Theoretical and empirical comparison of water chemistry and karst denudation in conduit and diffuse aquifers, and implications for global karst denudation models
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Twelve months of data, from June, 1999 to May, 2000, show that under similar climatic conditions, mean water hardness (118 ppm) in the Lost Cove, Tennessee, conduit aquifer system was significantly lower than hardness (220 ppm) in the Pocket Branch, Georgia, diffuse aquifer system. Estimated long-term annual mean denudation is 39 mm/ka at Pocket Branch and 30 mm/ka at Lost Cove. This significant difference suggests that aquifer type needs to be given more attention in assessing global variations in water chemistry and denudation in karst terrains. This is because the difference in denudation due to aquifer type is as large as differences between cold, temperate and tropical regions due to differences in temperature and soil carbon dioxide. There are both relict and recent tufa deposits along Pocket Branch. Present deposition is influenced by algae with calcite tubes forming around each algal cell. Tufa is deposited at most times of the year except during high streamflow conditions when some erosion occurs. This suggests that in other parts of the world tufa deposition may also be determined by streamflow conditions not by climate alone. About 65% of tufa deposition occurred during the winter and spring wet season, with 35% being deposited in the summer and fall dry season. Relict tufas in Pocket Branch are of early-Holocene age. The large size of the relict tufa at Pocket Falls suggests that in the early to middle Holocene rainfall and ground water was more substantial than today. It is possible that increased summer (monsoonal) rainfall in this region caused massive tufa deposition at Pocket Falls in the past. Climate warming at the end of the Little Ice Age (ca. AD 1850) may explain whey there is tufa deposition at Pocket Branch today.