Greenland was nearly ice-free for extended periods during the Pleistocene

Abstract

The Greenland Ice Sheet (GIS) contains the equivalent of 7.4 metres of global sea-level rise1. Its stability in our warming climate is therefore a pressing concern. However, the sparse proxy evidence of the palaeo-stability of the GIS means that its history is controversial (compare refs 2 and 3 to ref. 4). Here we show that Greenland was deglaciated for extended periods during the Pleistocene epoch (from 2.6 million years ago to 11,700 years ago), based on new measurements of cosmic-ray-produced beryllium and aluminium isotopes (10Be and 26Al) in a bedrock core from beneath an ice core near the GIS summit. Models indicate that when this bedrock site is ice-free, any remaining ice is concentrated in the eastern Greenland highlands and the GIS is reduced to less than ten per cent of its current volume. Our results narrow the spectrum of possible GIS histories: the longest period of stability of the present ice sheet that is consistent with the measurements is 1.1 million years, assuming that this was preceded by more than 280,000 years of ice-free conditions. Other scenarios, in which Greenland was ice-free during any or all Pleistocene interglacials, may be more realistic. Our observations are incompatible with most existing model simulations that present a continuously existing Pleistocene GIS. Future simulations of the GIS should take into account that Greenland was nearly ice-free for extended periods under Pleistocene climate forcing.

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Figure 1: The GISP2 bedrock core and GIS deglaciation.
Figure 2: 10Be and 26Al data for the GISP2 bedrock.
Figure 3: Exposure–burial scenarios consistent with the data.

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Acknowledgements

We gratefully acknowledge help from the National Ice Core Laboratory (NICL) and thank the GISP2 steering committee for providing the bedrock core samples. J.M.S. acknowledges support by the Lamont Climate Center and the Comer Family Foundation. R.B.A. acknowledges support by the NSF (AGS 1338832), as do J.M.S., J.P.B. and N.E.Y. (PLR Arctic System Science Program number 1503959). We thank J. Fyke for assistance in producing Fig. 1b. G.B. acknowledges support from the Ann and Gordon Getty Foundation. M.W.C. acknowledges support from the US National Science Foundation, grant EAR-1153689. The pioneering cosmogenic-nuclide study of the GISP2 bedrock core under the lead of Kuni Nishiizumi in the late 1990s motivated our study. This is LDEO publication number 8068.

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Contributions

J.M.S. and R.C.F. initiated and coordinated the project, were in charge of the data production and wrote the first draft of the manuscript. G.B. provided the data analysis. R.B.A. participated in the GISP2 project, and provided glaciological expertise and model perspective. A.J.G. was part of the first scientist team at the GISP2 camp when the bedrock core was retrieved, examined the rock core and provided stratigraphic ice-bedrock expertise. M.W.C. measured the samples for Al isotopes. R.S. processed all the rock samples and did the Al and Be extraction. J.P.B. and N.E.Y. provided Arctic glacier expertise and prepared final figures. All authors read and edited multiple versions of the manuscript.

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Correspondence to Joerg M. Schaefer.

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The authors declare no competing financial interests.

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Nature thanks P.-H. Blard, G. Milne and the other anonymous reviewer(s) for their contribution to the peer review of this work.

Extended data figures and tables

Extended Data Figure 1 10Be and 26Al concentrations compared to production rates.

10Be (n = 5) and 26Al (n = 2) concentration–depth profiles compared to the depth dependence of nuclide production rates (including both spallogenic and muon production). In both a and b, the red and blue boxes represent 10Be (red) and 26Al (blue) measurements. The vertical dimension of each box represents a distinct segment of core, and vertical lines connect multiple core segments that were amalgamated for each 10Be or 26Al analysis. In most cases amalgamated segments were adjacent to each other, but in some cases (for example, the uppermost two core segments), they were separated by gaps. The width of the boxes shows measurement uncertainty (1σ; see Extended Data Table 1) on nuclide concentrations. In a, we attempt to fit the observations by assuming that the bedrock surface is the land surface, the erosion rate is zero, and by allowing the duration of a single period of exposure to vary. For this exercise we treat 10Be and 26Al separately, that is, the predictions are not forced to obey the production ratio. The continuous thin black lines show predicted nuclide concentrations for this model, and the discontinuous, darker, black bars show predicted nuclide concentrations averaged over depth ranges corresponding to each analysis. The black bars, therefore, are the model predictions that we compare to the measurements. This model cannot be fitted to the data, because observed nuclide concentrations do not decrease as rapidly as they would if the bedrock surface were the land surface during exposure. In b, we perform the same fitting exercise, but also include a thickness of shielding mass above the bedrock surface as an additional fitting parameter. This removes the systematic misfit shown in a and makes it possible to fit the observations. A cover thickness of 350 g cm−2 best fits the observations.

Extended Data Figure 2 Two-stage exposure histories fit to 10Be and 26Al measurements.

