The stable isotope record of environmental and climatic signals in modern terrestrial microbial carbonates from Europe

Abstract

Stable carbon and oxygen isotope data from over 80 samples of Recent freshwater microbial carbonates from western Europe, confirm that these deposits record environmental and climatic information. Our sample area tested whether recent microbial carbonates record environmental signals over large regions with differing δ18O compositions for rainfall (δw), particularly in the Alps where δw is influenced by lower condensation temperatures caused by the orographic effect. Microbial crusts in Alpine areas are clearly distinguished, i.e., have isotopically lower δ18O values by up to 4‰, from those forming in lowland areas on the east side of the mountains. Bavarian lakes and rivers which receive runoff from the Alps also have δ18O compositions that reflect the Alpine meteoric water input. Microbial crusts in the higher Alpine sites have δ13C values around −4‰, which are between 2 and 6‰ higher than values from lowland sites. This difference probably reflects a smaller soil-zone carbon component in the mountain sites where soils are thin, poorly vegetated, or absent. Oxygen and carbon isotope values do not vary significantly between different types of microbial precipitate (e.g., undifferentiated crusts, Rivularia colonies, moss tufa, etc.) at a site. The oxygen isotope compositions of freshwater lacustrine mussel shell aragonite and associated (sometimes shell encrusting) microbial carbonates differ by <1‰, and both are probably close to equilibrium isotopic values. However, δ13C values in mussel shell aragonite are consistently lower, typically by 5‰, than the associated microbial carbonate value. This suggests that the δ13C of the microbial carbonate is affected by the microenvironmental photosynthetic processes of the microbes. These microenvironmental effects are only evident at lake shore sites where water flushing rates are low. These data suggest that selected ancient microbial carbonates may contain clear records of palaeoclimate (particularly relative changes in temperature) and environmental change. Pools behind fluvial barrage tufas are probably the best targets for reasonably continuous, long, dateable records with isotopic conditions least affected by microenvironmental processes.