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Glacial episodes of a freshwater Arctic Ocean covered by a thick ice shelf

Abstract

Following early hypotheses about the possible existence of Arctic ice shelves in the past1,2,3, the observation of specific erosional features as deep as 1,000 metres below the current sea level confirmed the presence of a thick layer of ice on the Lomonosov Ridge in the central Arctic Ocean and elsewhere4,5,6. Recent modelling studies have addressed how an ice shelf may have built up in glacial periods, covering most of the Arctic Ocean7,8. So far, however, there is no irrefutable marine-sediment characterization of such an extensive ice shelf in the Arctic, raising doubt about the impact of glacial conditions on the Arctic Ocean. Here we provide evidence for at least two episodes during which the Arctic Ocean and the adjacent Nordic seas were not only covered by an extensive ice shelf, but also filled entirely with fresh water, causing a widespread absence of thorium-230 in marine sediments. We propose that these Arctic freshwater intervals occurred 70,000–62,000 years before present and approximately 150,000–131,000 years before present, corresponding to portions of marine isotope stages 4 and 6. Alternative interpretations of the first occurrence of the calcareous nannofossil Emiliania huxleyi in Arctic sedimentary records would suggest younger ages for the older interval. Our approach explains the unexpected minima in Arctic thorium-230 records9 that have led to divergent interpretations of sedimentation rates10,11 and hampered their use for dating purposes. About nine million cubic kilometres of fresh water is required to explain our isotopic interpretation, a calculation that we support with estimates of hydrological fluxes and altered boundary conditions. A freshwater mass of this size—stored in oceans, rather than land—suggests that a revision of sea-level reconstructions based on freshwater-sensitive stable oxygen isotopes may be required, and that large masses of fresh water could be delivered to the north Atlantic Ocean on very short timescales.

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Fig. 1: A selection of 230Thex and 10Be records from the Arctic Ocean and the Nordic seas.
Fig. 2: Properties of sediment cores PS72/396-3 and -5 from the Amerasian Basin.

Data availability

The datasets used and generated during the current study are available in the PANGAEA data repository, https://doi.org/10.1594/PANGAEA.914629, where the previously published datasets are also linked and referenced.

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Acknowledgements

We thank D. Bethke and L. Schäfer for the lab work on the sediment cores. This work was funded through the research programme of the Helmholtz Foundation. Sample requests should be directed to the curator of the Polarstern core repository at the Alfred Wegener Institute (e-mail: curator@awi.de).

Author information

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Authors

Contributions

W.G. developed the concept and wrote the main text. Together with I.S., he developed the analytical scheme and led the elemental and radioisotope analyses. J.M. contributed data and text regarding the lithology and the synthesis of data. J.W. contributed data and text regarding the micropalaeontological situation, oxygen and carbon isotopes of core PS72/396-3. R.S. led the sampling efforts, described and developed the lithological units of the studied sediment cores and contributed data and text. All authors contributed to the discussion and to Methods.

Corresponding author

Correspondence to Walter Geibert.

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The authors declare no competing interests.

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Extended data figures and tables

Extended Data Fig. 1 Depth profile along the transect A–B along the Greenland–Scotland Ridge (GSR) shown in Fig. 1.

The shaded bars are schematic examples of assumed reductions of water depths at peak glacials (sea-level reduction, 120 m; sea ice, 60 m; shelf ice, 450 m). The sum of depths does not equal the thickness shown because icebergs are embedded and floating within water and sea ice, not below it. Additional adjustments would be necessary to reflect isostatic changes of water depths. Please note that the thickness of sea ice shown here is an estimate, assuming permanent sea-ice cover and brackish or fresh water at the sea surface in peak glacials. The assumed thickness of potential iceberg/ice-shelf reach is very conservative, less than one half the deepest observed plowmark.

