Rapid growth of meter-scale calcite speleothems in the Mission Tunnel, Santa Barbara, CA

摘要

Groundwater seeping from the unlined arkosic sandstone sections in the Mission Tunnel, completed in 1912, has resulted in large calcite speleothems including stalactites and flowstone (1.5m), and thick deposits on wooden beams and cables. Oxygen and carbon isotope values for the calcite speleothems range from —4.5 to —20 and —4 to -33per thousand (PDB), respectively and they show increasing values with decreasing age. The oxygen isotopic composition in the most recent calcite is about 2 per mil higher than expected for isotopic equilibrium with the present 26°C seep water. Oxygen and carbon isotopes show a systematic covariation trend with a slope of δ~(13)C/δ~(18)O = 1.7, which is intermediate to that calculated for the model of Bigeleisen & Wolfsberg (1958) and Slater (1953) for kinetic non-equilibrium fractionation. The calcite forms during rapid degassing of CO_2 charged ground water entering the tunnel.