Comptes Rendus

Hydrology, Environment (Surface Geochemistry)
Late Eocene to present isotopic (Sr–Nd–Pb) and geochemical evolution of sediments from the Lomonosov Ridge, Arctic Ocean: Implications for continental sources and linkage with the North Atlantic Ocean
Comptes Rendus. Géoscience, Volume 347 (2015) no. 5-6, pp. 227-235.


New geochemical and isotopic (Sr, Nd, Pb) data are presented for a composite sedimentary record encompassing the past 50 Ma of history of sedimentation on the Lomonosov Ridge in the Arctic Ocean. The sampled sediments encompass the transition of the Arctic basin from an enclosed anoxic basin to an open and ventilated oxidized ocean basin. The transition from anoxic basin to open ventilated ocean is accompanied by at least three geochemical and isotopic shifts and an increase in elements (e.g., K/Al) controlled by detrital minerals highlighting significant changes in sediment types and sources. The isotopic compositions of the sediments prior to ventilation are more variable but indicate a predominance of older crustal contributions consistent with sources from the Canadian Shield. Following ventilation, the isotopic compositions are more stable and indicate an increased contribution from younger material consistent with Eurasian and Pan-African crustal sources. The waxing and waning of these sources in conjunction with the passage of water through Fram Strait underlines the importance of the exchange of water mass between the Arctic and North Atlantic Oceans.

Supplementary Materials:
Supplementary materials for this article are supplied as separate files:

Published online:
DOI: 10.1016/j.crte.2015.02.008
Keywords: Arctic Ocean, Lomonosov Ridge, Radiogenic isotopes, Basin evolution, Sediment provenance, Anoxic environments

Ross Stevenson 1; André Poirier 1; Alain Véron 2; Jean Carignan 3; Claude Hillaire-Marcel 1

1 Geotop and Département des sciences de la terre et de l’atmosphère, Université du Québec à Montréal, P.O. Box 8888, Station Centre-Ville, Montréal, Québec H4V 1Y7, Canada
2 CEREGE, CNRS UMR6635, Université Aix-Marseille-3, Europôle de l’Arbois, BP 80, 13545 Aix-en-Provence cedex 4, France
3 CRPG-CNRS, 15, rue Notre-Dame-des-Pauvres, 54501 Vandoeuvre-lès-Nancy, France
     author = {Ross Stevenson and Andr\'e Poirier and Alain V\'eron and Jean Carignan and Claude Hillaire-Marcel},
     title = {Late {Eocene} to present isotopic {(Sr{\textendash}Nd{\textendash}Pb)} and geochemical evolution of sediments from the {Lomonosov} {Ridge,} {Arctic} {Ocean:} {Implications} for continental sources and linkage with the {North} {Atlantic} {Ocean}},
     journal = {Comptes Rendus. G\'eoscience},
     pages = {227--235},
     publisher = {Elsevier},
     volume = {347},
     number = {5-6},
     year = {2015},
     doi = {10.1016/j.crte.2015.02.008},
     language = {en},
AU  - Ross Stevenson
AU  - André Poirier
AU  - Alain Véron
AU  - Jean Carignan
AU  - Claude Hillaire-Marcel
TI  - Late Eocene to present isotopic (Sr–Nd–Pb) and geochemical evolution of sediments from the Lomonosov Ridge, Arctic Ocean: Implications for continental sources and linkage with the North Atlantic Ocean
JO  - Comptes Rendus. Géoscience
PY  - 2015
SP  - 227
EP  - 235
VL  - 347
IS  - 5-6
PB  - Elsevier
DO  - 10.1016/j.crte.2015.02.008
LA  - en
ID  - CRGEOS_2015__347_5-6_227_0
ER  - 
%0 Journal Article
%A Ross Stevenson
%A André Poirier
%A Alain Véron
%A Jean Carignan
%A Claude Hillaire-Marcel
%T Late Eocene to present isotopic (Sr–Nd–Pb) and geochemical evolution of sediments from the Lomonosov Ridge, Arctic Ocean: Implications for continental sources and linkage with the North Atlantic Ocean
%J Comptes Rendus. Géoscience
%D 2015
%P 227-235
%V 347
%N 5-6
%I Elsevier
%R 10.1016/j.crte.2015.02.008
%G en
%F CRGEOS_2015__347_5-6_227_0
Ross Stevenson; André Poirier; Alain Véron; Jean Carignan; Claude Hillaire-Marcel. Late Eocene to present isotopic (Sr–Nd–Pb) and geochemical evolution of sediments from the Lomonosov Ridge, Arctic Ocean: Implications for continental sources and linkage with the North Atlantic Ocean. Comptes Rendus. Géoscience, Volume 347 (2015) no. 5-6, pp. 227-235. doi : 10.1016/j.crte.2015.02.008. https://comptes-rendus.academie-sciences.fr/geoscience/articles/10.1016/j.crte.2015.02.008/

