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Digital archive of theses discussed at the University of Pisa

 

Thesis etd-07032023-142419


Thesis type
Tesi di laurea magistrale
Author
BORRACCINI, GIANLUCA
URN
etd-07032023-142419
Thesis title
Multiple cosmogenic nuclides to decipher the complex exposure history of the East Antarctic Ice Sheet
Department
SCIENZE DELLA TERRA
Course of study
SCIENZE E TECNOLOGIE GEOLOGICHE
Supervisors
relatore Prof. Baroni, Carlo
correlatore Prof.ssa Salvatore, Maria Cristina
controrelatore Prof. Vacchi, Matteo
Keywords
  • antarctic ice sheets
  • cosmogenic nuclides
  • geomorphology
  • glacial history
Graduation session start date
21/07/2023
Availability
Withheld
Release date
21/07/2026
Summary
The aim of this thesis is the Surface Exposure age Dating (SED) of samples collected in deglaciated areas of Victoria Land during several expeditions of the Italian National Antarctic Research Program (PNRA 1997-98, 2004-05, 2011-13) by the supervisors of this thesis (Proff. C. Baroni and M.C. Salvatore, University of Pisa). In particular, the samples were collected to contribute to the reconstruction of the Pleistocene history of the Antarctic glacial system and the subsequent stages of the retreat of the Antarctic icesheets. The analyzed samples were taken from erratic boulders of granitoid rocks, deposited during repeated Pleistocene advances of the Antarctic glacial system on Ross Island, a volcanic island located in the West Ross Sea at the edge of the East Antarctic Ice-Sheet. In fact, during the Last Glacial Maximum (MIS 2) the grounding line of the Ross Platform advanced northward to the edge of the continental shelf, causing a relevant increase in the glaciers’ thickness in coastal areas.
The SED method relies on the calculation of the concentration of cosmogenic nuclides that have formed on a surface since it was exposed to cosmic radiation. Secondary cosmic rays (mainly neutrons) interact by spallation reaction with atoms contained in target minerals on the surface, accumulating the cosmogenic isotopes. Through laboratory procedures, from the target minerals are extracted the nuclides to analyze. With an AMS is then calculated the ratio between the concentration of the cosmogenic isotope of interest and that of the corresponding stable isotope, from which the concentration of the single cosmogenic nuclide is obtained. Thanks to the known production rate and half-life of the isotope, and to other factors to consider, the surface exposure age is calculated from the concentration.
It was possible to prepare and process samples for this kind of analysis thanks to the work carried out in specialized universities and research centers, under the guidance of experts in the field. At the Department of Earth Sciences of Pisa, the samples were selected, crushed, and sieved to extract the fraction 250 << 500 µm. At Victoria University in Wellington, the physical and mechanical processes for the separation and extraction of quartz (the target mineral of the SED method used) were conducted (under the supervision of Dr. J. Stutz and Prof. K. Norton); subsequently, quartz was treated with acids (HNO3, HCl, HF) to obtain pure crystals. At the laboratories of the University of Wollongong (under the supervision of Dr. A. Codilean), quartz was dissolved, and all chemical procedures were made to separate the elements of interest (Be and Al) from the rest of the solution, precipitating them in the form of hydroxides, then converted into oxides (BeO e Al2O3). The final steps of the preparation were carried out at the Australian Nuclear Science and Technology Organisation (ANSTO) and involved the transfer of Al and Be samples to cathodes for analysis with AMS (under the supervision of Dr. R. Fulop). At ANSTO part of the samples were prepared for the calculation of the in-situ 14C concentration which, unlike the other isotopes, was purified and extracted as a gas before being converted to solid form and then pressed into cathodes.
The calculated exposure ages for these erratic boulders have allowed the identification of some phases of the middle-late Pleistocene glacial history of Ross Island. Thanks to the analysis of the spatial and chronological distribution of the erratics with 10Be ages that pre-date the LGM, it was possible to hypothesize that this Antarctic sector has been affected by repeated advances of the ice-sheet, defining then a complex exposure history. This evidence was confirmed by the combined multi-nuclide analysis (10Be -14C) obtained from two very old samples (10Be ca. 150 ka - 14C 5.5 ka; 10Be > 150 ka - 14C 7.8 ka), which showed a clear 10Be cosmogenic inheritance derived from different exposure and burial phases. A complex exposure history (due to repeated phases of glacial advance and retreat) has also been discussed for other Antarctic regions close to Ross Island along the southern Victoria Land and in the Terranova Bay area, where numerous SED and 14C dates confirmed this evidence. The results obtained on the study area also refer to the reconstruction of the main steps of thinning and retreat of the ice-sheet from Ross Island after the LGM. The beginning of deglaciation postdates ca. 20 ka, and the final retreat of glaciers was completed at about 5.8 ka (and certainly before 3.6 ka).
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