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Abstract. In 1966, drilling at Camp Century, Greenland, recovered 3.44 meters of sub-glacial material from beneath 1350 meters of ice. Although prior analysis of this material showed that the core includes glacial sediment, ice, and sediment deposited during an interglacial, the sub-glacial material had never been thoroughly studied. To better characterize this material, we analyzed 26 of the 30 core samples remaining in the archive. We performed a multi-scale analysis including X-ray diffraction, micro-computed tomography, and scanning electron microscopy to delineate stratigraphic units and assign facies based on inferred depositional processes. At the macro-scale, quantitative X-ray diffraction revealed that quartz and feldspar dominated the sediment and that there was insignificant variation in relative mineral abundance between samples. Meso-scale evaluation of the frozen material using micro-computed tomography scans showed clear variations in the stratigraphy of the core characterized by the presence of bedding, grading, and sorting. Micro-scale grain size and shape analysis, conducted using scanning electron microscopy, showed an abundance of fine-grained materials in the lower part of the core and no correspondence between grain shape parameters and sedimentary structures. These multiscale data define 5 distinct stratigraphic units within the core based on sedimentary process; K-means clustering analysis supports this proposed unit delineation. Our observations suggest that ice retreat uncovered the Camp Century region exposing basal till, covered with a remnant of basal ice or firn (Units 1 and 2). Continued ice-free conditions led to till disruption by liquid water causing a slump deposit (Unit 3) and the development of a small fluvial system of increasing energy up core (Units 4–5). Analysis of the Camp Century sub-glacial material indicates a diverse stratigraphy preserved below the ice that recorded episodes of glaciated and deglaciated conditions in northwestern Greenland. Our physical, geochemical, and mineralogic analyses reveal a history of deposition, weathering, and sediment transport preserved under the ice and show the promise of sub-glacial materials to increase our knowledge of past ice sheet behavior over time. 

Abstract We present new data from the debris-rich basal ice layers of the NEEM ice core (NW Greenland). Using mineralogical observations, SEM imagery, geochemical data from silicates (meteoric¹⁰Be, εNd,⁸⁷Sr/⁸⁶Sr) and organic material (C/N, δ¹³C), we characterize the source material, succession of previous glaciations and deglaciations and the paleoecological conditions during ice-free episodes. Meteoric¹⁰Be data and grain features indicate that the ice sheet interacted with paleosols and eroded fresh bedrock, leading to mixing in these debris-rich ice layers. Our analysis also identifies four successive stages in NW Greenland: (1) initial preglacial conditions, (2) glacial advance 1, (3) glacial retreat and interglacial conditions and (4) glacial advance 2 (current ice-sheet development). C/N and δ¹³C data suggest that deglacial environments favored the development of tundra and taiga ecosystems. These two successive glacial fluctuations observed at NEEM are consistent with those identified from the Camp Century core basal sediments over the last 3 Ma. Further inland, GRIP and GISP2 summit sites have remained glaciated more continuously than the western margin, with less intense ice-substratum interactions than those observed at NEEM.