Rock weathering may counteract CO₂ emissions from thawing permafrost

A new study published in the journal Nature reveals that the thawing of permafrost soils does not only release carbon, but may also promote its removal from the atmosphere.
Climate warming contributes to the thawing of permafrost soils all over the world. As a consequence, large volumes of organic carbon are exposed and converted into carbon dioxide (CO₂) on land and in rivers – a positive feedback mechanism that is further accelerating climate change.

A study recently published in the journal Nature has now suggested that an effect which is only rarely considered in permafrost studies could significantly diminish this vicious circle: The weathering of rocks.

“Minerals within and underneath permafrost soils that are exposed due to permafrost thaw can weather and directly impact the carbon cycle,” explains Aaron Bufe, Professor of Sedimentology at LMU and one of the lead authors of the new study.

The way that biological and geological processes interact and shape the CO₂ dynamics in rivers in permafrost regions was largely unknown until now. “In our study, we quantified how the balance of CO₂ sequestration and CO₂ release in rivers changes when the permafrost thaws,” says Bufe.

Study area: The Tibetan Plateau

To achieve their aim, the participating researchers from Germany, China, the UK, USA, Sweden and Switzerland studied the CO₂ balance of rivers on the Qinghai-Tibet Plateau – the largest continuous permafrost landscape outside the Arctic and Antarctic.

The team of researchers combined measurements of CO₂ emissions with chemical analyses in 50 rivers that constitute the headwaters of the largest river systems in Asia. The study area covers roughly 780,000 square kilometers and extends over altitudes ranging from 1,650 to 4,820 meters above sea level.

Some parts of the study area are underlain by continuous permafrost, but in other parts, permafrost occurs only sporadically or has disappeared entirely. The researchers analyzed the spatial distribution of their data to quantify how permafrost thaw affects the biogeochemistry of rivers on the Tibetan Plateau over hundreds of years.

The impact of chemical weathering is significant

The surprising finding: Carbon fluxes from chemical weathering could become increasingly important as permafrost cover declines and may even exceed CO₂ emissions from the conversion of organic carbon in rivers.

“Across the whole region that we studied, 35% of CO₂ emissions from rivers are offset by sequestration of carbon from rock weathering,” explains Dr. Liwei Zhang, biogeochemist at East China Normal University, who led the study alongside Aaron Bufe. “What is interesting, is that this ratio depends very much on the nature of the permafrost.”

According to the study, weathering compensates for only 15% of the CO₂ emissions in areas with continuous permafrost. In regions with sporadic permafrost cover, this number can increase to more than 100%. This finding indicates that rock weathering plays an increasingly important role as permafrost thaws.

“However, the influence of rock weathering on the carbon cycle also depends on the types of minerals that are exposed,” explains Bufe. Where silicate weathering is dominant, for example over large parts of the Qinghai-Tibet Plateau, rock weathering can counteract the release of carbon from permafrost soils. In contrast, weathering of sulfide minerals, such as pyrite, can emit CO₂. “Sulfide-weathering dominates in the south-eastern part of our study area, for example.”

Better understanding the feedback mechanism

According to the researchers, the geochemical patterns in the Qinghai-Tibet Plateau shed new light on the role of permafrost thaw for the balance of carbon sources and sinks.

Liwei Zhang summarizes: “The results of our multi-disciplinary approach show that rock weathering could increase with declining permafrost cover and sequester significant amounts of CO₂, linking inorganic and organic carbon cycles on human-relevant timescales.”

So, will rock weathering be able to counteract man-made climate change? “Unfortunately, the answer is no,” says Bufe. “Annual CO₂ emissions by humans are about 100-times greater than the amount of CO₂ sequestered by weathering. A bit of an increase in weathering rates with permafrost thaw will not affect this huge difference. The most helpful thing for us to do would be to drastically reduce emissions.”

The researchers argue that future analyses should extend beyond the current focus on biotic-organic carbon processes and holistically consider the balance of all mechanisms to better understand the net impact of permafrost thaw on the global carbon cycle and ultimately on the direction of the climate feedback.