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A multimillennial Alpine ice core chronology synchronized with an accurately dated Arctic Pb record
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DOI：10.5194/cp-22-1037-2026.png)
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Abstract
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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 timescale (TC2016) based on absolute time markers such as a peak in 3H activity; from 210Pb dating; and 14C dating of carbonaceous particles and organic remains provided evidence of one of the oldest Alpine ice core records; extending back to the last Northern Hemisphere Climatic Optimum and spanning the last ∼ 7000 years. Here we present additional time markers that corroborate the multimillennial nature of the Alto dell'Ortles ice cores and significantly reduce 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 δ18O and dust annual variations to refine the dating for the 20th 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 with Markov chain Monte Carlo simulation (COPRA model). CP2025/1 then served as the basis for temporal synchronization of a novel Pb concentration record obtained from the Alto dell'Ortles cores to a well-dated (±5 years) Pb record from an array of Arctic ice cores (AN); with synchronisation performed for the period from 175 BCE to 1755 CE. Possible ties for matching the two Pb records were thereby constrained by the requirement that resulting age shifts remained within the range of overlap between the 1–2σ dating uncertainty of CP2025/1 and the uncertainty estimates of the selected tie-points (1–2σ; in the ancient part; 1σ; 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. Most importantly for this study; the synchronization resulted in a further refined; final timescale with a strongly reduced dating uncertainty (CP2025/2). Investigation of CP2025/2 using a simple 1-D ice flow model suggests that non-steady-state conditions; particularly changes in net accumulation rates; must be considered to fully explain the obtained age-depth relationship. The new Alto dell'Ortles CP2025/2 chronology of improved accuracy and precision will allow to constrain Central European Holocene climatic and environmental histories emerging from this high-altitude glacial archive. 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 which typically suffer from larger dating uncertainties when compared to well-dated polar records.
Keywords:
Alpine ice core
chronology
lead (Pb) record
radiocarbon dating
ice flow model
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