A Re‐Evaluation of Facies on Great Bahama Bank II: Variations in the δ 13 C, δ 18 O and Mineralogy of Surface Sediments

Abstract

In order to investigate the spatial distribution of  13 C and  18 O of modern carbonate sediments on Great Bahama Bank, ~290 surface samples were collected from a grid of stations approximately 10 km apart between 2001 and 2004. These samples were classifi ed using a modifi ed Dunham scheme, physically separated into six size fractions and subsequently analysed for their mineralogy (aragonite, low-Mg calcite and high-Mg calcite) and  13 C and  18 O values. A striking feature of these data is the relatively positive  13 C values of all the samples. Based on measurements of  13 C and  18 O of the dissolved inorganic carbon and the water, most of the sediments can be considered to be in C and O isotopic equilibrium with the ambient waters. The high  13 C values are suggested to arise from isotopic enrichment of the dissolved inorganic carbon pool by photosynthesis of seagrasses, benthic algae and cyanobacteria on the platform and through the fractionation of HCO 3 − during the precipitation of calcium carbonate. Sediments that are not in C and O isotopic equilibrium are dominated more by skeletal material. The data showed an absence of signifi cant spatial variation in  13 C of the sediments on the Great Bahama Bank and no clear spatial patterns relative to the margin of the platform. The  18 O of the sediment showed more variation, with the interior sediments being isotopically enriched relative to the platform margin. The absence of signifi cant variations in the  13 C in the modern surface sediments of Great Bahama Bank irrespective of facies type suggests that in the case of Great Bahama Bank, downcore variations in  13 C cannot be related to changes in facies.