the positive shift around 1 Ma. This latter shift lags the INHG (2.7 Ma), seen clearly in the North Atlantic records, by >1.5 Ma (see text). The main features of the Quaternary data are the large negative excursions seen at the onset of glaciation. These features are interpreted to reflect changes in the terrestrial weathering regime in north Eurasia (see text).

The Quaternary leachate median 206Pb/204Pb value of 18.63 is similar to most of the Quaternary bulk sediment values, and is distinctly higher than the Neogene median described above (18.47) (Figure 6). The most obvious features of the Quaternary 206Pb/204Pb record are the prominent deviations toward less radiogenic values during glacial inceptions (Figure 6). Apart from these deviations, the Quaternary Pb isotope record is, like the Neogene section, remarkably constant, even between peak glacial and interglacial periods. Also similar to the Neogene section, the Quaternary Pb isotopic data are distinctly less radiogenic than the North Atlantic (Figure 6) [Foster and Vance, 2006].

4. Discussion

4.1. Strontium-Neodymium Isotope Composition of Bulk Sediment Over Time

The overall invariance of the Sr and Nd isotopic compositions of the Leg 302 sediments over the past 15 Ma (Figure 3) implies that the mechanism of sediment transport to the Lomonosov Ridge has been invariable as well, despite major tectonic and climatic changes in the Arctic region over this period of time [Winter et al., 1997]. This is not altogether surprising, if the dominant mechanism of sediment transport to the central Arctic Ocean has occurred via sea ice and ice-rafted debris (IRD) [Clark and Hanson, 1983]. That is, in agreement with previous studies [Moran et al., 2006], we suggest that ice-based transport of sediment, entailing continuous Arctic ice cover, has been in effect for the past 15 Ma [St. John, 2008; Krylov et al., 2008]. Furthermore, the dominant continental (shelf) sediment source region for the central Arctic Ocean has obviously remained constant over this period of time as well.

Comparison with the core top data of Tütken et al. [2002] demonstrates that the “Eurasian shelf” has been the dominant source for sediments on the Lomonosov Ridge, which is especially striking in light of the isotopic range of potential source provenance rocks (Figure 3). These “Eurasian shelf” sediments have been transported to the North Pole in shelf ice or sea ice by the Trans-Polar Drift (TPD) (Figure 1). This is in agreement with the interpretation of Winter et al. [1997] for the sediments of the Alpha Ridge (Canadian Basin) prior to 1.7 Ma. Moreover, the ACEX data cluster in the same range as those for the modern sea ice sediments known to originate in the Laptev and Kara seas [Tütken et al., 2002].

Interestingly, at some point between 1 and 2 Ma, the Sr and Nd isotopic records of Winter et al. [1997] show that the Canadian Basin bulk sediments diverged from a relatively invariant composition, similar to that on the Lomonosov Ridge, toward more North American cratonic compositions [Winter et al., 1997]. This isotopic shift in the Canadian Basin sediments, while relatively small considering the vastly different isotopic compositions of the North American cratonic source rocks (Figure 3), is interesting in that it postdates the major Initiation of Northern Hemisphere Glaciation (INHG) at 2.7 Ma [e.g., Raymo et al., 1992; Zachos et al., 2001]. The implication is that there was a regional climatic shift between 1 and 2 Ma, one result of which was that the Canadian Basin began receiving greater amounts of sediment from the North American margin [Winter et al., 1997] while not significantly affecting sedimentation in the central Arctic (i.e., Lomonosov Ridge).

In summary, the Sr-Nd isotopic composition of the Neogene and Quaternary detrital sediments from the Lomonsov Ridge indicates that ice transport via the TPD has been operational for the past ∼15 Ma, as it is today, despite major changes in both the tectonics and climate of the Arctic. The TPD is forced through general atmospheric circulation above the Arctic Ocean [Moon and Johnson, 2005] and thus the TPD might be expected to have operated in a similar way over the past ∼15 Ma. The constancy of TPD also explains domination of Eurasian Shelf-sourced sediments (as indicated by Sr and Nd signatures) in the Arctic Ocean for the past ∼15 Ma: although there have clearly also been secondary additions of sediments from other sources (e.g., North America, Greenland, Svalbard) [Winter et al., 1997; Tütken et al., 2002; Darby, 2008].