Version originale du texte intégral

1 Introduction

In 2004, the Integrated Ocean Drilling Program (IODP) as part of the Arctic Coring Expedition (ACEX) drilled four sites atop the Lomonosov Ridge near 88°N and produced a composite sedimentary record measuring almost 430 m in length and comprising about 55 million years of sedimentary history of the Arctic basin (Backman et al., 2006, 2008). The core preserves the transition of the Arctic basin from an essentially warm anoxic body of water with variable salinity in the Late Cretaceous to the open Arctic Ocean of today (e.g., Pagani et al., 2006; Sluijs et al., 2006, 2008).

Initial studies of the composite sedimentary record revealed that the lower portion of the core consisted of a black shale-like sediment. The character of this sediment, the remnants of fresh water flora (Brinkhuis et al., 2006) and geochemical markers of low oxygen content (März et al., 2011; Poirier and Hillaire-Marcel, 2009, 2011) suggest that during the Paleocene and most of the Eocene, the Arctic basin was a deep anoxic freshwater to slightly saline, lake. In the Late Eocene, the sediments exhibit alternating black and light grey layers referred to as the “Zebra zone” (Backman et al., 2006, 2008). These sediments are interpreted to record the transition from the anoxic Arctic lake stage to a ventilated open ocean (ibid.). This transition interval is characterized by decreasing anoxic conditions and has been interpreted as recording a shallowing of the basin such that the ridge became subaerial for periods of time creating a depositional hiatus and erosion of the sediment on the ridge (Backman et al., 2006, 2008; März et al., 2011). This interpretation has been challenged by Poirier and Hillaire-Marcel (2009, 2011) who propose its deposition under strong current conditions due to the lake drainage/marine submergence following the opening of Fram Strait.

The length of depositional hiatus or even its presence during the transitional interval has thus been the subject of debate. Backman et al. (2006, 2008) interpreted a depositional hiatus of 26 Ma on the basis of dinoflagellate biostratigraphy. März et al. (2011) concluded that their mineralogical and geochemical proxies strongly supported the hiatus as described by Backman et al. (2006). In contrast, Poirier and Hillaire-Marcel (2009, 2011) argued on the basis of:

  • • Re-Os dating of the anoxic muds on both sides of this sedimentary gap;
  • • from initial Os isotope ratios of these muds that the Arctic basin did not experience a 26 Ma depositional hiatus.

They suggested that this sedimentary perturbation and marine incursion/ventilation of the Arctic basin lasted about 0.4 Ma at best. The transition from restricted anoxic basin to open Arctic Ocean occurred at 36 Ma. Another hiatus anywhere within the overlying oxic sediments would be invisible to the Re-Os system, because of the low Os (and Re) content of the oxic material, and because of the flatness of the marine osmium evolution curve for that time interval (Poirier and Hillaire-Marcel, 2011).

Radiogenic isotope studies (Sr, Nd, Pb) of sediments from the ACEX core (Fig. 1) encompassing the past 15 million years reflect a mixture of sources from North American and Eurasian sources that show little variation despite repeated glacial periods over the same period (Haley et al., 2008). In contrast, Hillaire-Marcel et al. (2013) demonstrated that the last glaciation was marked by an isotopic (Sr, Nd) excursion during the Youger Dryas that correlated with the drainage of Lake Agassiz through the Mackenzie River system. März et al. (2011) described the mineralogical and geochemical changes that accompanied the transition of the Arctic basin from a restricted anoxic basin to an open ventilated ocean. Less well understood is whether these changes reflect wholesale changes in the sediment sources as a result of the opening of the Arctic basin to the open ocean. We present new geochemical (major and trace elements) and isotopic data (Sr, Nd, Pb) for sediments from the composite sedimentary core of the Lomonosov Ridge encompassing the past 50 million years. They provide information on sedimentological and geochemical changes in the Arctic basin from its anoxic to oxic stage. They also shed light on a multi-step transition between these two types of environments.

Fig. 1

(Color online.) Map showing location of ACEX core. Also shown are locations of surface sediments and cores discussed in the text as well as the locations of potential sediment sources. BG: Beaufort Gyre; TPD: Trans-Polar Drift.

Modified from Maccali et al., 2012.

