Abstract
Abstract. A recent coherent chronology has been built for four Antarctic ice cores and the NorthGRIP (NGRIP) Greenland ice core (Antarctic Ice Core Chronology 2012, AICC2012) using a Bayesian approach for ice core dating (Datice). When building the AICC2012 chronology, and in order to prevent any confusion with official ice core chronology, the AICC2012 chronology for NGRIP was forced to fit exactly the GICC05 chronology based on layer counting. However, such a strong tuning did not satisfy the hypothesis of independence of background parameters and observations for the NGRIP core, as required by Datice. We present here the implementation in Datice of a new type of markers that is better suited for constraints deduced from layer counting: the duration constraints. Estimating the global error on chronology due to such markers is not straightforward and implies some assumption on the correlation between individual counting errors for each interval of duration. We validate this new methodological implementation by conducting twin experiments and a posteriori diagnostics on the NGRIP ice core. Several sensitivity tests on marker sampling and correlation between counting errors were performed to provide some guidelines when using such a method for future dating experiments. Finally, using these markers for NGRIP in a five-core dating exercise with Datice leads to new chronologies that do not differ by more than 410 years from AICC2012 for Antarctic ice cores and 150 years from GICC05 for NGRIP over the last 60 000 years.
Highlights
The reference timescale for Greenland ice cores, GICC05, has been obtained by layer counting back to 60 ka
We focus on how this new version of Datice modifies the NGRIP and the four Antarctic ice core chronologies compared to AICC2012
The 1σ uncertainty of the GICC05 ice core is considered as half the maximum counting error (MCE). Following this approach means that errors for duration constraints at 40, 60 or 80 years will be derived by summing up the GICC05 20-year-window MCE 2, 3 and 4 times, respectively, in the case of full correlation within the time windows associated with the chosen sampling rate
Summary
The reference timescale for Greenland ice cores, GICC05, has been obtained by layer counting back to 60 ka (thousands of years before 1950; Vinther et al, 2006; Rasmussen et al, 2006; Andersen et al, 2006; Svensson et al, 2008) This chronology is absolute, with an increasing associated uncertainty with depth, reaching more than 2.6 at 60 ka. To force the NGRIP AICC2012 chronology to fit the GICC05 age scale, the modelled thinning function and accumulation rate of the GICC05 chronology (hereafter DJ– GICC05 scenarios; Vinther et al, 2006; Rasmussen et al, 2006; Andersen et al, 2006; Svensson et al, 2008) have been imposed as background scenarios in Datice. We focus on how this new version of Datice modifies the NGRIP and the four Antarctic ice core chronologies compared to AICC2012
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