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Task 2: Ice core drilling and dating

by Vincent Favier - 23 February 2016 ( maj : 15 November 2016 )

The area is characterized by accumulation rates of about 200 mm w.e. a-1. 4 firn cores between will be collected at depths between 50 m and 100 m to obtain punctual SMB estimates and 4 firn cores at depth of 25m (in the lowest accumulation zones). Dating will be performed by retrieval of horizons corresponding to anthropogenic radionuclides and to volcanic eruption fallouts (when possible). SMB point data will be interconnected thanks to continuous estimates from ground penetrating radar. When possible, information from Task 3 will be used to identify volcanic horizons (where marine intrusions are minimum and do not mask the volcanic signals). Dating of cores will be performed at LGGE before July 2018 thanks to a radiochemistry analysis. The SMB values extracted from the cores will be interpolated using GPR data collected during the traverse. GPR data will be processed in 2017.

Olivier Magand will manage the ice chronology, and E. Le Meur will manage GPR experiments.

Proposed method: Artificial radioisotopes resulting from atmospheric thermonuclear tests carried out between 1953 and 1980 were deposited in Antarctica after transport in the upper atmosphere and stratosphere, creating distinct radioactive reference levels in the snow. The dates of arrival and deposition in this polar region are well known and therefore provide a means to estimate Antarctic snow accumulation rates or describe air mass circulation patterns (Magand, 2009 and references associated). The 1955 and 1965 radioactivity peaks provide two very convenient horizons for dating snow and ice layers and measuring accumulation. Special techniques have been developed over the last 40 years to detect and measure artificial and natural radionuclides present in the ice sheets (Magand, 2009 and references associated). In Antarctica, total beta counting and gamma spectrometry remain the most frequent (and complementary) measurements used to date recent period (last century) and consequently to determine the average rate of snow accumulation. Total beta counting allows primarily to detect the radioactive absolute reference level(s) in the snow/firn column and gamma spectrometry allows to precisely distinguish the major radionuclide deposition times. For example, 241Am (associated to 241Pu) and 137Cs radionuclides constitute two well-known debris products of atmospheric thermonuclear tests between 1955 and 1980. 137Cs is characteristic of both well-known reference layers in snow (1955 and 1965 peaks), corresponding to the arrival and deposition of artificial radionuclides in Antarctica. 241Pu, on the other hand, is more prominent in the older reference level (1955). Indeed, the Yvy and Castles series of tests conducted in 1953 produced more 241Pu than the atmospheric thermonuclear tests in the sixties. Both radionuclides, detected by gamma spectrometry, therefore make it possible to distinguish between the two test series.