Coarse-grained cryogenic aragonite as end-member of mineral formation in dolomite caves

Abstract

Cryogenic carbonate particles (single crystals and aggregates) have recently been recognized as important witnesses of permafrost dynamics, as they can be used to estimate the thickness and timing of development of permafrost. In this study, the petrography and geochemistry of coarse-grained cryogenic aragonite (CGCAr) identified in Zoolithen Cave in NE Bavaria, Germany, are described in detail for the first time. We provide a sequence of carbonate precipitation during freezing, where CGCAr mark the final crystallization phase in freezing water on ice. Our results support the notion that host rock composition, solution chemistry, and cave ventilation play an important role for aragonite formation in caves.Using petrographic and geochemical evidence we distinguish aragonitic spherulites, and precursory spherulitic and rhombohedral aggregates of magnesian calcite. The aragonitic particles are characterized by very low oxygen isotope (δ18O = −17.1‰ to −17.8‰) and relatively high carbon isotope values (δ13C = +1.0‰ to +0.8‰). Among the magnesian calcites the spherulitic forms show more negative oxygen and more positive carbon isotope values, compared to rhombohedral crystals (δ18O = −17.2‰ to −15.0‰ vs. −16.2‰ to −9.9‰; δ13C = +0.4‰ to −3.5‰ vs. −0.8‰ to −4.8‰). This sequence is mirrored also in mineralogical/geochemical data from the spherulitic CGCAr, where the calcite fibers show an increase in Mg from 1.5–2.5 mol-% to 7.5–10 mol-% MgCO3, before aragonitic spherulites are formed.U-series dates from cryocalcites formed prior to the identified CGCAr in Zoolithen Cave place the formation of the former into the last glacial to between 28.75 ka and 30.63 ka. Aragonitic spherulites on cryocalcites represent the end-member of the succession cold water calcite – cryocalcite – cryoaragonite. The sampling depth at 30 m below surface represents a minimum thickness of last glacial permafrost in this location in Central Europe.