A re-evaluation of facies on Great Bahama Bank II: variations in the δ13C, δ18O and mineralogy of surface sediments

In order to investigate the spatial distribution of ö13C and 8180 of modern carbonate sedirnents on Great Baharna Bank, -290 surface sarnples were collected from a grid of stations approximately 10 km apart between 2001 and 2004. These samples were classified using a modified Dunham scheme, physically separated into six size fractions and subsequently analysed for their mineralogy (aragonite, low-Mg calcite and high-Mg calcite) and 813C and 8180 values. A striking feature of these data is the relatively positive 813C values of all the samples. Based on measurements of 813C and 8180 of the dissolved inorganic carbon and the water, most of the sediments can be considered tobe in C and O isotopic equilibrium with the arnbient waters. The high ö13C values are suggested to arise frorn isotopic enrichment of the dissolved inorganic carbon pool by photosynthesis of seagrasses, benthic algae and cyanobacteria on the platform and through the fractionation of HC03 - during the precipitation of calcium carbonate. Sediments that are not in C and O isotopic equilibriurn are dorninated more by skeletal material. The data showed an absence of significant spatial variation in 813C of the sediments on the Great Bahama Bank and no clear spatial patterns relative to the margin of the platform. The 8180 of the sediment showed more variation, with the interior sediments being isotopically enriched relative to the platform margin. The absence of signi.ficant variations in the ö13C in the modern surface sediments of Great Baharna Bank irrespective of facies type suggests that in the case of Great Baharna Bank, downcore variations in ö13C cannot be related to changes in facies