Changes in the CO2 dynamics in near-surface cavities under a future warming scenario: Factors and evidence from the field and experimental findings

Abstract

This study is based in a field monitoring of a cave-soil-atmosphere system validated with laboratory experimentation. CO2 and 222 Rn dynamics in the cavity demonstrate the dependence on the climatic parameters, mainly on the differences between outdoor and indoor temperatures. Within the cave, the annual cycles are characterized by two outstanding moments: cave gaseous recharge and ventilation when cave acts as gaseous sink or source respectively. Relationship with soil above the cave exists permanently. Soil temperature and moisture are responsible for CO2 production at different time scales.Soil CO2 in Rull site reaches values higher than 3000 ppm in April-May, which falls to nearly 1000 ppm during the summer time. Due to CO 2 diffusion, maximum values in the cave are reached in the warmest months and are in accordance to soil CO2 values. Within the cave, maximum CO2 concentration is, in average, 3470 ppm while minimum is 623 ppm. Cave temperature and relative humidity remain quite stable with mean values of 16.1 ºC and 97.9% respectively. To describe field findings, CO2 production and diffusion experiments are developed related to soil behaviour. Results show that soil CO2 production increases as soil temperature and moisture increase according to a calculated logarithmic equation until water content in soil exceeds the saturation values. In addition, CO2 diffusion in Rull soil is reduced around 60% when water content in soil increases from 0 to 30%. Soil-produced CO2 reaches Rull cave, by diffusion. We estimate 57 kg of emitted CO2 from the cave to the atmosphere in an annual cycle, considering a volume of 9915 m 3 . Finally, projections of future climate in the study site confirm a general tendency for annual-mean conditions to be warmer and drier, which will directly affect soil CO2 production. Under this situation, Rull cave will experience changes in the stored and then exchanged annual amount of CO2.

Role of soil pore structure in water infiltration and CO2 exchange between the atmosphere and underground air in the vadose zone: A combined laboratory and field approach

“…However, previous studies that examined the annual isotopic cycle (Pla et al, 2016a) The highest CO2 concentration observed as a direct consequence of the rainfall was achieved between March 28 and 29, some days after the last rainfall on March 24. After this last episode, soil VWC started decreasing but always remained higher than the value observed before the first rainfall episode (March 18).…”

“…After this event, CO2 concentration inside the cave increased from March 24 to March 29. Under other circumstances, i.e., without rain, the natural CO2 trend inside the cave would have decreased during this period (Pla et al, 2016a(Pla et al, , 2016b. During the studied event, the role of carbonate host rock dissolution as a contributing source of CO2 to the cave air (SerranoOrtiz et al, 2010) is considered to be negligible because little water was observed dripping into the cave after the rainfall event.…”

“…Particularly for Rull cave, the heavier δ 13 CO 2 values are caused by the strong influence of the external atmosphere, which is most pronounced during the degassing stage (i.e., during the coldest months). Lighter values of δ 13 CO2 in the cave indicate that soil-produced CO2 dominates, as occurs in the recharge stage during the warmest months (Pla et al, 2016a).…”

“…The soil is mainly composed by quartz, related to the presence of the conglomerates. The soil is classified as a silty -silty loam soil (Pla et al, 2016a). The only differentiated horizons are the O horizon (a thin soil layer dominated by organic matter) and the A horizon (a mineral horizon near surface with accumulation of humified organic matter mixed with the mineral fraction).…”

“…The δ 13 CO2 of approximately −27‰ is the result of the combination of equilibrium and kinetic fractionations that occur during the photosynthesis of C3 plants and the decomposition of C3 biomass (Amundson et al, 1998;Cerling et al, 1991). In both caves, the δ 13 CO 2 isotopic signal fluctuates cyclically, and this behaviour is linked to gas dynamics (Garcia-Anton et al, 2014a;Pla et al, 2016a;Sanchez-Moral et al, 2010). Particularly for Rull cave, the heavier δ 13 CO 2 values are caused by the strong influence of the external atmosphere, which is most pronounced during the degassing stage (i.e., during the coldest months).…”