4.2. Strontium Isotope Evolution of Arctic Seawater

Only six Neogene leachate samples slightly exceed the estimated 2% clay contamination limit in their Sr isotope composition (Figure 4), and all of them are older than 8 Ma. A somewhat higher clay contamination may well explain this observation, but there is no evidence for a significant change in sediment composition [Backman et al., 2006] and thus there is no a priori reason for such an increase only in the samples older than 8 Ma. If the data do indeed represent the composition of past Arctic seawater, the deviation from the global seawater 87Sr/86Sr curve may indicate that the Arctic basin did not have “unrestricted exchange” with the global oceans during periods of time prior to ∼8 Ma. “Unrestricted exchange” with the open ocean is, of course, a matter of definition; from a geochemical point of view, a suitable indicator is correspondence with the isotopic compositions with the well-homogenized global ocean 87Sr/86Sr signature. The Arctic divergences from this global record pre-8 Ma may thus reflect alternations in the mixing between the global ocean 87Sr/86Sr and a continental 87Sr/86Sr derived from Arctic riverine input. That is, as an intermittently isolated basin, the interaction between a shortened Sr residence time (associated with brackish salinities) and the high 87Sr/86Sr of the riverine inputs would make the Arctic more sensitive to changes in its Sr isotopic signature. In this case, it may be that Arctic-Atlantic exchange evolved from an initial condition of dominantly unidirectional (outward) surface flow prior to 17 Ma [Jakobsson et al., 2007], to a period of alternating open and restricted exchange reflecting a complex interaction of tectonic changes, climatic conditions, sea level and ocean circulation, and finally to permanently open exchange, probably established by <11.5 Ma, when the Arctic Sr isotope composition falls consistently on the global ocean Sr isotope curve (Figure 4). Of course, the timing of the incorporation of the Arctic into the Global Oceans is critical in paleoceanographic/paleoclimatic studies of the Neogene, and much more work will be done before this issue is resolved.

4.3. Lead Isotope Evidence for Past Continental Weathering Inputs

The Pb isotope record of AIW at the Lomonosov is distinct from that of the North Atlantic Intermediate and Deep Waters (NAIW; NADW) [Reynolds et al., 1999; Burton et al., 1997, 1999; Foster and Vance, 2006], but similar to the bulk sediment compositions at the Lomonosov Ridge (Figure 6). Given the short residence time of Pb in the oceans, it is not surprising that the Pb isotopic signatures of NAIW/NADW and AIW have been different, even though these water masses currently share similar origins of their weathering inputs [Schaule and Patterson, 1981]. Moreover, and in agreement with its short oceanic residence time, the similarity between the Arctic seawater (leachate) and the sediment (bulk digest) data (Figures 2, 5, and 6) corroborates the idea that the Arctic seawater Pb isotope signature has been derived locally from exchange with regionally weathered inputs. That is, the bulk digest and leachate data presented here are consistent with the idea that the Pb isotopic signal of deeper water (e.g., AIW) is derived from partial exchange with the sediments settling through the water column [Sherrell and Boyle, 1992; Winter et al., 1997; Jones et al., 2000; Ling et al., 2005; Klemm et al., 2007]. The same conclusion was reached for sediments and seawater of the Canadian basin of the Arctic Ocean [Winter et al., 1997]. Therefore, in sections 4.3.1 and 4.3.2, we will interpret the Pb isotopic record of the leaches as a reflection of changes in the Pb isotope composition of sea ice sediment.

As mentioned previously, the Lomonosov Ridge samples plot further within the nonradiogenic field delimited by the Putorana Basalt Pb isotopic data, whereas the Canadian Basin samples [Winter et al., 1997] are more similar to the fields defined by North Atlantic Pb isotopic data (Figures 1 and 5). The Putorana basalts (or Siberian Traps, or Siberian Flood basalts) represent an enormous (estimated at originally >1.5 × 106 km3 by volume) Phanerozoic (248 Ma) flood basalt province in northern Siberia [Sharma et al., 1991, 1992]. Centered roughly on the “PB” in Figure 1, the present aerial exposure is >300,000 km2 [Sharma et al., 1991], and the erosion products of these basalts are readily identified by their high smectite contents in the Kara Sea [Wahsner et al., 1999].

On the basis of comparison of the Pb isotopic fields bounding the Putorana basalts, the Lomonosov Ridge data and the Canadian Basin data, we conclude that the dominant sediment input sources to the central Arctic Ocean over the past 15 Ma have been the Laptev-Kara Sea regions, and that the Canadian Basin has been more influenced by inputs from the continental cratons of North America [Winter et al., 1997], consistent with the Nd-Sr isotopic data of the bulk sediments presented here and by Winter et al. [1997] (Figure 3).

