Effect of water excess on soil carbon dioxide, seepage water chemistry, and calcite speleothem growth: An experimental and modelling approach

Abstract

The effect of the water excess in soil on soil gaseous carbon dioxide concentrations (c CO2) was studied based on field experiments. The gradual water addition of 15 and 30 L m −2 to leptosols and anthrosols, simulating 15 and 30 mm precipitation, respectively, caused the overall c CO2 increase of 1.53 × 10 −1 mol m −3 (increase by 60%) and 1.61 × 10 −1 mol m −3 (increase by 112%) in the soil airs. The effect of the c CO2 increment on seepage water, cave dripwater chemistry, and calcite speleothem overgrowths was deduced from geochemical modelling. It showed that the c CO2 increments may lead to the increments in total dissolved carbon, aqueous calcium, and dissolved solids of 1.10 × 10 −3 mol L −1 (increase by 35%), 4.45 × 10 −4 mol L −1 (increase by 30%), and 1.55 × 10 −3 mol L −1 (increase by 34%), respectively. After the total degassing of CO 2 in the cave, the increment in the saturation index of dripwater, SI, could reach up to ΔSI = 0.31, which means an increase by hundreds of percent. The water excess of 5 L m −2 following a dry period would cause the increment in saturation index ΔSI = 0.17. The modelling further showed that the c CO2 increase associated with the 30 L m −2 water excess could induce the calcite overgrowth up to 1 μm thick per 1 m 2 surface area. The effect of water excess with additional water supplies gradually weakens, probably due to reduced CO 2 diffusivity and soil microorganism activity. It can be assumed that the most contrasting peaks in dripwater chemistry are associated with the individual precipitation events after short dry periods. The increased supersaturation of dripwater is expected to lead to faster growth of speleothem and changes in calcite textures. K E Y W O R D S calcite speleothem overgrowth, field experiment, geochemical modelling, seepage water chemistry, soil CO 2 concentration, soil water excess Abbreviations symbols and acronyms: [Ca] T , total concentration of dissolved calcium; c CO2 , concentration of gaseous carbon dioxide; CO 2 , carbon dioxide; DLI, daylight illuminance; H 2 O, water; K, equilibrium constant; L, litre; P, barometric pressure; P CO2 , carbon dioxide partial pressure; P CO2(cave) , carbon dioxide partial pressure in cave atmosphere; P CO2(soil) , carbon dioxide partial pressure in soil atmosphere;