Bernard Lauriol

Kinetic enrichment of stable isotopes in cryogenic calcites

The 13C and 18O contents of cryogenic calcites formed by expulsion during the freezing of bicarbonate groundwaters are examined. Samples from karst caves within the permafrost region of northern Yukon, Canada, have δ13C-values as high as + 17.0‰, representing the most isotopically enroched freshwater carbonates yet reported. To account for such enrichments, calcium bicarbonate solutions were frozen and sublimated under controlled laboratory conditions. The rapid rate of reaction is shown to effectively preclude isotopic equilibration during bicarbonate dehydration, resulting in a kinetic partitioning of 13C between CO2 and CaCO3. We find a value of 31.2 ± 1.5‰ for 1000ln13αKIE(13αKIE = 1.032), which is considerably greater than the equilibrium fractionation factor (13ϵCaCO3/1bCO2) of 10.3‰ at 0°C. This kinetic isotope effect (KIE) represents the ratio of the absolute reaction rate constants (13kd12kd) for the two isotopic species during the dehydration of dissolved bicarbonate. Similar results for δ18O-values confirm that the reaction proceeds without isotope exchange. The KIE of 18O is determined to be 1.006 for this reaction at 0°C. These data are compared with the KIE which occurs during the reverse reaction: CO2 hydroxylation by reaction with OH− in hyperalkaline waters.

Dynamics of the Late Wisconsin Ice Sheet in the Ungava Peninsula Interpreted from Geomorphological Evidence

Radiocarbon evidence, isostatic uplift data, and glacial landforms in the Ungava Peninsula have provided important insight into the nature and dispersal pattern of the late Wisconsin ice sheets in ...

Fissure calcretes in the arctic: a paleohydrologic indicator

Abstract The effects of modern climate warming are expected to be amplified in northern periglacial regions. Paleohydrological reconstructions in the arctic provide important insights into natural climatic variations during the Pleistocene and Holocene with which to compare predictions of modern impacts. Here the authors describe a new material, calcrete found in fissures in arctic carbonate terrains, which record hydrogeological and climatic conditions during climatic optimums in the north. 234 U and 230 Th activity ratios suggest growth during northern insolation maxima. These fissure calcretes strongly resemble biogenic calcite precipitates (endostromatolites), and grow on the external face within fissures in carbonate bedrock in periglacial environments. In thin section, samples are finely laminated and are composed of diverse fabrics including a uniformly present basal layer with regularly spaced columns from several microns to 3 cm in length. Their surface morphology is often preserved in the relief of the adjacent fissure face in a micro-lapies texture strongly suggestive of a dissolution-reprecipitation mechanism whereby the fissure widens in lock-step with column growth. Their highly enriched δ 13 C (0 to +8.4‰) composition suggests that methanogenenic bacteria, supported by a substrate of soil-derived organic matter, control growth in a network of groundwater-saturated near-surface fissures extending to a depth of 3 to 5 m. Accordingly, δ 18 O holds valuable paleotemperature information.

The distribution of diatom flora in ice caves of the northern Yukon Territory, Canada: relationship to air circulation and freezing

In the late 1980’s and early 1990’s, various media in karst environments in the Northern Yukon Territory were examined for their diatom content. Cryogenic cave calcite powders, grus and various ice formations (ice plugs, ice stalagmites and floor ice) were collected from three freezing caves and one slope cave to make an inventory of the diatom content, and to explain the spatial distribution of the diatoms within the caves. The results show that approximately 20% of diatoms in the caves originate from external biotopes and habitats (e.g., river, lake, stream), with the remaining 80% of local origin (i.e., from subaerial habitats near cave entrances). The results also indicate that the greater abundance of diatoms is found in the larger caves. This is explained by the fact that the air circulation dynamics are much more important in caves that have a larger entrance. Also grus, ice plugs and ice stalagmites have the lowest diatom diversity, but greater relative abundance, indicative of growth in specific habitats or under specific conditions. Overall, these results are a contribution to the study of particles transport in ice caves.

