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Earth Notes: Melting Ice Caves

Bogdan Onac

An ice-filled lava tube in western New Mexico may have meant survival for Ancestral Puebloans almost two thousand years ago.

Early residents were living and farming in the area, and traveling along the well-established Zuni-Acoma Trail across the rough basalt of the “bad lands.”  But in very dry times, they turned to cave ice as a water source, according to geoscientists.

Nearly a hundred ice-containing lava tubes have been found in El Malpais National Monument. One in particular, Cave 29, has the right seasonal conditions to keep ice frozen year-round deep inside it.  

Bogdan Onac, geoscientist at the University of South Florida and his team, ventured about 500 feet into the back of Cave 29.  They took an ice core sample and found charcoal in it. It was then radiocarbon dated and correlated with the tree-ring record for the area, so the timing of major droughts could be pinpointed fairly closely.

The burned wood, along with ceramic finds and reworked cave floor, convinced the researchers that people likely built small fires to melt the ice and collect pure water for drinking or ritual uses. And they relied on that practice to see them through at least five, extensive drought periods--from as early as AD 150 to as late as 950.

Now, the Southwest’s warming climate is melting that ice.  The loss is concerning to researchers and land managers. They urge people entering caves to leave things where they find them so valuable climate and archeologic information can still be recovered.