Obsidian artifacts can record ancient climate.

The layer of material that forms on the surfaces of arrow points, spearheads, and certain other natural-glass objects, as they age can be used to estimate the temperatures that the artifacts have experienced, a new analysis suggests.

The researchers focused on ancient objects made of the volcanic glass called obsidian. As soon as such an object breaks, its freshly exposed surfaces begin to absorb water, or hydrate. The rates at which those surface layers thicken depend on the temperature and humidity of the object's surroundings, says Lawrence M. Anovitz, a geochemist at the University of Tennessee in Knoxville. Now, he and his colleagues at the Oak Ridge (Tenn.) National Laboratory have turned that trait into a climate-monitoring tool.

To date the objects in their study, the researchers used obsidian artifacts found alongside objects that could be carbondated. They assumed that the obsidian and the other artifacts in a particular sediment layer are the same age. Given that age, they could deduce the environmental conditions from the thickness of the hydrated layer on the obsidian, Anovitz notes.

The researchers studied artifacts recovered at the Chalco site southeast of Mexico City. Analyses of the obsidian objects created between 900 and 450 years ago indicate that soil temperatures there have been about 21.3[degrees]C since the objects' manufacture. However, hydrated layers on the artifacts that were produced between 1,450 and 1,350 years ago indicate that soil temperatures then were about 5[degrees]C warmer. Other indicators of the region's climate history have suggested similar changes in temperature, the researchers note in the July Geology.