How do you date the disappearance of something? That is what Curtin University geologist Milo Barham asked, summarizing the dilemma that has fascinated scientists for years as they try to solve the mystery of Western Australia’s Pinnacles Desert limestone pillars. For many years, the origin of these bizarre karst spires five-meter-high columns scattered throughout the Nambung National Park in Western Australia was a mystery, with wildly divergent age estimates ranging from 25,000 to more than 200,000 years.
In recent years, a union of geochemical detective science and cutting-edge dating methods provided the final answer, and in the process, uncovered a verdant history of the Earth’s climatic past and its bearing on the future. The key was one of the tiniest of hints: tiny iron-rich nodules on the columns of limestone. The minute nodules, almost impossible to see with the naked eye, are geologic timekeepers. As outlined in the Science Advances research, the scientists utilized (U-Th)/He geochronology, a technique that measures the helium atoms that have built up as a result of the radioactive decay of uranium and thorium in the iron nodules.
By looking at the proportion of these isotopes, the researchers could accurately determine when the nodules and therefore the Pinnacles themselves were formed. The result is breathtaking: the Pinnacles were formed during a short, but immensely wet period some 102,000 years ago, the area’s most moist in the last half-million years. It was a time when rain, probably due to variations in atmospheric circulation or the very ancient Leeuwin Current, was so heavy that it dissolved vast parts of the Tamala Limestone to form the spires that characterize the current landscape.
“We found this period was locally the wettest in the past half-million years, distinct from other regions in Australia and far removed from Western Australia’s current Mediterranean climate,” said Dr. Matej Lipar, lead author of the study, in a press release. Karst landscapes like the Pinnacles result from the dissolution of soluble rocks limestone, dolomite, gypsum by water, which forms features like caves, sinkholes, and towers. The process is climate-sensitively dependent, with wet periods accelerating dissolution and reshaping entire regions with it. But, as Barham noted, the very character of karst defined by what has passed means that it is notoriously difficult to date.
Traditional methods, such as bracketing for underlying and overlying ages of material, yield only broad estimates and obscure the true chronology of climatic events. Application of (U-Th)/He dating of ferricrete nodules is a technical improvement. The researchers carefully selected and characterized more than forty pieces from six nodules using methods including x-ray diffraction, scanning electron microscopy, and isotope dilation mass spectrometry to determine the mineralogy and elemental association. Their conclusion, a statistically consistent age of 102.8 +10.6/−11.4 thousand years, precisely dates the wettest interglacial Marine Isotope Stage 5c of the region’s recent geological past.
This age is supported by optically stimulated luminescence and U-Th ages of the enclosing aeolianite and cements, providing an added level of confidence to the method. Aside from deciphering a geological enigma, the research is of far-reaching significance to paleoclimatology. Karst features are not exclusive to Western Australia; such karst formations occur on a global basis, from the Mediterranean to the Caribbean and South Africa. Having the potential to date with high precision karstification events by iron nodules creates new horizons for reconstruction of climate change in the past at high resolution.
As Dr. Lipar described, “Studying them within an accurate timeline helps us understand how Earth’s geological systems respond to climate shifts.” The duration and timing of the wet phase responsible for creating the Pinnacles are very different from the aridity of the region today. Today, southwestern Australia is Mediterranean in climate, having severe droughts and reduced rainfall a definite indication of the active range of natural climate variability. The study findings provide important data to climate models to improve projections of how landscapes and ecosystems may respond to future change.
As Barham noted, “This new knowledge will enhance our understanding of global environments and ecosystems, helping us prepare for, and mitigate the impacts of, a warming planet.” The (U-Th)/He method’s technical authority also eliminates long-standing limitations in dating features in the karst. Previous methods, such as paleomagnetism or U-Th dating of ferricretes, often yielded results with huge uncertainties or were compromised by low uranium and thorium concentration. The new approach, including selective sample selection, high-resolution microscopy, and strong statistical analysis, enables scientists to separate truly formation ages from anomalously old ages caused by mineral inclusion. The methodological breakthrough will likely revolutionize landscape evolution and paleoclimate research, not just in Australia but worldwide.
In parallel with the Pinnacles study, speleothem research secondarily deposited cave calcite has generated equally accurate records of climate history based on other isotopic systems. U-Th and U-Pb speleothem ages at sites such as the Nullarbor Plain have yielded hydroclimatic records extending several millions of years, giving concrete reality to the potential of the carbonate system as a climate proxy. Taken together, these methods make up a valuable toolkit with which to disentangle the interrelationship between climate, landscape, and life on a timescale encompassing geology. As climate change gathers pace, the record preserved in the Pinnacles’ iron nodules and limestone pillars becomes increasingly important. They remind us that the environments of the Earth are formed by periods of cataclysmic change, some gradual, some abrupt, and that history is the best teacher of the secrets of the future.
