INTRODUCTION

A primary goal of Ocean Drilling Program (ODP) Leg 166 was to examine fluid flow and carbonate diagenesis associated with the carbonate platform margin. Investigations of the pore-fluid geochemistry are useful in setting limits on the degree of diagenesis and fluid movement based on the magnitude and shape of pore-fluid profiles (Kastner et al., 1990; Swart et al., 1993; Gieskes et al., 1993). Sites 1003-1007 of Leg 166 were drilled through periplatform carbonate sediments along an off-bank transect adjacent to the western margin of the Great Bahama Bank in the Santaren Channel (Fig. 1). The five sites span a distance of 25 km with water depths ranging from 350 m at the most proximal site (Site 1005) to 658 m at the distal site (Site 1006). The deepest site (Hole 1007C) penetrated upper Oligocene sediments at a depth of 1238 meters below seafloor (mbsf). Sites 1003-1007 were drilled along an extension of a Western Geophysical seismic line, which extends from the Great Bahama Bank into the Straights of Florida (Eberli, Swart, Malone, et al., 1997). The remarkable continuity of the sedimentary sequences from the proximal to the distal sites allows for a detailed correlation of the pore-fluid chemistry profiles. In addition, the depth to which pore fluids were collected through semilithified sediments provides a particularly good data set for understanding large-scale hydrogeochemical processes within a carbonate platform margin.

The objective of this paper is to synthesize shipboard pore-fluid results for the Bahamas Transect and to integrate them with trace element and oxygen stable isotope data presented in DeCarlo and Kramer (Chap. 9, this volume) and Swart (Chap. 8, this volume). Results from solid-phase minor and trace element data for Sites 1007 and 1005 are also presented along with stable ¹³C data from pore-fluid dissolved inorganic carbon (DIC). An emphasis is placed on discussing fluid flow and other diagenetic processes highlighted by the pore-fluid chemistry.

A solid sequence stratigraphic framework is important for understanding and interpreting pore-fluid chemistry profiles.

The sedimentary sequences of all sites drilled during Leg 166 are strongly influenced by changes in sea level (Eberli, Swart, Malone, et al., 1997). The Neogene section of all recovered cores displays centimeter-to-meter cyclic alterations in the composition and degree of lithification. Sea-level highstands flooded the adjacent platform and resulted in a dramatic increase in platform-derived sediments and organic material (Fig. 2). Lowstands reduced the supply of platform sediments and created periods of nondeposition at the more proximal sites. At the distal sites, lowstands reduced the supply of platform material and organic matter and increased the supply of detrital material (clays) from exposed shelf areas further south. Calci-turbidites from bank tops and slope areas are observed in both highstand and lowstand periods, particularly at the toe of the slope (Site 1007). Turbidites have the effect of mixing sediments, which increases their diagenetic potential by (1) increasing their permeability through sorting, and (2) reoxidizing the turbidite sequence (Cranston and Buckley, 1990). Turbidite sequences in all cores show the highest degree of carbonate recrystallization and the lowest preservation of metastable aragonite.

The nature and magnitude of diagenetic reactions is influenced by the original composition of sediments. Sediments cored along the Bahamas Transect are composed mainly of carbonate (>90 wt%); however, important differences occur with regard to whether the carbonate is derived from neritic or pelagic sources, and in the abundance of organic and siliciclastic components (Fig. 3). These components vary among different depositional sequences and within individual sequences as a function of distance from the platform. Neritic material consists of skeletal fragments composed of metastable aragonite and high-magnesium calcite (HMC) derived from the platform during highstands. Pelagic material mainly consists of foraminifers composed of low-magnesium calcite (LMC). In general, neritic material is more common at the proximal sites, whereas pelagic material and siliciclastics are more common at the distal sites (Sites 1006 and 1007). As might be expected, diagenetic overprinting varies both as a function of distance from the platform margin and as a function of character of each lowstand or highstand deposit. The upper Pliocene-Pleistocene sequences were deposited during a rimmed platform, and have a distinctive mineralogy gradient away from the platform margin. Sediments are characterized by an abundance of aragonite (>70 wt%), with minor amounts of HMC, LMC, and dolomite (Fig. 3). The amount of deposited insoluble clays increases laterally away from the platform and is highest in the on-lapping drift deposits (Fig. 3). In contrast, the Miocene sediments were deposited in a ramp setting and show less lateral variation away from the platform (Eberli, Swart, Malone, et al., 1997).