Toward Solving the Mystery of Elevated Tectonic Degassing in South Eastern Europe: Insights From Gas Discharges Along the Vardar Suture (North Macedonia)

Abstract

Tectonic carbon degassing is an important contributor to the global carbon cycle. South Eastern Europe is an active extensional tectonic region. This is the result of intense geodynamic events related to the closure of the Tethys Ocean, whose remnants include an ophiolite orogenic belt and the Vardar megasuture. In North Macedonia, regional active fault systems, seismic activity, Cenozoic-Quaternary volcanism, large-scale degassing, and low-enthalpy geothermal resources are widespread. Nonetheless, a geochemical characterization of the gas manifestations is missing from the literature. Toward this contribution, we report the first characterization of chemical and isotopic compositions of fluids from the main geothermal and cold gas manifestations of North Macedonia, and we explore their origins, the processes controlling their chemistry, and their relationships with the regional geodynamic situation. Gas samples were collected along the whole country, and were analyzed for both their chemical (He, H2, H2S, O2, N2, CO2, CH4, Ar) and isotopic composition (δ13C-CO2, He, Ar). Based on their chemistry, samples can be subdivided into three groups: (a) N2-dominated, (b) CO2-dominated, and (c) H2S-rich, which are geographically well separated, following regional distributions. The CO2-dominated group is the most widespread, highlighting the importance of geogenic carbon degassing in the study area. Its origin is prevailingly crustal (δ13C-CO2 = −4.6–+1.0‰ vs. V-PDB; R/RA = 0.1–1.8), but a significant, up to 25%, mantle contribution can be inferred from the He isotope composition.