2 Methods

The sedimentary material was obtained from the IODP repository in Bremen, Germany. The samples were dried in an oven at 45 °C and subsequently ground to powder using an alumina mortar and pestle. The powder was divided into sub-fractions for major and trace element (at CRPG; Nancy, France) and isotopic (Sr, Nd, Pb) analyses.

The powders for isotope analyses were washed and decanted with distilled water and subsequently leached for 3 hours in a solution of 1 M acetic acid and 0.5 M hydroxylamine hydrochloride; a modified version of the leaching procedure described by Gutjahr et al. (2007). This leaching procedure served to remove carbonate (that may be of authigenic origin) and metals bound to authigenic minerals or adhering to detrital phases. The residues were washed in distilled water and dried in preparation for dissolution, chromatography and isotopic analysis. One fraction of the sediment residue was used for Pb–Pb isotope analysis while the remaining fraction was used for Sr isotope and Sm–Nd isotope analysis.

The fraction for Sr and Nd isotopes (0.1 g) was dissolved in a Teflon beaker using a mixture of concentrated hydrofluoric and nitric acids. Details of the Sr and Nd chemistry protocol can be found in Maccali et al. (2013). The Sr and Nd isotopes were analysed on a GV Sector-54 mass spectrometer in dynamic collection mode. The Sr fraction was loaded onto a single Re filament with a Ta oxide activator and the Nd was measured using a double Re filament mode. Repeated measurements of the JNdi-1 Nd standard (Tanaka et al., 2000) yielded a value of 143Nd/144Nd = 0.512106 ± 7 (2σ, n = 37). Sm and Nd concentrations and 147Sm/144Nd ratios have an accuracy of 0.5% that corresponds to an average error on the initial ɛNd value of ± 0.5 epsilon units. Repeated analysis of the NBS 987 Sr standard yielded a value of 87Sr/88Sr = 0.710263 ± 17 (2σ, n = 7). Blanks for Sr, Sm and Nd were typically < 150 pg.

For the Pb isotope analyses, about 20 to 40 mg of dried homogenized sediment fractions were dissolved and the Pb separated following the protocol of Maccali et al. (2012). Procedural Pb blanks were always below 40 × 10−12g. The lead isotope analyses were performed on a Micromass Isoprobe MC–ICP–MS at the GEOTOP laboratories of the Université du Québec à Montréal, using an Aridus desolvating membrane. Data were corrected for instrumental mass bias using thallium doping technique (Belshaw et al., 1998). Long-term reproducibility of the internal standard NBS981standard was better than 0.03% (1σ standard deviation) for all Pb isotopic ratios.

3 Results

3.1 Major and trace element data

Major and trace element data are presented as Supplementary material, Table A1 (data repository). The major element compositions of the sediments fall within three groups that mirror the physical division of the sediments into black muds, zebra muds and grey muds. The black muds are characterized by lower overall totals for the major elements reflecting the high organic matter content of these muds and are depleted in Al, Mg and K compared to the zebra muds and the grey oxic muds. Trace elements that partition into detrital minerals mirror the variations found for the major elements. Elements affected by the reducing conditions in the lower part of the core show large swings in concentrations. März et al. (2011) provide an excellent summary of the different enrichments of the elements in the ACEX composite core. We provide a few examples to demonstrate the concordance of our data suites and to corroborate changes in sedimentary provenance indicated by the radiogenic isotope data below (Fig. 2; see also Fig. 4 below).

Fig. 2

Changes in the trace element concentrations of uranium, thorium and cerium. Note the strong changes in concentrations of these elements across the transition interval following ventilation and oxidation of the Arctic basin.

Fig. 4

(Color online.) Geochemical and isotopic excursions in the ACEX sediment core. The three horizontal lines reflect changes in sediment compositions that correlate with the depositional intervals of März et al. (2011) and the red arrows reflect three possible distinct sedimentary sources/events. The small crosses in the K/Al plot that overlap with our data are the analyses from März et al. (2011) and demonstrate that both data groups are in agreement.

3.2 Isotope data (Sr, Nd, Pb)

The isotope data are present in Table 1 and Fig. 3. The Sr isotope ratios of the sediment residues presented on Fig. 3 were not corrected for in-situ decay of 87Rb because the mobility of Rb and Sr in the sediments prohibits the application of the Rb/Sr ratio to correct the measured Sr isotope ratio with any confidence. In any case, the overall age correction would be small as seen with the age-corrected Nd isotope compositions (Table 1). The strontium isotope compositions of the sediments decrease slightly from higher values in the restricted lake phase of the Arctic basin through the transitional period of the zebra muds and flatten out in the ventilated ocean stage.

Table 1

Sr, Nd and Pb isotope data for ACEX composite sediment core.

