New study chronicles Central Asia's "de-greening"

A new study illustrates the dramatic climatic shifts wrought by the rise of new mountain ranges and chronicles how Central Asia dried out over the last 23 million years into one of the most arid regions on the planet.

While researchers have long cited the uplift of the Tibetan Plateau and the Himalayan Mountains around 50 million years ago for blocking rain clouds' entry into Central Asia from the south, killing off much of the region's plant life, the new study paints a more nuanced picture of Central Asia's desertification.

"While Central Asia was probably never lush and verdant, it was certainly greener 23 million years ago and probably even greener in the more distant past," said Jeremy Kesner Caves, a doctoral student at Stanford University's School of Earth, Energy & Environmental Sciences and the lead author of the study published online in the journal Geology.

The researchers arrived at their conclusions after measuring a particular isotope, or version, of carbon in buried, ancient soil samples. The isotope speaks to the dryness of conditions at the time of the soil's deposition. Wetter, rainier conditions allow for greater numbers of organisms, including plants and soil-dwelling bacteria, to thrive and pull carbon out of their surroundings to fuel their growth and metabolism, leaving telltale isotopes in their environment.

Caves and his co-authors looked at samples over extensive geographic and temporal spans in order to draw a fuller portrait of the climatic changes influencing soil composition. "Our paper is the first-ever attempt to present maps of carbon isotopes over a geologic time frame of more than a million years," he was quoted as saying in a news release from Stanford.

He and several co-authors traveled to Mongolia, eastern Kazakhstan and northern China to collect the bulk of 171 new soil samples, while Russian co-authors collected samples near Lake Baikal. The new specimens were considered alongside more than 2,200 previously collected samples. Because most of those existing samples originated from the Tibetan Plateau, the research team plugged a gap in the geographical coverage by going to little-studied northern Central Asia.

Overall, the samples were well-distributed from 23 million to 2.6 million years ago during a geological period known as the Neogene. The Earth's climate cooled off substantially as the Neogene wore on, setting the stage for an Ice Age when glaciers crept from polar regions into lower latitudes. The samples "are honestly pretty boring," Caves said. "They look and feel like dirt." But the rocky outcrops exposing the old, hardened soil chunks can dazzle. "The outcrops are striped deep purple, red and green, and they often erode in crazy patterns."

Upon analysis, the samples' carbon isotope values revealed an exceptionally arid region deep in Asia's interior going back 23 million years, initially ringed by areas of higher rainfall. Starting about five million years ago, however, that dry region expanded to the north and west, as new mountain ranges reached heights sufficient to block westerly winds from delivering moisture.

"One way to think about this change is that when viewed from space today, Central Asia appears very brown because of its expansive deserts," Caves said. "If viewed from space 23 million years ago, though, Central Asia would have looked somewhat darker, simply due to there being considerably more leaves and vegetation." Endit