Dating of the basal ice using physical and biological methods

Basal ice can be dated using a combination of physical1 and biological2 methods.

Picture modified from (WILLERSLEV ET AL. SCIENCE 2007): A combination of biological dating methods (Amino acid racemization, AAR and molecular clock, COI) and physical dating methods (cosmogenic, 10Be/36C and luminescence, OSL) were combined to provide a consensus age estimate of the basal ice and the biological material within it.

At the Centre for Ice and Climate researchers use three physical methods:
-  Cosmogenic isotope dating of 10Be/36Cl
-  Uranium recoil dating using 238U/234U
-  Optically stimulated luminescence dating (OSL).

The biological methods used are:
- Racemization of amino acids
- Molecular clock dating of invertebrate COI mtDNAsequences

The consensus age, obtained by a combination of physical and biological methods, shows a time span covering the period within which the ice is formed and the impurities, from which the degraded DNA has been identified, are incorporated. Impurities may include soil particles that degraded DNA from ancient plants and animals can be bound to and thus preserved.The age corresponds to the minimum age during which the area was most recently ice-free and when the ice began to build up. The methods cannot be expected to provide exactly the same age, as they are dating different kinds of processes in the ice.

1. Physical methods:
10Be/36Cl give an age for when the precipitation fell and was incorporated into the ice sheet or glacier. The 238U/234U –dating estimates how long time the impurities or soil particles have been sitting in the ice.
The OSL-dating estimates when the minerals in the ice were last exposed to sunlight.

2. Biological methods:
Racemization of amino acids indicates how long it has been since the organisms they come from died and when the decaying began.
Molecular clock analysis is used on short DNA fragments in the gene sequence COI in invertebrates and tests:

i) Whether it is different from modern closely related invertebrates.
ii) Whether the genetic differences between the DNA sequences that are preserved in the ice are comparable to modern related DNA sequences. The result shows how long it has been since they existed. The method is useful against the background of existing estimates for how rapidly the genetic region develops. By using a statistical model and comparing the old and the modern sequence, it is possible to estimate a time interval for when the old sequence comes from.

A combination of physical and biological methods dated the basal ice in the DYE-3ice core to a minimum age of 450,000 years, seemap of Greenland for the different deep ice cores (image). These methods will also be used to determine the age of the basal ice at Camp Century and NEEM. The methods are continuously optimized as more basic ice processes are identified.