A study published Monday in the journal Nature Geoscience found that two lakes within the basin – Lac Mai Ndombe and its smaller neighbor, Lac Tumba – are releasing carbon in the form of planet-warming carbon dioxide (CO2). While some CO2 comes from recently produced plants, up to 40% stems from the basin’s ancient peat.
The swamps and peatlands of the Congo Basin cover only 0.3% of Earth’s land surface, but they contain 30 billion metric tons of carbon—one third of the amount stored in all tropical peatlands. Scientists have long believed that this carbon would remain locked inside peat for millennia, but these new findings suggest otherwise.
“As far as what it means for the sustainability of peat, it’s a question of 30 billion tonnes!” lead author Travis Drake, carbon biogeochemist at ETH Zurich, told Gizmodo in an email. “It is entirely possible that this is a natural, balanced cycle: vast peatlands slowly release carbon from below while sequestering a comparable amount from above, resulting in no net loss,” explained Drake. “However, the more worrying possibility is that climate or land-use changes are actively destabilizing the system, leading to the loss of its stored carbon.”
150 gigatons of ancient carbon each year
The role of the peatlands of the Congo Basin in regulating the global carbon cycle and thus climate is poorly understood. This is largely because it is difficult for researchers to access the central part of the basin due to the lack of road infrastructure. To overcome this, Drake and his colleagues used natural waterways as their highways.
Traveling on a large ship that served as both their living quarters and a floating laboratory, they tried to reach the southern point of Lake Mai Ndombe by crossing the Fiumi River – a large tributary of the Kasai.
Mai Ndombe and Tumba are both large, shallow black water lakes surrounded by swampy forests and thick peat deposits at the bottom. “Blackwater” is a colloquial term for a river or lake that has a high concentration of dissolved organic matter, which gives the water a dark brown color similar to strong tea, Drake explained. The subsurface peat layer has accumulated over thousands of years as plant material sank to the wetland floor and partially decomposed.
Researchers collected and analyzed water samples from both lakes, revealing that 39% of the carbon in Lake Mai Ndombe and 40% of the carbon in Lake Tumba came from peat. This suggests that the breakdown of long-stored peat is an important source of CO2 emissions from these lakes. Researchers estimate that Lake Mai Ndombe alone could release more than 150 gigatons of ancient carbon into the atmosphere each year.
A possible climate feedback loop
How this carbon is being released from peatlands is not clear, but Drake’s team believes it may be related to microbial activity deep within this organic layer.
As microorganisms feed on stored carbon, they convert it into methane through a process called methanogenesis. Researchers suspect that this subsurface methane moves into the lake via deep soil flow pathways, where it reacts with oxygen to produce CO2.
“Although we found isotopic evidence supporting this in the lake, we still need to investigate internal peat dynamics to confirm the full pathway,” Drake said.
It is possible that climate change is also playing a role in carbon sequestration from peat. Drake explained that as rising global temperatures cause more frequent and prolonged drought, it could partially dry out peatlands, exposing them to more oxygen and promoting faster decomposition.
“There is actually palaeoenvironmental evidence from regional peat cores that shows that similar climate-driven instability events have occurred in the past, causing large-scale losses of organic carbon,” he said.
If a similar event were to occur today due to human-induced warming, a feedback loop is taking shape. Drake explained, “Naturally, the CO2 released from such an event today would exacerbate climate change, although still to a lesser degree than the anthropogenic emissions that are currently driving the rapid build-up of CO2 in our atmosphere.”
He and his colleagues worry that rising temperatures and land use changes could turn the Congo Basin’s blackwater lakes into sources of greenhouse gases, but how close they are to reaching this potential tipping point is unclear. Their next project, which will examine the mechanisms behind their findings and how these carbon emissions have evolved over the past 12,000 years, may provide some insight.
“Ultimately, our goal is to better control the carbon budget of these peatlands, establish a baseline to assess future changes, and determine their current sustainability,” Drake said.
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