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Photo of a rocky pinnacles in a yellow desert landscape beneath a blue sky.
Limestone pinnacles of the Nambung National Park karst. Matej Lipar

Limestone and iron reveal puzzling extreme rain in Western Australia 100,000 years ago

Almost one-sixth of Earth’s land surface is covered in otherworldly landscapes with a name that may also be unfamiliar: karst. These landscapes are like natural sculpture parks, with dramatic terrain dotted with caves and towers of bedrock slowly sculpted by water over thousands of years.

Karst landscapes are beautiful and ecologically important. They also represent a record of Earth’s past temperature and moisture levels.

However, it can be quite challenging to figure out exactly when karst landscapes formed. In our new work published today in Science Advances, we show a new way to find the age of these enigmatic landscapes, which will help us understand our planet’s past in more detail.

Photo of a complex eroded cave landscape
Flowstones, stalactites and caverns within Jenolan Caves, NSW, Australia. Matej Lipar

The challenge

Karst is defined by the removal of material. The rock towers and caves we see today are what is left after water dissolved the rest during wet periods of the past.

This is what makes their age hard to determine. How do you date the disappearance of something?

Traditionally, scientists have loosely bracketed the age of a karst surface by dating the material above and beneath. However, this approach blurs our understanding of ancient climate events and how ecosystems responded.

Geological clocks

In our study, we found a way to measure the age of pebble-sized iron nodules that formed at the same time as a karst landscape.

This method has the technical name of (U/Th)-He geochronology. In it, we measure how much helium is produced by the natural radioactive decay of tiny amounts of the elements uranium and thorium in the iron nodules. By comparing the amounts of uranium, thorium and helium in a sample, we can very accurately calculate the age of the nodules.

Cartoon graphic showing the process of iron nodules binding radioactive elements during initial growth that over time decay to produce measurable helium that can act as a geological clock
How iron nodules can reveal their age. Milo Barham

We dated microscopic fragments of iron-rich nodules from the iconic Pinnacles Desert in Nambung National Park, Western Australia.

This world-famous site is renowned for its otherworldly karst landscape of acres of limestone pillars towering metres above a sandy desert plain. The Pinnacles form part of the most extensive belt of wind-blown carbonate rock in the world, stretching more than 1,000km along coastal southwestern WA.

Photo of lab equipment consisting of multiple grey containers.
The Western Australia ThermoChronology Hub (WATCH) ultra-high vacuum gas extraction line for measurements of radiogenic helium. Martin Danišik

We examined multiple microscopic shards of iron nodules that were removed from the surface of limestone pinnacles. These nodules formed in the soil that lay on top of the limestone during the period of intense weathering that created the karst. As a result, they serve as time capsules of the environmental conditions that shaped the area.

Greyscale electron microscope image of various shaded blobs.
A scanning electron microscope image of iron-rich cement (lighter grey in centre) binding darker grey, rounded quartz sand grains within an analysed nodule. Aleš Šoster

The big wet

We consistently found an age of around 100,000 years for the growth of the iron nodules. This date is supported by known ages from the rocks above and beneath the karst surface, proving the reliability of our new approach.

At the same time as chemical reactions caused growth of the iron-rich nodules within the ancient soil, limestone bedrock was rapidly and extensively dissolved to leave only remnant limestone pinnacles seen today.

Aerial photo of a landscape and another pic of two people standing next to a rocky pinnacle.
Left: a drone image of the Pinnacles karst landscape, Nambung National Park, Western Australia. Right: authors M. Lipar (L) and K. McNamara (R) discuss a limestone pinnacle. Sue Radford

From examining the entire rock sequence in the area, we think this period of intensive weathering was the wettest time in this part of WA over at least the past half a million years.

We don’t know what drove this increased rainfall. It may have been changes to atmospheric circulation patterns, or the greater influence of the ancient Leeuwin Current that runs along the shore.

Such a humid interval is in dramatic contrast to the recent droughts and increasingly dry climate of the region today.

Implications for our past

Iron-rich nodules are not unique to the Nambung Pinnacles. They have recently been used to track dramatic past environmental change elsewhere in Australia.

Dating these iron nodules will help to better document the dramatic fluctuations in Earth’s climate over the past three million years as ice sheets have grown and shrunk.

Understanding the timing and environmental context of karst formation throughout this time offers profound insights into past climate conditions, environments and the landscapes in which ancient creatures lived.

Photo of tiny dark blobs attached to a rocky pillar.
Dark iron-rich nodules attached to the side of the base of a limestone pinnacle in the Nambung National Park. Matej Lipar

Climate changes and resulting environmental shifts have been crucial in shaping ecosystems. In particular, they have had a profound influence on our ancient hominin and human ancestors.

By linking karst formation to specific climatic intervals, we can better understand how these environmental changes may have affected early human populations.

Looking forward

The more we know about the conditions that led to the formation of past landscapes and the flora and fauna that inhabited them, the better we can appreciate the evolutionary pressures that shaped the ecosystems we see today. This in turn offers valuable information for preparing for future changes.

As human-driven climate change accelerates, learning about past climate variability and biosphere responses equips us with knowledge to anticipate and mitigate future impacts.

The ability to date karst features with greater precision may seem like a small thing – but it will help us understand how today’s landscapes and ecosystems might respond to ongoing and future climate changes.

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