Introduction

Some of our most vivid glimpses into past lives are provided by archaeological sites and bodies that have been preserved through freezing. It is difficult to imagine coming closer to actually meeting people from the past than looking into remarkably well-preserved faces - the face of a man who lived in the Alps 5000 years ago, or the face of a teenaged girl who was a human sacrifice in the Andes 500 years ago, or the faces of sailors who perished 150 years ago during a doomed expedition seeking Canada’s Northwest Passage. In addition to bodies, freezing can also preserve a wide range of other materials that would not survive under most other archaeological conditions, especially organic materials. For these reasons, frozen sites and bodies are of immense archaeological interest.

In many environments outside the tropics, it is common for items to become frozen during the cold months of the winter. However, except under very special circumstances these items will invariably thaw again in the spring. Far from preserving something, repeated cycles of freezing and thawing will hasten the destruction of fragile organic materials. Thus, for something to be preserved by freezing for more than just a few months it must enter an archaeological context characterized by persistent cold temperatures. For that reason Arctic and alpine environments have yielded the greatest numbers of frozen sites and bodies. The high alpine environment is cold and supports year-round snow and ice cover in some topographic contexts. The Arctic environment is cold and is also characterized by ‘permafrost’, or permanently frozen ground. During the Arctic summer the surface layer of the earth thaws - this is referred to as the ‘active layer’. Depending on the latitude and on local conditions, the thickness of the active layer may vary from just a few centimeters to more than a meter. However, below the active layer is permafrost - a layer that remains frozen year round. Archaeological materials buried deep enough to lie within the permafrost layer will remain frozen indefinitely, although excavating in permafrost is an extremely laborious process.

The nature and degree of preservation resulting from freezing can vary. Organic items that become frozen for extended periods of time but which are exposed to the atmosphere can become freeze-dried - the water content of the items is lost through sublimation, leaving behind the dried soft tissues. Organic items retaining a greater portion of their original water content are the outcome of two different scenarios: burial within permafrost or burial within an ice field or glacier. Under these conditions, which involve stable temperature and humidity without the desiccating effects of exposure to the atmosphere, organic items may retain their water content and undergo fewer decompositional changes depending on how much time elapses between death and freezing. Dealing specifically with mummies (the generic term for preserved bodies), freeze-drying produces desiccated mummies whereas freezing without desiccation produces true frozen mummies. A much wider variety of analyses can be performed on frozen mummies but there are immense difficulties studying them both due to the problems of working with them in the frozen state, and due to the conservation concerns caused by temporarily thawing them (see Conservation and Stabilization of Materials).