Research Article

A warm and poorly ventilated deep Arctic Mediterranean during the last glacial period

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Science  14 Aug 2015:
Vol. 349, Issue 6249, pp. 706-710
DOI: 10.1126/science.aaa9554
  • Fig. 1 Sediment core locations.

    (A) Map showing core sites: red star, PS1243, at 2.7 km depth; blue star, RAPiD cores (1P, 4P, and 5P) collected on the South Iceland Rise (12). Gray arrows indicate modern Nordic Seas overflow (Denmark Strait Overflow Water, DSOW; Iceland-Scotland Overflow Water, ISOW). Orange arrows show surface inflow of the North Atlantic Current (NAC). The approximate extent of ice sheets during the LGM is indicated by the light blue dashed line (26). The dark blue dashed line indicates the extent of the Arctic Ocean (AO) at the shelf edge during the LGM, when sea level was lower. (B) Schematic cross section through the AM, as indicated by the black line in (A) (1). Deep and intermediate convection occurs in the Greenland and Norwegian Seas (thick blue arrows), whereas the deep Canadian basin may be ventilated by the sinking of small volumes of brine-enhanced shelf water (thin blue arrows) (1, 30). Dashed lines are schematic isopycnals. Deep-water exchange between the AM and the North Atlantic (gray arrow) is restricted by the Greenland-Scotland Ridge, with a depth of ~400 to 800 m.

  • Fig. 2 Deep Norwegian Sea radiocarbon reconstructions.

    (A) Δ14C from core PS1243, collected in the deep Norwegian Sea (black squares, benthic species; gray circles, planktic species), and from the South Iceland Rise (1.2 to 2.3 km depth; benthics, small gray diamonds) (12), shown with the IntCal09 radiocarbon age calibration curve (40). “Projection age” extrapolation for the sample at 23 ka is shown by the black dashed line. (B) C. wuellerstorfi δ13C measurements from core PS1243 (19). (C) Age model for core PS1243. White circles are tie points based on planktic 14C using the modern reservoir age of 400 years. Black circles are stratigraphic tie points based on the correlation of Neogloboquadrina pachyderma (s) δ18O data [shown in (D) by black arrows], the occurrence of the Vedde Ash, and an abrupt decrease in the percentage of N. pachyderma (s) (not shown) at the onset of the Holocene. (D) Correlation of N. pachyderma (s) δ18O measurements between core PS1243 (black) (19) and Norwegian Sea cores ENAM93-21 (solid gray) and MD952010 (dashed gray). The cores have been placed on the Greenland Ice Core Chronology 2005 age scale of the North Greenland Ice Core Project (NGRIP) (E) on the basis of their magnetic susceptibility (5, 41).

  • Fig. 3 Cross-plot of C. wuellerstorfi δ13C and benthic-atmosphere 14C ventilation ages (logarithmic scale), modified from (12).

    Black circles are new data from the deep Norwegian Sea; gray shapes are data from RAPiD cores previously collected on the South Iceland Rise (circles, core 5P; squares, core 4P; triangles, core 1P). Five-pointed stars are published estimates for HS1, and four-pointed stars are estimates for modern water masses.

  • Fig. 4 Cartoon of hypothesized changes in the circulation and ventilation of the Arctic Ocean, Nordic Seas, and Northeast Atlantic.

    North is to the right. Color shading indicates 14C ventilation. The pale blue layer indicates the varying extent of surface fresh water and sea ice or icebergs (white rectangles). Core locations are indicated by white circles. (A) Surface waters flow around the Nordic Seas and Arctic Ocean, losing buoyancy en route (SPMW, Subpolar Mode Water; LSW, Labrador Sea Water); ventilation of the intermediate and deep AM occurs mainly by convection in the Nordic Seas. (B) Reduction of deep convection and accumulation of poorly ventilated dense water in the deep AM, resulting in a sharp radiocarbon front (supplementary materials). (C) Entrainment and/or overflow of aged AM water into the North Atlantic during HS1.

  • Fig. 5 Deep Norwegian Sea temperature reconstructions.

    (A) Benthic δ18O data from core PS1243 (19). Measurements from Pyrgo depressa (gray squares; vital effect, –0.9‰), C. wuellerstorfi (gray circles), and Oridosarlis umbonatus (black diamonds; vital effect, –0.28‰) are shown with global sea level scaled to a 1‰ whole-ocean δ18O change. The low δ18O values of C. wuellerstorfi are discussed in the supplementary materials. (B) Glacial-to-Holocene temperature change in the deep Norwegian Sea. Shown are new clumped isotope data (Δ47, horizontal gray line; gray square, core top) from core PS1243, published ostracod Mg/Ca data (gray circles) from core PS1243 (35), and new benthic foraminifera Mg/Ca data (black diamonds) from core MD992276 (same site as PS1243) (supplementary materials). Dashed lines indicate the Holocene (lower) and glacial (upper) averages used for each proxy to calculate the glacial-to-Holocene temperature change (double-headed arrows, with ±1 SE); the Δ47 data set is also shown with ±1σ errors for each time interval (shading). The black arrow in the center highlights the deglacial release of heat from the deep Norwegian Sea. (C) Sample-mean Δ47 measurements from P. depressa (squares) and C. wuellerstorfi (triangles) in core PS1243, with averages (lines) and ±2 SE shading for the Holocene, deglacial, and glacial intervals. Core-top foraminifera Mg/Ca and Δ47 data points are from (42) and (43), respectively.

  • Table 1 Benthic 14C data from core PS1243.

    Intervals for benthic dates were selected based on local abundance peaks where available (supplementary materials). For some samples, a robust B-P age is not provided, because there was no planktic 14C date at the exact corresponding core depth. Foraminifera species used in this analysis include C. wuellerstorfi (Cw), P. depressa (Pyrgo), and Miliolida species.

    Time intervalDepth
    (m)
    Calendar
    age (ka)
    Species14C age
    (years)
    Error
    (years)
    B-P offset
    (years)
    Error
    (years)
    Benthic-atmosphere
    offset (years)
    Holocene0.288.52Cw829035170100570
    0.288.52Pyrgo809075–30110370
    0.3610.29Cw953040–70100340
    YD (Vedde)0.4512.03Cw18,00010073701407760
    YD0.4712.52Pyrgo17,80015064001607310
    BA0.5314.05Pyrgo/Cw14,250651810912030
    0.5414.30Miliolida15,350100N/A2930
    HS10.5615.15Pyrgo23,200250945029010,400
    0.57516.00Cw19,850130N/A6710
    0.5816.20Pyrgo13,35060–2300140172
    0.59517.00Pyrgo/Cw17,10095N/A3110
    Late glacial0.6218.80Miliolida24,00029065503508370
    0.64520.50Miliolida20,900160N/A3530
    0.6722.05Miliolida26,00027062103407640
    Early MIS 20.6923.35Pyrgo20,700660N/A1220
    0.7325.84Pyrgo21,800220N/A270
    0.7728.30Miliolida25,0002409503501520

Supplementary Materials

  • A warm and poorly ventilated deep Arctic Mediterranean during the last glacial period

    D. J. R. Thornalley, H. A. Bauch, G. Gebbie, W. Guo, M. Ziegler, S. M. Bernasconi, S. Barker, L. C. Skinner, J. Yu

    Materials/Methods, Supplementary Text, Tables, Figures, and/or References

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    • Materials and Methods
    • Supplementary Text
    • Figs. S1 to S10
    • Tables S1 to S5

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