Hydrology and atmospheric CO2 consumption by chemical weathering in a Mediterranean watershed

Chemical weathering of carbonates and silicates is an important sink of carbon as it consumes atmospheric CO₂ increasing river dissolved load. While both carbonate and silicate weathering contribute to short-term CO₂ uptake, only silicate weathering drives long-term CO₂ consumption.

The influence of lithologies with a non-dominant carbonate component on atmospheric CO₂ consumption has been explored in the literature. However, further research is needed to quantify the contribution of carbonate minerals within silicate rocks to weathering rates in comparison to those of pure carbonate rocks. Furthermore, the interactions that hydrological (e.g. streamflow variation) and geomorphological (e.g. erosion) processes have with chemical weathering remain controversial.

We measured at different streamflow conditions the dissolved and suspended loads (proxies of chemical weathering and erosion) in the Niccone watershed (Central Italy) composed mainly of siliciclastic sedimentary rocks. The dissolved load was estimated by measuring stream water alkalinity and electrical conductivity, and the suspended load was measured by using the DH-59 sediment sampler. The aim of this work is to investigate and quantify how the small percentages of carbonate minerals, included in the mixed sedimentary rocks (i.e. non-pure carbonate) outcropping in the Niccone watershed, influence the atmospheric CO₂ consumption rates over short timescales and to evaluate the influence of hydrological and geomorphological factors, such as runoff and erosion, on these rates. To achieve this, we also estimated the percentage of carbonate minerals present in the outcropping rocks within the stream watershed.

We found a chemostatic behavior in the ionic concentration, with much less variation compared to the flow rate. Chemical weathering and erosion exhibit a linear scaling at lower erosion rates given the relatively larger quantity of material for reactions exposed by erosion. At higher erosion rates, a gradual increase in weathering products is observed, as minerals are provided in excess relative to the rate at which they undergo reactions. We also estimated the variation with runoff of the atmospheric CO₂ consumed by chemical weathering.

Data enabled us to quantify a non-negligible carbonate component (less than 20%) in the Niccone watershed, significant from a CO₂ consumption perspective, despite siliciclastic rocks predominating across most of the study area. This finding may challenge some large-scale estimates of atmospheric CO₂ consumption based solely on lithological information.