Groundwater Contributions to Discharge in a Permafrost Setting, Big Fish River, N.W.T., Canada

Groundwaters, surface runoff and river discharge have been studied at the Fish Hole area of the Big Fish River catchment, near Aklavik, N.W.T., to quantify the seasonal variations in groundwater contributions to baseflow in a permafrost basin. Geochemical and isotope methods are used to distinguish three principal water types: (1) subpermafrost Na-Cl thermal waters (16°C) discharging from bedrock along the river, (2) shallow, Ca-SO4 groundwaters, and (3) low-salinity, Ca-HCO3, suprapermafrost drainage. Cl and SO4 concentrations in river water show that baseflow is largely derived from groundwater sources (Na-Cl and Ca-SO4 components), with less than 30% contributed from surface water runoff in any season. Thermal groundwaters discharge year-round at a calculated 1.6 m3 s−1. The shallow, Ca-SO4 groundwater is the dominant component of baseflow in summer but is absent in winter baseflow. Discharge analysis based on a single flow measurement (25.5 m3 s−1 in June, 1997) and Cl-dilution allows calculation of total river discharge. This is shown to vary from a low of 2.2 m3 s−1 in the winter, to 1050 m3 s−1 during early spring melt.

Weathering regime and geochemical conditions in a polar desert environment, Haughton impact structure region, Devon Island, Canada

This study examines the physical and geochemical properties of near-surface sediments, as well as the geochem- ical and stable O-H-C isotope composition of (ground)surface waters in and around the Haughton impact structure region (Devon Island, Nunavut) to determine the types of weathering (mechanical, (bio)chemical) and their relative contribution in this polar desert environment. The surface sediments collected from the Allen Bay and Thumb Mountain formations surrounding the impact crater are dominated by sand-sized particles; whereas the impact melt breccias inside the crater have a greater abundance of silt-sized particles. The subsurface sediments in the Allen Bay formation show a near equal amount of sand- and silt-sized particles. However, the micromorphologies of the sand-sized particles collected at the sur- face revealed that these grains, irrespective of the local geology, were heavily fractured. By contrast, fractures and rounded pits are observed on the surface of the sand grains located within the active layer; whereas those located just above the permafrost table have only rounded cavities on their surface. The (ground)waters also show variations in their solute con- centration with depth; the highest concentrations being found in the groundwaters near the top of permafrost. Taken to- gether, these observations suggest that there is a progressive evolution from a mechanically dominated weathering regime near the surface, to increasing chemical weathering with depth. The transition from mechanical weathering near the surface to increasing chemical weathering with depth can be attributed to the decreasing frequency and intensity of mechanical weathering processes (i.e., frost action, wetting-drying, thermal dilation) with depth, and to the presence of permafrost, which allows a greater availability of water for chemical aqueous reactions at the base of the active layer.

Microbial Diversity in Endostromatolites (cf. Fissure Calcretes) and in the Surrounding Permafrost Landscape, Haughton Impact Structure Region, Devon Island, Canada

In recent years, endostromatolites, which consist of finely laminated calcite columns that grow orthogonally within millimeter- to centimeter-thick fissures in limestone bedrock outcrops, have been discovered in dolomitic outcrops in the Haughton impact structure region, Devon Island, Canada. The growth mechanism of the endostromatolites is believed to be very slow and possibly intertwined with biotic and abiotic processes. Therefore, to discern how endostromatolites form in this polar desert environment, the composition of the microbial community of endostromatolites was determined by means of molecular phylogenetic analysis and compared to the microbial communities found in the surrounding soils. The microbial community present within endostromatolites can be inferred to be (given the predominant metabolic traits of related organisms) mostly aerobic and chemoheterotrophic, and belongs in large part to the phylum Actinobacteria and the subphylum Alphaproteobacteria. The identification of these bacteria suggests that the conditions within the fissure were mostly oxidizing during the growth of endostromatolite. The DNA sequences also indicate that a number of bacteria that closely resemble Rubrobacter radiotolerans are abundant in the endostromatolites as well as other Actinobacteria and Alphaproteobacteria. Some of these taxa have been associated with calcite precipitation, which suggests that the endostromatolites might in fact be microbially mediated. Bacterial communities from nearby permanently frozen soils were more diverse and harbored all the phyla found in the endostromatolites with additional taxa. This study on the microbial communities preserved in potentially microbially mediated secondary minerals in the Arctic could help in the search for evidence of life-forms near the edge of habitability on other planetary bodies.