The tundra is primarily always frozen. As you point out freeze/thaw cycles are terrible for biochemical stability, but at the tundra below the surface (where ea

The tundra is primarily always frozen. As you point out freeze/thaw cycles are terrible for biochemical stability, but at the tundra below the surface (where earth acts as an heat insulator) I believe this effect is minimized DNA has some remarkable stability properties when it comes to preservation. This is one of the reasons people are so interested in DNA storage technologies. DNA chemistry allows for information to be preserved for millenia at sub-zero centigrade temperatures. Robert N. Grass, Reinhard Heckel, Michela Puddu, Daniela Paunescu, Wendelin J. Stark. Robust Chemical Preservation of Digital Information on DNA in Silica with Error-Correcting Codes. Angewandte Chemie International Edition, 2015; DOI: 10.1002/anie.201411378