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Krypton and xenon in air trapped in polar ice cores : paleo-atmospheric measurements for estimating past mean ocean temperature and summer snowmelt frequency


Krypton and xenon are highly soluble noble gases. Because they are inert, they do not react biologically or chemically, and therefore can trace purely physical processes. By taking advantage of both the inert nature of these gases and their high solubilities, krypton and xenon can be used to reconstruct past ocean temperature variations and summer snow melt frequency. Ocean temperature is a fundamental parameter of the climate system. It plays a vital role in the transport and storage of heat, and may play a role in regulating atmospheric CO₂ , but its past variations are poorly constrained. This is due to the ambiguous nature of the benthic [delta]¹⁸O record in ocean sediments, which reflects both deep water temperature and the [delta]¹⁸O of the water itself (which depends on the extent of ice sheets on land). Recent studies have better constrained localized ocean temperature, but there is still need for global mean ocean temperature reconstructions. Krypton (Kr) and xenon (Xe) are highly soluble and more soluble in colder water. The total amount of Kr and Xe in the atmosphere and ocean together are essentially constant through time, so variations in mean ocean temperature would therefore modulate atmospheric Kr and Xe abundances. Kr and Xe, measured as ratios to nitrogen (N₂), are measured in air bubbles in ice cores to reconstruct atmospheric Kr/N₂ and Xe/N₂ histories, which can then be interpreted in terms of past mean ocean temperature. These Kr/N₂ and Xe/N₂ data and their derived mean ocean temperature (noble gas temperature index, NGTI) reconstructions are presented in Chapters 2 and 3. In Chapter 2, the initial Kr/N₂ data from the LGM indicate that mean ocean temperatures were 2̃.7°C colder at that time, which is consistent with other estimates of local deep ocean temperatures. In Chapter 3, [delta]Kr/N₂ and [delta]Xe/N₂ time series during the last glacial termination and inception are presented. The reconstructed mean ocean temperatures (NGTI's) are consistent with our earlier measurement and those of other studies. Additionally, these mean ocean temperature reconstructions appear to vary in step with atmospheric CO₂. Because Kr and Xe are highly soluble, they can also be used as an indicator of ice that has melted and refrozen. Visual identification of melt layers is been used as a proxy for exceptionally warm summers temperatures, but this type of melt layer identification becomes difficult as air bubbles form air clathrates at deeper depths. The use of Kr and Xe, measured as ratios to argon (Ar), is examined in Chapter 4. Seasononality may play a role in climate change, so a proxy of summer temperatures may prove to be a powerful constraint on climate change mechanisms that invoke seasonality

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