A multimillennial Alpine ice core chronology synchronized with an accurately dated Arctic Pb record

Abstract. A low latitude-high altitude Alpine ice core record was obtained in 2011 from the glacier Alto dell'Ortles (3859 m, Eastern Alps, Italy). A preliminary absolute timescale (TC2016) based on a peak in ³H activity, and ²¹⁰Pb and ¹⁴C dating of carbonaceous particles and organic remains provided evidence of one of the oldest Alpine ice core records spanning the last ~7000 years, back to the last Northern Hemisphere Climatic Optimum. Here we provide an additional number of time markers that corroborate the multimillennial nature of the Alto dell'Ortles ice cores while significantly decreasing the uncertainty of the chronology. First, ¹⁴C dating of an additional organic fragment (a charred spruce needle) discovered next to the basal ice provided an age (232 ± 126 BCE) which agrees with previous ¹⁴C dates in the oldest part of the record. Second, novel seasonally resolved pollen records from the upper firn/ice portion of the Alto dell'Ortles cores were combined with d¹⁸O and dust annual variations to refine the dating for the 20^th century by means of an automatic algorithm (Straticounter; between 1927 and 2011 CE) and visual counting (from 1900 to 1926 CE). The new and previous time markers were combined into a revised intermediate timescale (CP2025/1) by fitting using Markov chain Monte Carlo simulation (COPRA model). CP2025/1 was used for synchronizing a novel Pb concentration record obtained from the Alto dell'Ortles cores with a Pb record from an array of Arctic ice cores (AN), well-dated (±5 years) for the ~200 BCE to ~1900 CE period. The ties used for matching the two Pb records were within the uncertainty of CP2025/1 and of the selected tie-points (1- to 2-sigma, in the ancient part; 1-sigma, in the recent part). The correlation obtained after synchronization is 0.44 (Pearson's r, p < 0.001), demonstrating that these two distant atmospheric Pb records share a large portion of their variability back to 200 BCE. The synchronization of CP2025/1 with AN resulted in a, further refined, final timescale (CP2025/2). An investigation of CP2025/1 and CP2025/2 by means of a simple 1-D flow model suggests that non-steady-state conditions, in particular changes in past net accumulation rates, need to be considered to provide a full physical explanation of the age-depth relationship obtained. The new Alto dell'Ortles CP2025/2 chronology of improved accuracy will allow to constrain Holocene climatic and environmental histories emerging from this high-altitude glacial archive of Central Europe. The novel combination of methodologies used may also be adopted to build, or improve, the chronologies of other ice cores extracted from-low latitude/high-altitude glaciers that typically suffer from larger dating uncertainties compared with well dated polar records.