At about 4,600 meters above sea level, a patch of blue water perched on the Tibetan Plateau reflects the cloudy sky. The rolling hills surrounding Jiang Co, as the high-elevation lake is known, are mostly barren aside from spiky clumps of grass intermingled with cobbles.
But between 600 and 800 CE, the hills may have been greener, the flats grassier. Incoming rivers may have roared, with animals and even a village possibly sharing the banks. That’s according to paleoclimatologist Juzhi Hou, who, with colleagues, is finding new records of the Tibetan Plateau’s human and climate history buried in Jiang Co.
“We’re trying to understand why, against the background of climate at such high elevations, this civilization emerged and then fell.”
According to historical documents, small, independent tribes resided on the Tibetan Plateau prior to 600, consistent with a likely cool and dry climate that couldn’t support larger societies, Hou said. However, during the next 2 centuries—the Sui-Tang Warm Period—the Tibetan Empire conquered much of the Tibetan Plateau and beyond and controlled extensive branches of the Silk Road that connected eastern Asia to the West.
Then, the empire all but disappeared.
“We’re trying to understand why, against the background of climate at such high elevations, this civilization emerged and then fell,” said Hou, who is with the Institute of Tibetan Plateau Research at the Chinese Academy of Sciences. Aside from written records kept by the Chinese courts, details of the Tibetan Empire are hard to find, he said. “We don’t have [much] hard evidence.”
In new research to be presented at AGU’s Annual Meeting 2023, postdoctoral researcher Kejia Ji, Hou, and colleagues wove together proxies sourced from a precisely dated meter-long sediment core from Jiang Co that archives 2,000 years of high-elevation history. Their findings illuminate an unusually warm and wet period, which would have played an important role in a flourishing Tibetan Empire between 600 and 800.
Titanium, Oxygen, and Alkenones
Geochemical proxies can help scientists reconstruct climate history. For instance, the element titanium cannot be generated within a lake, originating instead in the surrounding rocks, Hou said. Transported into lakes by runoff, more titanium means more weathering and therefore more precipitation. The Jiang Co core’s titanium concentrations between 600 and 800 CE show just that.
Unlike titanium, which provides information about what was happening outside the lake, oxygen isotopes can provide a look within. Heavy oxygen prefers to stay in water, whereas light oxygen favors evaporation. By measuring these isotopes in calcite formed directly from lake water, Hou and colleagues explored whether or not there was a lot of evaporation. The core’s oxygen isotopes hint at high humidity between 600 and 800.
For temperature, the team turned to organic molecules called alkenones, which at Jiang Co are produced by certain algae, Hou said. Alkenones typically feature 37 carbon atoms strung together with a combination of single and double bonds. The more double bonds there are, the bendier the molecule is. The ratio of different alkenones—bendy or straight—can be converted to water temperature. Between 600 and 800, conditions were comparatively warm, according to these analyses.
But just because it was warm and wet doesn’t mean humans were present. One way to find out is to search for something humans and animals always leave behind: feces.
Unique Diet, Unique Poop
Fecal stanols are organic molecules excreted by many mammals. Consumers of meat have higher levels of certain stanols that result from processing cholesterol in the gut, whereas herbivores encounter and process a different sterol (beta-sitosterol, a plant sterol that’s similar to cholesterol), producing a distinct stanol that’s also left behind, Ji said. Fecal stanols can persist in soils for thousands of years, and tracing the ratio between omnivore and herbivore stanols can help scientists track human populations.
Those yaks, sheep, and other animals (such as the kiang, the largest species of wild ass) are herbivores unique to this region, and their poop should reflect that.
Though past studies have established fecal stanol proxies elsewhere, Ji is leading an effort to create a proxy for the Tibetan Plateau. Thus far, it includes more than 100 modern samples from both animals and humans. “The Tibetan diet heavily relies on meat and dairy from yaks and sheep and includes less vegetable intake,” he said. Those yaks, sheep, and other animals (such as the kiang, the largest species of wild ass) are herbivores unique to this region, and their poop should reflect that. This regional database is currently dominated by domesticated herbivores, but Ji is working to include more wild animal dung.
Preliminary analysis of fecal stanols in the Jiang Co core did not show direct evidence of intense human activity between 600 and 800; this result may have been influenced by the small catchment size, Ji said. However, herbivorous stanols increased during this time, “suggesting that vegetation provided more food for domestic and wild herbivores under warm and humid conditions,” he said.
More Proxies, More Poop
Shoring up these signals from the Tibetan Plateau is especially important because the Sui-Tang Warm Period does not appear to be a global signal, said Xiaozhong Huang, a palynologist at Lanzhou University who was not involved with this study. Instead, this temperature spike is largely confined to studies conducted on and around the Tibetan Plateau. Applying additional proxies, such as certain lake-sourced microfossils that tend to grow larger as temperature increases, might help, said Huang.
In addition, fungal spore analysis could point to a pastoral society, as has been done for lakes in the northeastern part of the plateau and northern China. To better understand human activity, Ji will continue to refine the Tibetan Plateau fecal stanol proxy populated by modern poop. He and his colleagues will also look to other lakes with archaeological sites, of which they have found several.
“If we have more proxies, we can have [a] more comprehensive understanding of the environment,” Huang said, “to reconstruct the whole picture of the past climate.”
—Alka Tripathy-Lang (@DrAlkaTrip), Science Writer