Although in 206,207,208Pb/204Pb space the central Arctic leach data are generally similar to the sediment digest data, important changes occurred in the AIW Pb isotope composition (Figure 6). Our interpretation of these variations is better constrained for the Quaternary record than for the Neogene record, which is partly due to the availability of supporting evidence and data. As such, in reverse chronological order, our interpretations of the Quaternary data will be used to help understand the Neogene record in sections 4.3.1 and 4.3.2.

4.3.1. Quaternary Pb Isotope Record of Seawater

It has been demonstrated that the Pb fraction leachable from young granitic soils is consistently more radiogenic than the source bedrock itself, and evolves toward the bedrock Pb isotope composition as the soils age [Erel et al., 1994; Harlavan et al., 1998]. The most pronounced occurrences of this “incongruent weathering” are observed during interglacial chemical weathering of postglacially exposed old U- and Th-rich bedrock on the North American craton [von Blanckenburg and Nägler, 2001]. However, this mechanism, which explains how a radiogenic Pb isotope signal can be released from nonradiogenic source rocks (e.g., of the North American craton [von Blanckenburg and Nägler, 2001; Foster and Vance, 2006]), cannot explain our observed shift toward more nonradiogenic Pb isotopes of AIW early during glacial ice sheet growth (Figure 6). Instead, we suggest here that the peaks of less radiogenic Pb isotope ratios observed early in glacial periods (Figure 6) reflect glacial remobilization of the erosion products of the Putorana basalts that were transported to the Siberian shelves and deposited there during the preceding interglacial. That is, early glacial advance mechanically remobilized these nonradiogenic interglacial sediments from the shelves, which, through incorporation into sea ice or as IRD, were then delivered to the Lomonosov Ridge. During the settling of these sediments, partial exchange imparted the nonradiogenic (basaltic) Pb isotope signal onto AIW [Sherrell and Boyle, 1992; Winter et al., 1997; Jones et al., 2000; Ling et al., 2005; Klemm et al., 2007], which was then recorded by the metal oxide coatings. By the time of peak glaciation, the shelf sediments were largely removed and thus exposed shelf bedrock was mechanically weathered and transported to the central Arctic Ocean by sea ice or as IRD. However, now containing much smaller amounts of Putorana Basalt-derived material, this erosion process only delivered sediment with a more radiogenic Pb isotope composition to the Lomonsov Ridge.

The scenario presented above can explain the apparent paradox that during peak glacials the Pb isotopic signature of AIW was similar to the signature during interglacials. That is, there were only small amounts of Putorana sourced sediment delivered to the Lomonsov Ridge, via sea ice and as IRD, at both peak glacial and interglacial times. In the latter, interglacial, period, Putorana Basalt erosion predominantly led to deposition in the Kara Sea, providing the material for remobilization during the next glacial inception. This hypothesis is consistent with north Eurasian glaciations having a significant coverage over the north Eurasian shelves [e.g., Svendsen et al., 2004]. Indeed, the scenario presented here supports early ice formation in the Kara and Laptev Sea regions, directly upstream of the TPD. We suggest “early” advance to indicate that the glacial remobilization we envisage occurred well before the full glacial maximum, during the early period of glaciations, as seen from δ18O records (Figure 6). Unfortunately, evidence for “early glacial” ice extents, in particular from the presently submerged shelf regions, is highly limited because any such evidence was largely removed during the subsequent full glacial maximum [e.g., Svendsen et al., 2004].

An alternative possible mechanism to explain a nonradiogenic Pb isotopic excursion is via loess input, which has been shown to release nonradiogenic Pb during weak acid leaches [Jones et al., 2000; Ling et al., 2005; Klemm et al., 2007]. Indeed, it has been argued that Chinese loess has been deposited in Greenland ice during glacial maxima [Biscaye et al., 1997; Burton et al., 2007]. This mechanism for generating a nonradiogenic Pb isotope excursion is, however, unlikely for several reasons. The most important one is that the loess source would then have to be excluded from the Arctic seawater during peak glacials, to explain the return of the Pb isotopes to more radiogenic values (Figure 6). In addition, it is hard to imagine that such a distal source could contribute more dominantly to the Pb isotope record than the vast proximal sources, such as the sediments from the extensive Eurasian shelves.

