Five million years of climate change preserved in one place

The researchers focused their investigation on the Pliocene and Pleistocene periods. The Pliocene, five to 2.6 million years ago, represents the best analogue for the climatic conditions of the Anthropocene: this geologic time period was the last time concentration of carbon dioxide in the atmosphere was comparable to today, around 400 parts per million (ppm). "That's why our insights from the Charyn Canyon sediments are so essential for understanding future climate," Prud'homme says.

Until now, little has been known about the role Central Asia plays in global climate evolution past and present. Earth's climate evolution over the past five million years has been understood mainly from the perspective of marine mechanisms. In contrast, the significance of climate feedbacks that originated on land -- rather than in the oceans, lakes or ice cores -- has remained largely unexplored. The international research team has filled this gap with their field research in Charyn Canyon.

Interactions between mid- and high-latitude climatic systems

The geographical location of the study site in the middle of Central Asia was of key importance to the team: "We needed to find a place that was inland and as far away from the ocean as possible," Kathryn Fitzsimmons, Group Leader of the Terrestrial Paleoclimate Reconstruction Research Group at the Max Planck Institute for Chemistry, explains. "We could hardly find a more continental situation than at Charyn Canyon in southeastern Kazakhstan." The semi-arid climate of the canyon, and its landscape, was shaped by the interaction between the mid-latitude westerlies and the high-latitude polar fronts, and by sediment transported from the nearby Tien Shan mountains. Charyn Canyon is ideal, according to Kathryn Fitzsimmons, for studying long-term land-climate feedback mechanisms.

The researchers examined the 80-meter-thick sedimentary succession and sampled by abseil to ensure continuous coverage of the record. By measuring the relative concentrations of isotopes within soil carbonates, they reconstructed the changing availability of moisture in the soil through time. A combination of paleomagnetic analyses and absolute uranium-lead dating of soil carbonates established the age and accumulation rates of the sediment record. The soil samples revealed a region characterized by ever-increasing aridity over the last five million years. In the early Pliocene, the soil was significantly wetter than in subsequent epochs or than today's climate. This process of aridification was not linear, however; it was interrupted by short-term climate fluctuations which provide insights into the interaction between the mid-latitude westerly winds and the Siberian high-pressure system.

Interaction between the Siberian high and rain-bringing westerlies

The research at Charyn Canyon enabled the scientists to investigate the long-term interaction of the Siberian high with the rain-bringing westerlies. Kathryn Fitzsimmons says: "We're confident that the changes in soil moisture we found at our site can also be used as a proxy for Siberian river activity further north." The hydroclimate at Charyn Canyon reflects that of the steppe to the north, from where a number of large Siberian rivers, such as the Irtysh and Ob, flow, she says. These are similarly influenced by the dynamics of the Siberian high and westerly air masses. One particular phase where this link is important stands out: a sustained period of wet conditions at Charyn Canyon just prior to the first major global glaciation around 3.3 million years ago. It is likely that these wet conditions extended to the Siberian rivers to the north, whose outflow of fresh water to the Arctic ocean may have breached a tipping point for widespread increased sea ice formation.

The information from this most complete terrestrial climate archive for the past five million years provides a very valuable basis for future climate models. Charlotte Prud'homme literally says, "We have opened a door."