FORMATION OF SEASONAL ICE BODIES ANDASSOCIATED CRYOGENIC CARBONATES IN CAVERNE DEL'OURS, QUEBEC, CANADA: KINETIC ISOTOPE EFFECTSAND PSEUDO-BIOGENIC CRYSTAL STRUCTURES

摘要

This study examines the kinetics of formation of seasonal cave ice formations(stalagmites, stalactites, hoar, curtain, and floor ice) and the associated cryogenic calcitepowders in Caverne de l'Ours (QC, Canada), a shallow, thermally-responsive cave. Theseasonal ice formations, which either formed by the: (1) freezing of dripping water (icestalagmite and stalactite); (2) freezing of stagnant or slow moving water (floor ice andcurtain ice) and; (3) condensation of water vapor (hoar ice), all (except floor ice) showed kinetic isotope effects associated with the rapid freezing of calcium — bicarbonate water. This was made evident in the 6D, 6180 and d (deuterium excess) compositions of theformed ice where they plot along a kinetic freezing line.The cryogenic calcite powders,which are found on the surface of the seasonal ice formations, also show kinetic isotopeeffects. Their 613C and 6180 values are among the highest measured in cold-climatecarbonates and are caused by the rapid rate of freezing, which results in strong C-0disequilibrium between the water, dissolved C species in the water, and precipitatingcalcite. Although the cryogenic calcite precipitated as powders, diverse crystal habitswere observed under scanning electron microscope, which included rhombs, aggregatedrhombs, spheres, needles, and aggregated structures. The rhomb crystal habits wereobserved in samples stored and observed at room temperature, whereas the sphere andneedle structures were observed in the samples kept and observed under cryogenicconditions. Considering that the formation of cryogenic calcite is purely abiotic (freezingof calcium — bicarbonate water), the presence of spherical structures, commonlyassociated with biotic processes, might represent vaterite, a polymorph of calcite stableonly at low temperatures. It is therefore suggested that care should be taken beforesuggesting biological origin to calcite precipitates based solely on crystal habits becausethey might represent pseudo-biogenic structures formed through abiotic processes.