Very high-frequency and seasonal cave atmosphere PCO2 variability:: Implications for stalagmite growth and oxygen isotope-based paleoclimate records

Cave air P-CO2 at two Irish sites varied dramatically on daily to seasonal timescales, potentially affecting the timing of calcite deposition and consequently climate proxy records derived from stalagmites collected at the same sites. Temperature-dependent biochemical processes in the soil control CO2 production, resulting in high summer P-CO2 values and low winter values at both sites. Large Large-amplitude, high-frequency variations superimposed on this seasonal cycle reflect cave air circulation. Here we model stalagmite growth rates, which are controlled partly by CO2 degassing rates from drip water, by considering both the seasonal and high-frequency cave air P-CO2, variations. Modeled hourly growth rates for stalagmite CC-Bil from Crag Cave in SW Ireland reach maxima in late December (0.063 mu m h(-1)) and minima in late June/early July (0.033 mu m h(-1)). For well-mixed 'diffuse flow' cave drips Such as those that feed CC-Bil high summer cave air, P-CO2 depresses summer calcite deposition, while low winter P-CO2 promotes degassing and enhances, deposition rates. In stalagmites fed by well-inixed drips lacking seasonal variations in delta O-18, integrated annual stalagmite calcite delta O-18 is unaffected; however, seasonality in cave air P-CO2 may influence non-conservative, -geochemical climate proxies (e.g., delta C-13, Sr/Ca), Stalagmites fed by 'seasonal' drips whose hydrochemical properties vary in response to seasonality may have higher growth rates in summer because soil air P-CO2 may increase relative to cave air P-CO2 due to higher soil temperatures. This in turn may bias stalagmite calcite delta O-18 records towards isotopically heavier summer drip water delta O-18 values, resulting in elevated calcite delta O-18 values compared to the 'equilibrium' values predicted by calcite-water isotope fractionation equations. Interpretations of stalagmite-based paleoclimate proxies should therefore consider the consequences of cave air P-CO2 variability and the resulting intra-annual variability in calcite deposition rates. Crown Copyright (C) 2008 Published by Elsevier B.V. All rights reserved.