Catching an Ancient Raindrop

water-cannon-gold-rush-era.jpgThey were after gold, these miners who shot streams of water through a cannon against Sierra Nevada mountain faces, cracking them open in hopes that treasure would spill out. Armed with these hydraulic hammers, miners could blast away “half a mountain in a few minutes,” according to a historian of the Gold Rush era. It was the kind of environmental assault that a fever for wealth often would inspire during this country’s first two centuries.

The crushing blows from the water cannons exposed soil and rock from the Eocene era — 40 to 50 million years ago. Embedded in those minerals are ancient raindrops. A team of geological researchers from Stanford University conducted chemical analysis of those raindrops, and concluded that the mammoth granite mountain range that cradles Yosemite Valley is much, much older than commonly believed.

From Science Blog:

(I)n a study published in the July 7 edition of the journal Science, Chamberlain and Stanford colleagues Andreas Mulch and Stephan A. Graham present strong evidence that the Sierra Nevada range has stood tall–7,200 feet (2,200 meters) or higher–for at least 40 million years.

“An elevation profile drawn across the northern Sierra Nevada 40 to 50 million years ago would not look much different than today’s profile,” said Graham, the Welton Joseph and Maud L’Anphere Crook Professor of Applied Earth Science at Stanford.

vernal_fall_2_yosemite.jpg“Those mountains probably have persisted since the Mesozoic Era–more than 65 million years ago–until today,” Chamberlain added. Back then, according to many scientists, California was split by an ancient subduction zone–a region of constant geologic upheaval, where a plunging oceanic tectonic plate continuously pushed the continental North American plate higher and higher to create the Sierra Nevada range.

This version of events is in sharp conflict with the “recent uplift” scenario, which argues that the Sierra rose from sea level to 7,200 feet about 3 million to 5 million years ago after an enormous block of the Earth’s crust broke off and fell into the mantle. According to this hypothesis, the crust was then replaced by hot, buoyant mantle material that eventually raised the mountains. Although the Science study found no evidence to support this scenario, data revealed that a modest uplift of 1,100 to 2,000 feet (350 to 600 meters) did occur as recently as 3 million years ago.

How do you catch an “ancient raindrop?” How do you get that raindrop to tell you its secrets?

(T)he scientists used an increasingly popular research tool that combines geology and chemistry to create a record of prehistoric rainfall patterns dating back millions of years. This technique relies on the fact that in nature, hydrogen and other atoms occur in different sizes called isotopes. Deuterium, for example, is a slightly heavier form of hydrogen, and drops of rainwater that contain deuterium isotopes often fall at lower elevations.

“If you have a cloud coming in and dropping out water, as it climbs the mountain its preference is to first drop the heavy water that’s rich in deuterium,” Chamberlain said. “As you go up in elevation, the raindrops become lighter and lighter. Therefore, the rainwater becomes gradually depleted of deuterium the higher up the mountain range it falls.”

Over time, some raindrops are incorporated into molecules of clay and other minerals that form on the ground. These clays provide scientists with a geologic record of ancient precipitation, which can then be compared with samples of modern precipitation collected at the same altitude. If the comparison reveals similar isotopic ratios, then the elevation of the mountain must have been similar in ancient and modern times.

The Stanford researchers believe the crest of the Sierras was once the western edge of the Great Basin of Nevada and Utah — before it was a basin. It was a large plateau that “basically collapsed,” according to Chamberlain.

Can the study of ancient raindrops help us understand global climate patterns? Yes, and that’s one reason why the Stanford team is engaged in this research. To model future climate change, Chamberlain says,

“There are basically six large mountain ranges climatologists need to know the history of–western North America, the Himalayas, Antarctica, Greenland, the spine down Africa and the Andes,” Chamberlain noted. “To get an idea of what’s going to happen if carbon dioxide levels double in the future, you’d have to go back 20 or 30 million years in time. If you knew what the topography of these six mountain ranges was then, you could include that in your computer models and see how they respond when you double the carbon dioxide.”