Dead trees in the Arctic store massive amount of carbon

Arctic rivers play a crucial role in transporting fallen trees from forests to oceans, inadvertently contributing to long-term carbon storage as logs accumulate along the waterways’ twists and turns. 

Recently, a groundbreaking study has mapped the largest known woody deposit in the Mackenzie River Delta, Nunavut, Canada, spanning 51 square kilometers (20 square miles) and storing an estimated 3.4 million tons (3.1 million metric tons) of carbon.

According to Alicia Sendrowski, the research engineer who led the study at Colorado State University, this staggering quantity of carbon is equivalent to the emissions produced by approximately two and a half million cars in a year. 

Despite its significance, our understanding of this “large carbon” in the form of wood remains limited compared to other forms of carbon, such as dissolved or particulate organic carbon. However, recent developments in research are beginning to address this gap in knowledge.

For many years, scientists have been aware of the mobility of driftwood in the Arctic, but only recently have they started to quantify the amount of wood and the potential loss of carbon storage due to climate change. 

The region’s cold, often dry or icy conditions allow trees to remain preserved for tens of thousands of years. As Sendrowski explained, a tree that fell a thousand years ago might appear just as fresh as one that fell the previous winter.

Virginia Ruiz-Villanueva, a fluvial geomorphologist at the University of Lausanne who was not involved in the study, highlights the importance of this emerging research area: “There’s been a lot of work on fluxes of carbon from water and sediment, but we simply didn’t pay attention to the wood until very recently. This is a very young field of research that is developing quite fast.” 

Ruiz-Villanueva further emphasizes the significance of studying these woody deposits, not only in the context of the carbon cycle but also to better understand the functioning of natural fluvial systems and the mobilization and distribution of wood by rivers.

How scientists measured this giant logjam

The research team sought to map and quantify woody deposits in the Arctic. They focused on the Mackenzie River, which boasts exceptionally high-resolution imagery and is known for its sizable wood deposits. The river’s delta, the third-largest in the world by land area, drains approximately 20% of Canada. The team examined around 13,000 square kilometers (5,000 square miles) of delta in the largest attempt to map woody deposits to date.

Over a three-week period, Sendrowski and her colleagues from Colorado State University measured river driftwood, mapped logjams, and sampled wood for radiocarbon dating. Utilizing remote imagery, Sendrowski identified wood on the river’s surface and estimated the areal extent of the logjam. She then calculated the volume of wood within the logjam and the amount of carbon it stored based on her field measurements.

The deposit, which comprises over 400,000 miniature caches of wood, stores approximately 3.4 million tons (3.1 million metric tons) of carbon. The largest single deposit, covering an area equivalent to around 20 American football fields, holds 7,385 tons (6,700 metric tons) of carbon. 

However, Sendrowski believes that the actual amount of carbon stored in the delta’s wood could be roughly twice as large, considering the logs buried in soil, submerged underwater, and hidden under vegetation from aerial imagery.

Although the Mackenzie River Delta is a “hotspot” of carbon storage due to its highly carbon-rich soils, the logs’ carbon storage represents a relatively small fraction of the delta’s total carbon storage – around 3 quadrillion grams of carbon. 

“But we think it’s still important because as changes in the basin occur, like logging or damming, and as climate change alters precipitation patterns and warming, wood preservation will decrease. It’s a significant amount of carbon, so there’s a potentially significant loss of carbon storage,” said Sendrowski.

The study reveals that the Mackenzie logjam is only one of many such basins in the Arctic. With at least a dozen deltas larger than 500 square kilometers, the combined large woody deposits throughout the region could constitute a significant carbon storage pool – one that remains largely unexplored.

The researchers also investigated the lifespan of trees in the Arctic, a crucial factor when modeling how “active” a carbon pool is, meaning how quickly material is moved around. Carbon dating revealed that while many sampled trees started growing around or after 1950, some were much older, dating back to around 700 CE. One study from the 1960s even carbon-dated wood from a tree preserved in an icy mound to approximately 33,000 years ago.

According to Sendrowski, the Mackenzie River Delta proved an ideal starting point for the study. “The exciting aspect for me isn’t just the scale, but also the potential to apply this to other places where large wood hasn’t been focused on,” she said. 

As a burgeoning field, there is still much to be learned about the role of large woody deposits in carbon storage and the Arctic environment.

This groundbreaking study, which sheds new light on the role of Arctic rivers in carbon storage, was published in Geophysical Research Letters, a journal known for high-impact, short-format reports with immediate implications across all Earth and space sciences.

More about logjams around the world

It is difficult to determine the exact number of massive logjams around the world, as they can form and dissipate in various river systems over time. Logjams, also known as wood jams or driftwood accumulations, are naturally occurring phenomena that result from the transport and deposition of large woody debris by rivers and streams. The size and location of logjams can change due to factors such as river flow, precipitation, vegetation, and human intervention.

Massive logjams have been documented in several major river systems worldwide, such as the Mackenzie River Delta in Canada, the Amazon River in South America, and the Lena River in Siberia. However, without comprehensive studies and monitoring, it is challenging to provide an accurate count of massive logjams across the globe.

Research on large woody deposits and their role in carbon storage and ecosystem functioning is still a burgeoning field. As more studies are conducted, scientists will likely gain a better understanding of the distribution, dynamics, and impact of massive logjams worldwide.

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