Sample 302-2A- Depth Age model 2a 87Sr/86Sr 2 s error Nd Sm 147Sm/144Nd 143Nd/144Nd 2 σ error ɛNd(0) ɛNd(t) TDM Pbb Ub Thb 238U/204Pbc 232Th/204Pbc 206Pb/204Pb 207Pb/204Pb 208Pb/204Pb 206Pb/204Pb 207Pb/204Pb 208Pb/204Pb 208Pb/206Pb
(mcd) (Ma) residue ppm ppm residue residue ppm ppm ppm mu kappa measured ratios corrected ratiosd
302-2a-1X-1W-2-3cm 1.25 0.09 492 2.2 10.2 0.3 1.3 18.871 15.671 38.405 18.871 15.671 38.405 2.035
1X 1W 12-13 cm 1.35 0.09 0.71831 0.00001 23.4 4.20 0.1086 0.512065 0.000010 -11.2 -11.2 1.6 23.34 2.1 9.8 5.6 26.7 18.755 15.608 38.649 18.755 15.608 38.649 2.061
1X 1W 22-23cm 1.45 0.10 18.243 15.558 38.162 18.243 15.558 38.162 2.092
1X 1W 32-33cm 1.55 0.11 0.71697 0.00001 18.438 15.579 38.403 18.438 15.579 38.403 2.083
1X 1W 42-43cm 1.65 0.11 18.536 15.577 38.523 18.536 15.577 38.523 2.078
1X 1W 62-63cm 1.85 0.13 0.71701 0.00001 18.532 15.583 38.631 18.532 15.583 38.631 2.085
1X 1W 92-93cm 2.15 0.15 18.324 15.572 38.288 18.324 15.572 38.288 2.089
11X 2W 75-76 cm 50.69 3.5 0.71592 0.00001 26.6 4.79 0.1088 0.512090 0.000010 -10.7 -10.7 1.5 14.13 2.8 10.1 12.2 45.7 18.505 15.564 38.609 18.499 15.564 38.602 2.087
23X 2W 22-23 cm 101.55 7.0 0.71679 0.00002 36.6 9.06 0.1497 0.512089 0.000012 -10.7 -10.7 2.5 12.85 2.9 10.5 14.1 52.3 18.544 15.578 38.638 18.529 15.577 38.620 2.084
34X 2W 22-23 cm 150.72 12.3 0.71637 0.00002 35.7 6.84 0.1159 0.512119 0.000011 -10.1 -10.0 1.6 37.16 2.3 10.3 3.8 17.7 18.477 15.568 38.555 18.470 15.567 38.545 2.087
35X 2W 18-19cm 154.45 14.0 0.71630 14.76 2.4 10.6 10.1 45.8 18.566 15.580 38.654 18.544 15.579 38.622 2.083
37X- 2W 27-28cm 164.28 19.2 0.71620 0.00018 18.38 2.6 11.1 8.9 38.5 18.553 15.589 38.661 18.526 15.587 38.625 2.085
38X 2W 27-28cm 167.16 20.7 0.71414 0.00001 33.1 6.56 0.1196 0.512087 0.000010 -10.8 -10.6 1.7 19.72 2.5 10.4 8.0 33.8 18.565 15.590 38.761 18.539 15.589 38.726 2.089
40X 2W 27-28cm 174.85 24.8 0.71483 0.00001 23.03 2.9 12.1 7.7 33.5 18.542 15.581 38.668 18.512 15.579 38.627 2.087
42X 2W 27-28cm 184.26 29.7 0.71485 0.00002 40.7 8.20 0.1218 0.512093 0.000010 -10.6 -10.3 1.8 22.77 2.6 12.0 7.1 33.6 18.492 15.574 38.628 18.459 15.573 38.578 2.090
43X 2W 22-23 cm 189.20 32.3 0.71427 0.00001 36.6 7.08 0.1170 0.512072 0.000010 -11.0 -10.7 1.7 24.92 2.3 12.6 5.6 32.4 18.448 15.563 38.629 18.420 15.561 38.577 2.094
