1,600 years ago, climate change hit the Australian Alps. We studied ancient lake mud to learn what happened


Shutterstock

Zoë Thomas, UNSW; Haidee Cadd, University of Wollongong, and Larissa Schneider, Australian National UniversityIf you’ve ever visited Australia’s highest peak — Mount Kosciuszko — you might remember the long uphill trek to the summit past some of Australia’s most picturesque and rugged landscapes. Vibrant snow gums, boardwalks with meadows of exquisite alpine plants, and blinding patches of snow.

As you approach the summit, a quartet of stunning blue lakes appear, created by glaciers during the last ice age that carved new valleys out of the mountain.

Lakes like these are windows to the past, offering an opportunity to understand how our climate and environment has changed over hundreds to thousands of years. One such lake, Club Lake — so-named for its resemblance to a suit in a deck of cards — was the focus of our new study.

After studying the lake’s sediment, we learned the Australian Alps experienced a sudden climate change about 1,600 years ago that brought a long spell of warmer conditions. What makes this sudden warming event particularly interesting is that it bears striking similarity to today.

Climate change in the Australian Alps

The Australian alpine region is the traditional home of a number of Aboriginal groups, including the Ngarigo, Walgalu and Djilamatang people. It is also home to highly diverse flora and fauna that occur nowhere else, from billy buttons (Craspedia costiniana) known for their vibrant yellow rosette of tiny flowers, to the broad-toothed rat and its chubby cheeks.

But this unique wildlife is under immense threat from climate change.

By 2100, Australia may warm by at least 4℃, with bushfires becoming more frequent and devastating. The fragile alpine ecosystem will be hit particularly hard by these changes.




Read more:
Meet the broad-toothed rat: a chubby-cheeked and inquisitive Australian rodent that needs our help


Many of Australia’s alpine species are already near their climatic limits, and are constrained by altitude. They’re at risk of becoming regionally extinct if their climatic thresholds are exceeded. As the temperature warms, treelines move upslope to cooler temperatures, pushing alpine flora and fauna to higher elevations. At some point they can go no higher — they’re squeezed out of their niche.

The critically endangered mountain pygmy-possum, for example, relies on the seasonal snowpack for winter hibernation, but increased temperatures are limiting this habitat.

A dip into the past

Our study showed Club Lake holds vital clues to the link between rising temperatures, loss of native plant species and more frequent fires in the Snowy Mountains.

Lake sediments are used all over the world as indicators of climate and environmental change because of the unique way they trap material. A body of water can act as a seal that ensures sediments are largely undisturbed over time.

We extracted sediments from the bottom of Club Lake to a depth of 35 centimetres. This equates to about 3,500 years of history, approximately 100 years for each centimetre.

Club-shaped lake in the mountains
Club Lake in Mt Kosciuszko.
Shutterstock

To work out how temperatures have changed over this time, we looked for the presence of molecular fossils, called “lipid biomarkers”. Analysing these biomarkers in the laboratory can tell us what the temperature in the environment was like, hundreds or thousands of years ago.

In the 3,500 years we examined, we detected a gradual warming trend. Superimposed on this, we found a sudden warming event that started 1,600 years ago, and lasted about six centuries. We suspect it was due to an atmospheric phenomenon linking higher tropical sea surface temperatures to southeastern Australia.

We’re not yet sure how much of Australia was affected by this warming, but other research from 2018 measured similar temperature changes in stalagmites from the Yarrangobilly caves 50 kilometres away.

Alpine snow gums (Eucalyptus pauciflora)
Zoë Thomas

What happened during this climate change?

During this unusual warmth, alpine herbs and shrubs declined, while the abundance of trees, particularly eucalyptus, increased. We know this by looking at grains of pollen preserved at different depths within the lake sediment samples, which indicates what types of plants were growing nearby.

We also found small particles of charcoal, produced by bushfires, embedded within the sediment layers. This showed the changes in vegetation also coincided with greater fire activity.

What surprised us most, however, was discovering a large increase in mercury at this time.

Mercury, which occurs naturally in the environment, is the only metal that’s liquid at room temperature, and is particularly sensitive to temperature changes. Higher temperatures enhance mercury deposition from the atmosphere, and our study shows a five-fold increase in mercury flux 1,600 years ago.

Alpine herbfields.
Nicola Pain

Industrial activities over the last 150 years, such as burning coal, have increased the abundance of mercury significantly. Our findings suggest future climate change is likely to increase the risk of mercury exposure not just in cities, but also in the seemingly remote Australian alpine environment.

Mercury contamination is a significant public health and environmental problem. At certain levels it’s poisonous to the nervous system, and it does not easily degrade.




Read more:
A tale of two valleys: Latrobe and Hunter regions both have coal stations, but one has far worse mercury pollution


What can we do?

Insights from the past can help governments, environmental agencies, and scientists come up with effective strategies to protect the vulnerable flora and fauna of the Australian Alps. But it’s not just changes in climate they’ll have to contend with in future.

