The Dolomites constitute the southernmost margin of the Eastern Alps. In the high Dolomites catchments, the chronological and spatial characterization of glacial fluctuations occurred during the Late Pleistocene remains still today not well constrained; this despite the recent improvement in investigation methods offered by the development of new dating techniques and by the current possibility to integrate field surveys with high resolution remote sensing data. The present work illustrates new data that allow the reconstruction of paleogeographic and paleo-environmental conditions occurred during the Late Pleistocene – a period characterized by global millennial and sub-millennial climate changes – in an area of the Eastern Dolomites, the Alta Badia valley. Despite the intense slope dynamics which characterize the valley, well preserved glacial deposits have been identified and mapped. They have been attributed, on the basis of geomorphological evidence, to the Last Glacial Maximum (LGM) and to the Lateglacial periods. The research took advantage of the study of these glacial deposits to reconstruct the extent and volumes of the ice field that during the LGM occupied the valley, as well as the advance phases of the local glaciers occurred during the Lateglacial. The dating of these deposits by means of the 36Cl Cosmic Ray Exposure (CRE) technique, applied for the first time in Alta Badia and neighbouring valleys, was fundamental to constrain in space and time paleo-glaciers fluctuations within the valley. The geometry of the glaciers (areal extensions, paleo-topography, volumes) were reconstructed by means of the analysis of aerial photographs and Digital Terrain Models, as well as through geomorphological field surveys and management and processing of data using Geographic Information Systems. The Lateglacial Equilibrium Line Altitudes (ELAs) were estimated in relation to different phases of glacial advance. The ELAs have been calculated for every single glacier, providing the opportunity to carry out paleoclimatic correlations on a regional scale for the period considered. The geomorphological evidence and the results of CRE dating on superficial deposits allowed to constrain the minimum age for the withdrawal of the LGM ice field that was at ca. 18-17 ka. In addition, several phases of glacial advance occurring between 16-12 ka were recognized. These results allowed to improve the Alpine chronological dataset relevant to the period ranging from the LGM to the end of the Lateglacial. The research took also advantage of the study of subsurface deposits exposed owing to eight continuous coring boreholes and an excavation wall on a former glaciated plateau, Pralongià, at ca. 2000 m a.s.l. Radiocarbon dating performed on organic sediment – collected from the boreholes and from the excavation wall at different depths – provided ages ranging between 45 and 38 ka BP (MIS 3), which are unique within the framework of the paleogeographic and paleoclimate reconstructions in high Alpine catchments. Stratigraphic evidence and radiocarbon dating results allowed to conclude that the Pralongià plateau was ice free during most of the MIS 3. From a paleoclimatic viewpoint, it is then confirmed the difficulty, during MIS 3, of ground ice persistence, which was probably even more amplified by the peculiar orographic conditions of the Alta Badia valley. The study revealed also that between 38 ka and the onset of full glacial conditions (~ 27 ka) gravitational clastic deposits, originating from the Dolomites massifs surrounding the valley, were emplaced on the plateau. This interpretation is supported by many examples of large landslides that occurred during and after the LGM in the Italian Alps which are reported in literature.
Le Dolomiti rappresentano il margine più meridionale della catena Alpina orientale. Nelle alte valli dolomitiche la caratterizzazione cronologica e spaziale delle fluttuazioni glaciali precedenti l’Olocene rimane ancora oggi poco documentata, nonostante le molteplici possibilità di indagine offerte dallo sviluppo di nuove tecniche di datazione assoluta e alla recente possibilità di integrare le indagini sul terreno con l’analisi di immagini telerilevate ad alta risoluzione. Il presente lavoro illustra nuovi dati che permettono la ricostruzione paleo-geografica e paleo-ambientale del Tardo Pleistocene – periodo caratterizzato da variazioni climatiche millenarie e sub-millenarie a scala globale – di un’area delle Dolomiti Orientali, l’Alta Val Badia. Qui, nonostante l’intensa dinamica di versante, sono stati identificati mappati depositi glaciali ancora ben preservati e attribuiti, sulla base delle evidenze geomorfologiche, all’Ultimo Massimo Glaciale (UMG) e al Tardoglaciale. La presente ricerca si è avvalsa dello studio di questi depositi per ricostruire l’estensione e i volumi dell’ice field che durante l’UMG occupava la valle e le fasi di avanzata dei ghiacciai locali durante il Tardoglaciale. La datazione di questi depositi mediante la tecnica delle superfici esposte (36Cl Surface Exposure Dating), applicata per la prima volta in Alta Badia e valli limitrofe, è risultata fondamentale per vincolare temporalmente e spazialmente le fluttuazioni dei paleo-ghiacciai della valle. Le geometrie dei ghiacciai (estensioni areali, paleotopografie, volumi) sono state ricostruite sulla base dell’analisi di fotografie aeree e Modelli Digitali del Terreno, di rilievi geomorfologici e della gestione ed elaborazione di dati tramite Sistemi Informativi Geografici. Le stime delle Linee di Equilibrio tardoglaciali, relative a diverse fasi di avanzata, sono state calcolate per ogni singolo ghiacciaio, fornendo così l’opportunità di effettuare correlazioni