EVIDENCE OF LATE HOLOCENE SOLAR INFLUENCE ON ALPINE CLIMATE FROM SPELEOTHEMS

Three active stalagmites were removed from different parts of the cave known as Grotta di Ernesto (1167 m a.s.l., 45°58¢37², Long. 11°39¢28²) in the Italian Alps. In all stalagmites, the late Holocene shows annual growth laminae consisting of a winter clear calcite layer and a summer-fall film of brown calcite. Here, we compare lamina thickness time-series data, indicative of annual growth rate variations, and carbon isotope profiles.

Shifts to more negative d¹³C values can be ascribed to enhanced soil activity and CO₂ production, under unlimited water and C3 vegetation conditions. Thicker laminae correlate with more negative d¹³C values. Lamina thickness variability is here considered a proxy for soil CO₂ production.

Annual growth rates were compared to mean monthly surface temperature (T) and precipitation (P) instrumental series starting from 1763. A significant positive correlation has been found with T, and in particular with winter T, while the correlation with P is never significant. Late Holocene soil CO₂ production in the studied site was thus probably also controlled by winter temperature through the duration of soil activity.

Spectral analysis of lamina thickness time-series data revealed periodic components at ca. 3, 7-8, 11 and 20-25 yrs, indicative of the influence of wintertime NAO and solar variability. Intervals of very thin laminae occurred during historical minima of solar irradiance, a further indication of solar forcing.

General Circulation Models (GCMs) suggest that variations in solar irradiance trigger NAO mode shifts at multidecadal timescales, resulting in winter temperature changes in Europe (Shindell et al., 2001, Science 294:2130-2136). Rapid diminution of lamina thickness during the Maunder Minimum and other observations supports atmospheric amplification of solar forcing predicted by the GCMs. At the 11 yrs sunspot cycle scale, we suggest that solar output modulated low cloud cover (Tinsley, 2000, Space Sci. Rev. 94:231-258), and thus insolation, which is known to influence the rate of microbial activity and soil CO₂ production.

Alpine stalagmites thus provide evidence of solar forcing, and of periodic phenomena connected to North Atlantic climate.