PRESERVATION BIAS IN SPELEOTHEM PROXY RECORDS DUE TO SEASONAL VENTILATION OF CAVES

Calcite deposited in caves (speleothems) is an important carrier of proxy information about paleoclimate and paleohydrology as recorded by growth rate, isotopic and elemental variations. If speleothem calcite deposition is not uniform throughout the year, however, proxy measurements will be biased towards the periods of fastest deposition. One variable that influences the rate of deposition of calcite is the contrast between CO₂ levels in the cave air and the CO₂ levels of vadose water entering the cave. Ongoing measurements of modern speleothem growth in Texas caves reveal an inverse correlation with cave-air CO₂ concentrations, with peak growth rates in winter, decreasing to zero in summer. Given a drip water with 100 ppm Ca at 21°C, theoretical growth rate calculations suggest that cave-air CO₂ in excess of approximately 2.7% is sufficient to stop growth. Cave-air CO₂ levels in this range are common. We hypothesize that the process reducing CO₂ levels in many caves that open upward to the surface is ventilation of cave air caused by density contrasts with surface air. To assess the controls on this process, we developed a model of air density in caves versus the surface. Air temperature is the main variable affecting air density, with barometric pressure and humidity exerting secondary effects. The model indicates that when temperatures at the surface remain above those in the cave, CO₂ accumulates as it out-gasses from cave drip water during calcite deposition. When temperatures at the surface drop below those in the cave, CO₂-rich cave air is displaced by cooler, denser outside air with low, atmospheric CO₂ levels (0.04%). The model reproduces measured CO₂ fluctuations in several Texas caves. Conditions in any cave can be modeled assuming that a cave's temperature approximates average surface temperature, and knowing the daily high and low surface temperatures. At mid and high latitudes, summer is typically the period of maximum cave-air versus surface-air stability. This period of potential CO₂ accumulation is when speleothem growth rates may slow and bias proxy preservation. For example, speleothem d¹⁸O measurements interpreted to represent the mean temperature over several years or decades might only represent an average of the cool seasons.