Scientific history, sampling approach, and physical characterization of the Camp Century sub-glacial sediment core, a rare archive from beneath the Greenland Ice Sheet
Authors: Bierman Paul R. , Christ Andrew J. , Collins Catherine M. , Mastro Halley M. , Souza Juliana , Blard Pierre-Henri , Brachfeld Stefanie , Courville Zoe R. , Rittenour Tammy M. , Thomas Elizabeth K. , Tison Jean-Louis , Fripiat Francois 
Published: January 2024

Abstract. Basal materials in ice cores contain information about paleoclimate conditions, glacial processes, and the timing of past ice-free intervals, all of which aid understanding of ice-sheet stability and its contribution to sea-level rise in a warming climate. Only a few ice cores have been drilled through ice sheets to the underlying sediment and bedrock, producing limited material for analysis. The Camp Century ice core, which the US Army drilled in northwest Greenland from 1960–1966 CE, recovered about 3.5 meters of sub-glacial sediment. Here, we document the scientific history of the Camp Century sub-glacial sediment, and present our recent core-cutting, sub-sampling, and processing methodology and results for what remains of this unique archive. In 1972, curators in the Buffalo Ice Core Laboratory cut the original core segments into 32 segments each about 10-cm long. Since then, two segments are unaccounted for, two were thawed, and two were cut as pilot samples in 2019. With the exception of the two thawed segments, the rest of the extant core remained frozen since collection. In fall 2021, we documented, described, and then cut each of the remaining frozen archived segments (n=26). We saved an archival half and then cut the working half into eight oriented sub-samples under controlled temperature and light conditions for physical, geochemical, isotopic, sedimentological, magnetic, and biological analyses. Our approach maximized sample usage for multi-proxy analysis, minimized contamination, and preserved archive material for future analyses of this legacy sample material. Grain size, bulk density, sedimentary features, magnetic susceptibility, ice content, as well as pore-ice pH and conductivity, suggest that the basal sediment contains five stratigraphic units. We interpret these stratigraphic units as representing different depositional environments in sub-glacial or ice-free conditions: from bottom to top, a diamicton with sub-horizontal ice lenses (Unit 1); vertically-fractured ice with dispersed fine-grain sediments (< 20 % in mass) (Unit 2); a normally graded bed of pebbles to very fine sand in an icy matrix (Unit 3); bedded very fine to fine sand (Unit 4); and stratified medium to coarse sand (Unit 5). Plant macrofossils are present in all samples and most abundant in Units 3 and 4; insect remains are present in some samples (Units 1, 3, and 5). Our approach provides a working template for future studies of ice-core basal sediment because it includes intentional planning of core sub-sampling, processing methodologies, and archiving strategies in order to optimize the collection of paleoclimate, glacial process, geochemical, geochronological, and sediment properties from archives of limited size. Our work benefited from a carefully curated and preserved archive, allowing for the application of techniques not available in 1966. Preserving uncontaminated core material for future analytical capabilities is an important consideration for rare archive materials such as these from Camp Century.