43X 2W 22-23 cm dup. 189.20 32.3 0.71396 0.00002
44X 3W 60-61 cm 189.58 32.5 21.44 8.1 13.4 23.4 39.8
44X 1W 76-77 cm 192.75 34.2 0.71026 0.00002 36.4 6.93 0.1150 0.512105 0.000010 -10.4 -10.0 1.6 24.06 1.7 11.3 4.4 30.1 18.485 15.573 38.617 18.462 15.572 38.566 2.089
44X 1W 76-77 cm dup. 192.75 34.2 0.71017
44X 2W 95-96 cm 194.44 35.1 0.71380 0.00001 49.8 10.2 0.1243 0.512171 0.000010 -9.1 -8.8 1.7 29.69 8.9 16.6 18.5 35.6 18.556 15.582 38.627 18.455 15.577 38.565 2.090
44X 1W 106-107 cm 194.44 35.1 29.85 5.3 12.3 11.0 26.3
45X 1W 3-4 cm 195.96 35.9 0.71525 0.00001 19.90 8.0 12.5 24.9 40.0
45X 1W 57-58 cm b 196.5 36.2 0.71278 0.00002 41.1 10.7 0.1570 0.512188 0.000010 -8.8 -8.6 2.6 18.20 22.2 75.6 18.934 15.612 38.675 18.508 15.592 38.675 2.090
45X 1W 57-58 cm dup. b 196.5 36.2 0.71223 0.00001 18.20 22.2 12.0 75.6 42.3 18.892 15.599 38.618 18.466 15.579 38.543 2.087
45X 1W 57-58 cm b 196.5 36.2 18.20 22.2 75.6 18.848 15.598 38.620 18.422 15.579 38.620 2.096
45X 1W 57-58 cm b 196.5 36.2 18.20 22.2 75.6
45X 1W 57-58 cm dup. b 196.5 36.2 19.10 21.6 13.8 70.2 46.2
45X 1W 110-111 cm 197.03 36.2 19.24 17.6 14.1 56.6 46.7
46X 2W 131-132 cm 199.81 36.6 0.71503 0.00002 20.7 3.89 0.1137 0.512081 0.000009 -10.9 -10.5 1.6
46X 2W 131-132 cm dup. 199.81 36.6 24.11 9.9 9.1 25.4 24.0 18.802 15.597 38.694 18.657 15.591 38.651 2.072
46X 2W 132-133 cm 199.82 36.6 0.71455 0.00001 14.9 2.91 0.1181 0.512111 0.000008 -10.3 -9.9 1.7 22.24 25.7 10.3 71.6 29.6 18.898 15.597 38.607 18.490 15.578 38.554 2.085
48X 2W 21-22 cm 208.41 37.7 0.71942 0.00001 16.2 2.99 0.1117 0.51206 0.00005 -11.3 -10.9 1.6 24.58 7.8 9.1 19.7 23.7 18.820 15.604 38.735 18.705 15.598 38.690 2.068
51X 2W 6-7 cm 220.00 39.1 0.71793 0.00003 13.46 5.4 6.8 24.8 32.1 18.752 15.582 38.644 18.601 15.575 38.582 2.074
51X 2W 6-7 cm 220.00 39.1
54X 1W 22-23 cm 230.22 40.3 0.71377 0.00009 6.42 6.7 3.0 64.5 29.9 18.878 15.597 38.685 18.474 15.579 38.625 2.091
56X 2W 95-96 cm 241.18 41.6 0.71688 0.00001 19.8 3.79 0.1155 0.512217 0.000007 -8.2 -7.8 1.4 7.86 11.2 5.7 88.3 46.0 19.060 15.612 38.703 18.488 15.586 38.608 2.088
56X 2W 95-96 cm dup. 241.18 41.6
58X 2W 60-61 cm 252.00 42.9 0.71839 0.00003 7.18 6.4 4.4 55.1 39.2 19.032 15.606 38.744 18.664 15.589 38.661 2.071
58X 2W 60-61 cm dup. 252.00 42.9 0.71853 0.00003

