Karst landscapes, with their sculptural appearance, cover nearly one-sixth of the Earth's surface. These natural formations, shaped by water over thousands of years, are as impressive as they are difficult to date.
Their importance goes beyond their beauty. Karsts reveal crucial information about Earth's past climates, but determining their age has long been complicated for researchers.
Pinnacles: limestone pillars, measuring up to 13 feet (4 meters) tall, in Nambung National Park.
The national park is located in Western Australia, near the town of Cervantes.
Image Wikimedia
These landscapes form through the dissolution of rocks by water. However, this dissolution makes it difficult to estimate their precise age, as it involves dating what has disappeared. Until now, scientists have relied on elements above and below these formations, a method often imprecise.
A major breakthrough has been made thanks to (U/Th)-He geochronology, which measures the isotopes of helium, uranium, and thorium in microscopic iron nodules. These nodules form at the same time as the karst, thus allowing for precise dating of their formation. This method was tested in the Pinnacle Desert, in Western Australia.
The Pinnacles site, famous for its giant limestone formations, provides an ideal field for this study. Fragments of iron nodules found in the soil above these rocks were analyzed, and their age was estimated at around 100,000 years. These results are consistent with the ages of the surrounding rocks.
This formation period corresponds to one of the wettest periods in Western Australia's recent history. Researchers still do not know what caused this peak in precipitation, but changes in atmospheric circulation or the influence of the Leeuwin Current, a tropical ocean current, are being considered.
These iron-rich nodules are not limited to the Pinnacles. They are present in other Australian regions and thus offer a way to trace global climate variations over the past three million years. This scientific advancement could help better understand the connections between karst formation and the climatic conditions that influenced ancient ecosystems, including those in which our hominid ancestors lived.
The study of karsts and past wet periods also reveals crucial information for predicting how our current landscapes and ecosystems will respond to ongoing climate change. Understanding past developments helps us anticipate and mitigate the future impacts of these upheavals.