The last glacial period exhibited abrupt Dansgaard-Oeschger climatic oscillations, evidence of which is preserved in a variety of Northern Hemisphere palaeoclimate archives. Ice cores show that Antarctica cooled during the warm phases of the Greenland Dansgaard-Oeschger cycle and vice versa, suggesting an interhemispheric redistribution of heat through a mechanism called the bipolar seesaw. Variations in the Atlantic meridional overturning circulation (AMOC) strength are thought to have been important, but much uncertainty remains regarding the dynamics and trigger of these abrupt events. Key information is contained in the relative phasing of hemispheric climate variations, yet the large, poorly constrained difference between gas age and ice age and the relatively low resolution of methane records from Antarctic ice cores have so far precluded methane-based synchronization at the required sub-centennial precision. Here we use a recently drilled high-accumulation Antarctic ice core to show that, on average, abrupt Greenland warming leads the corresponding Antarctic cooling onset by 218 ± 92 years (2σ) for Dansgaard-Oeschger events, including the Bølling event; Greenland cooling leads the corresponding onset of Antarctic warming by 208 ± 96 years. Our results demonstrate a north-to-south directionality of the abrupt climatic signal, which is propagated to the Southern Hemisphere high latitudes by oceanic rather than atmospheric processes. The similar interpolar phasing of warming and cooling transitions suggests that the transfer time of the climatic signal is independent of the AMOC background state. Our findings confirm a central role for ocean circulation in the bipolar seesaw and provide clear criteria for assessing hypotheses and model simulations of Dansgaard-Oeschger dynamics.

We present successful 81 Kr-Kr radiometric dating of ancient polar ice. Krypton was extracted from the air bubbles in four ∼350-kg polar ice samples from Taylor Glacier in the McMurdo Dry Valleys, Antarctica, and dated using Atom Trap Trace Analysis (ATTA). The 81 Kr radiometric ages agree with independent age estimates obtained from stratigraphic dating techniques with a mean absolute age offset of 6 ± 2.5 ka. Our experimental methods and sampling strategy are validated by (i) 85 Kr and 39 Ar analyses that show the samples to be free of modern air contamination and (ii) air content measurements that show the ice did not experience gas loss. We estimate the error in the 81 Kr ages due to past geomagnetic variability to be below 3 ka. We show that ice from the previous interglacial period (Marine Isotope Stage 5e, 130-115 ka before present) can be found in abundance near the surface of Taylor Glacier. Our study paves the way for reliable radiometric dating of ancient ice in blue ice areas and margin sites where large samples are available, greatly enhancing their scientific value as archives of old ice and meteorites. At present, ATTA 81 Kr analysis requires a 40-80-kg ice sample; as sample requirements continue to decrease, 81 Kr dating of ice cores is a future possibility.geochronology | paleoclimatology | glaciology

Carbon-14 (14 C) is incorporated into glacial ice by trapping of atmospheric gases as well as direct near-surface in situ cosmogenic production. 14 C of trapped methane (14 CH 4) is a powerful tracer for past CH 4 emissions from "old" carbon sources such as permafrost and marine CH 4 clathrates. 14 C in trapped carbon dioxide (14 CO 2) can be used for absolute dating of ice cores. In situ produced cosmogenic 14 C in carbon monoxide (14 CO) can potentially be used to reconstruct the past cosmic ray flux and past solar activity. Unfortunately, the trapped atmospheric and in situ cosmogenic components of 14 C in glacial ice are difficult to disentangle and a thorough understanding of the in situ cosmogenic component is needed in order to extract useful information from ice core 14 C. We analyzed very large (≈1000 kg) ice samples in the 2.26-19.53 m depth range from the ablation zone of Taylor Glacier, Antarctica, to study in situ cosmogenic production of 14 CH 4 and 14 CO. All sampled ice is > 50 ka in age, allowing for the assumption that most of the measured 14 C originates from recent in situ cosmogenic production as ancient ice is brought to the surface via ablation. Our results place the first constraints on cosmogenic 14 CH 4 production rates and improve on prior estimates of 14 CO production rates in ice. We find a constant 14 CH 4 / 14 CO production ratio (0.0076 ± 0.0003) for samples deeper than 3 m, which allows the use of 14 CO for correcting the 14 CH 4 signals for the in situ cosmogenic component. Our results also provide the first unambiguous confirmation of 14 C production by fast muons in a natural setting (ice or rock) and suggest that the 14 C production rates in ice commonly used in the literature may be too high.

Many of the ice-coring objectives in the Ice Drilling Program Office (IDPO) Long Range Science Plan, such as those in the International Partnerships in Ice Core Sciences (IPICS) 2k array and 40k network, are attainable in many locations with an intermediate depth drill (IDD) that can collect core from a fluid-filled hole down to 1500 m depth. The Ice Drilling Design and Operations (IDDO)group has designed and is in the process of building an agile IDD to meet this objective. The drill tent, power distribution and core-processing systems are an integral part of the IDD, which can be deployed by small aircraft and assembled by hand to minimize logistic requirements. The new drill system will be ready for testing in Greenland beginning in late spring 2014. The first production drilling is scheduled for the 2014/15 field season at the South Pole.

A new drilling system was developed by the US Ice Drilling Program (IDP) to rapidly drill through overlying ice to collect subglacial rock cores. The Agile Sub-Ice Geological (ASIG) Drill system is capable of drilling up to 700 m of ice in a continuous manner. Intermittent ice core samples can be taken as needed. Ten-plus meters of subglacial bedrock and unconsolidated, frozen sediment cores can be drilled with wireline core retrieval. The functionality of the drill system was demonstrated in 2016–17 at the Pirrit Hills, Antarctica where 8 m of high-quality, continuous granite core was retrieved beneath 150 m of ice. The particulars of the drill system development, features and performance are discussed.