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

Joerg M. Schaefer,         Robert C. Finkel,             Greg Balco,        Richard B. Alley,              Marc W. Caffee,             Jason P. Briner, Nicolas E. Young,            Anthony J. Gow      & Roseanne Schwartz

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.

The possibility that future warming will cause destabilization of the GIS has motivated the use of geological records to estimate the climate sensitivity of the GIS. Terrestrial studies2, 3 have argued that the palaeo-environment of the Kap Kobenhavn Formation in north Greenland implied an ice-free Greenland, with temperatures nearly 6 °C above present persisting for about 20,000 years (20 kyr) from 1.8 million years (Myr) ago to 2.0 Myr ago. Marine sedimentary proxy data from sites off southwest Greenland5, 6 are interpreted to indicate a smaller GIS during both the Marine Isotope Stage (MIS) 5e (or Eemian; about 120 kyr ago) and MIS 11 (about 410 kyr ago) interglacial periods. Biomolecules in basal ice of the Dye-3 ice core in southern Greenland provide evidence for subarctic conditions (and thus a smaller GIS) sometime in the past million years or so7 and a recent review8 argues that the near-field and far-field data require major ice-sheet fluctuations, and allow (but do not require) near-total ice loss during the Pleistocene. On the other hand, data from the basal NEEM ice core9 indicate minor ice-surface lowering during MIS 5e despite temperatures several degrees warmer than present. The geochemistry of Greenland Ice Sheet Project Two (GISP2) silty basal ice has been interpreted as being consistent with the scenario of continuous ice cover for the past 2.6 Myr (ref. 4) and trapped air enclosed in the silty ice layer of the nearby Greenland Ice Sheet Project (GISP) core indicate basal ice ages exceeding 1 Myr (ref. 10; Methods).

The GIS survived mid-Holocene temperatures somewhat warmer than those of the past millennium and many model simulations show a relatively stable GIS over the interglacials of the recent geologic past11, 12. However, simulations also show that the warming required to remove most of the GIS is model-dependent and sensitive to external forcings and internal feedbacks, including insolation forcing, accumulation rate parameterization, and distribution and seasonality of temperature. Results imply temperature thresholds for ice-sheet stability between one11 and a few degrees Celsius above present temperatures (see review in ref. 8; also refs 13 and 14). Because the GIS sensitivity probably changes with increasing forcing temperature, model timescales for ice-sheet removal depend on the amplitude of the forcing: a temperature threshold of 2 °C with a 5,000-year response time given 3 °C warming was inferred by ref. 13, but more extreme temperature forcing allows for GIS removal within a few thousand or even several hundred years13. Thus, current model results remain ambiguous but do show that both the magnitude and the duration of warmth are important to ice-sheet deglaciation.

Overall, existing geological data and model experiments have not resolved the question of whether the GIS disappeared or shrank substantially in warm interglacial periods. Much of this uncertainty reflects the fact that the geological data mostly comprise inference from remote proxy records, since direct evidence, if it exists, is buried beneath the present ice sheet. Here we attempt to overcome this obstacle via cosmogenic nuclide analysis of sub-GIS bedrock.

On 1 July 1993, after five years of drilling and recovery of a 3,040.3-m-long ice core and a 13.1-m-long core of sediment-rich basal ice, the GISP2 project penetrated 1.55 m of bedrock15 (Figs 1 and 2). We describe measurements of cosmic-ray-produced in-situ 10Be and 26Al from this GISP2 bedrock core. 10Be and 26Al, with half-lives of 1.4 Myr (refs 16 and 17) and 0.7 Myr (ref. 18) respectively, are trace radionuclides produced in situ by nuclear interactions between cosmic-ray particles and rocks exposed at Earth’s surface. The cosmic-ray flux decreases exponentially with an e-folding length (1/e ≈ 0.37) of about 60 cm in rock or about 1.5 m in ice, so cosmogenic-nuclide production is negligible beneath ice sheets. The presence of any substantial in situ cosmogenic radionuclide concentration in subglacial bedrock indicates geologically recent near-surface exposure and thus ice-free conditions. Pioneering analysis in the 1990s, published as an abstract19, indicated detectable 10Be and 26Al in the GISP2 bedrock core, but overall uncertainties remained large enough to prevent unambiguous conclusions about past GIS change. Here we describe comprehensive new 10Be and 26Al measurements, a detailed analysis of the data, and their implications for past GIS dynamics.

Leia mais: http://www.nature.com/nature/journal/v540/n7632/full/nature20146.html

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