You are here: carbonstrat.html
O-C Strat, Oxygen-Carbon Isotope Stratigraphy
The carbon isotope composition of marine limestone fluctuates on a shorter time scale than 87Sr/86Sr ratios (see SrStrat) and the pattern of fluctuation tends to be confined to individual basins. Such relatively short term excursions in marine carbonate d13C values may reflect changes in organic productivity and burial rates of organic carbon. e.g. in the Jurassic and Cretaceous positive d13C "anomalies" may correspond with higher levels of carbon storage in organic-rich mudstone and rising sea levels, while negative d13C excursions may correspond with lower sea levels and/or sub-aerial exposure of shelf areas. This secular variation has been used to correlate limestone strata on a local scale where the original marine carbonate d13C signature has been preserved (e.g. Vahrenkamp 1996).
The oxygen isotope composition of marine carbonates also show some broad trends through geological time. However, the main controls on d18O of marine carbonate are temperature and d18O of sea water at any given location and time. Latitude, evaporation, waxing and waning of sea ice, etc. all have controlling effects on these parameters.