An astronomically calibrated 40Ar/39Ar age for the North Atlantic Z2 Ash: Implications for the Greenland ice core timescale

Timescales based on counted annual ice layers underpin the interpretation of late Quaternary rapid climatic oscillations recorded in the Greenland ice cores and their correlation to tropical and Antarctic climate archives. An inherent problem in the annual counted ice layer dating method is the large accumulated age uncertainties for the older part of the Greenland ice core timescale due to so called uncertain annual ice layers. Annual ice layers become thinner with depth due to increased compaction. Thus, the occurrence of uncertain annual layers becomes more frequent, such that by a depth of similar to 2150 m (43 ka) in the NGRIP ice core the accumulated uncertainty on the age is > 2%. Radio-isotopic age determination of interbedded volcanic ash layers can provide independent verification of the Greenland counted annual ice layer ages along with the possibility of improved precision for the older part of this important timescale. Here we report an astronomically calibrated 40Ar/39Ar age of 56.14 +/- 0.44 ka (2 sigma) for the widespread North Atlantic Z2 ash forming Icelandic Thorsmo euro rk peralkaline rhyolitic eruption. The Z2 ash is found at a depth of 2359.45 m in the NGRIP ice core and has an annual layer counted age of 55.38 +/- 2.37 ka (2 sigma, b2k). Our radio-isotopic age for the Z2 eruption is > 5 times more precise than the one based on counted annual ice layers and when referenced relative to b2k indicates that the oldest part of the GICC05 timescale is systematically too young by approximately 740 years. We use our Z2 eruption age to calculate refined astronomically calibrated ages for Greenland Interstadials (GI) 11 to 17 which results in reducing the age uncertainties on the majority of these events from the millennial to the centennial level. Our refined ages for GI 11-17 generally show good agreement with the NALPS U-Th stalagmite record that covers this same interval and together these high-precision radio-isotopic time -scales provide firmer constraints for testing for synchronicity, or leads and lags between different geographical parts of the late Pleistocene climate system. (C) 2022 The Authors. Published by Elsevier Ltd.

Automated summary

The age uncertainties of the Greenland ice core timescale are inherently problematic, and radio-isotopic age determination of volcanic ash layers can provide independent verification and improved precision for the older part of the timescale. The study found that the radio-isotopic age is more precise compared to the counted annual ice layers, indicating that the oldest part of the timescale is systematically too young by approximately 740 years. These results provide firmer constraints for testing synchronicity or leads and lags in the late Pleistocene climate system.