Knowing how the frequency and intensity of wildfires has changed over time offers scientists a glimpse into Earth’s past landscapes, as well as an understanding of the impacts of future climate change. To reconstruct fire records, researchers rely heavily on sediment records from lake bottoms, but this means that fire dates from arid regions are often ignored. Now, a new study shows that sand dunes can serve as repositories of fire history and help expand scientific understanding of fire systems around the world.
Posted May 11 in quadruple searchThis study is the first to examine sedimentary records preserved in the sediments of foot slopes in the dunes. The research team, led by Nicholas Paton, PhD, now a postdoctoral researcher at DRI, studied four sand dunes in the Cooloola Sand Mass in Australia. Australia is one of the world’s most fire-prone landscapes, with a long history of natural and cultural burning, and vast expanses without lakes or ponds to collect sedimentary records from. The researchers sought to prove that these dune deposits can be used to reliably reconstruct a history of fires spanning several millennia. These previously unrecognized archives of arid regions around the world can be used to fill knowledge gaps in the places where fire shapes the landscape.
“There are many records of fires and paleoclimate where there are a lot of bodies of water like lakes, peat and swamps,” Patton says. “Because of this, most global models already have a bias toward temperate regions.”
The Cooloola Sand massif consists of enormous sand dunes – up to 240 meters high – that pile up on the coast and gradually shift inland away from the force of the winds. By determining the age of the dunes using a technique called Optically stimulated brilliance Dating, or OSL, the four dunes Patton’s team found span across the Holocene, which represents roughly the last 12,000 years.
Once the dunes have settled, meaning they are no longer growing but slowly deteriorating, the force of gravity acts on the slopes of the dunes to collect fallen sand at the base, along with leftover charcoal from local fires that is deposited on the surface of the dunes. These deposits accumulate over time, creating seams of char from fire events that can be reliably identified using radiocarbon dating.
“We were digging holes in the soil at the base of the dunes, and we saw a lot of coal — more coal than we expected,” Patton says. “And we thought maybe we could use these deposits to reconstruct local fires within the region.”
Patton found that in younger dunes (500-year-olds and 2000-year-olds), the coal seams represent individual fires, because the steep slope of the dunes quickly buried each layer. However, older dunes (5,000-year-old and 10,000-year-old) have more progressive slopes that mix coals from different fires over time, providing a better understanding of increasing or decreasing fire frequency periods.
The dunes displayed local fire histories from within a radius of approximately 100 metres, so fire records differ somewhat between the four dunes spanning a distance of approximately 2 km. However, Patton’s team compared their findings to other fire records from the area found in lake and bog sediments. Similar to regional records, their findings show three main periods of fire activity over the past 7,000 years.
Similar records are likely preserved in sand dunes around the world, the researchers write, and that regions such as California and the southwestern United States could benefit from a better understanding of regional fire histories. Including fire logs not only includes information about natural wildfires, but also the way humans have affected fire systems.
“The history of fires is important for understanding how fire was used in the past for cultural purposes, whether it was to clear fields for farming or for hunting,” Patton says.
Patton hopes to continue this type of research in other dunes near the nearly million-year-old Cooloola Sand massif to get a long-term fire history for the area. Because Australia has had human societies for at least 60-70,000 years, and possibly longer, these records can help understand the relationship between humans and historical fire systems.
“These kinds of long-term records are not always available within lake sediments, but they may be available within these dune sediments,” Patton says. “This is very exciting.”