STABLE ISOTOPIC COMPOSITIONS OF RECENT FRESH-WATER CYANOBACTERIAL CARBONATES FROM THE BRITISH-ISLES - LOCAL AND REGIONAL ENVIRONMENTAL CONTROLS

Recent (< 50 years old) freshwater cyanobacterial carbonates from diverse environments (streams, lakes, waterfalls) throughout Britain and Ireland were analysed for their stable carbon and oxygen isotope compositions. The mean deltaO-18 value of -5.9 parts per thousand PDB for river and stream data represents calcite precipitation in equilibrium with the mean oxygen isotopic composition of precipitation in central Britain (-7.5 parts per thousand SMOW) assuming a mean water temperature of 9-degrees-C. The mean deltaO-18 of lake data, -4.5 parts per thousand PDB, is statistically different, reflecting the effects of residence time and/or variations in the oxygen isotopic composition of rainfall. Carbon isotopes have wide variations in both fluviatile and lake data sets (+ 3 to -12 parts per thousand PDB). These variations are principally controlled in the fluviatile samples by contribution of isotopically light 'soil zone' carbon relative to isotopically heavier carbon from limestone aquifer rock dissolution. Lake samples have the heaviest carbon isotope values, reflecting a trend toward isotopic equilibrium between atmospheric CO2 and aqueous HCO3-. We infer that isotopic compositions of ancient cyanobacterial carbonates should also record environmental information, although the effects of stabilization and diagenesis on primary deltaO-18 values will need careful consideration. Primary carbon isotope compositions should be well preserved, although in marine samples values will be buffered by the isotopic composition of aqueous marine bicarbonate.