In the event of a wildfire, flee into the cave. This is the official emergency policy of Oregon Caves National Monument and Preserve, a forested area protecting a labyrinth of passages dissolved in a rare marble formation high in the Siskiyou mountains. There hasn鈥檛 been a fire in the preserve for a century. But the potential for a conflagration in the dry forest is palpable. If a fast-moving wildfire were to burn through, the cave would be the safest place for park rangers to hide.
However, the 1.7-million-year-old cave is not entirely isolated from any fires burning on the surface. When a fire burns above, heat and smoke can alter the chemistry of the water that seeps down through the rock. As it drips into the cave, it can leave behind traces of fire in sheer layers of mineral residue. Over millennia, this builds up within weird cave structures known as speleothems, which protrude from every surface where water flows, including stalagmites on the cave floor and stalactites on the ceiling.
鈥淚t鈥檚 a snapshot,鈥� , a palaeoclimatologist at Oregon State University, told me when I joined her on a recent expedition into the cave. She is among a growing set of researchers using cave records of wildfires to extend our view of fire activity back hundreds of thousands of years, to a time when temperatures on Earth were even hotter than today. That, in turn, is offering a fresh view of how we can expect future wildfires to behave as a result of climate change this century.
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The speleothem record
Speleothems have long been used to understand Earth鈥檚 climate history. Like tree rings, they preserve in their many layers a chemical record of changes in the environment around a cave, such as in temperature or precipitation. They are especially valuable alongside other records like ice cores or lake sediments because they cover up to millions of years of climate history and can be precisely dated by analysing the slow decay of radioactive elements within them. But 鈥渓ooking at fire in speleothems is still really, really new鈥�, says at Vanderbilt University in Tennessee, one of Wendt鈥檚 collaborators. 鈥淭here鈥檚 a lot that we don鈥檛 know.鈥�
For decades, researchers have suspected that fires leave traces behind in speleothems. But only recently have they shown this is a reliable link. Important evidence has come from the Yonderup cave in south-western Australia. When a fire burned above the cave several years ago, researchers who there noticed a spike in certain metals and other compounds generated from burning plants. 鈥淭he chemistry completely changed after the fire,鈥� says at the Australian Nuclear Science and Technology Organisation.
McDonough and her colleagues later found that such changes in the cave. To demonstrate this, they showed that a record of fire in a stalagmite that had grown over the past 300 years matched fires known within the historical record. This work validated earlier instances of fire indicated in the stalagmite, revealing that fires became less frequent but more intense after Europeans colonised the continent. 鈥淚t opens up a whole new understanding of the relationship between climate and fires,鈥� says McDonough. Speleothems, she says, could do for understanding ancient fires what ice cores did for the study of ancient atmospheres.
鈥楥an I steal your job?鈥�
In early October, I joined Wendt and several of her colleagues on a descent into the cave in Oregon as part of their efforts to identify such 鈥減alaeofire鈥� signals in the fire-prone forests of the Pacific Northwest, a first for the region. After donning bodysuits and helmets with bright lights (everyone needed at least two backups), we ducked into the mouth of the cave.
Bulges of flowstone caked in a white mineral substance called moonmilk seemed to drip from the walls like the wax of a giant candle. The stream that carved much of the cave from the marble, named the River Styx in a nod to Greek mythology, burbled alongside the path, and the air smelled like concrete dust. We passed through spaces earlier spelunkers 鈥� cave explorers 鈥� had given names like Petrified Garden, Belly of the Whale and Banana Grove.聽The thick root of a Douglas fir wound far into the cave questing deep for water.
A school group passed. 鈥淐an I steal your job?鈥� one kid shouted, impressed by our suits. 鈥淛ust study geology,鈥� replied , a cave geologist at the preserve, and our guide.
About a kilometre in, we departed the main path at a 鈥渟queeze鈥�, a narrow channel that forced us to crawl headfirst, then shimmy through a tilted passage barely wide enough to fit our helmets through. After easing around some bat guano, we arrived in a room known as Niagara 2 because of the waterfall formation that dominated one wall. At 120 metres below the surface, this was one of the deepest parts of the cave. 鈥淲elcome to my office,鈥� said Heimel.
Wendt pointed out a broken stalagmite about the size of a bottle of wine lying on a shelf of rock (pictured below). Earlier this year, she explained, she and her colleagues found that this stalagmite grew between 132,000 and 123,000 years ago, dating the rock by measuring the ratio of uranium to its decay product thorium within it. This span of time was during a warm stretch between glacial periods called the Last Interglacial; temperatures in many parts of the planet then were slightly higher than they are today.
Fires on Earth have already become larger and more intense in recent years, as our climate has warmed. But there remain large uncertainties about how future fires will behave as temperatures continue to rise, and knowing how wildfires behaved the last time things were this hot could offer deeper insight.
The Oregon Caves鈥檚 record of palaeofires could help with this 鈥� but will probably take years. One painstaking step, for instance, involves using a laser to blast away layers from stalagmites a few molecules at a time. And even before that, researchers must understand how exactly speleothems form in their home cave systems, which requires months of carefully measuring drip water and spelunking trips like this. Wendt sometimes must wait beneath a stubborn speleothem for a quarter of an hour to collect a single drip.
But caves are already revealing secrets of ancient wildfire activity elsewhere. For instance, not far south, in California, Oster and her colleagues have to identify a period of increased swings between wet and dry years around 8000 years ago; burned plant material within the speleothem also enabled them to study changes in vegetation. They found this 鈥渃limate whiplash鈥� led to substantially increased fire activity, which prompted a shift to woodier plants. 鈥淲e can look at the relationship between climate and fire and vegetation change all in the same record,鈥� says Oster. This same whiplash pattern is exactly what the state is expected to see more of with current climate change.
Caves are also revealing the deep history of fire in the great boreal forests of Siberia, where megafires in recent years have raised alarm about more burning and emissions as the Arctic warms. at Northumbria University, UK, and her colleagues recently used a stalagmite from Botovskaya cave to fires in southern Siberia during the Last Interglacial, and compare this to fire activity over the past 10,000 years. The historical record of fires in the remote region only goes back a few decades, but 鈥渋f we have caves there, we can fill in these gaps鈥�, says Margerum.
Although temperatures were actually cooling during this period near the end of the Last Interglacial, the researchers found a large spike in fire activity. Based on compounds from burned plants recovered from the stalagmite, they found this jump coincided with a shift to a forest dominated by softwood trees as well as more extreme differences between winter and summer temperatures.
In the context of today鈥檚 rapidly warming climate, this ancient spike in Siberian fires is concerning, says Margerum, because it suggests large increases in fire can happen even when temperatures are cooling. 鈥淥nce we鈥檝e disentangled this, we can then help modellers work out what could happen in the future.鈥�
