Hydrogen and Carbon Stable Isotopic Compositions and Concentrations of Methane in Cave Air of Cueva de Villa Luz, Tabasco, Mexico

Abstract:

Cueva de Villa Luz (CVL) is a unique biogeochemical environment where microbial consortia are supported by hydrogen sulfide (H₂S) leading to sulfuric acid speleogenesis (SAS) which is thought to have generated the porosity and permeability of several petroleum reservoirs. Possible sources of the sulfur (S) include the Chichón Volcano and petroleum basins in the area. A better understanding of the source of the H₂S in CVL may help predict where else SAS may have occurred. Analysis of methane (CH₄) in CVL may provide a proxy to assess the source of S entering CVL.

We obtained 13 air samples in 1-L Tedlar^® bags from varying locations in CVL to assess the role of CH₄ in sulfide-rich karst systems. CH₄ and carbon dioxide (CO₂) concentrations were measured by gas-chromatography. The stable isotopic ratios of carbon and hydrogen were measured on a stable isotope-ratio mass-spectrometer.

CH₄ in the air of CVL ranged from 1.88 ± 0.10 ppmv to 3.7 ± 0.2 ppmv. CO₂ concentrations ranged from 400 ± 20 ppmv to 920 ± 50 ppmv. For comparison, the CH₄ and CO₂ concentrations in the outside atmosphere were 1.96 ± 0.10 ppmv and 430 ± 20 ppmv respectively. CH₄ and CO₂ were positively correlated in CVL (R² = 0.91, CH₄ = [0.0035 ± 0.0007] CO₂ + [0.4 ± 0.4], p >0.01). The highest concentrations were near springs. Keeling-style analysis showed that the CH₄ samples from CVL plot along a two-end member mixing model and suggest that CH₄ is outgassing from spring water with isotopic compositions δ¹³C_CH4 = -24 ± 3 ‰ and δ²H_CH4 = -40 ± 40 ‰. CO₂ did not plot along a two end member mixing model.

The proposed δ¹³C of CH₄ entering from springs does not closely match the δ¹³C_CH4 values from hydrocarbon basins in the area. This is likely due to oxidative loss of CH₄ as it ascends to CVL which may be partly driven by anaerobic methanotrophy coupled to sulfate reduction. Analysis of the spring water chemistry coupled to biogeochemical modeling may help quantify the amount of methanotrophy occurring in the subsurface.