Ancient speleothem giant preserved in a high-Alpine cave (Dolomites, N Italy): Rare insights into the Neogene

Conturines Cave (Dolomites, northern Italy) opens at 2775 m a.s.l. at the headwall of a former glacial cirque, some 700–800 m above the modern treeline. The approximately 200 m long cave comprises a single ascending paleophreatic conduit. Large parts of the cave floor are mantled by an extensive flowstone (locally up to 3.5 m), and a few prominent stalagmites are present in the inner part of the passage, where the flowstone deposition initiated. All major speleothems are presently inactive, partially corroded, and fractured. The hydrological catchment is a narrow ridge devoid of vegetation and soil, and present-day drip waters are undersaturated with respect to calcite. Several drill cores covering the entire stratigraphy of the flowstone were obtained along the gallery. Multi-proxy analyses of the two longest cores, drilled ∼5 m apart, show excellent replication and record several magnetic polarity reversals. U–Pb dating indicates that speleothem deposition began in the Late Miocene or earliest Pliocene (at ca. 5.80 ± 0.24 to 5.38 ± 0.24 Ma) and continued intermittently until 1.82 ± 0.17 Ma, making this the oldest radiometrically dated speleothem in the Alps. The main phase of flowstone deposition occurred before 4.21 ± 0.22 Ma and coincided with the formation of tall stalagmites. Fluorescent lamination and pollen grains preserved within the flowstone suggest a soil-covered catchment area that supported vegetation dominated by pine, indicating that the cave was once located near the treeline. This uplifted, exceptionally old speleothem archive provides a rare window into Neogene environmental conditions in the Southern Alps.