Talk by Christo Buizert, Centre for Ice and Climate – Niels Bohr Institute - University of Copenhagen

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Niels Bohr Institute > Calendar > 2011 > Talk by Christo Buizer...

Talk by Christo Buizert, Centre for Ice and Climate

Cosmogenic in situ C-14 production and ice flow at the Taylor Glacier Blue Ice Area, Antarctica.

Abstract
The C-14 signature of gases trapped in glacial ice contains a wealth of information. Potentially it could be used for determining the age of the gas through radiocarbon dating. Second, it bears the imprint of the fossil contribution to paleo-atmospheric budgets of carbon containing gas species. In particular, the C-14 variations in methane (CH4) over the last glacial termination can teach us how much destabilization of (C-14 depleted) methane clathrates contributed to the CH4 budget. The large sample size (>500 kg) required for  accurate methane C-14 measurements rule out the use of traditional ice core samples.

Old ice can not only be obtained from deep ice cores, but also at ice margins and Antarctic blue ice areas (BIAs) where it is being re-exposed by ablation. For paleoclimate studies this provides an interesting alternative to ice coring, as sample retrieval is less challenging both from a technological and a logistical point of view. Taylor Glacier is an outlet glacier of the East Antarctic Ice Sheet that originates at Taylor Dome, and terminates in the McMurdo dry valleys. In the ablation zone ice with estimated ages between 11.5 and 65 kyr is being exposed. The ice stratigraphy along the centre flow line can be dated by comparison of trace gas mixing ratios and gas stable isotope measurements to ice core records, and comparison of water stable isotopes to the Taylor Dome record. The large ice samples required for high precision C-14 measurements of trace gas species such as CH4 and carbon monoxide (CO) can easily be mined from near the glacier surface.

Efforts to interpret C-14 data are complicated by in situ production in ice by cosmogenic radiation. Because measurable amounts of cosmogenic C-14 are produced down to a depth of  ~200 m, the exposure history of ablating ice parcels is a function of their flow path in the glacier. The aim of this study is to combine 2-D flow line modeling with up-to-date cosmogenic exposure theory to obtain a best estimate for the C-14 activity of ablating Taylor Glacier ice. We present two ways to parameterize vertical strain rates with depth, and we evaluate which method provides the better description for Taylor Glacier. The model allows us to assess the influence of glacier valley topography and solar modulation of cosmic ray intensity on the in situ production; both of which have been neglected in studies so far.

The work presented here is a part of an ongoing campaign at Taylor Glacier to reconstruct the C-14 activity of atmospheric methane over the last deglaciation, and to constrain cosmogenic production rates better. The model can aid us in determining a successful field sampling strategy, and can be used for interpretation of measurements. Most importantly, it will serve as a framework for correcting future C-14 of methane measurements for the effects of in situ production, which will allow determination of the true atmospheric signal.