Alpine Climate Changes as Reconstructed from Speleothems

The use of δ18O signals from calcite is a major component of speleothem based climate reconstructions. However, the validity of these reconstructions depends on knowledge of the source water isotopic composition. The complex controls on source water composition in the modern climate were investigated during a four-year programme of monthly monitoring of meteoric water δOp along an E-W traverse in Trentino, South-Central Alps at stations from 180 to 2,200 meters above sea level. The monitoring indicates that the modern precipitation δ18Op signal is influenced by 1) the δOp of the air masses from western and southern provenance, 2) the deviation of water vapor transport trajectories due to mountain ranges, and 3) the elevation effect, whereby moisture content in westerly air-masses, which cross 3,000m mountain ranges, is more depleted than that in southerly air masses. For example, at the meteorological station set in the Brenta Dolomites (W-Trentino), mean annual δOp increases by as much as 1.5 ‰ when southern trajectories dominate over western trajectories. Because past δOp may have varied considerably as a consequence of circulation changes, it is not possible to construct a simple transfer-function for the Holocene, relating the oxygen isotopic composition of speleothem calcite (δOc) to mean air temperature. It is, therefore, difficult to reconstruct Holocene temperature and rainfall history in the Alps from oxygen isotope profiles of speleothem calcite alone. Current research on Holocene climate change in the Southern Alps focuses on analysis of multiple, independent, annual-resolution proxies within a given speleothem and their relationships to climate parameters. In addition, several stalagmites are generally sampled from different nearby caves. For example, at Grotta di Ernesto, at 1,160 meters above sea level (Fig. 1), paleoclimate research has been carried out for the past 6 years. High-resolution, alpine climate and environmental reconstruction from stalagmites has been obtained through the comparison of a δOc, δCc, Mg/Ca ratio and calcite fabric series dated by TIMS U/Th (Fig. 2). Each of these proxy indicators has either a different sensitivity to the same environmental process or records processes which are not registered by the other proxies. In particular, calcite fabrics in shallow subsurface caves such as Grotta di Ernesto are very sensitive to climate-related fluctuations in water availability, seasonal flow changes, and the presence of growth-poisoning particles (or dissolved ions) in dripwater. Comparison of the four proxy data series, combined with the information from the monitoring of modern day precipitation, allows an interpretation of major trends in the Holocene δOc profile in terms of regional circulation changes. Figure 2 shows Holocene profiles of four climate proxy indicators (oxygen and carbon stable isotopes, Mg/Ca ratio and fabrics) for stalagmite ER 76 from the Southern-Central Alps of Italy.