Evaluating the accuracy of the Greenland Ice-Core Chronology (GICC)

The full potential of palaeoclimate data relies on reliable time scales, i.e., a relation tying the physical dimension of the palaeoclimate archive with age. For many records, including ice cores, identification and counting of annual layers is the most direct and accurate way to obtain a time scale provided that high-resolution measurements of parameters showing annual variability are available. Annual-layer counting can provide very precise estimates of event durations and rates of change, but as errors accumulate with age, the accuracy decreases with depth/time. In contrast, radiometric methods often have good accuracy, and in principle, the two approaches can be combined to form highly accurate and precise time scales provided that the archives and their time scales can be robustly aligned. This can be done based on e.g. volcanic markers, common and correlatable features in cosmogenic isotope records, or on climatic wriggle-matching when the possible leads and lags between records is considered.

The Central Greenland ice cores are drilled in the interior areas of the ice sheet where precipitation rates are appropriate for the formation and preservation of annual layers, thereby allowing annual layers to be identified in the Holocene period and well into the last glacial period. The Greenland Ice-Core Chronology (GICC) is an attempt to derive a consistent, common time scale for the Greenland ice cores by combining data from multiple cores, using for each time period all available annually resolved data and then applying the time scale to the other cores by means of matching patterns of volcanic eruptions and other reference horizons. In this way, data from all the ice cores can be interpreted together on a common time scale (i.e., with very small relative dating uncertainty), greatly reducing the risk of artificial offsets due to misinterpretation of individual records. The first version of GICC, named GICC05, was published in 2006 and 2008, where the dating covered the time period back to 60 ka b2k, at which point the layers had thinned too much to continue with continuous annual layer counting.

Since then, high-resolution data from the newer Greenland ice cores NEEM and EGRIP have appeared, and comparisons to other palaeoclimate records on radiometric time scales have shown that in some sections, GICC05 was not as accurate as initially estimated, motivating a revision on the time scale. The top 3.8 ka of the time scale was recently revised, leading to changes in age of 10-15 years for the section older than 2500 years (see presentation by Giulia Sinnl). Here, we review the older sections of GICC05 in the context of other well-dated palaeoclimate archives and cosmogenic isotope records, as well as model-based estimates of climate leads and lags relevant when aligning climate records, and outline a plan for how to continue the revision of GICC05.