Klaus Oeggl

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.

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.

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.

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.

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.

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.

Paleoclimatic glacial archives in low-latitude mountain regions are increasingly affected by melt, which leads to heavy percolation and can remove snow and firn accumulated across months, seasons or even years. Proxy system models, used for improved interpretation of glacial proxies and paleoclimatic reconstructions, generally do not account for melt because they are optimized for sites where snow layer removal by melting is negligible. In this paper, we present a mass balance model applied to the Mt. Ortles drilling site, at 3859 m a.s.l. in the Eastern Italian Alps, with the aim of building a pseudo proxy of atmospheric conditions during the formation of snow layers survived to ablation. This pseudo proxy is useful for improved dating and environmental interpretation of firn layers (<15 m depth), affected by significant melt in the period 1996–2011, which includes the extremely warm summer 2003. Here we show that the model significantly improves the interpretation of the firn stratigraphy. This is fundamental for detecting melted layers and for refining the dating of the core based on traditional annual layer counting of stable isotope and pollen seasonal oscillations.

Paleoclimatic glacial archives in low-latitude mountain regions are increasingly affected by melt, which leads to heavy percolation and can remove snow and firn accumulated across months, seasons or even years. Proxy system models, used for improved interpretation of glacial proxies and paleoclimatic reconstructions, generally do not account for melt because they are optimized for sites where snow layer removal by melting is negligible. In this paper, we present a mass balance model applied to the Mt. Ortles drilling site, at 3859 m a.s.l. in the Eastern Italian Alps, with the aim of building a pseudo proxy of atmospheric conditions during the formation of snow layers survived to ablation. This pseudo proxy is useful for improved dating and environmental interpretation of firn layers (<15 m depth), affected by significant melt in the period 1996–2011, which includes the extremely warm summer 2003. Here we show that the model significantly improves the interpretation of the firn stratigraphy. This is fundamental for detecting melted layers and for refining the dating of the core based on traditional annual layer counting of stable isotope and pollen seasonal oscillations.