Speleothems from the High-Alpine Spannagel Cave, Zillertal Alps (Austria)

Abstract

Spannagel Cave is a fairly extensive cave system (currently nine kilometers of explored passages) which developed in Jurassic marbles sandwiched between gneiss in the western part of the Zillertal Alps, Tyrol. The cave system extends from 2195 m to 2521 m a.s.l. and is adjacent to the actively retreating Hintertux Glacier. Marginal moraines of the glacial advance during the “Little Ice Age” (about 1850) show that even fairly recently parts of the cave system were in a subglacial position. Despite its high altitude well above timber line, this cave hosts speleothems, some of which are actively forming today. Modern speleothem deposition includes stalactites, soda straws, stalagmites, moonmilk and flowstones, whereas the latter are the most abundant variety of ancient speleothems in this cave. Th-U TIMS dating demonstrates that speleothem deposition occurred repeatedly during the past few hundred thousand years at this site. Stable isotope data of modern and ancient speleothems yielded a large spread both in C (-11.1 to +10.8%o VPDB) and O (-15.5 to -6.2%o VPDB). The C isotope values are biased toward high and commonly positive values, reflecting a low proportion of soil-derived CO2 in the karst system, largely buffered by the host rock. C isotope values as high as +10.8%o indicate kinetically controlled fractionation. Speleothems of Holocene age show O isotopic compositions consistent with near-equilibrium fractionation from modern dripwaters. In contrast, fossil speleothems commonly show lower O isotope values and higher and more variable C isotope values, reflecting the impact of isotopically depleted glacier meltwaters and the sensitivity of the karst system to changes in the high-alpine vegetation cover.