Growth rate and stable isotopic character of modern stromatolites from San Salvador, Bahamas

14C, δ 18O and δ 13C stratigraphies were measured on the successive carbonate crusts within large mushroom-shaped stromatolitic heads from hypersaline Storrs Lake, San Salvador, Bahamas and also on the finely laminated soft sediment that enclose them. 14C dates show that the carbonate layers in these stromatolites range in age from ∼2310 ± 70 years B.P. at the base to modern at the surface. This suggests that the heads develop at an overall rate of 16 cm/1000 yr. Because there are 1–5 macroscopic laminae per cm, less than one crustose layer is preserved in the heads per decade. Thin section examination of the crusts reveals numerous, discontinuous micro-unconformities which suggest multiple intervals of accretion and erosion. These factor complicate any history of slow growth that may occur. Values of δ 18O in the carbonates suggest that the heads were formed in waters with isotopic and temperature conditions similar to those that presently exist in the lake. However, carbonate δ 13C values from the heads and sediments vary over 7% (−5.1–2.0% PDB) and are significantly out of equilibrium with atmospheric CO 2. The fluctuations in the δ 13C values within the heads could not be correlated between adjacent heads nor with adjacent sediment on the basis of 14C ages. Thus, the sampled macroscopic layers (mm-cm) in these calcified stromatolites do not appear to be recording simultaneous basin-wide events but rather local diagenetic effects and micro-environmental conditions in or under the microbial mats. These data do not encourage basin-wide interpretations of isotopic stratigraphies observed within ancient stromatolites where independent chronologies, like 14C, are not available.