Geological CO2 quantified by high-temporal resolution stable isotope monitoring in a salt mine

Alexander H Frank,Bettina Strauch,Martin Zimmer,Anssi Myrttinen,Robert Van Geldern,Johannes A C Barth

doi:10.1038/s41598-020-77635-5

Abstract

The relevance of CO2 emissions from geological sources to the atmospheric carbon budget is becoming increasingly recognized. Although geogenic gas migration along faults and in volcanic zones is generally well studied, short-term dynamics of diffusive geogenic CO2 emissions are mostly unknown. While geogenic CO2 is considered a challenging threat for underground mining operations, mines provide an extraordinary opportunity to observe geogenic degassing and dynamics close to its source. Stable carbon isotope monitoring of CO2 allows partitioning geogenic from anthropogenic contributions. High temporal-resolution enables the recognition of temporal and interdependent dynamics, easily missed by discrete sampling. Here, data is presented from an active underground salt mine in central Germany, collected on-site utilizing a field-deployed laser isotope spectrometer. Throughout the 34-day measurement period, total CO2 concentrations varied between 805 ppmV (5th percentile) and 1370 ppmV (95th percentile). With a 400-ppm atmospheric background concentration, an isotope mixing model allows the separation of geogenic (16–27%) from highly dynamic anthropogenic combustion-related contributions (21–54%). The geogenic fraction is inversely correlated to established CO2 concentrations that were driven by anthropogenic CO2 emissions within the mine. The described approach is applicable to other environments, including different types of underground mines, natural caves, and soils.

Highlights

The relevance of CO2 emissions from geological sources to the atmospheric carbon budget is becoming increasingly recognized
Diffusive degassing may be the quantitatively predominant process of geogenic CO2 losses to the atmosphere[2]. While contributions and their dynamics have been extensively studied near volcanos and in geothermal regions[8,14,15,16,17,18], short-term temporal and spatial dynamics of geogenic CO2-releases to the atmosphere have likely been underestimated[12], especially from regions without geothermal heat flow
The contact with water accessing into the geological faults may have led to isotopic exchange processes with especially the oxygen atoms of the encountered C O2, why, in this study, we focus on carbon isotopes

Summary

Introduction

The relevance of CO2 emissions from geological sources to the atmospheric carbon budget is becoming increasingly recognized. It has been increasingly acknowledged that the Earth’s mantle and crust CO2 emissions may account for a substantial quantity of the global atmospheric carbon cycle[11,12] This belated recognition might have been rooted in the challenging apportionment of such geogenic CO2 contributions. Diffusive degassing may be the quantitatively predominant process of geogenic CO2 losses to the atmosphere[2] While contributions and their dynamics have been extensively studied near volcanos and in geothermal regions[8,14,15,16,17,18], short-term temporal and spatial dynamics of geogenic CO2-releases to the atmosphere have likely been underestimated[12], especially from regions without geothermal heat flow. Different proportions of the two stable isotopes 13C and 12C in CO2 from geogenic and combustion-related sources, this ambiguity can be overcome by stable isotope measurements and the application of isotope mixing models[17,24]

Methods

Results

Discussion

Conclusion