Microbial carbonates in lacustrine settings : an investigation into the Carboniferous East Kirkton Limestone
The East Kirkton Limestone outcrops in the Bathgate Hills area of central Scotland. The deposit is Visean (Lower Carboniferous) in age and is the site of a tropical freshwater lake set within a richly vegetated volcanic terrain. The succession consists of an unusual sequence of laminated, spherulitic and massive limestone beds, interbedded with thin mudstones, siltstones and abundant volcaniclastic horizons. This study investigates the palaeoenvironment of the East Kirkton lake, and for the first time presents convincing evidence for the presence of hot springs at East Kirkton. Two distinct mounds of massive limestone are located at the north end of the East Kirkton quarry and are interpreted as hot spring vent deposits. Calcite samples from the Lower Mound have light δ18OPDB values consistent with precipitation at elevated temperatures ranging from 45°C to 80°C, whereas samples from the laminated lake sediments have heavier δ18OPDB values, consistent with precipitation at lower temperatures. The East Kirkton Limestone is unique because it contains a variety of unusual radial fibrous calcite (RFC) precipitates, including mm-sized spherules, oncoids, and laminated botryoidal accretions. These are found within the laminated limestone and also within the Lower Mound of massive limestone, and many of them contained filamentous and cellular microbial remains and are within the definition of microbial carbonates. This study provides good evidence for a microbial involvement in the formation of the East Kirkton laminated accretions. A study of modern stromatolites from Lake Tasek Dayang Bunting, Malaysia highlights close similarities with the East Kirkton accretions in terms of morphology and microstructure. Both have a nodular growth morphology and laminated interior consisting of layers of radial fibrous carbonate botryoids interspersed with organic-rich micritic laminae. The modern stromatolites are covered by a microbial community comprising filamentous bacteria and cyanophytes, plus diatoms and associated mucilage, and microbes are thought to have played an essential role in the morphogenesis of both the modern and ancient examples.