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A new source of fire records, hidden in the sands, gives us a bigger picture of the risks


Sand dunes are not an obvious place to find high quality fire records. For starters, anyone walking the forested sand dunes of South East Queensland will be impressed by the intensity of the ant activity at their feet. The ant nests extend to at least 2 meters below the surface. As the ants move materials around their nests, any charcoal from past fires preserved in the sand would be seriously disturbed.

Somewhat surprisingly, soil pits dug at the bottom of the slope of dune front walls (the leading edge of a dune) showed that several layers of sediment are preserved there. This shows that ant activity is not intense on the foot slopes. Undisturbed charcoal slabs can be found in this part of the dune.

Our newly released research focuses on four well-dated sand dunes. Unlike previous studies that extracted fire histories from sediment cores of lakes, marshes, and other organic sediments, we extracted fire records from these dunes. We believe this is a breakthrough that will vastly expand the areas for which we can extract fire histories.

Swamps and lakes are usually found in more humid areas and near the coast, while sand dunes are common in drier areas of Australia, including desert areas. This new source of fire histories can help us broaden our understanding of fire in Australia.

Collecting charcoal samples from the soil profile of a 10,000-year-old dune for radiocarbon dating.
Nick Patton, Author provided

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Why is a new source of fire registrations important?

Fire is important in the Australian landscape. Many ecosystems are not only designed to survive fire, but also require burns to survive and thrive.

However, recent years have seen an increase in the size and intensity of bushfires in Australia, culminating in the black summer of 2019-20. During that summer, areas that normally did not burn severely burned intensely. The fires caused long-term damage to vegetation and significant loss of human, animal and building life.

There are concerns that if the climate patterns associated with the Black Summer fires become more established, Australia’s ecology could be permanently altered and human activities in many regions could be severely affected.

Read more: 200 experts dissected the Black Summer wildfires in unprecedented detail. Here are 6 lessons to watch out for

There is an urgent need to better understand the role of fire in the Australian landscape, prompting a wave of research into both modern fire behavior and extracting fire histories from the landscape. These histories are crucial as they can help us identify and quantify the risk of fire. These studies can also show where climate and environmental changes have created new fire risk.

Scientists have previously relied on lake cores, marshes and other sources of organic sediments to uncover fire histories. The gradual accumulation of these sediments preserves charcoal from previous fires in layers. The layers can be dated, revealing the age of the charcoal and therefore when the fire started. This means that we can extract continuous records of past fires from these sediments.

However, due to the focus on organic-rich sediments, these fire histories are limited to moist areas, where swamps and lakes are present. Such sediments are usually located close to the coast. Fire risk is much more widespread in Australia.

maps of Australia and the world showing dryland distribution and paleofire records as well as coastal and continental dunes in Australia
(a) Orange areas show global distribution of arid regions (Sorensen, 2007) and white dots show published paleofire records from the Global Paleofire Database (Harrison et al., 2022). (b) View of Australia and the general locations of coastal (yellow) and continental (orange) dunes (Lees, 2006; Hesse, 2016). Much of Australia and the world is covered in dry areas as well as with no fire history.
Van Patton et al 2023/Quaternary Research, CC DOOR

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So what did the dune study find?

Us study focuses on the fire history of the Cooloola Sand Mass between Noosa and Tin Can Bay in South East Queensland. We examined four well-dated sand dunes from 500 to 10,000 years old.

In a Study from 2022, we showed that there are two distinct phases in the sediment records. These fit with a historical change in slope processes on the dunes.

For the first 1,000 years after the dunes stabilized, frequent but small flows of grains of sand along the front of the dune slowly built up sediments at the base of the dune. The sand deposited at the base includes the remnants of charcoal from local fires that have deposited on the dune surface. This sediment builds up over time, preserving charcoal layers from fires.

The different charcoal layers in the sand represent individual fire events. These charcoal layers can be reliably identified using radiocarbon dating.

image shows the deposition at the bottom of the dune of charcoal layers from fires
Charcoal deposited on the dune surface from previous fires collects in layers of sediment at the base of the dune.
Patton et al 2023/Quaternary research, CC DOOR

Read more: A dive into the deep past reveals that native burning helped suppress wildfires 10,000 years ago

After about 1000 years, the dune slopes became less steep. Slow soil creep, the gradual grain-by-grain movement of sand through the soil under the influence of gravity, became the dominant process. Charcoal is dispersed through the sediments. This means that individual fires cannot be recognized, but the overall fire activity is still properly recorded.

We compared the fire records of the sand dunes with local and regional fire histories. The records from the dunes matched the other records. Our data shows a connection between fire and strength El Niño Southern Oscillation (ENSO) periods – associated with more frequent drought – in South East Queensland.

Worldwide, there are very few fire histories from dry areas. And just like in Australia, extreme fires are on the rise in these regions, including California and the Mediterranean. We should now be able to better define natural fire hazards in these arid zones.

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