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Dating the formation of the Greenland ice sheet using luminescence

Christine Thiel1, Andrew S. Murray2, Astrid M.Z. Schmidt3, Jan-Pieter Buylaert1, Kristina J. Thomsen4, Jørgen Peder Steffensen5
1 Nordic Laboratory for Luminescence Dating, Aarhus University & Centre for Nuclear Technologies, Risø DTU
2 Nordic Laboratory for Luminescence Dating, Aarhus University
3 Niels Bohr Institute, Ice and Climate, University of Copenhagen & Natural History Museum of Denmark, Centre for GeoGenetic, Copenhagen
4 Centre for Nuclear Technologies, Risø DTU
5 Niels Bohr Institute, Ice and Climate, University of Copenhagen

V 7.2 in Freie Themen

19.09.2012, 08:50-09:10, H8

Major ice sheets are one of the best climate archives on Earth, but there are at present no reliable methods for determining when these ice sheets last began to form. Such information is of great importance to verifying climate models. Additionally it is of relevance for  and to other studies, which make use of fossil material such as ancient DNA preserved close to the bottom of the ice. Although the ice itself may not be datable, any mineral grains incorporated during initial ice build up can, in principle, be dated using luminescence techniques; this is the scope of the DFG-funded project (number TH1651-1) ‘Dating the Initiation of the Polar Ice Sheet Formation’. Here, we are presenting an update of the ongoing work on both the Camp Century and the NEEM ice core.

 

For Camp Century, equivalent doses for five basal ice samples from different depths close to the base of the ice sheet have been measured using an elevated temperature post-IR infra-red stimulated luminescence (IRSL) protocol. Due to the small amount of material available (200-400 mg per sample), mineral separation prior to luminescence measurement was not considered practical. As a result, equivalent doses were first measured on all grains that gave sufficient light. K- and Na-rich feldspar grains were then distinguished by scanning electron microprobe analysis, which furthermore allowed for (semi-) quantification of the K content of the individual grains. In total, 209 K-rich feldspar grains (K > 9%) could be identified. Additional LA-ICP-MS measurements were conducted to quantify uranium and thorium; these data were used to calculate an average dose rate. The observed equivalent dose distributions are skewed, with a tail towards large doses (>800 Gy). One cause for the skewed distributions could be incomplete bleaching of individual grains prior to deposition. However, bias towards lower doses due to light exposure during sampling and subsequent handling is also possible. To evaluate the signal loss during handling, we exposed feldspar grains to similar laboratory lighting and measured the remaining signal after different exposure times; using these data we estimated a signal loss of about 20% and minimum ages of ~250 ka.

 

In June 2012, drilling at the base of the borehole at NEEM is planned in order to derive basal ice which will not have seen any light; this is the first time, basal ice samples are specifically taken for luminescence dating. The new data from this core will not only allow for dating the base of NEEM but also help to understand the dose distributions derived from the Camp Century samples.



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Letzte Änderung 25.07.2012