Understanding the linkage between regional climatology and cave geochemical parameters to calibrate speleothem proxies in Madagascar

摘要

Robust reconstructions of paleodimate and paleoenvironmental changes using stalagmite proxy records critically depend on detailed observations of the transfer function between the regional climate/environment the karst aquifer hydrology, and finally the cave microclimate via monitoring, which is currently lacking in Madagascar. This paper reports the first monitoring study performed in Anjohibe Cave, in Mahajanga, NW Madagascar to understand the linkage between regional dimatological changes and cave responses to such changes. In this research, we monitored (1) the drip water pH, TDS, EC, temperature, δ~(13)C_(DIC), δ~(18)O_W, δ~2H_W. and elemental (Ca, Mg, Sr) composition, and (2) the cave atmosphere pCO_2, relative humidity (RH) and temperature. Three significant findings were drawn from the results. First, the data show that air-to-air transfer is fast and the internal parameters closely vary with the regional climatology. Second, rainfall to drip signal transfer is not immediate, and it can take few months to one season for the signals to be detected in the drip water due to the "epikarst storage effect". Lastly, CaCO_3 precipitation is likely to occur during the winter-summer transition, during which prior carbonate precipitation was detected. Since the growth of speleothems is influenced by numerous cave-specific factors, this study, although preliminary, indicates that Anjohibe Cave drip waters are capable of registering changes in its surrounding environment. Such information is ultimately archived in speleothems to reconstruct paleodimate and paleoenvironmental changes. Results from this research will be of high significance for those working on speleothems within Madagascar, and for those working on understanding the transfer of climatic variations to cave deposits.