THE STABLE-ISOTOPE RECORD OF ENVIRONMENTAL AND CLIMATIC SIGNALS IN MODERN TERRESTRIAL MICROBIAL CARBONATES FROM EUROPE

Stable carbon and oxygen isotope data from over 80 samples of Recent f reshwater microbial carbonates from western Europe, confirm that these deposits record environmental and climatic information. Our sample ar ea tested whether recent microbial carbonates record environmental sig nals over large regions with differing delta(18)O compositions for rai nfall (delta w), particularly in the Alps where delta w is influenced by lower condensation temperatures caused by the orographic effect. Mi crobial crusts in Alpine areas are clearly distinguished, i.e., have i sotopically lower delta(18)O values by up to 4 parts per thousand, fro m those forming in lowland areas on the east side of the mountains. Ba varian lakes and rivers which receive runoff from the Alps also have d elta(18)O compositions that reflect the Alpine meteoric water input. M icrobial crusts in the higher Alpine sites have delta(13)C values arou nd -4 parts per thousand, which are between 2 and 6 parts per thousand higher than values from lowland sites. This difference probably refle cts a smaller soil-zone carbon component in the mountain sites where s oils 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 t ufa, etc.) at a site. The oxygen isotope compositions of freshwater la custrine mussel shell aragonite and associated (sometimes shell encrus ting) microbial carbonates differ by <1 parts per thousand, and both a re probably close to equilibrium isotopic values. However, delta(13)C values in mussel shell aragonite are consistently lower, typically by 5 parts per thousand, than the associated microbial carbonate value. T his suggests that the delta(13)C Of the microbial carbonate is affecte d 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 a ncient microbial carbonates may contain clear records of palaeoclimate (particularly relative changes in temperature) and environmental chan ge. Pools behind fluvial barrage tufas are probably the best targets f or reasonably continuous, long, dateable records with isotopic conditi ons least affected by microenvironmental processes.