In a and d, the red and blue boxes represent 10Be (n = 5; red) and 26Al (n = 2; blue) measurements. The vertical dimension of each box represents a distinct segment of core, and vertical lines connect multiple core segments that were amalgamated for each 10Be or 26Al analysis. In most cases amalgamated segments were adjacent to each other, but in some cases (for example, the uppermost two core segments), they were separated by gaps. The width of the boxes shows measurement uncertainty (1σ; see Extended Data Tables 2 and 3) on nuclide concentrations. The thick black lines are nuclide concentrations predicted for each core segment by the best-fitting parameters of each model. b and e show the exposure history implied by the best-fitting parameters of each model compared to the LR04 oxygen isotope stack from ref. 30. Red bars represent periods of surface exposure and blue bars periods of cover by the ice sheet. c and f show observed nuclide concentrations compared to model predictions for samples in which both 10Be and 26Al were measured, normalized to production rates at their respective depths implied by each model, on a two-nuclide diagram29. Red ellipses are 68% confidence regions for the nuclide concentrations including measurement uncertainties only, and black dots are nuclide concentrations predicted by best-fitting model parameters. The solid black lines show the simple exposure region; darker dashed lines are isolines of burial in increments of 1 Myr, and lighter dotted lines are isolines of exposure time in increments of 0.1 Myr. a, b and c show the fit of model 1, the simplest possible model that fits the data, which includes a single period of surface exposure, a single period of burial, 350 g cm−2 of cover thickness above the bedrock surface during exposure, and zero surface erosion. In c, nuclide concentrations are normalized to production rates at sample depths below this additional cover thickness. This model provides a good fit to the measurements. The exposure history implied by the best-fitting parameters for model 1 (280-kyr exposure, 1.1 Myr burial) provides a maximum limiting constraint on the length of time the present ice sheet has been continuously present at the core site. d, e and f show the fit of model 1B, which includes a long period of continuous exposure with steady surface erosion, a single period of burial, and zero additional cover thickness above the bedrock surface. In f, nuclide concentrations are normalized to production rates at sample depths below the bedrock surface. This model cannot be adequately fitted to the observations.

Extended Data Figure 3 Two-stage exposure-burial models with four free parameters.

ad show parameter values for two-stage exposure-burial models with four free parameters (model 1C: free parameters are exposure time texp, burial time, surface erosion rate during exposure, and additional cover thickness above the bedrock surface during exposure periods) that yield acceptable fits to the observations. All panels show the same set of results; only the axes differ. Note that model 1 with best-fitting parameters (280-kyr exposure, 1.1 Myr burial, zero surface erosion, 350 g cm−2 cover thickness) is an endmember of this distribution.

Extended Data Figure 4 Many-stage exposure histories fitted to 10Be and 26Al measurements.

In a, c, e and f, the red and blue boxes represent 10Be (red) and 26Al (blue) measurements. The vertical dimension of each box represents a distinct segment of core, and vertical lines connect multiple core segments that were amalgamated for each 10Be or 26Al analysis. In most cases amalgamated segments were adjacent to each other, but in some cases (for example, the uppermost two core segments), they were separated by gaps. The width of the boxes shows measurement uncertainty (1σ; see Extended Data Tables 2 and 3) on nuclide concentrations. The thick black lines are nuclide concentrations predicted for each core segment by the best-fitting parameters of each model. b, d, f and h show the exposure history implied by the best-fitting parameters of each model compared to the LR04 oxygen isotope stack from ref. 30. Red bars represent periods of surface exposure and blue bars periods of cover by the ice sheet. a and b show model 2, which is a dynamic steady state model with 100-kyr cycles. This model has one free parameter, which is the fraction of each cycle during which the site is ice-free, and assumes zero surface erosion and 350 g cm−2 cover thickness above the bedrock surface. The best-fitting length of ice-free periods for this model is 8,000 years. All other panels show models with ice-free conditions during some or all middle and late Pleistocene interglaciations, and have two free parameters: the duration of ice-free conditions during interglaciations, and the length of an initial ice-free period in the middle or early Pleistocene. c and d show a model in which the core site is ice-free during all interglacials within the period of 100-kyr-long glacial–interglacial cycles after 1.1 Myr ago, during an (arbitrarily long) series of short interglacials during 41-kyr-long glacial–interglacial cycles before 1.1 Myr ago, and during an initial longer period of exposure in the middle Pleistocene. The best-fitting duration of ice-free conditions during interglacials for this model is 4,200 years. e and f show ice-free conditions during MIS 9, MIS 11 and MIS 13, an (arbitrarily long) period of short ice-free interglaciations during 41-kyr-long cycles before 1.1 Myr ago, and an initial period of continuous exposure in the middle Pleistocene. The best-fitting duration of ice-free conditions during interglacials for this model is 7,400 years. g and h show ice-free conditions during MIS 9, MIS 11 and MIS 13, with (arbitrarily located) occasional ice-free periods in the early and middle Pleistocene. The best-fitting duration of ice-free conditions during interglacials for this model is 18,200 years.

Extended Data Table 1 10Be and 26Al concentrations of the GISP2 bedrock core
Extended Data Table 2 GISP2 bedrock 10Be data
Extended Data Table 3 GISP2 bedrock 26Al data

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Schaefer, J., Finkel, R., Balco, G. et al. Greenland was nearly ice-free for extended periods during the Pleistocene. Nature 540, 252–255 (2016). https://doi.org/10.1038/nature20146

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