For past climate clues, ask a stalag-mite

Scientists have turned to mites fossilized in cave formations to show in a novel way that the American Southwest at times during the past few thousand years was much wetter and cooler than it is now.

Hidden Cave lies at an altitude of about 2,000 meters in the Guadalupe Mountains in southeastern New Mexico. A steep shaft leads about 25 m down from a wide entrance to a 100-by-150-m cavern with muddy floor. Stalagmites and other cave formations slowly grew on the floor and walls as water dripped into the cave and deposited dissolved minerals.

The moist, slick surfaces of those burgeoning limestone formations captured a variety of debris, including dust that had washed or blown into the cave, says Victor J. Polyak, a geologist at the University of New Mexico in Albuquerque. Also among that debris were the remnants of mites and other animals that now serve as clues to a previous ecosystem. Radioactive dating shows that the mites were entombed roughly between 3,200 and 800 years ago, Polyak notes. He and his colleagues report their findings in the July Geology.

Because the stalagmites also included partial remains of spiders and cave crickets, Polyak suggests there was a thriving ecosystem in Hidden Cave. At least 12 species of mites were fossilized in the cave’s formations. Most likely, these small arthropods had once lived in the soil and fallen leaves outside the cavern but had adapted to life underground.

None of the mites was parasitic, so it’s unlikely any rode into the cave on a host, says Polyak. Most were of fungus-grazing varieties, so they probably lived in the cave, were transported into the cavern through groundwater, or were washed or blown into the entrance. Of the 12 species fossilized in the formations, only one is among the 16 types that live in the cave now. More than 32 species of mites currently live outside the cavern, but only two of those also live inside.

The types of mites trapped in the Hidden Cave stalagmites are found today only miles away in

higher, cooler portions of the Sacramento Mountains, a range to the west of the Guadalupes.

This indicates that there’s been significant climate change in the area. The fossil mites suggest that the Guadalupe and the Sacramento mountain ranges were once ecologically linked during a cooler, wetter period, he says. That would have allowed the same mites to range throughout the region.

“Using fossil mites from stalagmites to look at past climate is an innovative approach,” says MaryLynn Musgrove, a geochemist at Harvard University. She notes that there are few methods available for investigating ancient land climates.

Key among those techniques is the measurement of the ratio of oxygen-16 and oxygen-18 isotopes deposited in stalagmites in the unchanging temperatures of deep caves. This ratio can provide a rough record of climate in the cave’s region because it depends on the average annual temperature there. Hidden Cave is small, however, so the ratio of oxygen isotopes in its stalagmites could have been affected by evaporation through the cave mouth. That’s why Polyak’s team turned to mites.

“This is an interesting way of going about [climate research],” says Cal Welbourn, a mite curator at the Florida State Collection of Arthropods in Gainesville. He notes that mites are often adapted to specific microhabitats and can therefore help researchers infer details about climate.