The fate of the Greenland Ice Sheet (GrIS) and Arctic ecosystems in response to climate warming remain uncertain. Terrestrial records of a GrIS smaller than today and the ecosystems that emerged in formerly ice-free areas are crucial for understanding ice-sheet and ecosystem sensitivity to warming, yet rare due to present ice cover. Basal materials from ice cores can be a critical archive of past ice-free periods. Collected in 1966 from northwestern Greenland, the Camp Century ice core was the first ice core drilled to the bed of an ice sheet and retrieved 3.5 m of sub-glacial sediment (Hansen and Langway, 1966). However, the subglacial sediment was incompletely studied and then misplaced for decades – until it was re-discovered in 2017. Now, we can apply many analytical techniques to the sediment that were unimaginable at the time of its collection nearly 60 years ago.

Here, we show that the upper- and lower-most sub-glacial sediment from Camp Century contains a multi-million-year record of vegetation and glacial history in northwestern Greenland (Christ et al., 2021). The sediment preserves lipid biomarkers and frozen plant macrofossils, including Arctic willow, twigs, moss leaves and stems, and fungal spores, that resemble tundra vegetation from modern ice-free areas of Greenland. Enriched ?¹⁸O values from sediment pore ice require precipitation at lower elevations and warmer temperatures that imply ice-sheet absence. Multiple geochronometric analyses suggest the upper sediment contains reworked material similar to the lower sediment. Cosmogenic ²⁶Al/¹⁰Be and infrared stimulated luminescence (IRSL) data bracket the burial of the lower-most sediment between <3.2 ± 0.4 Ma and >0.7 to 1.4 Ma. In the upper most sediment, multiple IRSL dating methods indicate that the last exposure to sunlight was soon after the peak warmth of the Marine Isotope Stage (MIS) 11 interglacial. The luminescence data are consistent with the cosmogenic ²⁶Al/¹⁰Be burial age of <1.0 ± 0.1 Ma, which together mandate no more than 13 kyr of exposure during the last ice-free event.

The subglacial sedimentary record from Camp Century documents at least two episodes of ice-free, vegetated conditions, each followed by glaciation. The lower sediment derives from an Early Pleistocene GrIS advance and contains the oldest record of Greenland vegetation recovered from beneath the ice sheet. The upper sediment provides direct terrestrial evidence for a smaller GrIS during MIS 11 and requires at least 1.8 m of sea level contribution from Greenland. At a wider scale, the ²⁶Al/¹⁰Be in the Camp Century upper sediment match those in GISP2 subglacial bedrock from central Greenland (Schaefer et al., 2016), which suggests a similar history of ice cover across Greenland. The GrIS survived most Pleistocene interglacials, but much of it melted at least once in the last million years. The loss of the northwestern GrIS and emergence of a tundra ecosystem during MIS 11 underscores ice-sheet sensitivity to prolonged interglacial warmth (Robinson et al., 2017; Irvali et al., 2020).