FIRST DISCOVERY OF CO2-DEPLETED WARM AIR VENTS IN CHRYSOTILE MILLING WASTE: SURFACE EVIDENCE FOR NATURAL CARBON SEQUESTRATION AT DEPTH

Warm air vents capable of melting snow have been found at the upper surface of a chrysotile milling waste pile holding 130 Mt of serpentinized ultramafic rocks, accumulated since 1953, at the Black Lake mine, Québec, Canada. The waste pile has an irregular conical shape up to 130 m high with a flat surface. The warm air vents are otherwise inconspicuous at the surface of the milling waste piles. The material has a heterogeneous grain-size varying from cobble to silt-size rock fragments. It has a surface area of about 1.7 km² and a maximum height of 130 m. The warm air vent temperatures ranged from 7.0 to 18.5 °C from March 2009 to July 2010, as measured with a thermocouple placed in the venting area identified by infrared photography. During this time, the temperature of ambient air ranged from -13.4 to 10 °C. The warm air vent is buffered to a small temperature variance by the thermal energy stored within the waste pile. A covariation between warm air temperature and CO₂ concentration (5 L sample bags; from <25 to 390 ppm CO₂) suggests a mixture between the vent air and open atmosphere during sample bag filling. Measurements of CO₂ concentration using a Li-Cor Li-8100 equipped with a 10-cm survey chamber yielded values as low as ca. 10 ppm CO₂ during winter 2010. In summer 2010, infrared surveys showed more diffuse temperature anomalies slightly offset from the winter active vents, and the minimum CO₂ concentration was 235 ppm. The CO₂ depleted warm air vents indicate that atmospheric CO₂ is captured by exothermic mineral carbonation reactions within the chrysotile milling waste pile. Lower CO₂ concentrations in vents during winter months are interpreted to indicate reduced wind-forced air infiltration along the steep flanks of the waste pile as a result of either snow cover or porosity occlusion by frozen interstitial water. A typical winter volumetric flux of a warm air vent is 6.8 L/m²/s. The vent CO₂ concentration of 10 ppm indicates that 380 ppm CO₂ have been stripped from the atmosphere with 390 ppm CO₂, such that a typical warm air vent captures approximately 0.45 kg/d/m² CO₂ during winter. Chysotile milling waste piles are naturally, and passively, permanently stripping carbon from the atmosphere by mineral carbonation reactions inside the waste piles, a new and previously unrecognized carbon store.