The GICC05 time scale
A new stratigraphical timescale for the Greenland ice cores NGRIP, GRIP, and DYE-3 has recently been published by the Centre for Ice and Climate. The new timescale is called the Greenland Ice Core Chronology 2005 (GICC05) and reaches back to 60,202 b2k. The notation "b2k" was introduced together with the GICC05 timescale and means years before A.D. 2000.
The special thing about the GICC05 timescale is that it is based on many overlapping data series from the three different cores, using for each time period the best available data and the most appropriate dating approach. Overlaps between the different sections ensure consistency between the different parts of the timescale.
Back to 7,900 b2k, the main data set used is the DYE-3 stable isotope data set. δ18O measurements comprise the majority of this data set, supplemented by new δD measurements. The annual layers are defined from the annual peaks in the δ18O and δD data. In the upper few thousand years, δ18O data from NGRIP and GRIP complement the DYE-3 data, but due to their lower resolution, DYE-3 data remains the main basis.
From 7,900 to 10,276 b2k, the best available data for annual layer identification are GRIP impurity data. Calcium and Ammonium data are supplemented by short sections of δ18O data. The formal tie-point between the sections based on DYE-3 and GRIP data, respectively, is a distinct volcanic layer at 8,236 b2k.
At 10,276 b2k, the NGRIP Continuous Flow Analysis data set starts. Together with measurements of the ice conductivity and visual stratigraphy, the CFA data set provides an impressive data set of up to 9 parallel data series with a strong annual signal. The Younger-Dryas-Holocene boundary is dated to 11,703 b2k, and the transition from the glacial into the Bølling interstadial takes place at 14,692 b2k. The annual layer identification has to date been continued to 60,202 b2k.
In the Holocene, the records from the three cores have been matched using the conductivity records (ECM), so that the timescale is common to all three cores. In the glacial parts, a similar technique has been used to transfer the GICC05 timescale to the GRIP and GISP2 records.
You can also read more about how GICC05 was created in the published papers that describe the different section of the timescale:
The dating of the period from today back to 7.9 ka b2k (thousand years before A.D. 2000) is based on high resolution measurements of stable isotopes in the DYE-3, GRIP and NGRIP ice cores. The results were published by Vinther et al. in Journal of Geophysical Research (opens in new window).
- The transition from the last glacial to the present interglacial.
The dating of the period between 7.9 ka b2k and the first abrupt warming after the cold glacial (the Bølling period) 14.7 ka b2k is based on high resolution measurements of chemical impurities, conductivity of the ice and visual stratigraphy from the GRIP and NGRIP ice cores. The results have been published in Journal of Geophysical Research by Rasmussen et al. (opens in new window).
- The last glacial back to 42 ka b2k.
The dating of the period from 14.7 to 42 ka b2k is based on the visual stratigraphy and measurements of the ice conductivity. The high resolution chemistry measurements are also used, but when the annual layers become thinner than about 1.5 cm they are hard to distinguish in the chemical measurements. The dating of this period and comparisons with comparable timescales were published in Quaternary Science Reviews by Andersen et al. and Svensson et al. (opens in new windows).
- The period of the last glacial from 42 to 60 ka b2k.
This is the oldest Greenland ice core section ever dated by annual layer counting. The results were published in Climate of the Past by Svensson et al.