Atmospheric CF4 trapped in polar ice – A new proxy for granite weathering

Peter.Koehler [ at ]


The reconstruction of continental weathering rates using trace elements and their isotopes measured on marine sediments and crusts is a vigorously growing field. Here, we use a novel approach using the ppt-level trace gas CF4, tetrafluoromethane, which can be analysed in air trapped in ice cores. CF4 is a trace impurity in granites and other plutonic rocks, and during weathering this gas escapes into the atmosphere. In preindustrial times, this release from granitic rocks was the only natural source of CF4. Because CF4 is inert to destruction processes in the tropo- and stratospheres, its only sink is destruction by UV radiation and radicals in the mesosphere. This chemical inertness is responsible for an exceptionally long atmospheric lifetime which is expected to range between 50 kyr and 400 kyr. Althoug the globally integrated CF4 emission flux from weathering is only a few tons per year, the exceptionally long lifetime allows to establish a long-term atmospheric concentration of about 33 ppt. We developed a vacuum melt-extraction system for ice core samples coupled to a mass spectrometry detector to precisely measure these trace amounts of CF4 found in past atmosphere and applied this method to ice over the entire Dome C ice core. During the last 800 kyr, atmospheric CF4 varied in a narrow band between 31 ppt and 35 ppt, i.e. only 10-15 % variability, providing a first estimate of the long-term weathering rate fluctuations. Our record shows that CF4 increases during interglacials and falls during the coldest, glacial phases. However, our CF4 record shows also a pronounced shift toward higher CF4 levels after 430 kyr (the Mid-Brunhes Event). With the beginning of Marine Isotope stage 11, we find a steep rise in CF4 that probably relates to intense weathering during the first full interglacial after a series of lukewarm interglacials.

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Conference (Invited talk)
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Goldschmidt Conference, 25 Aug 2013 - 30 Aug 2013, Florence, Italy.
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Schmitt, J. , Seth, B. , Köhler, P. , Willenbring, J. and Fischer, H. (2013): Atmospheric CF4 trapped in polar ice – A new proxy for granite weathering , Goldschmidt Conference, Florence, Italy, 25 August 2013 - 30 August 2013 .

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