First experiences of correlation between erosion and chemical weathering at basin scale. The case study of Niccone stream (Central Apennines, Italy)

Rivers' sediments can be classified as dissolved (ions transported in solution), suspended (small particles like clay and silt transported by the fluid's flow) and bedload (largest particles like sand, gravels and pebbles transported along the river bed). 

It is well known by the literature that chemical weathering of carbonate and silicate minerals consumes atmospheric CO₂, enriching the dissolved load. In the “short-term” (<1 My) both carbonate and silicate weathering consume atmospheric CO₂, while in the “long-term” only silicate weathering contributes to CO₂ consumption. Assuming that the only reactions that occur in the river basins are the dissolution of silicates and carbonates by chemical weathering, knowing the dissolved load, as well as runoff and lithology, it is possible to calculate the atmospheric CO₂ consumed by chemical weathering. 

Several authors highlighted that it is not clear the role in consuming atmospheric CO₂ of mixed-carbonate or non-purely silicate lithologies such as sandstone and claystone, as well as sedimentary rocks like calcarenites, marls, and interlayered sandstone and limestone, where carbonate is not dominant. Moreover, the interactions that hydrological and geomorphological processes, such as variation in water runoff and erosion, may have with chemical weathering processes remain controversial and poorly understood.

In this paper we measured both the dissolved and the suspended load in the Niccone stream, a right tributary of Tiber basin (Central Apennines, Italy) mainly composed of siliciclastic sedimentary rocks, at different streamflow conditions. The dissolved load was estimated measuring alkalinity and electrical conductivity in stream waters, while the suspended load was measured by using the DH-59 sediment sampler.

The results of the fieldwork allowed us to investigate the relationship between electrical conductivity and water alkalinity, as well as the behavior of dissolved and suspended load with water discharge during flooding events. Moreover, starting from the knowledge of river water alkalinity, we estimated the amount of atmospheric CO₂ consumed by chemical weathering and its variation with runoff. The comparison with literature data allowed us to suppose the presence of non-negligible carbonate components in the Niccone watershed, where lithology is mainly composed of siliciclastic rocks. 

The experimental activities carried out within the Niccone watershed represent a first step in understanding the extent to which atmospheric CO₂ consumption processes by chemical weathering are influenced by meteo-climatic events and subsequent erosional phenomena. Moreover, the work confirms that lithologies usually considered without carbonate content like sandstones and claystones, could have a non-negligible carbonate component. This suggests that the estimates of atmospheric CO₂ consumed by chemical weathering shown in the literature, should be slightly corrected, both in the “long-” and in the “short-term”.