The analysis of sedimentary rocks allows us to go back to the climatic past of our planet by reading its different layers, in particular their composition, and by estimating, among other things, the processes involved. Recently, researchers discovered an 80-meter-thick “sedimentary sequence” in the Charyn Canyon, Kazakhstan. Thus, this astonishing discovery makes it possible to go back to five million years ago in the climatic past of the Earth.
” The 80-meter-thick sedimentary sequence that we found at the Charyn Canyon in southeastern Kazakhstan provides us with a virtually continuous record of five million years of climate change. It is a very rare phenomenon on Earth! », Explains palaeologist Charlotte Prud’homme, who until recently worked at the Max Planck Institute for Chemistry, now a researcher at the University of Lausanne.
The alternation of dust and soil layers provides the first reliable evidence, in one location, of long-term interactions between major climate systems on the Eurasian continent. ” Over the past five million years, Eurasian land surfaces appear to have contributed more actively to the land-atmosphere-ocean water cycle than previously recognized. The sediments preserved in the Charyn Canyon served as a litmus test for the influx of fresh water into the Arctic Ocean, stimulating the transport of moist air masses from the North Atlantic to land via airflows. from the west », Explains Prud’homme, corresponding author.
The researchers focused their research on the Pliocene and Pleistocene periods. The Pliocene, which extends from 5 to 2.6 million years ago, represents the best analogy with the climatic conditions of the Anthropocene: this geological period was the last when the concentration of carbon dioxide in the atmosphere was comparable to that of today, around 400 parts per million (ppm). ” This is why the knowledge we have gained from the sediments of the Charyn Canyon is so essential to understanding the future climate. », Explains Prud’homme. The results of the study were published in the journal Communications Earth and Environment.
Until now, little was known about the role of Central Asia in shaping the global climate past and present. The evolution of the Earth’s climate over the past five million years has been understood primarily from the perspective of marine mechanisms. In contrast, the importance of climate feedbacks that originated on land – rather than in oceans, lakes or ice cores – has remained largely unexplored. The international research team filled this gap with their field study in the Charyn Canyon.
The importance of studying a place far from the ocean
” We needed to find a place inland and as far from the ocean as possible Says Kathryn Fitzsimmons, head of the Earth’s Paleoclimate Reconstruction research group at the Max Planck Institute for Chemistry. ” We could hardly find a more continental location than in Charyn Canyon in Southeast Kazakhstan “. The geographical location of the study site, in the middle of Central Asia, was therefore of paramount importance to the team.
The canyon’s semi-arid climate, and its landscape, was shaped by the interplay between mid-latitude westerly winds and high-latitude polar fronts, as well as sediments 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 sediment succession and took booster samples to ensure continuous coverage of the record. By measuring the relative concentrations of isotopes in soil carbonates, they reconstructed the evolution of moisture availability in the soil over time.
A combination of paleomagnetic analyzes and absolute uranium-lead dating of soil carbonates allowed the age and rates of sediment accumulation to be determined. Soil samples revealed an area characterized by increasing aridity over the past five million years. At the start of the Pliocene, the soil was significantly wetter than in later times or than the current climate. This aridification process was not, however, linear according to the researchers; it was interrupted by short-term climatic fluctuations that provide insight into the interaction between mid-latitude westerly winds and the Siberian high pressure system.
Changes in soil moisture: a potential indicator of river activity
This research has also made it possible to study the long-term interaction between the Siberian high pressure and rain-bearing westerly winds. ” We are convinced that the changes in soil moisture that we have seen on our site can also be used as an indicator of the activity of the Siberian rivers further north. Says Kathryn Fitzsimmons.
The hydroclimate of Charyn Canyon mirrors that of the steppe to the north, where a number of major Siberian rivers flow, such as the Irtysh and Ob, she says. These are also influenced by the dynamics of the Siberian high pressure and the westerly air masses. One particular phase where this connection is important stands out: a sustained period of wet conditions at Charyn Canyon just before the world’s first great glaciation, about 3.3 million years ago. It is likely that these wet conditions extended to the Siberian rivers to the north, whose freshwater flow to the Arctic Ocean may have passed a tipping point for widespread sea ice formation.
Information from this most comprehensive terrestrial climate archive for the past five million years provides a very valuable basis for future climate models. ” We opened a door », Says Charlotte Prud’homme. ” The response of land surface dynamics to Plio-Pleistocene climate change in Central Asia likely generated terrestrial feedbacks affecting oceanic and atmospheric circulation. This missing earth link helps elucidate the importance of land-water feedbacks for long-term global climate », Conclude the researchers.