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.

ABSTRACT. The deep ice-sheet coring (DISC) drill consists of four major mechanical drilling subsystems and four subsystems supporting on-surface activities. The mechanical drilling subsystems are a drill sonde, a drill cable, a tower and a winch. The drill sonde is the down-hole portion of the drill system and consists of six distinct sections: (1) the cutter head, (2) the core barrel, (3) the screen section, (4) the motor/pump section, (5) the instrument section and (6) the upper sonde, which includes anti-torques and drill cable terminations. The drill cable not only provides the means of supporting the drill sonde in the borehole, but also provides conduits for electrical power and data transmission. The tower tilts to allow the drill sonde to be serviced in the horizontal position without removing it from the tower. The winch provides a means of quickly raising the sonde from the borehole and providing the fine control necessary for coring operations.

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The Deep Ice Sheet Coring (DISC) drill, developed by Ice Coring and Drilling Services (ICDS) under contract with the US National Science Foundation, is an electromechanical drill designed to take 122 mm diameter ice cores to depths of 4000 m. The conceptual design of the DISC drill was developed in 2002/03 based on science requirements written by K. Taylor and the United States ice-coring community and on engineering performance objectives. Detailed design of the drill began in June 2003. Special attention was paid to building safety into the design and operation of the drill system. The drill was designed and manufactured by a team of engineers and technicians from the University of Wisconsin–Madison and various subcontractors with assistance from the science community, the European ice-drilling community and polar logistical support organizations. ICDS successfully tested the drill in Greenland in 2006 and will continue its development to meet the science objectives of the West Antarctic Ice Sheet Divide Ice Core Project.

ABSTRACT. The deep ice-sheet coring (DISC) drill was used for production ice-core drilling at WAIS Divide in Antarctica for six field seasons between 2007 and 2013. Continuous ice-core samples were obtained between the snow surface and 3405 m depth. During the 2012/13 austral summer, the DISC drill's newly designed replicate ice-coring system was utilized to collect nearly 285 m of additional high-quality core samples at depths of high scientific interest. Annual progress graphs are described, as well as milestones achieved over the course of the project. Drilling operations, challenges encountered, drill fluid usage, drilling results, and the drill crew's experiences with the DISC drill and replicate coring system during production drilling are described and discussed in detail. Core-processing operations are described briefly, as well as the logistical undertaking of the DISC drill's deployment to Antarctica.