Multimillennial synchronization of low and polar latitude ice cores by matching a time constrained Alpine record with an accurate Arctic chronology

Abstract. We present a novel application of empirical methodologies that significantly reduce the chronological uncertainty of a low latitude-high altitude Alpine ice core record obtained in 2011 from the glacier Alto dell’Ortles (3859 m, Eastern Alps, Italy). A preliminary absolute timescale based on a peak in ³H activity, and ²¹⁰Pb and ¹⁴C analyses on 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 combine three empirical methods that 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 analysis of an additional organic fragment (a charred spruce needle) discovered next to the basal ice provides an age (232 ± 126 BCE) which agrees with previous ¹⁴C dates in the oldest part of the record. Second, a new millennial-scale high resolution atmospheric Pb depositional record was used to synchronize the Alto dell’Ortles cores with an accurately-dated (±5 years) Pb record from an array of Arctic ice cores during the ~200 BCE to ~1900 CE time period. This match resulted in a shift of the initial Alto dell’Ortles timescale ~200 years earlier, but still within the initial time uncertainty. Third, 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 2011 and 1927 CE) and visual counting (1926–1900 CE). The time markers obtained by these three methods were combined in a continuous timescale by running a Montecarlo based fit (COPRA model). This Alto dell’Ortles revised chronology shows a significantly reduced uncertainty, between ±1 and ±4 years after 1927 CE, and between a maximum of ±100 years to a minimum of ±5 years between 1927 CE and 200 BCE by conservative estimates. An investigation of the revised chronology by means of a simple 1-D flow model suggests that non-steady-state conditions (e.g., changes in past snow accumulation rate) need to be considered to provide a full physical explanation of the age-depth relationship obtained. The new revised chronology will allow the constraint of the Holocene climatic and environmental histories emerging from this high-altitude glacial archive of Central Europe. The novel methodologies 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.