Carbon isotope values as paleoclimatic indicators. Study on stalagmite from Nerja Cave, South Spain

摘要

In theory, it should be possible to use C-isotope variations in speleothems as a monitor of past climate change in a manner analogous to O-isotopes. C isotope values are related to the terrestrial C cycle and provide information about soil and vegetation. Variations in the carbon isotopic composition (δ13C) values of speleothems reflect changes of the vegetation type in the vicinity of a cave and arise because of the differences in the photosynthetic pathways between C3 and C4 type vegetation. Carbon isotopes in speleothems may serve as indicators of vegetative change, climatic conditions, and karst processes. Carbon isotope variations in speleothems arise from a variety of causes from both surface environment (local climate, bioproductivity, vegetation type, etc.) and subsurface environment in which rain water passes through the soil zone and then the epikarst into cave voids. Even within the cave, environmental local processes such as evaporation, drip rates, and changes in cave air pCO₂ controlled by seasonal ventilation can influence the final δ13C values found in the speleothem record. Interpretation of δ13C records may require several consecutive processes. In the present study, the δ13C record of a stalagmite from Nerja Cave was examined to evaluate the roles of environmental processes and local cave conditions on speleothem δ13C values. Stalagmite consisted of aragonite crystals with mineralogical composition uniform. There was no observed evidence to indicate recrystallization of aragonite to calcite or the presence of alternating mineral laminae. The influence of vegetation on speleothem δ13C primarily reflects changes in the density of vegetative cover and biomass. Greater vegetative cover contributes to lower δ13C values as more biogenic CO₂ is dissolved into the seepage water. When vegetative cover is reduced by changes in climate or from deforestation, the vegetative contribution of biogenic CO₂ diminishes and speleothem δ13C values increase. Ventilation of Nerja Cave may also induce changes in cave air pCO₂ that contributed to the δ13C record. During cold periods, enhanced air exchange may have occurred between the cold, dense air outside and the internal cave atmosphere. Increased air exchange could have lowered pCO₂ of the cave air, increased degassing, and enriched the precipitating aragonite in 13C.