paleoclimatiche su scala regionale per il periodo considerato. Le evidenze geomorfologiche e le età di esposizione superficiale hanno permesso di vincolare l’età minima del ritiro dell’ice field in corrispondenza dell’UGM, intorno a ca. 18-17 ka. Inoltre sono state riconosciute diverse fasi di riavanzata tardoglaciale occorse tra 16 e 12 ka, permettendo così di ampliare il dataset cronologico alpino per il periodo che si estende dall’UGM fino alla fine del Tardoglaciale. La ricerca si è avvalsa inoltre dello studio di depositi sub-superficiali portati alla luce grazie a carotaggi effettuati a 2000 metri di quota sull’altopiano del Pralongià, coperto dall’ice field durante l’UMG. Datazioni radiometriche su materiale organico, prelevato dai carotaggi a diverse profondità, hanno fornito età comprese tra 45 e 38 ka (MIS 3), uniche nel quadro delle ricostruzioni paleo-geografiche e paleo-climatiche alle alte quote alpine. Lo studio di questi depositi ha permesso di concludere che durante il MIS 3 il Pralongià era libero dai ghiacci. Dal punto di vista paleo-climatico è confermata la difficoltà, durante questo periodo, di una persistenza del ghiaccio al suolo, che qui in Val Badia, vista la peculiare conformazione orografica, è stata probabilmente ancor più amplificata. Lo studio ha inoltre rivelato che tra 38 ka e l’instaurarsi delle condizioni pleniglaciali (~27 ka) depositi clastici gravitativi, provenienti dai massicci dolomitici che orlano la valle, sono stati messi in posto sull’altopiano. Questa interpretazione trova riscontro in analoghi esempi di grandi frane occorse durante e dopo l’UMG nelle Alpi italiane riportate dalla letteratura.
La storia glaciale di un’alta valle delle Dolomiti: nuovi dati sulle relazioni tra clima e processi geomorfologici del Tardo Pleistocene
VANDELLI, VITTORIA
2020
Abstract
The Dolomites constitute the southernmost margin of the Eastern Alps. In the high Dolomites catchments, the chronological and spatial characterization of glacial fluctuations occurred during the Late Pleistocene remains still today not well constrained; this despite the recent improvement in investigation methods offered by the development of new dating techniques and by the current possibility to integrate field surveys with high resolution remote sensing data. The present work illustrates new data that allow the reconstruction of paleogeographic and paleo-environmental conditions occurred during the Late Pleistocene – a period characterized by global millennial and sub-millennial climate changes – in an area of the Eastern Dolomites, the Alta Badia valley. Despite the intense slope dynamics which characterize the valley, well preserved glacial deposits have been identified and mapped. They have been attributed, on the basis of geomorphological evidence, to the Last Glacial Maximum (LGM) and to the Lateglacial periods. The research took advantage of the study of these glacial deposits to reconstruct the extent and volumes of the ice field that during the LGM occupied the valley, as well as the advance phases of the local glaciers occurred during the Lateglacial. The dating of these deposits by means of the 36Cl Cosmic Ray Exposure (CRE) technique, applied for the first time in Alta Badia and neighbouring valleys, was fundamental to constrain in space and time paleo-glaciers fluctuations within the valley. The geometry of the glaciers (areal extensions, paleo-topography, volumes) were reconstructed by means of the analysis of aerial photographs and Digital Terrain Models, as well as through geomorphological field surveys and management and processing of data using Geographic Information Systems. The Lateglacial Equilibrium Line Altitudes (ELAs) were estimated in relation to different phases of glacial advance. The ELAs have been calculated for every single glacier, providing the opportunity to carry out paleoclimatic correlations on a regional scale for the period considered. The geomorphological evidence and the results of CRE dating on superficial deposits allowed to constrain the minimum age for the withdrawal of the LGM ice field that was at ca. 18-17 ka. In addition, several phases of glacial advance occurring between 16-12 ka were recognized. These results allowed to improve the Alpine chronological dataset relevant to the period ranging from the LGM to the end of the Lateglacial. The research took also advantage of the study of subsurface deposits exposed owing to eight continuous coring boreholes and an excavation wall on a former glaciated plateau, Pralongià, at ca. 2000 m a.s.l. Radiocarbon dating performed on organic sediment – collected from the boreholes and from the excavation wall at different depths – provided ages ranging between 45 and 38 ka BP (MIS 3), which are unique within the framework of the paleogeographic and paleoclimate reconstructions in high Alpine catchments. Stratigraphic evidence and radiocarbon dating results allowed to conclude that the Pralongià plateau was ice free during most of the MIS 3. From a paleoclimatic viewpoint, it is then confirmed the difficulty, during MIS 3, of ground ice persistence, which was probably even more amplified by the peculiar orographic conditions of the Alta Badia valley. The study revealed also that between 38 ka and the onset of full glacial conditions (~ 27 ka) gravitational clastic deposits, originating from the Dolomites massifs surrounding the valley, were emplaced on the plateau. This interpretation is supported by many examples of large landslides that occurred during and after the LGM in the Italian Alps which are reported in literature.File | Dimensione | Formato | |
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https://hdl.handle.net/20.500.14242/78998
URN:NBN:IT:UNIMORE-78998