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AROUND 11,500 YEARS AGO the methane content of the atmosphere increased by about 50% in as little as 200 years. Scientists who study the natural variability of methane are especially interested in this event because it coincides with a period of rapid climate reorganization known as the Younger Dryas to Preboreal transition (YD-PB). In order to study the YD-PB and other events like it, scientists have used deep ice cores from Greenland and Antarctica. Because these cores are only about 5 inches in diameter, the amount of ice available to make measurements of extremely rare gases or special isotopic tracers is limited. These difficult measurements are important because they can tell scientists why gases such as methane have varied in the past. For reference, the YD-PB transition in methane in the Dome C ice core in East Antarctica is at about 400 meters below the surface. At WAIS Divide, West Antarctica, the transition will likely be at ~2000 meters depth. For most other ice cores ice from this time period occurs somewhere between these two depths. The total amount of YD-PB ice ever recovered by all deep ice cores is approximately 2,000 kg. While that may sound like a lot of ice, it is only a "drop in the bucket" for scientists looking to make ambitious measurements such as the radiocarbon content of methane, which requires a 1,000 kg sample. To make measurements like this, which are either impossible or very difficult in traditional deep ice cores, scientists have begun looking for areas of ablation on the ice sheet where glacier flow has exposed old ice on the surface; often referred to as blue ice zones. In these regions the flow of the ice combined with ice removal by sun and wind have done the work of the drill and brought the old ice to the surface. One group who pioneered work on ancient air from ablation zones in Greenland recently completed a successful two-month season working on a new site on the Taylor Glacier in Antarctica. The science team, led by co-PIs Jeff Severinghaus (Scripps Institution of Oceanography) and Ed Brook (Oregon State University) included Vasilii Petrenko (University of Rochester), Hinrich Schaefer (National Institute of Water and Atmospheric Research in New Zealand), Daniel Baggenstos (SIO), Thomas Bauska and James Lee (OSU). Tanner Kuhl and Robb Kulin (Ice Drilling Design and Operations) led the drilling operation and Paul Rose provided invaluable science support.
To understand the stratigraphy of the site the team made over 300 methane measurements in the field. By comparing the methane variations found in the Taylor Glacier ice with the atmospheric history of methane known from deep ice cores, the team was able to identify layers of ice spanning the last glacial termination (~20,000 to 8,000 yrs BP). This work resulted in the identification of the YD-PB layer in a steeply dipping four-meter layer of ice directly underfoot. This outcropping layer can be followed up and down the glacier for hundreds of meters, essentially yielding an unlimited amount of ice.
The main focus of this past season's work, however, was to recover air samples from much older ice (~50,000 years old) to understand how carbon-14 is produced in ice by cosmic rays. These samples were drilled with the recently developed Blue Ice Drill (BID) designed by IDDO of the University of Wisconsin. This drill, capable of recovering a 24 cm diameter core with a length of >1.0 m, drilled a total of 580 meters of core from about 30 different holes. Volumetrically, ~26 cubic meters of ice were drilled. For comparison, the entire 3300 m WAIS Divide core volume is ~39 cubic meters. The 1,000 kg ice samples selected for carbon-14 measurements were melted and the air was extracted on site.
The team was also able to recover ample amounts of ice that will allow for very high-precision measurements of the stable isotopes of carbon dioxide and methane during the last glacial termination. Daniel Baggenstos lead a heroic effort to chainsaw a 40 meter trench in order to recover samples for dust, trace element analysis and water isotopes across the initiation of the last deglaciation. Reconnaissance samples were taken to investigate the possible presence of Eemian age ice (~125,000 years ago) and the stratigraphy of ice older than the last glacial maximum (~20,000 years ago).
Next season, the team will sample the YD-PB for carbon-14 of methane. These measurements will be used to test theories about the origin of rapid variations in methane during the last deglaciation. The team hopes to establish the Taylor Glacier as an ancient air and ice sample archive that can be utilized for future research.
This work is funded by the National Science Foundation under Grant Numbers ANT-0839031 (Scripps Inst. of Oceanography) and ANT-0838936 (to Oregon State University).