During Leg 166 of the Ocean Drilling Program (ODP), scientists drilled a transect of sites along the extension of the Western Geophysical Seismic Line (Fig. 1) into the Straits of Florida to examine the development of the margin of the Great Bahama Bank and to examine potential fluid flow through a carbonate platform (Eberli, Swart, Malone, et al., 1997; Shipboard Scientific Party, 1997). The sediments at these sites, late Oligocene to Holocene in age, were principally composed of material derived from three sources: (1) aragonite and high-magnesium calcite from the Great Bahama Bank, (2) pelagically derived low-magnesium calcite and aragonite, and (3) sediments derived from the weathering of Cuba and Hispaniola. Variations in the contributions from these three sources were controlled by changes in sea level, with banktop-derived sediments dominating during highstands and pelagic and siliciclastic materials becoming more important during lowstands (Eberli, Swart, Malone, et al., 1997). The sites closest to the margin of the Great Bahama Bank—Sites 1005, 1004, and 1003—were more influenced by carbonate production on the platform top showing high rates of accumulation during highstands and reduced or little sedimentation during sea-level lowstands. Rates of deposition at Sites 1003 through 1005 were ~100 m/m.y. over the upper 100 m. Below this depth, rates declined (Shipboard Scientific Party, 1997). Rates of deposition were slightly reduced at Sites 1006 and 1007 in the upper 50 to 100 meters below seafloor (mbsf). Details of the sedimentology and rates of deposition were provide by Eberli, Swart, Malone, et al. (1997) and Shipboard Scientific Party (1997).
During the drilling of
these sites, geochemical measurements on interstitial water samples revealed the
presence of a zone in which there was an absence of changes in the
concentrations of both conservative and nonconservative minor and trace element
constituents in the pore waters (Shipboard Scientific Party, 1997). Although
less convincing, similar trends were noted in the temperature profiles measured
using the Adara and water-sampling temperature probe temperature tools (Eberli,
Swart, Malone, et al., 1997). The isochemical trends are well illustrated if the
concentrations of chloride (Fig. 2A)
and strontium (Fig. 2B) in the
pore waters in the upper 100 mbsf are examined. Both these elements exhibit a
region with no changes in the concentrations, overlying a zone in which there is
a rapid increase in concentration. In the case of chloride, the high
concentration at depth appears to be related to diffusion from an underlying
chloride-rich source such as evaporated seawater or an evaporite deposit, like
that encountered previously in the Bahamas (Austin, Schlager, et al., 1986).
This upper zone has been termed the flushed zone. Below the flushed zone, there
are rapid changes in all of the conservative and nonconservative constituents.
Changes in the nonconservative components such as Sr2+, Ca2+,
and Mg2+ are related to the recrystallization of metastable
carbonates (Baker et al., 1982; Swart and Guzikowski, 1988). Changes in the SO42-
and alkalinity are related to the oxidation of organic material. In addition to
the upper flushed zone, there is a region near the platform margin at Sites 1004
and 1005 where the chloride concentration suggests that normal marine water is
being drawn into the platform, presumably by some type of convection mechanism (Fig.
3). The purpose of this paper is to present data on the 
18O
of pore waters to (1) further constrain the origin of waters in the flushed
zone, (2) examine the origin of the increase in chloride concentration with
depth, and (3) examine the apparent chloride anomaly near the platform margin
and ascertain whether it is a result of bottom water being advected into the
platform.
