An ice core is a cylindrical sample recovered by drilling into a glacier or ice sheet. It preserves accumulated snowfall, chemical impurities, and trapped air, providing evidence of past climate and environmental conditions. Cores from Antarctica, Greenland, and mountain glaciers are important archives in paleoclimatology. Their distinctive value is that they contain both indirect indicators of past conditions and actual samples of the ancient atmosphere. (legacy.bas.ac.uk)
Formation of the archive
Where snowfall persists from year to year, successive deposits bury older snow. Compaction and recrystallization transform snow into firn, a porous intermediate material, and eventually into dense glacier ice. While pores remain connected, air can circulate and diffuse through them. As compaction closes these passages, air becomes isolated in bubbles. Bubble closure commonly occurs tens of metres below the surface, with depth depending on local conditions. (bas.ac.uk)
The ice and its enclosed air therefore have different ages. The ice dates from the original snowfall, whereas the air was trapped later and is generally younger. This gas-age–ice-age difference depends especially on temperature and snow accumulation. Establishing separate chronologies for ice and gas is essential when comparing atmospheric changes with climatic events. (courses.washington.edu)
Accumulation also affects temporal resolution. High-snowfall sites can preserve relatively thick annual layers, allowing detailed reconstructions of short-lived events. Low-accumulation sites may contain much longer records within a given ice thickness, but individual years become harder to distinguish. Burial and ice deformation further thin older layers. (cp.copernicus.org)
Drilling, handling, and analysis
Electromechanical coring drills recover ice in successive cylindrical sections. Different systems serve shallow, intermediate, and deep investigations; one established drill design produces cores 104 millimetres in diameter from depths of approximately 400 metres. Deep boreholes commonly require drilling fluid to limit closure and maintain core quality. Fluid entering cracks can contaminate samples, making contamination checks part of laboratory analysis. (icedrill.org)
Recovered sections are documented, transported frozen, and divided among analytical programmes while material is retained for future work. Long-term cold storage preserves these finite archives: the United States National Science Foundation Ice Core Facility maintains its storage freezer at −36°C. Archived cores can subsequently be investigated with improved techniques or new scientific questions. (icedrill.org)
Laboratory methods include chemical measurements, isotope analysis, particle detection, and gas extraction. In continuous-flow analysis, a section is melted progressively so that multiple properties can be measured along its depth. Gas laboratories use dry extraction or melt–refreeze procedures, together with chromatography and spectroscopy, to determine concentrations and isotopic compositions. (bas.ac.uk)
Information preserved in ice
The relative abundances of isotopes in water, particularly oxygen and hydrogen isotopes, provide indicators of past climatic conditions. These measurements are climate proxies, rather than direct thermometer readings: interpreting them requires consideration of the processes that influence precipitation and its isotopic composition. diffusion within firn can smooth the original isotope signal, reducing the preservation of very rapid variations. (bas.ac.uk)
Trapped air permits direct measurement of past carbon dioxide, methane, and other greenhouse gases. This distinguishes atmospheric gas records from proxies that infer conditions indirectly. Measurements of gas concentrations and isotopic composition help investigate interactions between climate and the carbon cycle. (bas.ac.uk)
Impurities record additional environmental processes. Dust, chemical species, and volcanic material provide evidence concerning terrestrial dust sources, atmospheric transport, sea ice, biological activity, and pollution. Sulfate deposition associated with volcanic eruptions can also supply chronological markers, linking environmental evidence with the dating of a core. (bas.ac.uk)
Dating and synchronization
An ice-core chronology assigns ages to measured depths. Where seasonal variations remain distinguishable, researchers count annual layers using visual features and chemical signals. Recognizable volcanic deposits provide reference horizons and allow different cores to be synchronized. These methods can be combined with numerical ice-flow models to develop an age–depth relationship. (legacy.bas.ac.uk)
Dating precision varies within a core. Thin layers, weak seasonal signals, and years of very low accumulation can prevent reliable annual counting. Matching a core to another, better-resolved record may therefore provide a stronger chronology than independent counting alone. Shared volcanic signals are particularly valuable for comparing Greenland and Antarctic records without assuming that their temperature changes occurred simultaneously. (cp.copernicus.org)
Major records and research applications
Greenland cores document repeated abrupt warming events during the last glacial period, including temperature increases exceeding 10°C within about 40 years. Antarctic records show a different pattern, helping researchers investigate interhemispheric relationships and the role of ocean circulation in climate change. Such comparisons provide evidence for testing mechanisms represented in climate models. (legacy.bas.ac.uk)
The European Project for Ice Coring in Antarctica, or EPICA, established an approximately 800,000-year record at Dome C. In January 2025, Beyond EPICA completed a roughly 2,800-metre core at Little Dome C. Subsequent analysis reported a continuous sequence estimated to extend beyond 1.2 million years, although detailed age determination remained dependent on integrating the analytical data. The extension targets the Mid-Pleistocene Transition, when dominant glacial-cycle durations shifted from roughly 41,000 to 100,000 years. (bas.ac.uk)
The age of individual ice samples should not be confused with the duration of a continuous record. At Little Dome C, the deepest basal ice is heavily deformed and may include mixed or refrozen material. Such sections require separate interpretation rather than automatic treatment as an undisturbed chronological continuation. (bas.ac.uk)