The rise and fall of cave pearl pools: Highly variable growth, recrystallization and demise of a mine pearl site

Abstract

AbstractSpeleothems form layered archives of the climate and local cave conditions during their formation. The origin of layering in cave pearls, however, is not well‐understood. Cave pearls grown in two adjacent mine sites between 2006 and 2014 elucidate the complexity of speleothem growth and recrystallization. Site A cave pearls grew under an active drip, while Site B cave pearls grew in small rimstone‐dam pools that filled with reverse‐graded fitted pearls between about 2009 and 2014. Despite the variation in pool setting, all samples are layered in grey and/or brown laminations and dendrites. The order and number of these layers varies widely, even between pearls growing millimetres apart in the same pool. However, stable isotope values reflect homogenized local precipitation. The variability between adjacent samples supports control by very local factors within each pool, likely related to CO2degassing at the water–air interface and water flow within the confined space of each pool. Recrystallization of calcite to calcite occurs resulting in triangular microspar patches and much less obvious bladed calcite. Laminations of brown or grey 1 to 5 µm calcite crystals recrystallize to bladed calcite up 100 µm long, all the while retaining a memory of the original layers in the form of ‘ghost’ layers, as revealed by gentle acid etching. Pearls at the top of rimstone‐dam pools grew faster than those just a few millimetres deeper, resulting in reverse grading. This model is applicable to reverse grading in marine and lacustrine pisolites. This study suggests that cave pearls in active flow regimes (drips or currents) are similar and largely abiogenic, in contrast to other locations with less flow, where more biological input is common. Recrystallization of calcite to calcite proceeds not only to equant spar (classic Ostwald ripening), but also to bladed calcite. Thus, bladed calcite in speleothems needs to be carefully evaluated for recrystallization even when aragonite is absent.