This report provides the results of geochemical analyses performed on Miocene sediments recovered during Ocean Drilling Program (ODP) Leg 166 at Sites 1006 and 1007. Sites 1006 and 1007 are the most distal of five sites along the Bahamas Transect drilled into prograding carbonate sequences along the western margin of the Great Bahama Bank (GBB). Site 1007 is located on the toe-of-slope of the western GBB in ~647 m of water. At Site 1007, a nearly complete 900-m-thick Miocene section consisting of a succession of bioturbated, periplatform limestone (e.g., Schlager and James, 1978) with interbeds of turbiditic packstone, grainstone, and floatstone (Eberli, Swart, Malone, et al., 1997) was recovered. Site 1006, the most distal site along the Bahamas Transect, was drilled ~30 km from the platform edge of the GBB in 658 m of water. Approximately 334 m of middle and upper Miocene sediments were recovered at this site, which is positioned in a nearly continuous sequence of Neogene drift sediments that onlap and interfinger with prograding bank slope deposits (Eberli, Swart, Malone, et al., 1997). Because of its more distal location, the Miocene section at Site 1006 has relatively few turbiditic deposits and a significantly higher pelagic component, and it consists primarily of nannofossil chalk and limestone.
The Miocene section at Site 1007, which contains a relatively high proportion of bank-derived components, has been affected by diagenesis. Sediments are fully lithified below ~300 meters below seafloor (mbsf) and exhibit burial compaction features including anastomosing solution seams at depths greater than ~1120 mbsf. By contrast, Miocene sediments at Site 1006, composed primarily of pelagic carbonates, do not become fully lithified until a depth of ~675 mbsf. It is well known that periplatform carbonates undergo accelerated rates of diagenesis relative to low-Mg calcite (LMC)-dominated pelagic counterparts, primarily because of the metastable nature of the platform-derived aragonite and high-Mg calcite (HMC) component in deep, cold seawater (e.g., Schlager and James, 1978; Mullins et al., 1985; Dix and Mullins, 1988, 1992; James and Choquette, 1990). However, to what extent the early onset of lithification in periplatform carbonates influences further alteration during burial is poorly known. In this regard, the suite of geochemical data derived from the sequences of pelagic and periplatform carbonates at Sites 1006 and 1007, respectively, provide a foundation for the examination of the influence of initial sediment composition on patterns and processes of burial diagenesis in fine-grained, shelf carbonates.
Bulk sediment samples (~10-20 cm3) were collected aboard the JOIDES Resolution during Leg 166. A representative range of lithologies from the Miocene sections at Sites 1006 and 1007 was chosen at semi-regular intervals on the basis of core descriptions and visual observations. Approximately one fourth of each bulk sample was powdered and dried in a 75°C oven; in cases where individual sediment samples were lithologically diverse, an equivalent amount of each lithology was prepared. Bulk powders were subsequently analyzed for total carbon and carbonate contents, carbonate mineralogy, trace metal contents, and carbon and oxygen isotope compositions. The selective removal and analysis of individual petrographic components in Site 1007 samples were commonly precluded by the dense cementation and the fine-grained nature of most lithologies. However, an absence of cement in intermittent clay-rich intervals at Site 1007 allowed for the disaggregation of selected samples and the isotopic analysis of individual petrographic components. These samples were disaggregated by soaking in a 5.25% sodium hypochlorite (NaOCl) solution for 24 hr and rinsing repeatedly in deionized water. Disaggregated samples were then washed with deionized water and oven dried at 50°C. Additional carbon and oxygen isotope analyses were performed on individual petrographic components from these separates, which were selected with the aid of a binocular microscope. Data for Holes 1006A and 1007C are reported in Table 1 and Table 2, respectively.