Appalachian Mountains Rock Ice Age
The Appalachian mountains are seldom mentioned among the world's great mountain ranges. Consisting mostly of low gentle ridges, when compared with the snow-capped peaks of the Himalaya, Andes, or Alps many would hesitate to call them mountains at all. But they are a large and ancient range, stretching over 1500 miles along the eastern portion of North America. The time of their formation has been dated back to the Paleozoic, with major uprisings occurring 650 million years ago. Then, about 460 million years ago, during the Ordovician period, they were the site of one of the most violent volcanic outbursts in Earth's history. New research reveals that, following that bout of vulcanism, weathering of Appalachian rock may have triggered one of Earth's major ice ages—a relatively brief frigid period that ultimately killed two-thirds of all species on the planet.
As the eastern edge of what became the North American continental plate overrode the basin of an ancient ocean, volcanoes sprang up in what are today the Taconic Mountains of New York state and New England. These volcanoes produced enough lava to form mountains as high as the Alps and also emitted a tremendous amount of carbon dioxide (CO2), perhaps more than at any time in Earth's past. This outgassing created greenhouse-gas levels 20 times higher than they are today.
North America during the late Ordovician. Credit: Dr. Ron Blakey, NAU Geology.
That much CO2 should have kept the climate warm for eons, but a short 10 million years later atmospheric levels of the gas began to plummet. 5 million years after that, Earth entered a severe ice age. Seth A. Young et al., writing in the October issue of Geology, have proposed that weathering of rock in the Appalachian chain triggered the dramatic climate change. Their hypothesis is based on studying three sedimentary rock formations in Nevada, where sediments had washed down from the Appalachians 460 million years ago.
Previous research based on oxygen isotope excursions indicate that the end-Ordovician event lasted only about 500,000 years. The rapid drop in sea levels is an indication of how quickly continental ice-sheets formed, ushering in the End-Ordovician Ice Age. This caused a worldwide reduction in sea level of approximately 260 feet (80 meters), which dried up and exposed the extensive shallow-water continental shelves that existed throughout the world at that time. This led to the extinction of large numbers of species who depended on this shallow water environment. Scientists count the extinction at the end of the Ordovician as the second worst in terms of lost marine species after the great Permian-Triassic extinction 251 mya.
More than 60% of sea life went extinct during the End-Ordovician Ice Age.
In the new study, the major clue as to what happened came from analyzing isotopes of the element strontium. Most sediments sport traces of seven parts strontium-87 (87Sr) to 10 parts strontium-86 (86Sr), a ratio of 0.7. But the Nevada limestone showed a ratio of 0.6, the biggest disparity ever recorded. According to the researchers, the most likely reason the sampled basalt is rich in 86Sr is heavy limestone runoff from the Appalachians. As described in the paper's abstract:
A large drop in seawater 87Sr/86Sr during the Middle Ordovician was among the most rapid in the entire Phanerozoic. New 87Sr/86Sr measurements from Nevada indicate that the rapid shift began in the Pygodus serra conodont zone of the upper Darriwilian Stage. We use a numerical model to explore the hypothesis that volcanic weathering provided the flux of nonradiogenic Sr to the oceans. A close balance between volcanic outgassing and CO2 consumption from weathering produced steady pCO2 levels and climate through the middle Katian, consistent with recent Ordovician paleotemperature estimates. In the late Katian, outgassing was reduced while volcanic weathering continued, and resulted in a cooling episode leading into the well-known end-Ordovician glaciation.
Young and colleagues propose the following scenario: As CO2 induced acid rain fell on the rocks, it formed limestone that washed into the Nevada sea, locking away huge amounts of carbon from the atmosphere. Then, as the volcanism ended about 450 million years ago, the sequestering continued, thinning CO2 levels significantly. Back then the Sun was dimmer and could not keep the atmosphere warm without the high levels of CO2. Plummeting CO2 levels hastened the eventual onset of the ice age. All this took place 350 million years before the Rocky Mountains began to form.
