INTRODUCTION
The extent to which
carbonate lithologies in the flanks of long-drowned shallow-water carbonate
platforms are open or closed geochemical systems is not clearly known.
Interstitial waters may still be nearly identical to primary connate waters,
heavily altered by exchange with the host rock, or flushed by many volumes of
seawater since their formation. Here, we assess the extent of fluid movement
through the eastern flanks of the Florida-Bahama Platform using strontium as a
tracer.
Many active carbonate
platforms and atolls are known to have very fast flushing rates. Active
circulation cells that involve seawater and may involve mixing with freshwater
are common on the flanks of carbonate platforms (Kohout, 1967; Hanshaw and Back,
1980; Whitaker and Smart, 1990). Fluids in deep boreholes on Anewetak and
Pikinni atolls oscillate on a tidal frequency (Swartz, 1958; Ladd and Schlanger,
1960; Aharon et al., 1987). Apparently, the interstitial waters deep within
these emergent atolls are in open communication with the ocean. Boreholes into
the tops of guyots in the Mid-Pacific Mountains indicate that the permeable
parts of these long-drowned carbonate platforms are also flushed with modern
seawater (Paull et al., 1995; Wilson et al., 1995).
Conversely, the interiors
of some carbonate platforms contain fluids with distinct compositions that
require restricted circulation for them to develop. For example, the Mesozoic
rocks from the subaerial parts of the Florida-Bahama Platform contain pore
waters with salinities of more than 200
(Manheim and Horn, 1968). Dense, brine-rich fluids believed to have been derived
from the interior of the Florida-Bahama Platform are seeping out of the western
flank of this platform along the base of the Florida Escarpment (Paull et al.,
1991). The Florida Escarpment seeps are believed to be part of a large
circulation cell that involves both seawater entering along the escarpment face
and exiting the platform along the base of the escarpment. Similar circulation
may occur along the face of the Blake Escarpment.
Many of the driving forces
that stimulate fluid flow in emergent atolls or larger carbonate platforms
(e.g., tidal forces, wave pumping, small changes in thermocline structure,
hydrostatic heads associated with freshwater aquifers, and drainage associated
with changes in sea level [Whitaker and Smart, 1990]) do not apply to their
drowned counterparts. However, flow may be stimulated within drowned platforms
by (1) lateral thermal differences that are developed by heat flow warming the
interior of the platform (Kohout, 1967); (2) mixing with denser brines in the
interior of the platform; (3) perturbations in the oceanic water column
structure related to currents that flow around and over these structures; (4)
paleoceanographically induced changes in the adjacent oceanic water column
structure; and (5) diagenetic changes that alter pore-water density.
During Ocean Drilling
Program (ODP) Leg 171B, five sites were drilled along an ~40-km-long transect
following the crest of the Blake Spur in water depths between 1344 and 2657
meters below sea level (Fig. F1)
(Norris, Kroon, Klaus, et al., 1998). Middle to late Eocene-age pelagic
carbonates covered with a thin phosphorite pavement crop out along the drilling
transect. Most drill holes penetrated an expanded early Paleogene-age pelagic
carbonate sequence and ended in Cretaceous-age carbonates.
The Blake Spur is a
prominent east- to west-oriented submarine headland that has developed between
1.5- and 5-km water depths along the otherwise north- to south-trending Blake
Escarpment (Land et al., 1999). The slopes on the seaward edge of the Blake Spur
between 2750- and 4800-m water depths are extremely steep (60°-80°). The Leg
171B transect (Fig. F1)
extends from near the upper edge of the steep carbonate cliff updip toward the
Blake Plateau and is associated with more gentle seafloor slopes (<5°).
The rocks exposed on the
face of the Blake Spur (sampled between 2800 and 4000 m of water depth using the
submersible Alvin) are composed of horizontally bedded Early
Cretaceous-age carbonates of a shallow-water facies (Dillon et al., 1985).
Regional seismic reflection profiles clearly show that these strata extend
laterally under the Blake Spur and are in fact continuous with those underneath
the Blake Plateau (Dillon and Popenoe, 1988).
Strontium is incorporated
as a minor component (2000-10,000 ppm) during the formation of biogenic
carbonates (Morse and MacKenzie, 1990). During normal burial diagenesis the
strontium concentration in carbonate minerals decreases as they recrystallize.
Thus, most ancient limestones have strontium concentrations of 200-600 ppm (Bathurst,
1975). The strontium lost by carbonate minerals undergoing recrystallization is
transferred to the pore waters. Thus, carbonate diagenesis is one mechanism to
elevate strontium concentrations in pore waters above the ~8 ppm strontium
concentration that is characteristic of modern seawater. The isotopic
composition of the strontium added to pore water by carbonate recrystallization
will be the same as the biogenic carbonate from which it formed. Thus, by
measuring differences between the 87Sr/86Sr values in pore
waters and their host carbonates of known age, one can assess how much of the
interstitial strontium is original and how much has been transported into the
system (e.g., Baker et al., 1991; Elderfield et al., 1993).
Strontium isotope
stratigraphies are based on the observation that the ocean is well mixed with
respect to strontium and that carbonates, which form in ocean waters, reflect
the isotopic composition of seawater. At the time of deposition, carbonates and
the original pore water have the same isotopic values as the contemporaneous
seawater (e.g., Burke et al., 1982; Hess et al., 1986; Ludwig et al., 1988).
However, long-term changes in global chemical budgets slowly modify the
strontium isotopic composition of the ocean (Brass, 1976). The general trends of
seawater strontium isotopic composition (87Sr/86Sr) are
well known back through the Cretaceous (Hess et al., 1986) with values of Early
Cretaceous-age seawater (as monitored by the carbonates) <0.707 and modern
seawater values >0.709.
The transect of sites
drilled during Leg 171B (Fig. F1)
is well suited for using strontium isotopes as a tracer of fluid circulation
because (1) most of these carbonate-rich rocks have low clay content; thus,
other sources of strontium that can make the interpretation of strontium isotope
data equivocal (Elderfield and Gieskes, 1982) are reduced or eliminated; (2) the
strontium isotopic compositions of the Lower Cretaceous-age carbonates and
modern seawater are very distinct, and the change between these two time periods
has been essentially unidirectional; and (3) the steep northern, eastern, and
southern sides of the Blake Spur (Land et al., 1999) have exposed the strata
within the spur to potential lateral flushing with seawater.
