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.

A Re‐Evaluation of Facies on Great Bahama Bank I: New Facies Maps of Western Great Bahama Bank

“…1b; established using GPS) and to quantify not only the types of sediments present, but also the mineralogy, grain-size distribution and skeletal and non-skeletal content. This paper together with a companion paper (Swart et al, 2009) takes a new look at the surface facies and geochemistry of sediments from Great Bahama Bank.…”

Bio-chemostratigraphy of the Barremian-Aptian shallow-water carbonates of the southern Apennines (Italy): pinpointing the OAE1a in a Tethyan carbonate platform

“…The bias of local palaeoenvironmental conditions could have been partly counteracted by the fact that micrite, or the micritic matrix in case of grainy facies, were analyzed instead of bulk samples. On the other hand, the lack of a strict relationship between facies type and δ 13 C has been observed also in the recent sediments of the Great Bahama Bank (Swart et al, 2009).…”

Whiting-related sediment export along the Middle Miocene carbonate ramp of Great Bahama Bank

“…This observation is in agreement with Cloud (1962a), Lowenstam and Epstein (1957), and Shinn et al (1989), who all have shown an increase in seawater salinity from west to east across modern GBB. The relationship between d 18 O w and salinity on GBB is rather complex but Lowenstam and Epstein (1957), Shinn et al (1989) as well as Swart et al (2009) suggested an overall positive correlation between d…”

“…The elevated d 13 C of the GBB modern sediments is perhaps also explained by a high seawater d 13 C DIC due to fractionation during photosynthesis (preferential removing of 12 C causing an enrichment of waters in 13 C; Immenhauser et al 2008;Swart et al 2009). For this reason, organic or inorganic carbonates formed in the shallow realms of Great Bahama Bank tend to record higher d…”

“…the Bahamas (Weber 1967;Swart and Eberli 2005;Swart et al 2009), Australia (Weber and Woodhead 1969;Dix et al 2005), and selected places at Belize, Maldives, and Kuwait (Gischler et al 2009). The origin of the 13 Cenriched values of aragonite on GBB is not well understood.…”