Ventilation and cave air PCO2 in the Bunker-Emst Cave System (NW Germany): implica-tions for speleothem proxy data

摘要

Cave air pCO2 (carbon dioxide partial pressure) is, along with drip rate, one of the most important factors controllingspeleothem carbonate precipitation. As a consequence, pCO2 has an indirect but important control on speleothem proxydata (e.g., elemental concentrations, isotopic values). The CO2 concentration of cave air depends on CO2 source(s) andproductivity, CO2 transport through the epikarst and karst zone, and cave air ventilation. To assess ventilation patternsin the Bunker-Emst Cave (BEC) System, we monitored the pCO2 value approximately 100 m from the lower entrance(Bunker Cave) at bi-hourly resolution between April 2012 and February 2014. The two entrances of the BEC systemwere artificially opened between 1860?1863 (Emst Cave) and 1926 (Bunker Cave). Near-atmospheric minimum pCO2dynamics of 408 ppmv are measured in winter, and up to 811 ppmv are recorded in summer. Outside air contributes thehighest proportion to cave air CO2, while soil, and possibly also ground air, provide a far smaller proportion throughoutthe whole year. Cave air pCO2 correlates positively with the temperature difference between surface and cave airduring summer and negatively in winter, with no clear pattern for spring and autumn. Dynamic ventilation is driven bytemperature and resulting density differences between cave and surface air. In summer, warm atmospheric air is entrainedthrough the upper cave entrance where it cools. With increasing density, the cooled air flows toward the lowerentrance. In winter, this pattern is reversed, due to cold, atmospheric air entering the cave via the lower entrance, whilerelatively warm cave air rises and exits the cave via the upper entrance. The situation is further modulated by preferentialsouth-southwestern winds that point directly on both cave entrances. Thus, cave ventilation is frequently disturbed,especially during periods with higher wind speed. Modern ventilation of the BEC system—induced by artificiallyopenings—is not a direct analogue for pre-1860 ventilation conditions. The artificial change of ventilation resulted in astrong increase of δ13Cspeleothem values. Prior to the cave opening in 1860, Holocene δ13Cspeleothem values were significantlylower, probably related to limited ventilation due to the lack of significant connections between the surface and cave.Reduced ventilation led to significantly higher pCO2 values, minimal CO2 degassing from drip water and low kineticisotope fractionation. Both modern and fossil speleothem precipitation rates are driven by water supply and carbonatesaturation, and not by cave air pCO2. Today, pCO2 variability is too small to affect carbonate precipitation rates and thesame is likely true for pCO2 variability prior to artificial opening of the cave. Thus, fossil speleothems from BEC Systemare likely more sensitive to temperature and infiltration dynamics. The Bunker-Emst Cave System, therefore, representsdifferent ventilation patterns and their influence on speleothem proxy data in an exemplary manner, and it may serveas a template for other cave systems.