Glacial Oscillations and Climate Variability in the NE Greenland

Abstract

[eng] The remote coastal ice-free areas of Northeast Greenland National Park are considered a unique laboratory to examine the chronology of past cold-climate geomorphological processes and associated climatic conditions. This dissertation endeavors to employ a multidisciplinary approach, leveraging various natural archives and employing complementary techniques. The overarching goal is to enhance our understanding of the glacial history and climate variability within the specified region. Four valley environmental reconstructions are presented here, combining geomorphological mapping and a geochronological dataset of 84 10Be cosmogenic radionuclide exposure (CRE) dating samples (in some cases complemented with OSL dating and historical data) to reconstruct glacial oscillations. In addition, a multiproxy characterization of lake sediment records is provided, using seven different but complementary paleolimnological methods together with 13 14C dating, to unravel climate variability of the last millennia, with a focus on local temperature and precipitation regimes. The highest CRE-dated samples indicate that these valleys were fully glaciated before the global Last Glacial Maximum (LGM) at ~80-50 ka. After the LGM, a rapid deglaciation process with massive ice thinning occurred, and most of the currently ice-free areas of the region were deglaciated at the end of Termination-1. The complete sequence of moraine ridges distributed across the slopes surrounding the Zackenberg Valley floor revealed ice thinning between ~13.7 and 11.2 ka, followed by accelerated glacier retreat at ~10-11 ka, when the valley glaciers disconnected from the main ice tongues. Deglaciation of the fjord entrance valleys and terminal valleys occurred during the Early Holocene at ~10-8.5 ka, due to the long-term decrease of precipitation. This long-term recession was interrupted by periods of glacial stillstand/ advance of the valley glaciers that favored the formation of the moraine ridges on the slopes and valley bottoms of the different valleys (as during short periods of cooling, such as the ~11.5 and 9.2 ka events). The prolonged recession exposed the rock slopes and the moraine ridges, favoring rapid paraglacial slope readjustment. The disappearance of the ice has led to the formation of the present landscape through a variety of postglacial processes, mainly periglacial dynamics, glacial-isostatic uplift and redefinition of the coastlines, permafrost aggradation, and nivation processes. As in most areas of NE Greenland, there are no glacial records from the mid-Holocene to the beginning of the Late Holocene (~8.2- 4.2 ka), and therefore no glacial oscillation data were obtained from any of the four valleys. his phase was parallel to low precipitation regimes from ~5.2 to ~3 cal. ka BP in the area. At this time, from ~3 ka BP forward, as conditions became wetter and warmer, new evidence of glacial advances are detected in some valleys (as at ~1.3, ~0.6 and ~0.3 ka BP). The Little Ice Age (LIA) (1300-1850 CE) constitutes the last phase of glacier expansion in Greenland driven by solar minima that favored lower global temperatures. Paleolimnological reconstructions show that the beginning of the LIA was characterized by a marked decrease in air temperatures, followed by a decrease in precipitation, which was lowest at the end of the period. Since the last major LIA advance, the glacier fronts have experienced minor oscillations, as shown by historical images of the 1930s, with glaciers close to LIA moraines. The last 50 years of the Aucella Lake and ZAC1906 Lake records are characterized by abrupt temperature increases, with the highest peaks of the last ~11 cal. ka BP that are associated with anthropogenic global warming. In conclusion, the main findings of this PhD thesis constitute new relevant contributions to understanding climate variability and glacial response in NE Greenland from the last glacial cycle to the present, considering the limitations of using CRE dating in areas where paraglacial dynamics are very active. However, they have also raised new questions and uncertainties that need to be investigated in the future.

[cat] Les remotes zones costaneres sense gel del Parc Nacional del Nord-est de Groenlàndia es consideren un laboratori únic per a examinar la cronologia dels processos geomorfològics del passat en climes freds i les seves condicions climàtiques associades. Aquesta tesi pretén emprar un enfocament multidisciplinari, aprofitant diversos arxius naturals i emprant tècniques complementàries. L'objectiu global és millorar la nostra comprensió de la història glacial i la variabilitat climàtica en aquesta regió. Aquí es presenten quatre reconstruccions ambientals de valls, combinant cartografia geomorfològica i un conjunt de dades geocronològics de 84 mostres de datació per exposició a radionúclids cosmogènics (en alguns casos complementats amb datació OSL i dades històriques) per a reconstruir les oscil·lacions glacials. A més, s'ofereix una caracterització multiproxy dels registres de sediments lacustres, utilitzant set mètodes paleolimnològics diferents però complementaris juntament amb 13 datacions 14C, per a desentranyar la variabilitat climàtica dels últims mil·lennis, amb especial atenció als règims locals de temperatura i precipitació.