Another argument is that the early glacial “excursions” toward less radiogenic values, are just the opposite. That is, the data (Figure 6) could be interpreted as representing more prolonged positive “excursions” from a baseline of 18.47, equivalent to the preceding Neogene median value. This may seem initially attractive given that the AIW record could then be interpreted in a way more consistent to that of the neighboring North Atlantic [Foster and Vance, 2006]. However, such an explanation would depend on soil formation to generate more radiogenic Pb [Erel et al., 1994; Harlavan et al., 1998], and it seems highly unlikely that soil formation would begin during peak glaciation in the high Arctic. Moreover, the contemporaneous bulk sediment Pb isotope signature of the sediments is unambiguously characterized through the bulk digests (Figure 6), which demonstrate that the nonradiogenic Pb isotope peaks are truly negative deviations from a more radiogenic “congruent” baseline.

What remains problematic for our preferred hypothesis, invoking Putorana-sourced sediment contributions, is that the Nd and Sr isotopic records of the same bulk sediment are essentially constant, and do not appear to change toward a Putorana Basalt composition, as shown in Figure 3. This may, however, be simply a mass balance issue in that the bulk digest Pb isotope signal may be more sensitive to the Putorana Basalt influence than either Nd or Sr.

In summary, the Quaternary Pb isotope records imply that the north Eurasian ice sheets grew quite rapidly at the start of a glacial cycle, developing to an extent that the interglacial sediments deposited on the shallow shelf areas were removed during the first 30 – 50 ka of glacial/interglacial transitions. These sediments, containing a significant Putorana Basalt source component, were remobilized through glacial erosion and then transported to the central Arctic Ocean via the TPD within sea ice or as IRD. Ultimately, the AIW Pb isotope signature above the Lomonosov Ridge was imparted through partial exchange with this sediment as it settled through the water column, creating the nonradiogenic Pb isotope signals at the beginning of glacial periods.

4.3.2. Neogene Record

The interpretation of the Neogene Pb isotope variability observed in the ACEX sediments (Figure 6) is more speculative than for the Quaternary part, mainly because there is little supporting evidence from independent data. We have tried to fulfill broad assumptions, such as ascribing million-year variability to tectonic or other long-term changes, but even such tectonic changes themselves are not well constrained.

In comparison to the North Atlantic records, the Pb isotopic changes in AIW during the Neogene (pre-2 Ma) were similarly small, and always less radiogenic [e.g., Winter et al., 1997; von Blanckenburg and Nägler, 2001]. Indeed, as discussed above, the Pb input into the Arctic Ocean has generally been more “congruent.” This is supported by the overall similarity of the Neogene AIW data, from the leachates, and the bulk sediment Pb isotope compositions (Figures 5 and 6).

The slightly more radiogenic Pb isotope compositions between ∼15 and 9 Ma in the AIW Pb record (Figure 6) are potentially consistent with a more radiogenic Pb input to the Arctic from Pb leaching during soil formation [von Blanckenburg and Nägler, 2001]. However, it seems more likely that this small deviation reflects a slight change in the Pb isotopic composition of the “congruently” eroded bedrock being delivered to the deep central Arctic, where partial exchange with AIW occurs.

The subsequent excursion to less radiogenic values, centered around 5 Ma and lasting ∼2 Ma (Figure 6), may have been the result of enhanced erosion or remobilization of Putorana basalt-sourced material, and subsequent export to the Arctic Ocean. Thus we consider that this period of nonradiogenic AIW Pb reflects a change in the erosion regime in north Eurasia; for example, rerouting of drainage, or change in surface exposure of the Putorana basalts. However, any such change must not have greatly affected the Sr and Nd isotopic compositions of the bulk sediment (Figure 3), and thus this Pb isotopic change probably reflects only a minor change in total overall north Eurasian erosion. Moreover, the similarity of the median values of the Neogene record, and those of the early glacial “excursions” (Figure 6) implies that there was a continuous supply of Putorana sourced sediment to the Lomonosov Ridge before 2 Ma.

Despite the inherent uncertainties we currently face with the interpretation of the Neogene record, our record of the Pb isotope evolution of AIW can still offer insights to improve our understanding of Neogene climatic and oceanographic evolution. For instance, the key feature of the North Atlantic Pb isotope evolution over the past 15 Ma is the conspicuous inflection in the trend at ∼3–2 Ma: i.e., at the time of the INHG (Figure 6) [Reynolds et al., 1999; Burton et al., 1997]. In contrast, no such dramatic change in the Pb isotope composition of AIW around 2.7 Ma is observed in the central Arctic Ocean. Indeed, our Pb isotope data indicate that the period between 4 and 1 Ma was a period of stable Arctic (Eurasian) erosion and weathering (Figure 6). That is, unless our sampling between 3 and 1 Ma coincided exactly with the Pb isotopic minima characteristic of the early glacial transitions, the latest period of the “Neogene” section was devoid of the larger (>0.1 in 206Pb/204Pb) variability seen in association with glacial-interglacial cycles (Figure 6).

