Northwest Greenland During Two Pleistocene Ice-Free Intervals: Macrofossil and Geochemical Evidence from the 1966 Camp Century Ice Core Sub-Glacial Material

The 1390-meter Camp Century deep ice core, drilled in Northwest Greenland from 1960-1966, contains 3.44 meters of frozen sub-glacial material. Initial analyses of this material revealed plant, fungal, and invertebrate macrofossils preserved within the uppermost and lowermost samples of the sub-glacial core. Dating showed that the sediment was deposited during two Pleistocene ice-free periods (1.4-3.2 million years ago, and ~416,000 years ago). Terrestrial records of Greenland’s ecological history during warm climate intervals before the last glacial maximum (~20,000 years ago) are rare and often inaccessible. To address this knowledge gap, we isolated and analyzed organic material from the entire remaining Camp Century sub-glacial archive (n=28 samples). We used macrofossil analysis to identify, quantify, and compare the assemblages of taxa represented in each sample, and employed stable isotopic (δ13C, δ15N) and elemental ([C], [N], C:N) analysis in both isolated organic material and bulk sediment to characterize the origin and degree of degradation of this material in the core. We found that macrofossil remains were preserved throughout the entire core, and the taxa we identified are all found on Greenland today. In the lowermost section of the core (Unit 1), macrofossils (e.g., Tomentypnum nitens, Lepidurus cf. arcticus, Cenococcum geophilum), likely derived from local and regional sources, reflected the presence of cold-tolerant biota in wetland, freshwater, and upland environments. The ice-rich section of the core (Unit 2) contained remains of hardy Arctic species likely incorporated from the material below (e.g., Pogonatum cf. urnigerum, Oreomecon), with invertebrate remains found only at the interface with the overlying unit. Assemblages in the upper section of the core (Units 3, 4, and 5) were better-preserved and richer in plant and invertebrate taxa (e.g., Dryas octopetala, Betula nana, Paludella squarrosa, Daphnia cf. pulex) than those below, suggesting a mosaic of wetland, upland, and aquatic environments in proximity to the core site. δ13C, δ15N, and C:N ranges in both isolated organic material and bulk sediment were consistent with mixed terrigenous and aquatic sources. Carbon and nitrogen concentrations in bulk sediment were low throughout the depth of the core, and similar to those observed in other deep ice core basal materials. Characterization of the organic material within the Camp Century sub-glacial archive provides the first direct terrestrial evidence for sustained ice absence and an environment supporting several plant functional types, resembling modern Arctic tundra ecosystems, in Northwest Greenland during two Pleistocene interglacial intervals. Our analyses reveal the ecological history preserved below the Greenland Ice Sheet; future recovery of similar basal materials will refine our understanding of Arctic biosphere response to interglacial and ice-free conditions.