Carbonate precipitates were sampled in August 2017 from Obstanser Eishöhle, which is located 3 km south of Kartitsch (46.6875°N, 12.4932°E), close to the Austrian-Italian border. Aragonite was actively precipitating in this cave from drip water containing Mg2+/Ca2+ > 1.5 and occurred as snow-white flowstones and stalactites in a close vicinity to hydromagnesite and calcite (26). We collected precipitates both (i) from the stalactite drip water and (ii) from the surface of aragonite flowstone covering the cave wall (fig. S1). (i) To sample drip water precipitates, we drilled the bottom of a TEM grid box so that drip water could flow through the hole during carbonate precipitation. We inserted lacey-C–coated Cu TEM grids (Agar Scientific) into the grid box and placed it under the drip water for 20 hours. (ii) To collect precipitates from the surface of the aragonite flowstone, we placed a TEM grid into self-locking tweezers and gently touched the aragonite surface three times. This sample collection procedure permitted the study of the freshly deposited and loosely bound carbonates. Taking the studied flowstone’s aragonite growth rate of 34 μm/year (26) into account and using a maximum thickness estimate of ~10 to 20 μm (crystals shown in the TEM figures are from the thin part of the sample) for the material on the TEM sample holder, the sampled carbonate of the flowstone surface likely corresponded to less than 1 year of precipitation. We analyzed the drip water and flowstone precipitates using TEM.

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