The effect of freeze/thaw cycles on archaeological bone

Repeated freezing and thawing may cause mechanical damage to wet porous materials. In Arctic regions, archaeological materials in the uppermost soil layers are subjected to freezing and thawing as the seasons change, and climate change is currently increasing the thickness of these active layers. However, the effects on buried archaeological materials is largely unknown. The state of preservation of 824 faunal bones from Nunalleq in Southwestern Alaska was examined visually and graded on a preservation scale and the preservation was correlated to the find location. To simulate freeze/thaw cycles, archaeological reindeer rib bones were placed in closed containers with soil (n=24) or air (n=4) and cycled between −20°C and +5°C. After 95 cycles and 200 cycles the bones were evaluated visually by monitoring pre-existing and new microscopic and macroscopic cracks before and after the freeze/thaw cycles. Oxygen consumption was measured before and after freeze/thaw cycles, to monitor potential changes in reactivity caused by microcracks that may increase the porosity of the bone. The investigations showed that bone preservation at Nunalleq was very good, with well-preserved bones concentrated in areas with deeper cultural layers, and poorly preserved bones concentrated in areas with thinner deposits. The visual evaluation of the bone post-freeze/thaw showed that damage to bone buried in soil was minimal but slightly higher for bones exposed to air. Additionally, there was no change in oxygen consumption before and after the freeze/ thaw process, suggesting minimal if any change in porosity and microbial activity as a result of these cycles.