Two stalagmites from Anjohibe Cave have annual layers made up of inclusion-rich calcite over inclusion-free calcite or of darker aragonite over clear aragonite. Geochemical evidence indicates that the basal units are deposited slowly in the wet season and the upper units more rapidly in the dry season. For the period with rainfall and temperature data (ad 1951–1992), layer thickness correlates well with the Southern Oscillation Index (SOI), as well as rainfall, water surplus, and actual evapotranspiration (AET) at nearby Majunga. Com parison of the layer record for one stalagmite with 1866–1994 SOI data indicates that layer thickness correlates best with the frequency and intensity of warm, low-phase SO (El Niño) events, not with average SOI conditions. In addition, the 415-year layer thickness time-series from that speleothem agrees remarkably well with historical records of El Niño frequency, with Galápagos (Ecuador) coral records of sea-surface temperature in the eastern Pacific, and with accumulation rates on the Quelccaya Ice Cap of Peru, which are lower at times of high El Niño frequency.

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In this paper, I develop global karst chemical denudation models using chemical equilibrium equations. Theoretical karst chemical denudation rates are calculated as soil carbon dioxide concentration varies from 10 Ϫ 3.5 atm to 10 Ϫ 1.0 atm, temperature varies from 0 Њ C to 30 Њ C, and annual runoff varies from 500 mm to 3,000 mm. Both open and closed karst solution systems are examined. The Drake (1980) and the Brook-Folkoff-Box (1983) soil pCO 2 equations are used to develop chemical denudation models for different carbonate rocks (limestone and dolomite), climate (tropical, temperate, and arctic/alpine terrains), and karst solution type (open and closed systems). The major conclusion is that the karst solution type, least known in the past karst studies, is an important factor in controlling chemical denudation rates.

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