Model timescales for the GRIP core

The first publication of the entire GRIP isotopic record used a model time-scale—now called ss09 (Dansgaard et al., 1993). The ss09 time-scale was based on a model for thinning of the annual layers and an empirical relation between the past surface snow accumulations and the isotopic values in the core. Further the ss09 time-scale was constrained by fixing the age of the end of YD and the Eemian to 11.5kyrBP and 110 ka, respectively; the first date was taken from the stratigraphical dating of the GRIP core and the second was based on information from other records.

The ss09 scale was later corrected for the influence of the iso-topic composition of the ocean water and was then called ss09sea (Johnsen et al., 2001). Recently a paper by Genty et al. (2003) on speleothems in France claims to be more accurate, but in my opinion the speleothem time-scale has not yet been verified sufficiently.

The main problem is that the various time-scales deviate several thousand of years over the last glacial stage, which is unsatisfactory because the abrupt and strong climatic changes demand a better time control in order to analyse the causal relationship between the various records. Whereas the dating of the Holocene is in principle solved with a sufficient accuracy, at least for some ice cores, the glacial chronology still needs to be scrutinized.

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