“Our model shows that these Atlantic volcanoes were spewing carbon into the atmosphere at the same time the Appalachians were removing it,” explains study co-author Mathew Saltzman. “For nearly 10 million years, the climate was at a stalemate. Then the eruptions abruptly stopped, and atmospheric carbon levels fell well below what they were in the time before volcanism. That kicked off the ice age,” he said.
Bear in mind that, when it is reported that CO2 levels plummeted, it was from levels not seen since that age. Estimates are that roughly half of the CO2 in the atmosphere was sequestered, leaving levels only 10 times today's. In more recent times, the only spike in atmospheric CO2 that compares was the Pliocene Eocene Thermal Maximum (PETM) around 56 mya. That event only elevated CO2 levels to 4 or 5 times modern levels.
Volcanic ash from Appalachian volcanoes created the dark line in this rock formation in Tennessee.
Credit: M. Saltzman, Ohio State University.
Geologist Curtis R. Congreve of the University of Kansas, has called the End- Ordovician an ice age in the middle of a greenhouse. CO2 levels did not dip to modern levels until the formation of the great Carboniferous coal swamps around 300 million years ago, 150 million years after the End-Ordovician Ice Age (see my article “The Grand View: 4 Billion Years Of Climate Change”).
“What is really neat” about the current study, says University College London geochemist Graham Shields in the journal Science, is that it solves the long-standing mystery about the timing of the ice age and the end of the Ordovician (see “The Mountains That Froze the World”). One idea was that the Appalachian volcanoes themselves sparked the big chill by spewing sun-blocking ash into the air. But that didn't jibe with the fact that the ice age started 5 million years after the eruptions stopped. The new study shows, Shields says, how the cooling effect “really kicks in” when the volcanism ends.
By the time humans first walked their slopes, the Appalachian mountains had worn down from prehistoric peaks over 16,000 ft (5,000 m) high to 6,500 ft (2,000 m) today. Today, the plant and animal life of the Northern Appalachians is still recovering from the last glacial period, which ended 10,000 years ago. The Southern Appalachians were never buried beneath the great ice-sheets of that frigid time, providing refuge for native species. Often spreading a couple hundred miles wide, the chain's north-south alignment allowed easy migration of animals fleeing the glaciers. Had these mountains been aligned east-west, like the European Alps, they would have presented a barrier to migration, a trap that would have ensured mass extinctions.
The Appalachian Mountains are a long system of mountains, stretching from Newfoundland in Canada all the way to Alabama in the southern United States. In my youth I hiked portions of the Appalachian Trail, a popular destination for nature lovers. Many make hiking the entire length of the trail a lifetime goal, which is a significant undertaking as the full trail in the United States is 2,174 miles long.
What lessons can be learned about climate change from events nearly a half billion years ago, when life on our planet was much different than it is today? To start with, the dramatic swings in global temperature during the last half of the Ordovician required massive amounts of CO2, some 20 times the amount in our atmosphere today. To generate that much CO2, our planet had to undergo one of the greatest volcanic outbreaks in the history of life on Earth. Mountain ranges were raised and then eroded, with their remains forming sediments at the bottom of the sea. Yet life carried on.
Granted, it would not have been pleasant to live through the events of 450 million years ago. They should serve as a reminder of the gigantic forces that are required to cause dramatic changes in the world's climate. Some have claimed that this study reinforces the notion that CO2 levels in the atmosphere are a major driver of Earth's climate. CO2 can cause global warming, but only at 10-20 times today's levels. Anyone who thinks that man's puny efforts can push Earth's climate system into irreversible and unprecedented change does not understand the history of our planet. Anyone who thinks that anthropogenic global warming caused by human CO2 emissions is a big deal need only gaze upon the worn down bones of the Appalachians, a mountain range once as tall and as rugged as the Alps, to understand mankind's insignificance.
Be Safe, enjoy the interglacial and stay skeptical.