Freeze thaw cycles and their impact on lithic artifact displacement and microscopic surface altérations : a review of experimental insights

Pleistocene sediments across Eurasia and the artifacts they contain were frequently altered by freeze-thaw cycles. These processes can cause artifacts to move both vertically and laterally, producing microscopic surface alterations that are sometimes difficult to distinguish from human-made wear. Improving our ability to characterize and recognize microscopic wear caused by freeze-thaw cycles would enhance reconstructions of post-depositional artifact movements at archaeological sites. It would also enable a more informed reconstruction of stone tool use by hominins through micro-wear analysis. Here, we review experiments conducted by geologists, geoarchaeologists and micro-wear analysts in both natural (in vivo) and controlled (in vitro) environments to evaluate the effects of freezing and thawing on archaeological artifacts and more specifically on their displacement and micros-copic surface alterations. Our synthesis shows that numerous interacting variables influence published experiments, often leading to ambiguous results. The influence of artifact shape on displacement remains poorly understood: available data suggest that elongated objects tend to move upwards more quickly than larger and less elongated objects but the role of artifact thermal conductivity in this process remains unclear. Microscopic surface alterations on lithic artifacts observed in experiments are generally minimal; however, the number of freeze-thaw cycles in most studies is typically far fewer than those experienced by archaeological stone tools. We identify four key variables that should be controlled in future experiments: (1) sediment type and permeability, (2) temperature variations within the sediment, (3) water saturation, and (4) artifact shape, size, and thermal conductivity. To better isolate the factors influencing artifact displacement and better predict micro-wear formation resulting from freeze-thaw cycles, we recommend long-term, systematic experiments replicating hundreds of freeze-thaw cycles in climatic chambers while rigorously controlling the four variables identified above.