17 Nov 2021

17 Nov 2021

Review status: this preprint is currently under review for the journal CP.

A multi-ice-core, annual-layer-counted Greenland ice-core chronology for the last 3800 years: GICC21

Giulia Sinnl1, Mai Winstrup2, Tobias Erhardt3,4, Eliza Cook1, Camilla Jensen4, Anders Svensson1, Bo Møllesøe Vinther1, Raimund Muscheler5, and Sune Olander Rasmussen1 Giulia Sinnl et al.
  • 1Physics of Ice, Climate, and Earth, Niels Bohr Institute, University of Copenhagen, Denmark
  • 2DTU Space, National Space Institute, Technical University of Denmark, Kongens Lyngby, Denmark
  • 3Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, Bremerhaven, Germany
  • 4Climate and Environmental Physics & Oeschger Center for Climate Change Research, Physics Institute, University of Bern, Switzerland
  • 5Quaternary Sciences, Department of Geology, Lund University, Sweden

Abstract. Ice-core timescales are vital for the understanding of past climate; hence they should be updated whenever significant amounts of new data can contribute to improvements. Here, the Greenland ice-core chronology was revised for the last 3835 years by synchronizing six deep ice-cores and three shallow ice-cores from the central Greenland ice sheet. A layer-counting bias was found in all ice cores because of site-specific signal disturbances, and a manual comparison of all ice cores was deemed necessary to increase timescale accuracy. A new method was applied by combining automated counting of annual layers on multiple parallel proxies and manual fine-tuning. After examining sources of error and their correlation lengths, the uncertainty rate was quantified to be one year per century.

The new timescale is younger than the previous Greenland chronology by about 13 years at 3800 years ago. The most recent 800 years are largely unaffected by the revision, while the slope of the offset between timescales is steepest between 800 and 1000 years ago. Moreover, offset-oscillations of about 5 years around the average are observed between 2500 and 3800 years ago. The non-linear offset behavior is attributed to previous mismatches of volcanic eruptions, to the much more extensive data set available to this study, and to the finer resolution of the new ice-core matching.

In response to volcanic eruptions, averaged water isotopes and layer thicknesses from Greenland ice cores provide evidence of notable cooling lasting for up to a decade, longer than reported in previous studies of volcanic forcing. By analysis of the common variations of cosmogenic radionuclides, the new ice-core timescale is found to be in alignment with the IntCal20 curve. Radiocarbon dated evidence found in the proximity of eruption sites such as Vesuvius or Thera was compared to the ice-core dataset; no conclusive evidence was found regarding if these two eruptions can be matched to acidity spikes in the ice cores. A hitherto unidentified cooling event in the ice cores is observed at about 3600 years ago (1600 BCE), which could have been caused by a large eruption which is, however, not clearly recorded in the acidity signal. The hunt for clear signs of the Thera eruption in Greenland ice-cores thus remains elusive.

Giulia Sinnl et al.

Status: open (until 12 Jan 2022)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • AC1: 'Notification of edit in figure 10', Giulia Sinnl, 19 Nov 2021 reply

Giulia Sinnl et al.

Giulia Sinnl et al.


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Short summary
A new Greenland ice-core timescale, covering the last 3800 years, was produced by layer counting of ice core data, using the machine learning algorithm StratiCounter. By aligning ice core data on a common timescale, we were able to study the average impact of volcanic cooling in Greenland, finding that it can last for up to 10 years. Our new timescale has a better alignment with radiocarbon data, improving paleoclimate studies across the Northern Hemisphere.