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

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

doi:10.5194/egusphere-egu2020-8197

Abstract

&lt;p&gt;Salt deposits may be affected by post-depositional CO&lt;sub&gt;2&lt;/sub&gt; intrusions. In central Germany, such CO&lt;sub&gt;2&lt;/sub&gt; contributions from the mantle may originate from Tertiary Rh&amp;#246;n- and Vogelsberg-volcanism. The intrusion of those gases may cause technical and operational implications for storage caverns and salt mines.&lt;/p&gt;&lt;p&gt;Carbon isotope compositions of CO&lt;sub&gt;2&lt;/sub&gt; are useful tools to differentiate between sources and are expressed as &amp;#948;&lt;sup&gt;13&lt;/sup&gt;C values in &amp;#8240; versus an international standard known as the Vienna Pee Dee Belemnite (VPDB). Typical average endmember values for CO&lt;sub&gt;2&lt;/sub&gt; from the mantle are -5.1 &amp;#8240;, while background air and anthropogenic influences range around averages of -11.9 &amp;#8240; and -29.8&amp;#8240;. Detection of fluctuations between these endmembers can be challenging with discrete sampling. This can be overcome by high-temporal resolution and long-term monitoring.&lt;/p&gt;&lt;p&gt;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 &amp;#948;&lt;sup&gt;13&lt;/sup&gt;C and concentrations of CO&lt;sub&gt;2&lt;/sub&gt; were generated close to a site where mantle CO&lt;sub&gt;2&lt;/sub&gt; 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&lt;sub&gt;2&lt;/sub&gt; concentrations ranged between 700 and 1600 ppmV, while &amp;#948;&lt;sup&gt;13&lt;/sup&gt;C values ranged between -21.5 &amp;#8240; and -11.5 &amp;#8240;. Lower concentrations coincided with more positive isotope values and occurred around weekends when anthropogenic influences in the mine were less.&lt;/p&gt;&lt;p&gt;While influences of fresh air venting may have caused these weekly shifts, the admixture of mantle CO&lt;sub&gt;2&lt;/sub&gt; 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&lt;sub&gt;2&lt;/sub&gt;.&lt;/p&gt;

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