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

Abstract

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.

Magnitude, frequency and climate forcing of global volcanism during the last glacial period as seen in Greenland and Antarctic ice cores (60–9 ka)

“…The list of NH volcanism contains only volcanic deposits larger than 20 kg km −2 in any of the applied Greenland ice cores, and the list containing the Southern Hemi-sphere includes all deposition events larger than 10 kg km −2 for the Antarctic ice cores, which biases our list to larger eruptions. This cutoff is necessary because of the highly variable sulfate background in the Greenland cold periods of the last glacial predominantly associated with mineral dust (e.g., gypsum; Svensson et al, 2000) (Table S2). A sulfate deposition of 20 kg km −2 corresponds to half the Greenland deposition from the 1815 CE Tambora eruption (Sigl et al, 2015), and thus only quite large events in terms of total sulfur injections into the atmosphere are detected in this work.…”

“…2007; Robock, 2000). Large volcanic eruptions injecting sulfuric gases into the stratosphere and forming sulfate aerosols have a global or hemispheric cooling effect of several degrees lasting for several years after the eruption (Sigl et al, 2015;Sinnl et al, 2021).…”

High resolution aerosol data from the top 3.8 ka of the EGRIP ice core

“…The high temporal resolution in turn can enable the generation of annual-layer counted age models from the aerosol records (e.g. Sinnl et al, 2021). Aerosol records of both shallow and deep ice cores are routinely measured at high depth resolution using continuous melting and analysis setups, i.e., using so-called continuous flow analysis (CFA) (e.g.…”

“…We also provide a discussion of the uncertainties related to these changes covering analytical precision, depth-assignment and resolution. This paper accompanies the publication of the revision of the Greenland ice core chronology (GICC21) presented in Sinnl et al (2021), that makes extensive use of the data sets shown and discussed here.…”

“…The error of absolute depth assignment of the data may be on the order of 1 cm, however relative depth offsets between the records of the individual species is only on the order of 1 mm. The presented data has sub-annual resolution over the entire depth range and has already formed part of the data for an annually layer-counted time scale for the EGRIP ice core used to improve and revise the multi-core Greenland ice-core chronology (GICC05) to a new version,GICC21 (Sinnl et al, 2021).The data is available in full 1-mm resolution and decadal averages on PANGAEA (…”