Journal of Conference Abstracts

Speleothem Deposition in the Spannagel Cave: A High-Altitude, Periglacial Carbonate Record from the Eastern Alps

Christoph Spötl (christoph.spoetl@uibk.ac.at)¹, Stephen J. Burns², Norbert Frank³ & Augusto Mangini³

¹ Institut f. Geologie, Universität Innsbruck, Innrain 52, 6020 Innsbruck, Austria

² Geologisches Institut, Universität Bern, Baltzerstrasse 1, 3012 Bern, Switzerland

³ Heidelberger Akademie der Wissenschaften, Institut für Umweltphysik, Im Neuenheimerfeld 229, 69120 Heidelberg, Germany

The Spannagel Cave is a fairly extensive cave system (currently seven kilometers of explored passages) which developed in Jurassic marbles sandwiched between gneisses in the western part of the Zillertal Alps, Tyrol. The cave system extends from 2195 m to 2521 m a.s.l. and is directly adjacent to the actively retreating Hintertux Glacier. Marginal moraines of the glacial advance during the Little Ice Age show that even fairly recently parts of the cave systems were in a subglacial position. Despite its high altitude well above the timber line this cave hosts locally abundant speleothems, some of which are actively forming today. Modern speleothem deposition includes stalactites, stalagmites, soda straws and locally minor flowstones, whereas the latter are the most abundant variety of ancient speleothems. All speleothems are exceptionally pure, commonly snow-white and of low-Mg calcite composition. The samples are composed of coarsely crystalline columnar calcite with individual crystals commonly reaching several centimeters in length. TIMS dating of ten samples from this cave demonstrates that speleothem deposition occurred repeatedly during the past 400 ka at this high-altitude site. Ancient flowstones record calcite precipitation during marine isotope stages 5e, 6, 7, 8 and 9. Of particular interest are three speleothem samples which formed during isotope stage 8 which is traditionally correlated with the extensive Mindel Glaciation of the Alpine glacial stratigraphy. The presence of these carbonates is a clear testimony that conditions had to be - at least temporarily - above the freezing point during this glacial period. As the present-day air temperature inside the cave is only slightly above freezing (+1.8°C), we are looking at a highly sensitive system. Stable isotope data of modern and ancient speleothems yielded a large spread both in carbon (-10.2 to +10.8 per mil VPDB) and oxygen (-15.5 to -6.4 per mil VPDB). The C isotope values are biased toward high and commonly positive values reflecting a high proportion of atmospheric relative to soil-derived carbon dioxide. C isotope values as high as +10.8 per mil in several ancient speleothems strongly suggest kinetically controlled fractionation, probably caused by freezing of water in the unsaturated zone above the cave and concomitant degassing of isotopically light carbon dioxide. Speleothems of Holocene and Eemian age show O isotopic compositions consistent with equilibrium fractionation from modern dripwaters. In contrast, speleothems deposited during glacial stages show generally lower O isotope values (and more variable C isotope values), presumably reflecting a combination of deposition from isotopically depleted, glacial precipitation, and repeated freezing of the karst water aquifer. Inasmuch as sedimentary archives in Alpine settings that extend back in time before the onset of the last deglaciation are exceedingly rare, this cave and its speleothems offer an interesting opportunity to examine environmental change from a high-altitude perspective where climate proxy data stored in carbonate formations survived multiple Pleistocene glaciations.

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