There are other perils, such as soil erosion and habitat fragmentation from the legacy of sheep and cattle grazing, and tourism. Invasive pests and pathogens are likely to further reduce the resilience of these alpine ecosystems.

Feral horses graze near a tree
Feral horses are a significant threat to native wildlife in Australia’s alpine region.
Shutterstock

Restoration programs over the last 50 years have aimed to revitalise the natural vegetation in the Kosciuszko National Park following 135 years of grazing — finally banned in 1969 — and the environmental damage caused by the Snowy River Hydro-Electric scheme.

More recently, the federal government has committed A$3.5 million towards recovery from the devastating 2019-2020 bushfires. Incorporating Aboriginal knowledge into mainstream fire management is essential for tackling future crises.

This is the critical time for climate action to protect this unique and iconic Australian landscape.




Read more:
Double trouble as feral horse numbers gallop past 25,000 in the Australian Alps


The Conversation


Zoë Thomas, ARC DECRA Fellow, UNSW; Haidee Cadd, Research associate, University of Wollongong, and Larissa Schneider, DECRA fellow, Australian National University

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

Melting ocean mud helps prevent major earthquakes — and may show where quake risk is highest



Shutterstock

Kate Selway, Macquarie University

The largest and most destructive earthquakes on the planet happen in places where two tectonic plates collide. In our new research, published today in Nature Communications, we have produced new models of where and how rocks melt in these collision zones in the deep Earth.

This improved knowledge about the distribution of melted rock will help us to understand where to expect destructive earthquakes to occur.

What causes earthquakes?

Giant earthquakes, such as the magnitude-9.0 quake in 2011 that caused the Fukushima nuclear disaster, or the magnitude-9.1 event in 2004 that caused the Boxing Day tsunami, occur at the collision zones between two tectonic plates. In these so-called subduction zones, one plate slides beneath the other.




Read more:
The Fukushima quake may be an echo of the 2011 disaster — and a warning for the future


The sinking plate acts as an enormous conveyor belt, carrying material from the surface down into the deep Earth. Earthquakes occur where the sinking plate gets stuck; strain builds up until it eventually quickly releases. Fluids and molten rocks in the system lubricate the plates, helping them slide past each other and stopping big earthquakes from happening.

When happens when ocean mud ends up inside Earth?

My colleague Michael Förster and I were interested in what happens to sediments when they are carried down into the deep Earth at a subduction zone. These sediments start out as thick layers of mud on the ocean floor but get carried down into the deep Earth as part of the sinking plate.

Michael took a sample of mud collected from the ocean floor and heated it up to the high temperatures and pressures it would experience in a subduction zone. He found the sediments melt and then react with the surrounding rocks, forming the mineral phlogopite and also saline fluids.

A puzzle solved

Geophysical models of subduction zones allow us to map out exactly where the molten rocks and fluids are. These measurements are like x-rays of Earth’s interior, helping us peer into places we cannot otherwise see.

We were particularly interested in models of the electrical conductivity of subduction zones. This is because the fluids and molten rock we were looking at are more electrically conductive than the surrounding rock. Models of subduction zones have long been enigmatic, because they show Earth is very conductive in regions where people did not expect to see a lot of fluids and molten rock.

Melting sediment from the seafloor helps tectonic plates slide over one another without creating major earthquakes.
Selway & Forster, Author provided

I calculated the electrical conductivity of the phlogopite, molten sediments and fluids that were produced in the experiments and found they matched extremely well with the geophysical models. This provides good evidence that what we see in the experiments is happening in the real Earth, and allows us to calculate where the molten rock and fluids are in subduction zones around the world.

Understanding where big earthquakes are likely to occur

Giant earthquakes are not likely to occur in the parts of the subduction zone where the sediments melt. All of the products of the melting — the molten rock itself, the saline fluids, and even the mineral phlogopite — help the two plates slide past each other easily without causing large earthquakes.

We compared our models with locations of earthquakes in subduction zones along the west coast of the United States. We found there were no large earthquakes where sediments were melting, but the movement of fluids from the melted sediments could explain some small, non-destructive earthquakes and very faint signals of tremor where the two plates easily slide past each other.




Read more:
Breaking new ground – the rise of plate tectonics


Earthquakes are a tangible reminder that we live on an active planet and that, deep beneath our feet, huge forces are making rocks flow and melt and collide. Accurately predicting earthquakes will be an ongoing goal of geoscientists for decades to come.

It requires intricate detective work to weave together all the tiny threads of information we have about processes that occur so deep in the Earth that we will never be able to see or sample them. Our results are one new thread in this puzzle. We hope it will contribute to one day being able to keep people safe from the risk of earthquakes.




Read more:
Underground sounds: why we should listen to earthquakes


The Conversation


Kate Selway, , Macquarie University

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