Microbes That Fed on Oil Carved Carlsbad Caverns, Scientists Say

The Big Room at Carlsbad Caverns, a room that could fit more than six football fields and reach the height of a 30-story building, inspires awe among visitors, but it’s a nagging puzzle for geologists.

For decades, visitors were told that the cave, with its gypsum formations, was formed by the relentless drip-drip-drip of carbonic acid eating away at the limestone.

But where did all that rock go?

In most caves formed that way, subterranean streams carry off the residue. But there are no underground streams in Carlsbad Caverns.

The story scientists had been telling tourists didn’t make sense to Carol Hill, a University of New Mexico geologist.

“What really puzzled us was the size of it: The Big Room in Carlsbad Caverns is the biggest cave room in North America. That’s a lot of limestone to be carried away. The room is just there, and it doesn’t go anywhere,” she said.

The answer was under their noses all along.

An old explanation was printed on guideposts for the 500,000 annual visitors along Carlsbad Caverns’ pathways: Carbonic acid was said to have seeped down to the limestone from rain runoff, slowly eating away all that rock.

But as Hill discovered, this old trickle-down theory was misleading.

It wasn’t trickle-down. It was bubble-up.

And it wasn’t just geology. It was biology and geology.

Diana Northup, a University of New Mexico biologist who worked with Hill, said single-cell microbes -- bacteria -- that fed on pools of petroleum under the Carlsbad region were the real cave carvers.

“The carbon compounds available in oil are eaten by the micro-organisms,” Northup said, “and the product they produce is hydrogen sulfide.”

This deadly gas rises through fissures until it reaches water -- and oxygen. Hydrogen sulfide reacts chemically with oxygen to produce sulfuric acid, which can dissolve whole stadiums of limestone.

This process leaves clues for geologists. Large blocks of gypsum, hard to overlook in Carlsbad Caverns and especially Lechuguilla Cave, within the park boundaries, are the chemical byproducts.

Like most detectives, Hill had to find the answers long after the culprit had left the scene. Carlsbad and Lechuguilla were formed 3 million to 4 million years ago.

Some answers were found in literature about other caves -- particularly the Lower Kane Cave in northern Wyoming and the Cueva de Villa Luz in Mexico. The work of J. Harlen Bretz, who challenged conventional views of cave formation in the 1940s, also helped.

Hill’s book, “The Geology of Carlsbad Caverns,” published in the 1980s, was fully accepted by scientists only within the last five years.

Hill and Art Palmer, a professor of earth sciences at the State University of New York at Oneonta, said caves like Carlsbad are among just 5% to 10% in the United States formed primarily by such sulfuric acid reactions. Hill’s work is regarded as very accurate, Palmer said.

“It’s just very exciting to have a totally different way to form caves,” Palmer said.

There remain doubters in the oil industry, he said, but “there are just so many smoking guns -- minerals that can only be produced by sulfuric acid” -- that the evidence gets stronger supporting Hill all the time.

Pete Modreski, a geochemist with the U.S. Geological Service in Denver, also endorsed Hill’s work, saying her findings have “advanced the science in quantum jumps.”

Hill’s comparative work with sulfur isotopes, skewed versions of the element, let her trace sulfur through the process and see how it worked. If her research weren’t enough, the process is still at work inside caves in Mexico and Wyoming.

Today, at Carlsbad Caverns, the brochures reflect the latest findings.