Fissure calcretes in the arctic: a paleohydrologic indicator

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. 234U and 230Th 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-lapiès texture strongly suggestive of a dissolution-reprecipitation mechanism whereby the fissure widens in lock-step with column growth. Their highly enriched δ13C (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, δ18O holds valuable paleotemperature information.