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Across the Arctic, fallen trees are making their way from forests to ocean to rivers. These logs can pile up as the river turns and turns, resulting in long-term carbon storage. A new study has mapped the largest known timber deposit, covering 51 square kilometers (20 square miles) of the Mackenzie River delta in Nunavut, Canada, and calculated that the logs store around 3.4 million tonnes (about 3. 1 million metric tons) of carbon.
“To put that into perspective, that’s about two and a half million car emissions per year,” said Alicia Sendrowski, a research engineer who led the study at Colorado State University. “It’s a significant amount of carbon,” she said, but it’s not a pool of carbon that we know much about. “We have a great knowledge of carbon in other forms, like dissolved or particulate organic carbon, but not what we call ‘big carbon’ – big wood.” It’s starting to change.
Scientists have known for decades that driftwood can really move around the Arctic, but they’re only just beginning to quantify how much wood there is and how much of its carbon storage we stand to lose to of climate change. The cold, often dry or icy conditions of the Arctic mean that trees can be preserved for tens of thousands of years. a tree that fell a thousand years ago may look as fresh as one that fell last winter, Sendrowski said.
“There’s been a lot of work on carbon fluxes from water and sediment, but we just haven’t paid attention to wood until very recently. It’s a very young area of research that is emerging. is developing quite rapidly,” said Virginia Ruiz-Villanueva, a fluvial geomorphologist at the University of Lausanne who was not involved in the study. “And it’s important to study this wood not just for the carbon cycle, but in general for our understanding of how these natural river systems work, how rivers mobilize and distribute wood.”
The study was published in Geophysical Research Letters.
To get a glimpse of the traffic jams, Sendrowski and his colleagues focused on the Mackenzie River, which has exceptionally high-resolution imagery and is known to have large timber deposits. Its delta is the third largest in the world and drains about 20% of Canada. The team surveyed about 13,000 square kilometers (5,000 square miles) of delta in the largest attempt to map woody deposits to date.
The researchers spent three weeks in the field measuring driftwood from the river with colleagues from Colorado State University, mapping traffic jams and sampling wood to date using radiocarbon dating. After the fieldwork, Sendrowski used remote imagery to identify wood on the surface of the river and estimate the extent of the jam. She then estimated the volume of wood in the jam and the amount of carbon it stores, based on her field measurements.
Sendrowski found that the deposit, comprising more than 400,000 miniature caches of wood, stores about 3.4 million tonnes (3.1 million metric tons) of carbon. The largest single deposit, which covers about 20 American football fields, stores 7,385 tonnes (6,700 metric tons) of carbon alone. But because there are still more logs buried in the ground, submerged underwater, and hidden from aerial footage under vegetation, the total amount of carbon stored in delta wood could be about twice as large, a she declared.
The Mackenzie River Delta is a carbon storage “hot spot” thanks to incredibly carbon-rich soils, Sendrowski said, so log carbon storage is a relatively small fraction of the delta’s total carbon storage, which is about 3 quadrillion grams of carbon. . “But we think it’s still important because as changes occur in the basin, such as logging or damming, and climate change alters rainfall patterns and warming, the preservation of the wood will decline. That’s a significant amount of carbon, so there’s a potentially significant loss of carbon storage,” she said.
The Mackenzie ice jam also reflects a single basin in the Arctic; at least a dozen deltas larger than 500 square kilometers dot the north, so all together large woody deposits throughout the Arctic could be a significant carbon storage pool, about which we know little.
The researchers were also interested in the lifespan of a tree in the Arctic, which is important when modeling the “activity” of a carbon pool, i.e. the the speed with which materials move. Carbon dating revealed that while many sampled trees started growing around or after 1950, some were much older, dating back to around 700 CE. (A study in the 1960s carbon-dated the wood of a tree preserved in a mound of ice about 33,000 years ago.)
The Mackenzie River Delta was a good starting point. “The exciting thing for me is not just the scale, but also the ability to apply this to other places where big wood hasn’t been concentrated,” Sendrowski said. It’s a growing field, she added, and there’s still a lot to learn.
Alicia Sendrowski et al, Timber-Based Carbon Storage in the Mackenzie River Delta: The World’s Largest Mapped River Timber Deposit, Geophysical Research Letters (2023). DOI: 10.1029/2022GL100913
Geophysical Research Letters
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