a Model age calculations from Poirier and Hillaire-Marcel (2009, 2011).

b Concentrations from Table A1.

c Mu and kappa ratios calculated from U, Th and Pb concentrations.

d Age corrected ratios calculated using above mu and kappa values and decay constants for 235U, 238U and 232Th.

Fig. 3

Plots of ɛNd (a), 87Sr/86Sr (b) and 206Pb/204Pb (c) isotope compositions of the ACEX sediments. Note that the x-axis reflects sediment age in order to accommodate the data from Haley et al. (2008). See text for details.

The Nd isotope compositions of the sediments show limited variation between −8 and −11.7, although there is a gradual shift in ɛNd values from a high of −7.7 at ca 50 Ma to a low of −12 at the top of the core (Fig. 3). Higher density sampling in the period covering the zebra muds indicates that sediment sources varied rapidly during this interval. The majority of the 147Sm/144Nd ratios cluster between 0.11 and 0.12 and compare well with those for average shales and sediments (Goldstein et al., 1984), indicating that the sediments were for the most part well mixed.

The lead isotopes are corrected for in-situ decay using the U and Th concentrations measured in the core. The lead isotope ratios at the top of the core are highly variable (both age-corrected and uncorrected ratios) and reflect the infiltration of anthropogenic lead into the upper layers of the sediment through natural and biological activity (Table 1). Below the zone of anthropogenic influence (ca 51 cmd), the correlation with the Sr and Nd isotope systems is more evident (Fig. 4). The lead isotope ratios in the Arctic lake phase are variable with a tendency to have higher values that the zebra muds and Arctic Ocean phase. More detailed sampling of the ventilation phase shows a decrease in lead isotope ratios in the zebra muds (with some fluctuation) followed by a slow increase of the lead isotope compositions into the Arctic Ocean phase (Fig. 3).

4 Discussion

Changes in the chemistry and source of the sediments from the Lomonosov Ridge have been the subject of a number of studies (Haley et al., 2008; Hillaire-Marcel et al., 2013; März et al., 2011; Poirier and Hillaire-Marcel, 2009, 2011). The change in the Arctic basin from a restricted anoxic basin to an open ventilated ocean basin is reflected in the nature of the sediments (Backman et al., 2006, 2008), the fauna and flora identified in the core (Brinkhuis et al., 2006; Moran et al., 2006; Onodera et al., 2008; Stickley et al., 2008) and in the geochemistry of redox-sensitive elements and isotope systems. Poirier and Hillaire-Marcel (2009, 2011) showed that the redox-sensitive Re was concentrated in the anoxic basin and decreased after the ventilation event at 36 Ma, with their discreet data points closely matching those of the continuous measurements of März et al. (2011) within the transition zone. Similarly the initial Os isotope ratios, at first very radiogenic in the lake/epicontinental sea zone, decreased to global ocean values after ventilation due to the injection of non-radiogenic marine Os to the sedimentary load (Poirier and Hillaire-Marcel, 2009, 2011). Figs. 2 and 4 show how the concentrations of selected redox-sensitive elements such as Th, U, and Ce, and detrital-sensitive elements (K/Al) change from pre to post-ventilation and that they are consistent with the findings of März et al. (2011). Although our sampling density over 220 mcd (50 Ma) is less than the sampling density of März et al. (2011) over 12 mcd, our findings are broadly similar in that:

  • • redox-sensitive elements show sharp changes in concentrations across the ventilation event;
  • • the concentrations of the redox-sensitive elements are more variable in the anoxic basin compared to the open ocean stage;
  • • elements such as K and Al that are largely controlled by detrital minerals also fluctuate more widely in the anoxic basin stage and become more constant in the open ocean stage.

The geochemical fluctuations in the anoxic basin stage of the Arctic basin suggest that the basin was not entirely restricted, but exchanged water mass with the open ocean (März et al., 2011). A critical aspect of sediment accumulation over the Lomonosov Ridge is the prominent role of sediments deposited from sea ice and iceberg rafting. St. John (2008) showed that sea ice rafting started very early, during the Mid-Eocene, whereas iceberg dispersal may only have been a factor over the past 2.7 Ma when large ice sheets developed over surrounding land and shelves (Raymo, 1994). Thus, sedimentary supplies to the Lomonosov Ridge were likely largely controlled by ice-streaming routes of ice sheets, under full glacial conditions, and by “sea ice factories”, throughout much of the history of the basin discussed in this paper.

Fig. 3 illustrates the individual isotope plots for Sr, Nd and Pb isotopes. Here again, the open ocean stage of the Arctic basin is characterized by rather stable isotope compositions whereas the transition era (zebra muds) and anoxic basin are characterized by more variable isotopic compositions to be discussed further below. The strontium and lead isotope compositions of our younger sediments agree well with those of Haley et al. (2008), although the younger Nd isotope analyses of Haley et al. (2008) show more variations than those of our study. This likely reflects the greater density of sampling by Haley et al. (2008). Overall, sediment isotope compositions are more variable in the anoxic basin phase suggesting that the variations in the sources of sediments stabilized post-ventilation except for glaciation-induced isotopic excursions (Haley et al., 2008; Hillaire-Marcel et al., 2013). The anoxic basin tends to have slightly more radiogenic Sr and Pb isotope compositions compared to the open ocean phase. The average pre-ventilation isotope ratios for 87Sr/86Sr and 206Pb/204Pb are 0.7195 and 18.547, respectively, compared to 0.71502 and 18.485 after ventilation. This may reflect a larger contribution by older sources during the restricted basin phase. The radiogenic nature of the lone Nd analysis at 41.6 Ma appears to argue against an older source, but the strong overall correlation between the Nd and Sr isotopes compositions (Fig. 5a) suggests that the Nd isotope composition at 46 Ma is likely an artefact of one of several isotopic excursions in the restricted basin phase. These isotopic excursions were likely produced by periodic water mass exchange with the open ocean (März et al., 2011).

