CF4 and CO2 - coupling weathering and carbon cycle


Contact
Peter.Koehler [ at ] awi.de

Abstract

The analysis of CO2 and its stable carbon isotopes from ice cores revealed large changes of atmospheric CO2 related to changes in ocean circulation, marine biological processes and contributions from the terrestrial carbon storage. These processes dominate the glacial/interglacial CO2 variations. Yet, CO2 is also modulated by the marine alkalinity balance. The net alkalinity influx to the ocean is driven by silicate weathering drawing down atmospheric CO2. Conversely, alkalinity is lost when CaCO3 is buried in marine sediments. On orbital time scales, these fluxes are assumed to be almost balanced as atmospheric CO2 and its climatic effects feedback on the weathering rates providing a negative feedback loop. Trace elements from marine sediments are widely applied to derive weathering rates or changes in the weathering style for a certain region. Here, we use a novel approach to provide a global weathering estimate using the ppt-level trace gas CF4 archived in polar ice cores. CF4 is found as a trace gas in granites, and during weathering it escapes to the atmosphere. Because CF4 is inert in the lower atmosphere, its only sink is destruction by UV radiation 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. We developed a vacuum melt-extraction system for ice core samples to precisely measure these trace amounts of CF4 and applied it 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 also shows a pronounced shift toward higher CF4 levels after 430 kyr. With the beginning of MIS 11, we find a rise in CF4 that probably relates to intense weathering during the first full interglacial after a series of lukewarm interglacials. This dataset lends support to a strong positive coupling of continental weathering and climate.



Item Type
Conference (Keynote)
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Primary Division
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Peer revision
Not peer-reviewed
Publication Status
Published
Event Details
Goldschmidt Conference, 25 Aug 2013 - 30 Aug 2013, Florence, Italy.
Eprint ID
34103
Cite as
Schmitt, J. , Seth, B. , Köhler, P. , Willenbring, J. and Fischer, H. (2013): CF4 and CO2 - coupling weathering and carbon cycle , Goldschmidt Conference, Florence, Italy, 25 August 2013 - 30 August 2013 .


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