RECONSTRUCTION OF LIMNOLOGY AND MICROBIALITE FORMATION CONDITIONS FROM CARBONATE CLUMPED ISOTOPE PALEOTHERMOMETRY: A TEST CASE FROM PAVILION AND KELLY LAKES

Quantitative tools for deciphering the environment of microbialite formation are relatively limited. For example, the carbonate-water geothermometer requires assumptions about the isotopic composition of the water of formation. This study explores the utility of using “clumped” isotope thermometry to study microbialite formation conditions. We measured freshwater microbialites from Pavilion and Kelly Lake (British Columbia) in order to determine the temperature of precipitation and the ¹⁸O/¹⁶O ratio of the waters of formation. Microbialites span the thermocline in both lakes, ranging from depths of 10-55 m in Pavilion Lake, and 10-25 m in Kelly Lake. Isotopic ratios were determined for each sample, and used to reconstruct limnology. Results were then compared to current limnological data from the lake in order to reconstruct the history of microbialite formation. Microbialites collected at shallow depths (~10 m) in Pavilion Lake and Kelly Lake yield clumped isotope-based temperature of formation of 20.8 ±2.8°C and 15.9 ±2.5 °C (1 s.e.) respectively. Similar temperatures are also found for microbialites collected from deeper parts of both lakes. The data for the shallower depths match the current measured temperature profiles, however the clumped isotope-based temperature estimates from the deeper microbialites do not match the present limnological characteristics. The deepest microbialites (~50 m) yield radioisotope ages indicating they primarily formed earlier in the Holocene, when pollen data and our reconstructed water ¹⁸O/¹⁶O ratios indicate a period of aridity, with lower lake levels. Given the deepest microbialites are receiving <1% of photosynthetically active radiation (PAR), it is likely that the deepest microbialites primarily formed when lower lake levels resulting in these microbialites being located higher in the photic zone.