Fig. 5

(Color online.) a: plot of the ɛNd values versus the 87Sr/86Sr isotope ratios of the ACEX sediments. There is a general overlap of the isotope data from sediments from various Arctic locations. Overall, the Nd and Sr isotope data suggest a mixture of older Precambrian-sourced sediments from the Canadian Shelf and younger sediments from Eurasian sources possibly including present-day to Tertiary and Permian basalts. See text for details; b: plot of the 208Pb/204Pb isotope versus 206Pb/204Pb isotope compositions of the ACEX sediments with comparison to the lead isotope compositions of sediments from other Arctic Ocean localities and from Fram Strait. Mackenzie, Red and Lena Rivers from Millot et al. (2004). Alpha Ridge data from Winter et al. (1997). The A and B trends reproduce those of Maccali et al. (2012) indicating that before ventilation, sediments to the Arctic basin were dominated by older sources, whereas younger sources were introduced following ventilation. See text for details.

Fig. 4 shows examples of these excursions and demonstrates that:

  • • our K/Al ratios are concordant with those of März et al. (2011);
  • • our geochemical and isotopic excursions across the transition interval suggest sporadic sediment supplies from distinct sources and, at least, three distinct sedimentary events (red arrows) that correlate with the depositional intervals of März et al. (2011).

The red arrows indicate changes in sediment sources. For example the opening of Fram Strait may have led to the addition of Pan-African-derived material from the Svalbard-East Greenland passage. Furthermore, the high ɛNd excursion marking the second sedimentary pulse of this transition might indicate supplies from younger material such as Tertiary basalts from Svalbard and possibly ridge basalts.

The sources of sediments to the Arctic basin are further investigated with the help of the Sr–Nd and Pb–Pb isotope plots on Fig. 5. Note the strong overlap in the Nd and Sr isotope compositions of the pre- and post-ventilation sediments (Fig. 5a), illustrating the continuity of sediment sources throughout the history of the Arctic basin. Also plotted for comparison are the isotope compositions for the Eocene to Holocene sediments of the Lomonosov Ridge (Haley et al., 2008; Hillaire-Marcel et al., 2013) and Holocene sediments from the Alpha Ridge (Winter et al., 1997). The strong overlap in the isotopic compositions of all these sediments indicates that although the relative proportions of the contributing sources may have varied over the past 50 Ma, the ultimate sources have remained largely unchanged. The isotopic composition of the older end member is represented by the Sr and Nd isotope composition of sediments from the Canadian Shelf/Mackenzie River which was ultimately largely shed from a Precambrian Shield (Hillaire-Marcel et al., 2013).

Fig. 5b shows a plot of the age-corrected lead isotope compositions from the ACEX sedimentary record. The lead isotope compositions of Miocene to Holocene sediments from the Lomonosov Ridge (Haley et al., 2008), surface sediments from the Alpha Ridge (Winter et al., 1997), core sediments from Fram Strait (Maccali et al., 2012) and the compositions of sediments from the Mackenzie and Lena Rivers (Millot et al., 2004) are plotted for comparison. Also included are the data fields for Arctic sediments from Gartside (1996) and Pan-African crust (Fagel et al., 2002). The lead isotope compositions of the Lomonosov sediments from this study overlap the Lomonosov sediment isotope compositions from Haley et al. (2008) and the most radiogenic pre-ventilation sediments from this study overlap those deposited in Fram Strait (Maccali et al., 2012). These latter sediments align with those from the Alpha Ridge (Winter et al., 1997), Arctic sediments (Gartside, 1996) and the Lomonsov Ridge (Haley et al., 2008) to form a trend that is identical to Trend A of Maccali et al. (2012). Also shown on Fig. 5b is a second trend (Trend B) that is defined by the near surface sediments in Fram Strait (Maccali et al., 2012). The Lomonosov Ridge post-ventilation sediments and least radiogenic pre-ventilation sediments from this study plot between these two trends. The shared trend (Trend A) of sediments from different parts of the Arctic basin indicates that the ultimate sources of the sediments for the Lomonosov and Alpha Ridge regions of the Arctic basin remained largely unchanged for the past 40–50 million years. Note that the lead isotope compositions of sediments from the Lena and Mackenzie rivers (Millot et al., 2004) plot at opposite ends of the Arctic sediment lead isotope trend, suggesting that these isotope compositions may represent the more extreme end-members of the sediments that are supplied to the Arctic basin. Maccali et al. (2012) interpreted Trend B to reflect the contribution of younger crustal material from Pan-African crustal terrains in Greenland whereas the low 206Pb/204Pb end of Trend A reflects the addition of contribution of younger Eurasian sources such as the Laptev Sea and Lena River (Millot et al., 2004; Tütken et al., 2002) as well as small possible contributions from present-day to Permian basaltic rocks in and around the Arctic basin. The Mackenzie and Red Rivers represents the supply of sediments shed from the Canadian Precambrian Shield that ultimately comprise much of the Canadian shelf. The predominance of higher 206Pb/204Pb isotope ratios prior to the ventilation event suggests that Canadian shelf sources dominated during the anoxic basin era (Fig. 3c).

