The Ancient Past, Alive
|Gypsum dunes in Cuatro Cienegas, Mexico. Click image for larger view.
Credit: Leslie Mullen
As we trekked into an alien world, our feet crunched into white gypsum sand that sparkled in the sun like ice. High winds pulled fine grains into the air and gave the landscape a dreamy appearance. Pillars of gypsum stood as lonely sentinels, trees embedded within them stretching out crooked black limbs to greet us.
Some of the larger mounds were similar to rocks in the desert southwest, with wind-carved stripes running across their smooth sides. The more intrepid members of our group scaled these heights and stood proudly at the peaks. The sun started to set, and the golden sky added an otherworldly glow to the haze while the wind started to freeze as well as bite. Later I would discover my face was covered in a fine layer of grit, and many of my fellow explorers sported sand-burned faces.
We were in Cuatro Cienegas, a butterfly-shaped valley in an ancient part of the Chihuahua desert of Mexico. The valley is surrounded by looming mountain ranges that are silent testament to the many contortions the region has undergone over millions of years and the still-active geothermal faults that lie beneath. In addition to the tall mountains and the unusual gypsum dunes, the arid region is speckled with a variety of spring-fed pools, or “pozas,” that vary in size, color, and species complexity.
|Poza Azul in the Cuatro Cienegas valley in Mexico. “Shelf” stromatolites can be seen living in the clear blue water. Click image for larger view. Credit: Mya Breitbart, University of South Florida|
The pozas have become living laboratories for astrobiologists. Not only are they home to stromatolites, a form of life that was widespread in ancient times but is rare today, but the chemical conditions of the region are in some respects reminiscent of the ancient Earth.
The earliest life on Earth was microbial, and life remained single-celled for most of Earth’s history. The earliest evidence for life crops up in the rock record 3.8 billion years ago, and life seems to have remained single-celled for over 2 billion years.
During that huge chunk of time, some organisms lived in communities, forming bacterial mats that trapped dirt and nutrients. When a mat became too packed with sediment or was rendered otherwise unlivable, the bacteria could just migrate upwards and build a new mat on top of the old. Over time, the multiple layers stacked up to form large stone-like structures called stromatolites.
Modern stromatolites are rare because more modern organisms compete with them for food and space, or even feed on the stromatolites themselves. Most stromatolites today only survive by living in extreme conditions that other organisms can’t tolerate. The famous Shark Bay stromatolites in Australia, for instance, live in water that is twice as salty as normal seawater.
Cuatro Cienegas stromatolites are unusual because they manage to live with others. Fish, snails, turtles and other organisms share the pozas and have evolved into unique species in the isolated, Galapagos-like conditions. The snails do feed on the stromatolites, but this particular species of mollusk is extremely small and doesn’t seem to be harmful to the stromatolites’ survival.
|Stromatolite producing oxygen bubbles. Scientists believe stromatolites helped produce the earliest atmospheric oxygen on Earth. Click image for larger view.
Credit: Mya Breitbart, University of South Florida
The earliest stromatolites were probably anoxic, but the Cuatro Cienegas stromatolites produce oxygen. Scientists taking core samples of one type of Cuatro Cienegas stromatolite have found that the top layer of the mat is composed of diatoms, and beneath them, oxygen-producing photosynthetic cyanobacteria. Purple sulfur bacteria lay buried in a third layer, and sulfur-reducing bacteria compose a fourth underlying layer. Each layer obtains and processes energy in a different way and produces different waste products, and yet it is an interactive community where each layer contributes to the survival of the mat as a whole.
Janet Siefert, of Rice University in Houston, Texas, is studying the Cuatro Cienegas stromatolites in order to learn about the more ancient anaerobic stromatolites. For instance, by figuring out what chemical conditions are necessary for the stromatolites to turn into stone and become fossils today, she thinks we can better understand what the conditions were like for the Earth’s earliest fossilized life.
“We can extrapolate back to an anaerobic system if we can figure out the basic energy dynamics that are needed,” says Siefert. “How much biomass production there needs to be, over what period of time, and what kind of phosphorus supply does there need to be – figuring out what are the basic things needed to produce stromatolites that can lithify.”
The water in the pozas is exceptionally low in phosphorus, which is vital for many functions of life and even forms the backbone of DNA. But life in Cuatro Cienegas has evolved to compensate for the lack of that element. Understanding how life is able to survive such conditions could help scientists understand the limitations of life in chemical environments elsewhere.
|Stromatolites and fish living in harmony in Rio Mesquites, a river in Cuatro Cienegas. Click image for larger view.
Credit: Mya Breitbart, University of South Florida
This scarcity of phosphorus also makes Cuatro Cienegas a good analogue of the Precambrian Earth. About 600 million years ago, the fossil record suddenly filled with a profusion of different life forms – an event referred to as the Cambrian explosion. There was also a spike of phosphorus in the rock record at that time, and some scientists think the two events might be related.
Jim Elser of Arizona State University has been traveling to Cuatro Cienegas for several years to try to understand how different levels of phosphorus in the environment affect life.
“The idea is that if this is some sort of analogue for what the Earth was like during that time, then maybe you could understand something about that period,” says Elser. “The fossil record is very good at telling us what was there in the past, but it’s very hard to infer what was happening in the past; how the different elements were interacting.”
The goal of using Cuatro Cienegas to study ancient times is now a race against time, however, because the pozas are drying up. Although the region was declared a National Protected Area in 1994, local alfalfa farmers, ranchers and others are still permitted to drill wells to tap the underground aquifer. We passed several of the alfalfa fields in our travels around the valley. After mile upon mile of the grayish browns, muted yellows and dusty greens of hardscrabble desert vegetation, suddenly, like a mirage, an alfalfa field appears — a circular patch of bright emerald green that’s as false to the eye as a child’s drawing in crayon.
|Valeria Souza holds a stromatolite that was pulled out of the waters of Rio Mesquites. Click image for larger view.
Credit: Leslie Mullen
Valeria Souza, an ecologist at the National Autonomous University of Mexico, is pressuring the Mexican government to protect the pozas of Cuatro Cienegas. Souza says the pozas have been around for at least 200 million years, but are disappearing in less than 20 years. She’s seen large swaths of land that were once underwater become drained, leaving behind dried-out, crumbling stromatolites and layers of evaporated minerals. There are also many cavernous sinkholes in the region, more evidence of the rapid water loss that is emptying the aquifer.
However, scientists can not say for certain if the loss of water is due to human practices or to climate change. There is evidence that some pozas have dried up in the past, before such intensive water use occurred, and some pozas have even migrated from one spot to another. Representatives for Mexico’s federal government currently deny that the pozas of Cuatro Cienegas are at risk.
Whatever the cause of the current water loss, scientists hope they have time to study this unique ecosystem before it disappears.
“I don’t think some of the pozas will last another summer,” says Souza. “If the puzzle of life dies, it’s hard to work on it.”
(Top banner photo credit: Mya Breitbart, University of South Florida))