Geochemical and microbiological variability of aerobic and anaerobic sediments along a bifurcated sulphate-saturated brine spring in western Canada

Abstract

A tufa mound developed at La Saline Lake, an oxbow of the Athabasca River in the Athabasca Oil Sands deposit of north-east Alberta, is characterised by an unusual emplacement of a gypsum caprock. This two-tiered architecture resulted from the bicarbonate-saturated groundwater flow along Upper Devonian limestone being redirected deeper and encountering the halite–anhydrite dissolution trend within the underlying Middle Devonian Prairie Evaporite Formation, 175 to 200 m below. Migrations up-section of sulphate-saturated brine resulted in the gypsum caprock on the tufa mound and discharge of a sulphate-saturated brine spring with total dissolved solids of ca 80,000 mg/L. The brine spring is bifurcated with flows to the south-west and north-west. Calcite–gypsum thrombolytic bottom sediments along the south-western branch were covered by a halite deposit and subsequently a gypsum crust with a microbial community dominated by the cyanobacteria Coleofasciculus chthonoplastes. The thrombolite contact zone with the halite–gypsum encrustation has a more diversified community with the cyanobacteria Dactylococcopsis. Cyanobacterial mats that wrap around the bulbous gypsum crust protuberances distributed along the brine pool bottom surfaces have significant eukaryotic diversity, represented by the heterotrophic Ochrophyte Paraphysomonas. In contrast, sediments accumulated along the adjacent spring branch flow to the north-west were thicker and clogged by abundant decomposed and fermented floral debris, unlike the deposit accumulated along the south-western branch. This resulted in an oxygen-depleted anaerobic environment dominated by sulphate-reducing bacteria resulting in a calcite-rich and sulphate-starved anaerobic sediment with ca 20% elemental sulphur and emanation of H₂S gas.