The Italian central Apennines as a source of CO2

They found that weathering in this region leads to an overall CO₂ uptake. However, these near-surface processes only determine the CO₂ balance in areas with a thick and cold crust. On the western side of the Central Apennines, the crust is thinner and the heat flow is higher. There, CO₂ outgassing from depths is up to 50 times greater than CO₂ uptake through weathering. All in all, the analysed landscape is a CO₂ emitter. The structure and dynamics of Earth's crust, therefore, controls the release of CO₂ here more strongly than chemical weathering. The study was published today in the scientific journal Nature Geoscience.

Background: The role of mountains in Earth's CO₂ budget

In addition to human-made CO₂ emissions, many natural processes -- both biological and geological -- also play a role in balancing the global CO₂ budget. Mountain landscapes strongly modulate the carbon cycle, and it is important to adequately consider the competition of CO₂ emission and CO₂ uptake occurring here in climate models.

On the one hand, rocks on the Earth's surface are weathered by chemical dissolution processes: erosion continuously exposes rock, which -- depending on the type of rock -- weathers at different rates and either absorbs or releases CO₂. Silicate minerals, for example, bind CO₂ and form limestone. In turn, the weathering of carbonate and sulphide-containing minerals releases CO₂.

A research team led by Aaron Bufe and Niels Hovius has investigated the competition of CO₂ release and drawdown from weathering in a further study published in the journal Science at the beginning of March. They analysed the influence of the erosion rate on the CO₂ balance using various mountain regions around the world as an example.

However, mountain building does not only influence erosion and weathering rates on Earth's surface. Where tectonic plates slide over each other, heating of carbonate rocks in the crust and mantle can lead to chemical reactions associated with CO₂ emissions.

"Previous studies have often focussed on a single process and have treated weathering on the surface and processes at depth separately. We wanted to change that," says Niels Hovius.

Investigations in the Apennines: CO₂ outgassing or storage -- which process dominates?

The competition between near-surface and deep-seated processes is now the focus of a new study by Erica Erlanger, post-doctoral scientist at the GFZ and the Université de Lorraine (France), Aaron Bufe, Professor of Sedimentology at the LMU Munich and former post-doctoral scientist at the GFZ, and Niels Hovius, Head of the Geomorphology Section at the GFZ and Professor at the University of Potsdam, together with colleagues from France, Italy, the USA and Switzerland.

The central Apennines in Italy prove to be a particularly suitable region for this study, as Erica Erlanger, first author of the study, explains: "This area is part of an active mountain range with closely spaced zones of thick, cold crust and thin, warm crust, allowing us to investigate the influence of subsurface activity. The climatic conditions as well as the topography and the rock types on the surface are similar throughout the area, so there should not be any large differences in weathering activity."

Sampling and analysis of CO₂ content

In the western central Apennines, the crustal thickness is around 20 kilometres and the heat flux is up to over 100 milliwatts per square metre, while the crust in the east is more than 40 kilometres thick, with a heat flux of around 30 milliwatts per square metre.

The researchers took a total of 104 water samples in the western Tevere and eastern Aterno-Pescara River systems, 49 of them in summer 2020 and 55 in winter 2021, covering the warmest and driest seasons and the wettest and coldest seasons to estimate the minimum (summer) and maximum (winter) CO₂ fluxes.

Water samples are suitable because rivers and springs transport carbon, which originates both from depths and from weathering reactions near the surface. The chemical analysis of the samples included determining the relative abundance of various carbon isotopes. These can provide information as to whether the carbon originates from a plant or from the atmosphere or was released from a subducted rock.

"On this basis, we were able to calculate the quantities of CO₂ released by weathering or from carbonates at depths, and the quantities of CO₂ bound by weathered silicates," explains Erlanger.

In order to estimate an overall balance for the CO₂ budget of the Apennines, the researchers also took into account estimates for inorganic CO₂ emissions from gas vents known from the western side of the Apennines, as well as from organic CO₂ exchange.

Result: Central Apennines as a net CO₂ source, but with a split CO₂ balance

The research team found that the weathering processes in the entire study area predominantly capture CO₂ and do not release it. Remarkably, however, where the crust is thin and the heat flow is high, CO₂ release from depths outpaces weathering-related CO₂ fluxes by a factor of 10 to 50. Overall, the region is, therefore, a CO₂ source.

"Importantly, fluctuations in CO₂ release from deep rock are much greater than fluctuations in chemical weathering fluxes. This means that the regional geodynamics in the central Apennines influences the carbon cycle most strongly by modulating the release of CO₂ from depth, and not by impacting weathering reactions," summarises Erica Erlanger. "Based on the geological evolution of the area, we estimate that CO₂ outgassing from the crust and mantle has probably occurred over the last 2 million years."

Further impacts: Better climate models and understanding of the delicate CO₂ balance over geological timescales

"Our investigations will contribute to a better understanding of the actual CO₂ balance for the atmosphere and, thus, to better long-term climate models," says Aaron Bufe. "They also help to clarify how our planet has maintained the narrow range of conditions that are conducive to life by balancing CO₂ outgassing and CO₂ storage processes over geological times."

Niels Hovius looks ahead: "If we want to investigate the role of mountains for Earth's carbon cycle in a more general sense, even seemingly simple geological questions will require a more holistic approach. Of particular interest are geologically young mountain belts at plate boundaries, where carbonate rocks are likely to predominate both near the surface and at depth. Today's Mediterranean region and other comparatively young mountain ranges, such as the Indonesian archipelago, exhibit geological conditions and rock types similar to the central Apennines. So, the next big question we face is whether outgassing in active tectonic areas could be a global phenomenon in space and time."