The Sr–Nd–Pb isotope plots of Fig. 5 suggest that sediments in the Arctic basin were derived from three major sources: one end member of the Arctic basin sediment mixture is controlled by the supply of sediments from the erosion of Precambrian terrains such as the Northwest Canadian Shield whereas the other two end-members are controlled by the erosion of younger Pan-African (Caledonian) or Eurasian crust (Laptev Sea and Lena River). This latter component could also contain sediments derived from the erosion of Phanerozoic to present-day basaltic crust (Permian Siberian Trapps, Tertiary Basalts and present-day mid-ocean ridge basalts: MORB). The slight predominance of radiogenic strontium and lead isotope ratios in the restricted anoxic basin stage provides evidence that sediments sourced from the Canadian Shield were predominant over the Eurasian sediment sources, although the Nd isotope compositions show no substantial difference between the pre- and post-ventilation eras (Fig. 5a). The decrease in the strontium and lead isotope compositions following the ventilation event indicates that open ocean circulation favoured a greater contribution from Eurasian sources and/or Pan-African sources. The variable Sr–Nd–Pb isotopic compositions identified in the pre-ventilation era reflect the waxing and waning of these two sources during periodic water mass exchanges with the open ocean as suggested by März et al. (2011) on the basis of the elemental geochemistry of the sediments.

Thus, the restricted basin phase of the Arctic basin appears to have favoured a greater contribution of older sediments (source #1; Fig. 1) on the Lomonosov Ridge whereas following ventilation, the establishment of stable sediment sources with a greater contribution from younger sources suggests that the passage of water through Fram Strait enhanced and stabilized erosion of the Eurasian shelf (sources #2 and possibly #3; Fig. 1). This is supported by the presence of two diverging trends (A and B) on Fig. 5b. This shared sediment source trends between sediments of the Lomonosov ridge and Fram Strait (Carignan et al., 2008; Maccali et al., 2012) underlines the importance of the flow of water through Fram strait in controlling and stabilizing the flow of water in the Arctic Ocean and ultimately the source of sediments deposited along the Lomonosov Ridge.

5 Conclusions

Geochemical and isotopic analyses of sediments from the composite ACEX sediment core obtained from the Lomonosov ridge in the Arctic Ocean provide constraints for the evolution of the Arctic basin from restricted anoxic basin to open ventilated ocean. The geochemical data illustrate:

  • • the depletion and concentration of redox-sensitive elements in the Eocene anoxic Arctic basin;
  • • the ventilation of the restricted basin and formation of the modern Arctic Ocean is reflected in the stabilization of both redox-sensitive elements and elements controlled by detrital minerals (this study, März et al., 2011; Poirier and Hillaire-Marcel, 2009, 2011).

The Sr, Nd and Pb isotope compositions of the sediments are also more variable in the pre-ventilation era compared to the post-ventilation era suggesting either the establishment of the open ocean stabilized sediment sources, and/or that enhanced supplies from sea ice factories became more important in response to the Cenozoic large-scale cooling trend (e.g., St. John, 2008). Nevertheless, the variability in the isotopic compositions during the restricted basin phase supports the notion that the basin might have experienced periodic injections of water from the open ocean (März et al., 2011). Although the isotopic data indicate that sediment sources were broadly constant throughout the history of the Arctic basin, strontium and lead isotope compositions suggest that older crustal sources attributed to sediments shed from the Canadian Shield dominated during the restricted basin phase. Younger crustal contributions increased following ventilation of the basin and the establishment of the modern Arctic Ocean. The similarity in isotopic compositions and sediment sources for sediments from the Lomonosov ridge and Fram Strait (Maccali et al., 2012, 2013) underscore the importance for the passage of water through Fram Strait for controlling both the circulation and sediment deposition within the Arctic Ocean and the importance the contribution of water masses to the North Atlantic Ocean.


This article is dedicated to Jean Carignan: protégé, peer, mentor and friend. This study was supported by NSERC grants to RS and CHM.


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