Modeling high temporal resolution climatic records perserved in speleothem deposits

This thesis develops a stalagmite growth and isotopic incorporation model that tests the relative influences that precipitation and temperature above the cave have on speleothem growth and isotopic incorporation. Model inputs include daily historical climate data, rock and soil thickness, soil porosity, rock δ13C values, relative abundance of dicots and grasses, solar radiative flux for the site, and cave PCO₂. The model compartmentalizes the soil and rock into distinct reservoirs. The hydrology within each reservoir has been simplified using a “leaky bucket” analogy in which the complex flow paths within each reservoir are simplified into single conduit compartment. Unknown parameters such as tortuosity, capillary forces, and dissipative energy-loss functions are integrated into a single parameter k. The operator specifies the catchment area over the cave, and the diameter of the openings (drip orifices) in the “leaky buckets”. These determine fluid storage capacity and transit time of fluids to and from the reservoirs.; The model calculates infiltration, soil CO₂ production, and soil fluid carbonate chemistry. The model calculates the percent of dicots and grasses that use the C₃ versus C₄ photosynthetic pathways to calculate the isotopic composition of soil respired CO₂. Exchange of soil and atmospheric CO₂ is used to calculate the carbon isotopic composition of the soil CO₂. Surface (air) temperature and empirical relationships between mean monthly temperature and oxygen isotopic composition of precipitation (Simpkins, 1995) are used to calculate the isotopic composition of precipitation. The model assumes that limestone dissolution occurs only in the rock compartment. Rock contribution to carbonate equilibria (calcium and CO₃=) and to fluid isotope composition are calculated immediately after soil fluids enter the rock reservoir. Calcite precipitation rates are calculated using kinetic growth equations. Isotopic equilibrium is assumed in the stalagmite 13C and 18 O calculations.; The model is calibrated using a speleothem from Mystery Cave in southeastern Minnesota. The model replicates measured growth and isotopic composition of the calibration sample to within the error of measured data sets. This model can be used to test interpretations of shifts in speleothem characteristics as being representative of paleoclimatic fluctuations.