Orbital tuning of deep ice cores using O2/N2 of trapped air

Abstract:

The chronology of the first Dome Fuji deep ice core (80,000-340,000 yr BP) was established by orbital tuning of measured O₂/N₂ ratios in trapped air to the past local summer insolation at the drill site (Kawamura et al., 2007). The O₂/N₂ ratios found in ice cores are generally lower than atmospheric ratio because of size-dependent molecular fractionation during bubble close-off. The magnitude of this gas fractionation appears to be influenced by snow metamorphism when the layer was originally at the surface, which in turn is controlled by local summer insolation (Fujita et al., 2009). The O₂/N₂ record has little 100,000-yr periodicity (strongest in climatic records), suggesting insignificant climatic influence in the orbital tuning. Agreement of the O₂/N₂ chronology with U-Th radiometric chronology of speleothems (within ~2000 yr) suggests that O₂/N₂ and summer insolation are indeed in phase. However, it may not be common to all ice cores that O₂/N₂ signal only records local summer insolation. For example, the GISP2 ice core (Greenland) has clear imprint of abrupt climate changes in the O₂/N₂ record, indicating climatic (non-insolation) signal in the record and the possibility of phase variability of O₂/N₂ relative to the past insolation (Suwa and Bender, 2008).

Here we present new O₂/N₂ record from the second Dome Fuji ice core with significant improvements in ice core storage practice and mass spectrometry. In particular, the ice core had been stored at about -50 ˚C until the air extraction except during transportations, which prevent fractionation due to gas loss during the core storage. The precision of the new O₂/N₂ data set is improved by a factor of 3 over the previous data, and we do not observe outliers (there were 15% outliers in the previous data). Clear imprint of local insolation is recognizable in the new O₂/N₂, which would enable us to generate a chronology with accuracy of ~2000 yr towards older periods. Samples from the first core after long storage (>10 yr) at -25 ˚C and -50 ˚C were measured as well, and the data show clear dependence of O₂/N₂ depletion on the storage temperature. We also plan to measure the NEEM ice core (Greenland) for the Eemian and the last glacial period in order to investigate the feasibility of O₂/N₂ tuning for the Greenland ice cores.