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

Nearly a sixth of the earth’s surface is covered in otherworldly landscapes with a name that may also be unfamiliar to you: karst. These landscapes resemble natural sculpture parks, with spectacular terrain dotted with caves and bedrock towers slowly sculpted by water over thousands of years.

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

However, it can be quite difficult to determine exactly when karst landscapes were formed. In our new work published today in Scientific advancesWe show a new way to determine the age of these enigmatic landscapes, which will help us understand our planet’s past in more detail.

The challenge

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

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

Traditionally, scientists have loosely defined the age of a karst surface by dating the material above and below it. However, this approach confuses 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 this paper, we measure the amount of helium produced by the natural radioactive decay of small amounts of the elements uranium and thorium in iron nodules. By comparing the amounts of uranium, thorium and helium in a sample, the age of the nodules can be calculated very precisely.

We have 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 consisting of acres of limestone pillars rising several meters above a sandy desert plain. The Pinnacles are part of the largest belt of windblown carbonate rocks in the world, extending more than 1,000 km along the southwest coast of Washington State.

We examined several microscopic fragments of iron nodules that were removed from the surface of limestone pinnacles. These nodules formed in the soil that covered 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 region.

The big wet

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

At the same time that chemical reactions were causing iron-rich nodules to grow in ancient soil, limestone bedrock was rapidly and largely dissolved to leave only the remains of limestone pinnacles seen today.

Looking at the entire rock sequence in the area, we believe this period of intense weathering was the wettest period in this part of Washington over the last half-million years.

We don’t know what caused this increase in precipitation. This may be due to changes in atmospheric circulation patterns or the increased influence of the ancient Leeuwin Current which runs along the shore.

Such a wet period contrasts dramatically with recent droughts and the region’s increasingly drier climate today.

Implications for our past

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

Dating these iron nodules will better document the dramatic fluctuations in Earth’s climate over the past three million years, as ice sheets expanded and shrank.

Understanding the timing and environmental context of karst formation throughout this period provides in-depth insight into past climatic conditions, environments, and landscapes in which ancient creatures lived.

Climate change and resulting environmental changes have played a crucial role in the formation of ecosystems. In particular, they had a profound influence on our ancient hominid 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.

Look forward to

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 to prepare for future changes.

As human-caused climate change accelerates, knowledge of past climate variability and biosphere responses allows us to gain the knowledge needed to anticipate and mitigate future impacts.

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

This article is republished from The Conversation under a Creative Commons license. Read the original article.