Not only does the frequency and amplitude of Pb isotopic variability appear to change sometime around 1 Ma, but there is also an apparent shift from a “Neogene” Pb isotopic median value (18.47) to a “Quaternary” value (18.63) at this time (Figure 6). It is unlikely that this represents a shift in the provenance or composition of the bedrocks eroded, because the Nd and Sr isotopic data measured in the bulk sediments are virtually invariant (Figure 3 and Table 2). It is more likely that weathering characteristics changed at this time, coincident with other changes seen in North America associated with the mid-Pleistocene climate revolution (MPR; the transition from the 41 to 100 ka glacial cycles) [e.g., Ruddiman et al., 1989; Helmke et al., 2005; Clark et al., 2006]. Unfortunately, the sampling interval and resolution of the data presented here are woefully insufficient to make any detailed comparison with the climatic changes occurring at the MPR. However, two observations remain salient: there is no evidence from Pb isotopes for an enhancement of north Eurasian glaciation at the time of the INHG (2.7 Ma), and the first indications for central Arctic glacial/interglacial cyclicity apparently coincide with the MPR (1 Ma). This, however, requires corroboration by future, higher-resolution studies either from the ACEX core or possibly from materials to be recovered in the future.

5. Summary

The Leg 302 ACEX sediments, for the first time, provide a means to study Neogene Arctic paleoceanography and paleoclimatology, although extracting such information from these essentially microfossil barren sediments requires new approaches and techniques. It is demonstrated here that the radiogenic isotope compositions of Pb and Sr of sediment leachates and Pb, Sr and Nd of bulk sediments are powerful tools for Arctic paleoceanographic research.

The bulk sediment radiogenic isotope compositions from the Lomonosov Ridge are remarkable in their constancy, showing almost negligible variations in Sr-Nd isotope space for the entire record of the past ∼15 Ma, including modern sea ice sediments. This implies that the provenance and transport mechanism of sediments via sea ice has been a long-standing and stable feature in the central Arctic Ocean. Moreover, this constancy adds to the growing evidence for the early existence of an Arctic sea ice cover [e.g., Moran et al., 2006].

Strontium isotope data from sediment leachates indicate that the Arctic Ocean may not have had a continuously unrestricted exchange with to the global oceans prior to 11.5 Ma. Indeed, variability in the Sr isotopic record derived from Arctic sediment leachates may reflect the latest stages of limited exchange between the Arctic Ocean and the global oceans, prior to the incorporation of the former into the latter. However, the small Sr isotope deviations from global seawater observed may also originate from contamination with Sr released from clays during the leaching procedure.

The Pb isotope composition of past deep seawater, generated through partial exchange with settling sediment, ultimately reflects the erosive regimes of the northern Eurasian continental sediment source regions. Similar to Sr and Nd isotope records of the bulk sediment, the Pb isotopes indicate that the Kara and Laptev Sea shelf sediments have been generally dominant in the central Arctic Ocean over the past 15 Ma. The most distinct temporal variations in the AIW Pb isotopic record are nonradiogenic pulses that coincide with glacial advances early in the transition from an interglacial to a glacial state during the Late Quaternary. These pulses imply that the growth of north Eurasian ice sheets over the Kara and Laptev Shelves occurred quite rapidly during glacial inceptions, remobilizing the interglacially deposited Putorana sourced sediments with their nonradiogenic Pb isotope composition. The invariance of the AIW Pb isotopic record across the INHG at 2.7 Ma indicates little impact of this major climate transition on Arctic Ocean sediment transport processes or the weathering regime of the Eurasian margins. Rather, our records imply that the time of pronounced Arctic climatic change was more coincident with the mid-Pleistocene climate revolution (MPR) at about 1 Ma.


We thank Jutta Heinze for her help in running the chemical preparations of the samples and Ana Kolevica for laboratory support. We greatly appreciate the invaluable insights and comments of J. D. Gleason and D. Vance, which improved the original manuscript enormously. Finally, we thank E. Rohling for editorial handling of this manuscript. The ACEX sediments were acquired through joint efforts of the IODP, ECORD, and the Swedish Polar Research Secretariat.