Carbonate deposits in polar regions: Origin, age and paleoclimatology. A geochemical and isotopic approach.

摘要

Given the growing interest in secondary carbonate deposits from polar regions as paleoclimatic proxies, this thesis evaluated if they could be effectively be used in paleoclimatic reconstructions. To facilitate comparison between studies, the cold-climate carbonate precipitates were classified into three categories: powders, crusts and speleothems. The carbonate powders include those that precipitated in relation to aufeis aggradation (cryogenic aufeis calcite) and in relation to the growth of annual and perennial ice formations in caves (cryogenic cave calcite). The carbonate crusts were further subdivided according to their lithic environment; those that precipitated on the upper surface of bedrock/clasts (i.e. subglacially precipitated calcite and evaporative calcite crusts); those that are located on the underside of clasts (i.e. active layer carbonates); and those that precipitated in rock outcrop fissures (i.e. endostromatolites). The speleothems consist of a group on their own because they are not restricted to polar regions and most are currently inactive due to the presence of permafrost.;To determine if the secondary carbonates in polar regions can be used as reliable paleoclimatic proxies, the chemical and isotopic (18O/ 16O; 13C/12C) partitioning that is occurring prior to and during the precipitation of carbonates and the water from which they precipitated was examined. The cold-climate carbonate precipitates, which were collected in distinct geological settings (carbonated vs. non-carbonated), have a δ18O composition between -6.5 and 36‰ and δ 13C values in the -5 to 17‰ range. The measurement of the difference (Δ) in stable C-O isotope composition of actively forming carbonate deposits and of the water from which they precipitated provided valuable insights into the formative mechanism that led to their precipitation. It was found that carbonates that precipitated under equilibrium physico-chemical conditions (i.e. cryogenic aufeis calcite powders, cryogenic cave calcite pearls, subglacially precipitated calcite) had a δ13C value that is in equilibrium with that of the parent water, while their δ18O compositions were more variable, as it is in part controlled by the temperature of reaction, by the δ18O and calcite saturation state of the parent water and formative mechanism. By contrast, carbonate deposits that precipitated under non-equilibrium physico-chemical conditions (i.e. cryogenic cave calcite powders, evaporative calcite crusts), their δ18O and δ 13C values are highly enriched relative to that of the parent water due to the faster rate of reactions which precludes isotopic equilibrium to be reached. In the case of biologically precipitated carbonate deposits (i.e. endostromatolites), their δ18O composition reflects that of the parent water, while its δ13C composition was enriched over that of the parent water because bacteria prefer to metabolize the light C (12C) in the DIC pool. These findings have significant implications regarding the use of cold-climate carbonate precipitates in paleoclimate studies as the δ18O signature preserved in most carbonates have been modified by freezing or other processes prior to their precipitation, which will modify δ18O composition of the carbonates.