Extended Data Fig. 2 Properties of sediment core PS51/038-4 from the Alpha Ridge, Amerasian Basin.

af, Isotopic and elemental composition of PS51/038-4. g, Photograph of sediment core PS51/038-4. White labels in the line scan indicate common stratigraphic units in western Arctic sediments (see Methods). h, Counts of planktonic foraminifera (green, bottom axis, in counts per gram, #g), sand content (>63 μm; blue, top axis) and the stable carbon (pink, bottom axis) and oxygen (blue, top axis) isotope composition of N. pachyderma. Blue shading indicates the two major low-230Thex intervals discussed in the text. Error bars denote one standard deviation precision; data on analytical reproducibility are in Extended Data Table 2. The precision of the stable isotope measurements has been found to be better than 0.06‰ and 0.08‰ (absolute) for δ13C and δ18O, respectively, over a one-year period.

Extended Data Fig. 3 Stable oxygen and carbon isotopes of N. pachyderma, abundance of planktonic foraminifera, and sand content for four locations in the Arctic22.

ad, Data for cores PS72/396-3 (a), PS51/038-4 (b), PS2200-5 (c) and PS2185-6 (d). For core PS72/396-3 (a) only, we also show the relative abundance of allochtonous foraminifera (orange, top axis). The low-230Thex intervals are indicated by blue shading. In c, d, indicated at the left of the y axis are the oldest corrected radiocarbon ages58. Please note that a reservoir age correction of 400 years was applied in the original reference. The precision of the stable isotope measurements has been found to be better than 0.06‰ and 0.08‰ (absolute) for δ13C and δ18O, respectively, over a one-year period.

Extended Data Fig. 4 Compilation of various parameters reported for sediment core PS1533-3 from Yermak Plateau (Eurasian Basin).

Left to right: 230Thex from ref. 47 as blue dots, recalculated (but not decay-corrected) and corrected for ingrowth from authigenic uranium (Uauth) using a published age scale22; Uauth calculated using a U/Th activity ratio of 0.6 as green/black dots, using 232Th from ref. 47 and 10Be from ref. 48 as small red dots. Magnetic volume susceptibility kappa (dimensionless in SI units) from ref. 85 is shown as yellow dots. Total organic carbon (TOC) as anthracite/black dots and calcium carbonate86 as small grey dots. Stable isotope values for 13C (magenta) and 18O (blue) in N. pachyderma from ref. 22. Abundance of planktonic foraminifera86 as green dots. Sand content as a blue dotted line from ref. 47. The ages for the respective boundaries of the low-230Thex intervals are shown as discrete numbers; the MIS boundaries shown as grey bars on the right follow ref. 22. The low-230Thex intervals identified throughout the Arctic are highlighted in blue (see Fig. 1). The MIS 2 interval only seen in rapidly accumulating cores is highlighted in pink. Analytical uncertainties are not shown; the values are available for the U-series isotopes and for 10Be in the publicly available dataset (see Methods section ‘Data availability’).

Extended Data Fig. 5 Schematic visualization of the situation during glacial fresh Arctic Ocean intervals.

Bering Strait and other outflows are blocked for inflowing seawater (green) by lowered sea levels. A floating Arctic ice shelf up to 1,000-m thick builds up, leaving grounding evidence on Lomonosov Ridge and on the GSR. Freshwater (light blue) from the entire drainage basin of the Arctic Ocean is forced through Fram Strait and eventually the narrow channels of the GSR (flow shown by blue dotted arrow). Seawater is prevented from flowing into this ice-dammed system with strong outflow. However, even a small change at the GSR can lead to seawater quickly filling the Arctic from the bottom (green arrow), displacing substantial amounts of freshwater into the Atlantic in a very short time period (for example, Heinrich event H6), triggering melting in the Nordic seas and possibly beyond. This figure is not drawn to scale and serves only to visualize the processes described in the text.

Extended Data Table 1 Core locations and origin of 230Th data shown
Extended Data Table 2 Details on analytical reproducibility for the U-series and elemental measurements on cores PS51/038-4 and PS72/396-5

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Geibert, W., Matthiessen, J., Stimac, I. et al. Glacial episodes of a freshwater Arctic Ocean covered by a thick ice shelf. Nature 590, 97–102 (2021). https://doi.org/10.1038/s41586-021-03186-y

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