Stable isotope composition of hypogenic speleothem calcite in Kalana (Estonia) as a record of microbial methanotrophy and fluid evolution

Abstract

Aeronian (Silurian Period) carbonate rocks in Kalana quarry in central Estonia contain cave and fracture structures filled with calcitic speleothem precipitates of atypical composition. Calcite crystals in dolomitized limestone cave walls have diverse shapes (equant-blocky, bladed and fibrous), but most of the cave walls and speleothems are covered with an up to 10 cm thick crust of microcrystalline botryoidal calcite. The morphology of precipitates suggests their formation in low hydrodynamic conditions in water supersaturated with calcite. Calcite in speleothems is associated with the mineralization of sulphur-bearing minerals, such as pyrite and abundant barite. Unlike that of speleothem calcite, the stable isotope composition of authigenic calcite shows extreme depletion in 13C and large variations in δ13CPDB from –11 to –56‰, whereas the δ18OPDB values range from –5 to –12‰, suggesting calcite precipitation from a 13C-depleted carbon source supplied by microbially mediated anaerobic oxidation of methane and/or other hydrocarbons at elevated temperatures (up to 70°C). Systematic variation in the δ13CPDB and δ18OPDB values of layered precipitates indicates a change from an initially biogenic methane source to an either thermogenic methane or hydrocarbon source in the low-temperature hydrothermal fluid. Calcite speleothems in Kalana possibly developed at the mixing front of sulphate-rich seawater or groundwater and low-temperature methane-bearing hydrothermal fluids in the phreatic zone of a hypogenic-hydrothermal (karst) system.

Speleothem Records from the Eastern Part of Europe and Turkey—Discussion on Stable Oxygen and Carbon Isotopes

“…The current karstwater table is near the surface in the Baltic region, creating phreatic conditions (i.e., completely submerged, or partly flooded) already in near surface cavities and subaerial speleothem formation has not been observed in known karst caves [61]. Speleothems were presumably formed in the Baltic region during the pre-Pleistocene [62], but the former vadose zone was denudated by extensive Pleistocene glaciation [63].…”

Geochemical Fingerprinting of Rising Deep Endogenous Gases in an Active Hypogenic Karst System

“…Most hypogenic caves formed by the movement of thermal waters in phreatic settings are now located far above the water table and are thus no longer active. One exception to this pattern may be represented by caves associated with active faulting and geothermal areas where there are high concentrations and releases of upwelling fluids of endogenous origin, which enhance the processes and distinctive features of hypogene karstification [8][9][10][11][12]. As an example, venting of subcrustal CO 2 has been described in a hydrothermal cave associated with an active, deep-rooted fault [13].…”

Illitization of the lower Cambrian (Terreneuvian) Blue Clay in the northern Baltic Palaeobasin

“…Using Ar diffusion modelling, they suggested that the BC in the Lontova Formation was heated to a temperature of 130-140 °C for a duration of 2-5 Myr or the temperature may have been as high as 170 °C during a shorter period of time (< 1 Myr), in relation to largescale fluid migration and dolomitization in the Silurian-early Devonian. Indeed, recent studies (Eensaar et al 2017a(Eensaar et al , 2017bGaškov et al 2017) have shown evidence of hydrothermal activity in the northern BP and have delineated a Zn-Pb mineralization event driven by the intrusion of highly saline low to moderate temperature (60-200 °C) fluids, the maximum age of which can be constrained by crosscutting relationships to the early Middle Devonian (Eifelian). However, this event is tied to thin crosscutting sphalerite-(galena)-calcite veins and scattered fracture zones with no evidence of significant wallrock alteration (Eensaar et al 2017a).…”