Hypothesis Links Extreme Volcanism To Global Cooling

SOCORRO, N.M. July 1, 2009 – New Mexico Tech scientist Dr. Steve Cather authored a brief article in the academic journal Geosphere that has generated a flurry of mainstream articles.

Cather, a senior field geologist with the Bureau of Geology, and his co-authors propose than a series of massive volcanic eruptions started a chemical and geologic domino effect on a global scale that led to the formation of the polar ice caps.

“That’s the upshot of the hypothesis – that episodes of major explosive volcanism led to ice ages,” Cather said. “After we first noted the association between explosive volcanism in southwestern North America with the beginnings of today’s icehouse climate, we wanted to see if there was similar correlation between ancient ice ages and silicic large igneous provinces, or SLIPs. It turned out to be a close correlation, so we decided to go forward with this publication.”

Cather’s co-authors for the paper in Geosphere include geochemist Dr. Nelia Dunbar, geochronologist and volcanologist Dr. Bill McIntosh and geologist Dr. Peter Scholle, all of New Mexico Tech, and geochronologist Dr. Fred McDowell of the University of Texas at Austin.

The bulk of the team’s research focused on a major series of explosive volcanic eruptions in what is now Mexico and the American Southwest from 50 to 15 million years ago. However, the hypothesis might also explain older ice ages known to have occurred on Earth. The team identified six periods of extreme volcanic activity and correlated them to the onset of the cooling cycles.

About 55 million years ago, the Paleocene-Eocene Thermal Maximum saw global warming of about 6 degrees C over a 20,000-year period. Roughly 25 million years later, the planet was solidly in a cooling cycle. The polar regions eventually became covered in ice as the global climate became much chillier.

Scientists have long been able to date ice ages using paleontology, geochemistry and geochronology; however, the cause of radical climate change has been elusive. Now, Cather and his colleagues at New Mexico Tech propose that explosive silicic volcanism may be a common factor among all the cool climate episodes in the last half billion years.

Silicic volcanic provinces are characterized by high levels of silica-rich volcanic rocks, like rhyolite, which is similar to granite in chemical composition. The planet experienced its most recent episode of silicic volcanism beginning about 50 million years ago.  Explosive volcanism continued for many millions of years, with massive eruptions happening on average every 15,000 to 40,000 years.

First, a series of cataclysmic eruptions spewed iron-rich ash into the atmosphere. Each eruption would have dwarfed anything recorded in human history – and filled the atmosphere with volcanic ash.

The first task of the researchers was to accurately determine when the planet entered the phase of intense volcanism and determine the size of the eruptions.

Previous geologic studies in Colorado have established that a serious of eruptions – averaging 120 times the size of the 1991 eruption of Mt. Pinatubo – occurred in a Silicic Large Igneous Province in southwestern North America.

Bill McIntosh has dated silicic volcanic deposits from the Sierra Madre Occidental of Mexico and in the southwestern U.S. using his Argon 40-Argon 39 lab at New Mexico Tech. He and Fred McDowell, two of the preeminent geochronologists in the country, were able to establish that a series of massive eruptions occurred between about 50 and 15 million years ago.

Nelia Dunbar, a geochemist, calculated the iron content in the silicic eruptions. She and Cather then determined the how much iron would need to settle in the oceans to stimulate enough algae growth to sequester enough carbon to cool the entire planet’s atmosphere.

“These eruptions started about 50 million years ago,” Cather said. “Each time, a certain amount of ash ends up in the ocean and stimulates extra growth of algae. Some of the carbon ends up at the bottom of the ocean – sequestered and buried.”

Over a period of years or decades after each eruption, the ash settled into the oceans, where the iron served as a sort of fertilizer, stimulating the growth of algal organisms, like tiny plankton. Similar to plants (but not classified as plants), algae need sunlight, consume nutrients and excrete oxygen. Algae need iron to make chlorophyll, which in turn allows photosynthesis.

As the algae drew CO2 from the oceans and the atmosphere, global temperatures fell. This series of eruptions episodically replenished the oceans with iron and stimulated algal growth over a period of 35 million years.

The team theorizes that nearly a half million cubic kilometers of iron-bearing ash and rock were ejected into the atmosphere. Some of the ash would have rained iron in the mid-ocean regions – areas that have been shown to be typically low in this critical nutrient that algae need to thrive. This volcanic material would have fertilized a global explosion of algae blooms. When thriving, algae create “carbon sinks” in the oceans – the opposite of the greenhouse effect. Using geochemical models, the team posits that the planet’s algae could have consumed enough carbon dioxide to cool the planet.

After the article first appeared in Geosphere, the Discovery Channel’s website published an article about Cather’s research. That report, which included an interview with Cather was then picked up by more than a dozen websites and blogs.

“I was kind of amazed at the reaction,” Cather said. “This is truly only a hypothesis, but I knew it would generate a buzz among scientists. I’ve been doing geology for decades and this is the first thing that has generated interest from more than just geologists.”

– NMT –

By Thomas Guengerich