Geochemical features of Volatile Organic Compounds (VOCs) in punctual and diffuse hydrothermal manifestations across the Sabatini Volcanic District (Latium, Italy)

Volatile Organic Compounds (VOCs) ubiquitously occur in hydrothermal gases emitted from both punctual vents and diffuse soil degassing. Their origin is mainly ascribed to the presence of organic matter embedded in the geological materials. The physicochemical features of hydrothermal reservoirs (e.g. T, redox, sulfur fugacity) shape the speciation of organic gaseous compounds. Once at shallow depth, deep-sourced VOCs encounter sharply different physicochemical conditions and microbial communities able to exploit VOCs as carbon substrates. Hence, in such areas, the composition of the released VOCs can be regarded as a chemical cocktail resulting from the co-occurrence of multiple physical, chemical and biological processes acting from the deep hydrothermal reservoirs up to the soil-atmosphere interface. The Sabatini Volcanic District (SVD; Latium, Italy) offers an ideal large-scale natural laboratory to investigate the response of VOCs speciation from punctual and diffuse emissions to variable conditions controlling the feeding hydrothermal reservoir. In the whole SVD, a regional hydrothermal system supplies the fluid discharges occurring from the Apennines to the coastline. Nevertheless, a geochemical gradient in the composition of both thermal waters and gas emissions was evidenced by Cinti et al. (2017) moving from the eastern to the western SVD sectors. In this study, we present the results from sampling campaigns of gas vents and soil gases from Caldara di Manziana (MZ) and Solforata di Nepi (NP), sited in the western and eastern SVD sectors, respectively. The study aimed to investigate and compare the VOCs speciation in fluids emitted at MZ and NP to highlight similarities and dissimilarities between the two areas, exploring the capability of soil gases to mimic the compositional features of the feeding hydrothermal system. Sharp differences in both the abundance and distribution of VOCs observed in punctual gas vents from the two sites were also reflected in the composition of soil gases. The MZ gas vent was enriched in CH4 and H2S, accompanied by high abundances of S-bearing compounds and alkenes, testifying reducing conditions and high temperature of the hydrothermal reservoir. Conversely, the NP gas vent showed an overall abundance of VOCs, mainly represented by alkanes (both short and long chains) and an enrichment in O-bearing compounds with respect to the MZ gas, pointing to the involvement of a larger fraction of fresh organic material and oxygen availability, in line with the hypothesis of a feeding system affected by larger mixing with recharging meteoric waters (Cinti et al., 2017). Despite the occurrence of shallow secondary processes that partially altered their composition with respect to punctual gas vents, soil gases collected at 40 cm depth were able to keep track of the peculiar compositional features of the feeding hydrothermal fluids in the two SVD sectors.