A Global Model for Seasonal Preservation Bias In Speleothem Proxy Records

CO₂ concentrations in many caves vary seasonally, commonly peaking in summer. These high CO₂ levels suppress cave calcite growth.  This seasonal growth rate variability can bias proxy environmental records preserved by speleothems. The bias potential is toward preserving records of cool-season versus warm-season conditions. Cave CO₂ levels represent a balance of inputs, typically from soils, and outputs, typically by exchange with outside air. CO₂ input from soil respiration is controlled by seasonal soil temperature and moisture. Loss of accumulated cave CO₂ is commonly through density-driven exchange (ventilation) with outside air. These density differences are controlled by differences in temperature, relative humidity, and air pressure between cave and surface air, parameters that cycle diurnally and seasonally. 

Historical weather data can be used to model cave and surface air densities to predict cave CO₂ accumulation and ventilation. This model predicts first-order geographic controls on speleothem growth. For example, as latitude increases, winter-summer differences in soil temperature and air density increase the magnitude and duration of summer CO₂ build-up. This trend results from the contrast between lower average annual temperatures and comparatively higher summer temperatures with increasing latitude. At low latitudes diurnal temperature (and thus density) variations are greater than seasonal variations and daily ventilation and diffusion control CO₂ levels.   Similarly, sites at all latitudes with a moderating maritime influence ventilate more frequently than latitude alone would predict. Thus, the potential for seasonally-biased records is greatest in speleothems from caves at mid to high latitudes and lowest at low latitudes and coastal sites. This first-order model ignores other controls on speleothem growth.  Global weather data can be used to map differences between average annual temperature and average high temperatures. Such maps suggest where CO₂ build-up in caves is most likely, which in turn suggests where seasonal preservation biases are most likely.

---