20-EPN-014: Constraining CO2 uptake and release through chemical weathering pathways in a young, active orogen

20-EPN-014: Constraining CO2 uptake and release through chemical weathering pathways in a young, active orogen.

Visit by Erica Erlanger, GFZ Potsdam (Germany) to TA2.10 Stable, Rare Gas and Radiogenic Isotope Facility at CRPG (France).
Dates of visit: 14-21 June 2021

Report Summary: Young, active orogens often retain an intact sedimentary cover that is composed of marine sequences, which can host large volumes of carbonate and sulfuric acid-producing minerals, such as pyrite. Unlike silicate weathering, which is responsible for CO2 drawdown over geologic timescales, sulfuric acid weathering of carbonates has the potential to release COinto the atmosphere that was previously trapped in rock. The goals of this study are to calculate the overall carbon budget for the Central Apennines, a young, active orogen, and to understand the mechanisms for the release and drawdown of CO2 in this landscape. 
Compiling a representative assessment of chemical weathering fluxes requires an understanding of the possible variability between seasons. To this end, the objective of my TA visit to the CRPG in Nancy, France was to process riverine water samples collected in winter of 2021 for δ34SSO4, δ18OSO4, and  δ13CDIC. These samples are replicate analyses of samples from summer 2020, and provide a direct comparison of isotopic signatures between the hot and dry summer versus the wet and cool winter. Preliminary results show that δ34S signatures are similar between winter and summer for spring and groundwater samples, whereas river samples are more enriched in summer. Further analysis and results from other isotopic systems will help elucidate the major sources of variability that we observe in the river samples. 

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20-EPN2-012: Discovering the origin of dissolved gases in CO2-rich mineral groundwaters from Aquae Spadanae

20-EPN2-012: Discovering the origin of dissolved gases in CO2-rich mineral groundwaters from Aquae Spadanae (Spa, eastern Belgium).

Visit by Agathe Defourny, University of Liège (Belgium), to TA2.10 Stable, Rare Gas and Radiogenic Isotope Facility at CRPG (France).
Dates of visit: 21 June – 02 July 2021

Report Summary: The visit at CRPG aimed at better assessing the origin of dissolved CO2 found in naturally sparkling groundwater springs from the east of Belgium. Previous analysis on δ13C had shown that the carbon could be either from mantellic or sedimentary (dissolved carbonates) origin, but a clear distinction between both could not be made. The goal of the stay at CRPG was then to focus on the analysis on other dissolved gases, in particular He and Ne. The combination of their isotopic signature, together with the isotopic composition of carbon is a powerful tool to highlight degassing from either crustal or mantel origin.

The results were really clear. The majority of the 4He/20Ne ratios stands between 50 and 500, indicating that more than 99% of the helium is not atmospheric and result from a mixture of crustal and mantellic gaz. Moreover, the ratio between CO2/3He (~109) versus δ13C (from -8 to -2 ‰) clearly shows that the dissolved COin theses springs is from mantellic origin. 

A few samples from non-carbogazeous springs from the same area were also collected and analysed and present a very different signature, with more negative δ13C values, and lower 4He/20Ne ratios. The measured value could be compared to different samples from the literature, particularly gas samples from the Eifel volcanic fields, at the border with Germany, showing very similar signatures. We can hence conclude with a high confidence level that the gases dissolved in the naturally sparkling spring from eastern Belgium come from the degassing of the Eifel mantellic plume, at a distance of about 100 km. 

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