Detection of mantle CO2 in an underground salt mine via long-term and high-resolution monitoring by laser-based isotope techniques

Abstract

<p>Salt deposits may be affected by post-depositional CO<sub>2</sub> intrusions. In central Germany, such CO<sub>2</sub> contributions from the mantle may originate from Tertiary Rhön- and Vogelsberg-volcanism. The intrusion of those gases may cause technical and operational implications for storage caverns and salt mines.</p><p>Carbon isotope compositions of CO<sub>2</sub> are useful tools to differentiate between sources and are expressed as δ<sup>13</sup>C values in ‰ versus an international standard known as the Vienna Pee Dee Belemnite (VPDB). Typical average endmember values for CO<sub>2</sub> from the mantle are -5.1 ‰, while background air and anthropogenic influences range around averages of -11.9 ‰ and -29.8‰. Detection of fluctuations between these endmembers can be challenging with discrete sampling. This can be overcome by high-temporal resolution and long-term monitoring.</p><p>Towards this purpose, a laser-based isotope system was set up in an active underground salt mine in central Germany. For 34 days, continuous measurements of δ<sup>13</sup>C and concentrations of CO<sub>2</sub> were generated close to a site where mantle CO<sub>2</sub> intrusions were suspected. A timer regularly switched intakes from two capillaries, of which one was placed inside a borehole and the other in ambient air of the mine. Measured CO<sub>2</sub> concentrations ranged between 700 and 1600 ppmV, while δ<sup>13</sup>C values ranged between -21.5 ‰ and -11.5 ‰. Lower concentrations coincided with more positive isotope values and occurred around weekends when anthropogenic influences in the mine were less.</p><p>While influences of fresh air venting may have caused these weekly shifts, the admixture of mantle CO<sub>2</sub> seemed to play a continuous role. This is because small differences between the capillary from the borehole and the one with ambient air existed throughout the time series. Our results indicate that short-term dynamics on the order of hours to days are overlain by admixtures mantle gas intrusions of CO<sub>2</sub>.</p>