Researchers say they have mapped the world’s largest known cumulative logjam in the Canadian Arctic and it holds millions of tonnes of carbon, representing an important but understudied part of the carbon cycle.
A study published in Geophysical Research Letters earlier this month describes how the scientists mapped almost 415,000 individual deposits of wood in the Mackenzie River Delta in the Northwest Territories, with the pileup stretching nearly 52 square kilometres.
The authors calculated the logs collectively store more than three million tonnes of carbon, equivalent to emissions from 2.5 million cars a year.
The single largest deposit covers 112,600 square metres, or around 20 American football fields in area, and stores nearly 6,700 tonnes of carbon.
“That’s the minimum,” said lead author Alicia Sendrowski, a research engineer with Michigan Technology University who led the study while working at Colorado State University. “(That) doesn’t include the wood that’s under canopy or buried or submerged.”
She said they mapped the massive logjam using high-resolution satellite imagery and machine learning. They also took around 80 samples and used radiocarbon dating to determine the age of the logs.
The study says the trees sampled ranged in date from AD 690 to 2015, but around 40 per cent of them began growing after 1955.
“I think one of the reasons potentially there’s that young wood is that the system is very active with delivering new trees each year as they flow up the river,” Sendrowski said. “So we’re just seeing kind of the surface layer, kind of this younger wood, but then potentially as you go deeper, as you get to wood that has been there for longer, there might be older pieces of wood.”
The Arctic’s cold and often dry conditions mean trees can be preserved for tens of thousands of years.
An unrelated 1962 study of buried driftwood pieces taken from the Ibyuk and Sitiyok pingos, or ice-cored hills, near Tuktoyaktuk, N.W.T., found those trees were between 12,000 and 33,000 years old.
Sendrowski said most of the wood in the delta comes from the Liard River, as well as from along the Mackenzie River, Peel River and Arctic Red River. The Mackenzie River exports significant amounts of carbon and large wood through the Beaufort Sea to the Arctic Ocean.
Sendrowski said the delta is a “hot spot” of carbon storage due to its carbon-rich soil. But she said there has been a lack of study on the age of logs in Arctic deltas and how much carbon they store, resulting in a gap in carbon-cycle estimates.
“I think maybe to some extent people don’t think it’s as an important component compared to soil carbon because there is less of it overall,” she said. “But as we’ve seen from the Mackenzie, there are some places that just store a pretty phenomenal amount of wood.”
The Mackenzie River Delta is the second largest Arctic delta in land area and the third largest in drainage basin size, totalling more than 13,000 square kilometres.
Sendrowski said the research is also important to help answer questions about how these systems could respond to climate change.