A series of holes was drilled on the western flank of the Great Bahama Bank (GBB) during Ocean Drilling Program (ODP) Leg 166. Of these, Sites 1003-1007, which are located in the Straits of Florida at the north end of the Santaren Channel (Fig. 1), comprise a transect from shallow to deep water that drilled through late Oligocene- to Holocene-age prograding carbonate sequences (Fig. 2). A major objective of Leg 166 was to investigate the significance of fluid flow through the margin of the GBB. This objective was met by a combination of in situ temperature measurements and a high-resolution interstitial water (IW) sampling program. Recovered fluids were exhaustively analyzed for a variety of major, minor, and trace constituents (Eberli, Swart, Malone, et al., 1997). Two other holes, Clino and Unda, which were drilled in 7 m of water during the Bahamas Drilling Project (1990), provided data from proximal sites on the western margin of the GBB that complement data collected during ODP Leg 166 and the subsequent postcruise research (Eberli, Swart, Malone, et al., 1997).
Water/rock interactions and large-scale fluid circulation within sedimentary deposits on continental margins represent the most important chemical transport mechanisms within margins. These processes contribute to the transfer of elements from the lithosphere to the hydrosphere and to global biogeochemical cycles. Microbial activity and inorganic chemical reactions within the sediments control the diagenesis of sediments and the concomitant alteration of fluid chemistry. Fluid flow enhances microbial activity deep in the sedimentary column and acts as a conveyor belt that imports a stream of reactants and exports the end products of organic matter degradation. Interactions between water, rock, and organic matter also alter rock porosity and permeability. Thus, diagenetic processes alter not only the chemical and physical properties of the sediments, but because fluid flow is frequently concentrated along distinct intervals within the sediments, the affected sedimentary sequences can often be converted into lithified horizons that define seismic sequence boundaries.
In this paper we present results of shore-based minor and trace element analyses of interstitial waters collected from Sites 1005, 1006, and 1007. Elements reported include Li, Rb, Sr, Ba, Fe, and Mn. The results of Fe, Li, and Sr analyses are compared to preliminary data generated aboard JOIDES Resolution during Leg 166, and are discussed in terms of constraints inherent in the analytical methods used to determine their concentrations. The concentrations of dissolved Li, Rb, Ba, Fe, and Mn reported herein are also discussed in terms of their geochemical significance, in particular their relationship to diagenetic reactions resulting from the oxidation of organic matter in a carbonate dominated environment.
The Bahamas Archipelago consists of a series of isolated carbonate platforms located on the southern tip of the eastern continental margin of North America. This margin has its beginning in the Jurassic period during the opening of the Atlantic Ocean. The southern portion of the margin collided with the Caribbean plate in the Late Cretaceous, reaching the greatest intensity in the Eocene (Ball et al., 1985; Walles, 1993; Denny et al., 1994). Upon a shift of the plate boundary to south of the island of Cuba at the end of the Eocene, tectonic activity subsided and Cuba remained attached to the southern portion of the American continental margin.
Significant debate continues regarding the foundation of the Bahamas platform. Two questions central to this debate are: (1) Is the Bahamian basement of continental or oceanic origin? and (2) Is the horst and graben topography that formed during the Jurassic rift stage still reflected in the modern platform pattern (Mullins and Lynts, 1977; Sheridan et al., 1981)? The seismic refraction evidence available to date does not allow unequivocal identification of the basement as being either continental or oceanic crust. Thus, drilling of the Bahamas platform margin was considered key to resolving the controversy between the horst and graben and megabank hypotheses.
Fluid flow or recharge in the GBB is evidenced from the nonsteady-state profiles of conservative and nonconservative species in the IW recovered from the uppermost 100 m of the sediments at Sites 1003-1007 (Eberli, Swart, Malone, et al., 1997). A "flushed zone" was identified to occur within the top 30-50 meters below seafloor (mbsf) through a general lack of geochemical gradients, with most IW constituents displaying concentrations near those of bottom seawater. Progressing seaward, the flushed zone decreases in thickness; and in the case of Sites 1006 and 1007, a small but significant gradient in the concentration of Sr is observed in pore water from this zone. It was, therefore, inferred that more subdued flushing occurs within a thinner zone (~25-30 m) at the two more distal sites relative to the GBB proximal sites. Below the flushed zone at each of the transect sites, sharp variations occur in concentrations of IW constituents. Gradients of nonconservative elements are nonsteady-state and reflect the occurrence of downward advecting fluids as well as local reaction zones. Large increases in Cl- concentrations occur at all sites as a function of depth and suggest the existence of a deep-seated high-salinity brine or evaporite deposits. Even within the predominantly conservative profiles of species such as Cl-, small perturbations in concentrations that cannot be attributed to analytical error are observed. These appear to correlate with changes in physical properties of the corresponding sediments and may reflect horizontal fluid migration along sequence boundaries.