Abstract
An interesting association of slope breccia, inactive and active tufa deposits, and speleothems is present in the central Vinschgau Valley, Italy. The occurrence of abundant carbonate cements in fractures and voids of crystalline basement rocks is unexpected considering the fact that this valley is among the driest spots in the entire Alps. Low annual precipitation of 440-530 mm coupled with frequent wind give rise to a semiarid climate and steppe vegetation along the south-facing slopes of the valley.
Springs in this area are mostly supersaturated with respect to calcite, and carbonate precipitation occurs locally as tufas and, less well known because of lack of accessibility, as speleothems in the shallow subsurface. The majority of the tufa deposits and speleothems, however, are fossil. Speleothems are composed of low-Mg calcite and calcite-aragonite, respectively. Delicate growth textures including presumable annual lamination caused by pronounced changes in fluorescence intensity are preserved in both calcite and aragonite. Most calcite is a primary precipitate, but small amounts of secondary calcite replacing aragonite are common in most aragonite-bearing samples.
The highly radiogenic Sr isotope composition, as well as high concentrations of U, Fe, Sr, and Mg, indicate that the groundwater from which these carbonates precipitated experienced intensive interaction with the host crystalline rocks. The very low tritium concentrations and the lack of a seasonal O isotope variation in modern spring waters, as well as their rather constant hydrochemical composition, also support this suggestion. S isotope data for dissolved sulfate and Ca and Mg sulfate precipitates indicate a sulfide source, i.e., oxidation of sulfide ore minerals in the aquifer, resulting in elevated sulfate and Fe concentrations. Th/U dating of speleothem samples using thermal ionization mass spectrometry yielded ages between 13,710 and 378 yr BP, with most ages falling in the early to middle Holocene. Although no isotopic dates are available for the tufa deposits, field evidence strongly suggests that speleothems, tufa deposits, and carbonate cements in the slope breccia were closely related. We therefore interpret the existence of these terrestrial carbonates as evidence of changes in climate since the middle Holocene. Their presence suggests a higher annual rainfall during the first half of the Holocene, possibly because of enhanced moisture transport from the Mediterranean.
