The
majority of samples analyzed in this study have clearly undergone some form of
diagenesis which has perturbed their U-Th systematics. This is most clearly
demonstrated by (230Th/238U)
values in excess of secular equilibrium, and is also shown by the anomalous 234U(T)
values of many samples.
The high (230Th/238U) values of Site 1005 indicate that either U loss or 230Th gain has occurred. Thorium is generally a highly immobile element, so U loss is the more likely case. Future discussion here will make the assumption that U loss has occurred, rather than 230Th gain, but if the situation were reversed much of the interpretation would stand unaltered.
Loss
of U makes the apparent age too great and therefore increases the 234U(T).
This loss can be investigated by constructing curves on a
234U(T)-vs.-age
diagram for each sample (Henderson et al., 1993) (Fig.
3). The position of the curve is controlled by the measured
234U(0)
value and samples move toward the bottom left as U is returned to the age
calculation to correct for U lost from the sample. Because U loss is unlikely to
cause a significant isotopic fractionation, the true age of each sample is
expected to lie somewhere on a curve derived in this way.
Similar curves can be constructed for some of the samples from Sites 1003 and 1006, but here U addition seems to have occurred rather than U loss (Fig. 3). In this case, the possibility exists of adding U with an isotopic composition different from the sediment; however, the amount of U addition required for these samples is sufficiently small that, unless the isotopic ratio of the added U is extreme, it is not expected to cause samples to deviate significantly from the curves shown in Figure 3.
The
percentage of U lost or gained can be calculated for each sample (although this
is somewhat dependent on the diagenetic model, discussed below [see "Diagenetic
Models" section]). Sample 5-2 requires a U loss of some 40%,
while other Site 1005 samples require progressively less U loss downcore to a
value of only
5%
for Sample 5-6 (Fig. 3). Much
smaller amounts of U addition are required for the Site 1003 and 1006 samples,
with the only significant additions being
2%
for Sample 6-3 and
3%
for Sample 6-4. Note that these percentages are calculated assuming that all
loss or gain occurred recently. Total U addition would need to be higher if some
of it occurred in the past.
There
are two diagenetic models with which to estimate true ages for these samples.
First, if straightforward gain or loss of U occurred, then the intersect between
the curves and the seawater value of 234U
(
149)
would provide the true age. This approach, however, yields ages that are
unreasonable given that these high-aragonite samples are expected to come from a
sea-level highstand. In particular, Sample 5-4 would yield an age of
270
ka and Samples 5-6 and 6-3 ages close to 560 ka—both times of sea-level
lowstands (Fig.
3).
The
second diagenetic model assumes that, in addition to the gain or loss of U, -recoil
of 234U
causes a continuous loss of pure 234U
from these U-rich samples. If the samples are assumed to come from a highstand
interval, then the intersection of the curves in Figure
3 with highstands (shown as gray bands) enables an assessment of the 234U
loss and of the possible age. Such intersections for all nine samples in Figure
3 define a linear trend toward decreasing
234U(T)
with age of sample (shown by the bold dashed arrow). This trend is as expected
for continuous loss of 234U,
and the fact that all samples fall on the same trend lends support to the idea
that
-recoil
is systematically effecting these samples.
Such
-recoil
is probably an internal reorganization within the sediment. Measurements
presented in this paper are on U-rich separates from the bulk sediment. This
U-rich material is expected to lose more 234U
by
-recoil
than it gains from the U-poor fraction of the sediment. The surprise is that
-recoil
can be so significant for 63- to 250-µm grain size separates as the recoil
distance is short compared to this grain radius. A possible explanation is that
U-rich organics on the grain surface play an important role in
-recoil
exchanges between grains.
Whatever
the process, there is clear empirical evidence that -recoil
is occurring for these samples and for similar samples from elsewhere in the
Bahamas. For instance, deep samples from Site 1003 have
234U(0)
less than zero, indicating that they have preferentially lost 234U.
Measurements from the Little Bahama Bank show that aragonite in slope sediments
is prone to loss of 234U
(G.M. Henderson, N.C. Slowey, and M.Q. Fleisher, unpubl. data). The rate of 234U
loss can be calculated from the deep Site 1003 samples and is similar to that
required to explain the
-recoil
trend shown in Figure 3.
In
general, the clear linear trend toward low 234U(T)
(Fig. 3), together with
independent evidence for the importance of
-recoil,
suggests that a diagenetic model involving both U loss/gain and continuous 234U
loss by recoil is reasonable for these sediments.
The diagenetic model favored above requires significant U loss from Site 1005 samples but slight addition of U to samples from Sites 1003 and 1006. The difference between the sites may be explained by their distance from the platform and the different fluid-flow history that they have, therefore, experienced.
Data collected during Leg 166 (Eberli, Swart, Malone, et al., 1997) suggest that fluid flow through the slopes of the Bahamas has two modes, depending on whether the banks are flooded or not (Henderson et al., 1999). This is illustrated in Figure 4 and is most clearly seen from the chlorinity data. At Site 1005, pore-water Cl concentrations show a clear inversion at depth because of lowstand circulation (Fig. 4, upper panel) and a flushed zone of seawater values above reflecting highstand circulation (Fig. 4, lower panel).
Close
to the banks the direction of fluid flow, therefore, probably changes from
outward flow caused by fresh-water compensation during low sea level to inward,
thermally driven flow when the banks are flooded. Site 1005 sediments,
therefore, experience long periods (90
k.y.) of outward flow of nonoxygenated pore water while the banks are exposed,
followed by a shorter periods (
10
k.y.) of downward flow of oxygenated seawater during peak interglacials (Fig.
3).
In
contrast, sites situated farther from the platform have always experienced
inflow of seawater. In these sites, U-rich organic material in the sediment is
remineralized relatively quickly by the continuous inflow of oxygenated water.
But at Site 1005, U-rich organic material may survive for considerable periods
while producing radiogenic 230Th
before oxidation during a highstand event. For instance, after 115
ka, sediments from Site 1005 were probably bathed in nonoxygenated pore waters
while producing radiogenic 230Th.
Then, at ~10 ka, the banks flooded, flow reversed, oxidation of organic material
began, and U was removed while leaving behind the immobile 230Th,
thus yielding apparent ages that are too old. This scenario is supported by the
fact that the percentage of U loss required to explain the measured U-Th ages of
the sediment decrease downcore for Site 1005. This is the result of deeper
sediment packages experiencing multiple phases of U loss at previous sea-level
highstands.
The small amount of U addition experienced by the samples from Sites 1003 and 1006 is probably explained by the downward advection of pore waters from oxidizing conditions to reducing conditions. As U is insoluble when reduced, the process of fluid flow moves small amounts of U downcore to precipitate in the more reducing conditions at depth.
The differences in fluid-flow regime experienced by the sediments of this study produce U diagenesis sufficiently complex that precise age information cannot be calculated for these samples. Nevertheless, there appears to be a coherent diagenetic explanation of the data such that each sample can be assigned to a single highstand event, enabling the production of age models for these cores.
Samples have been assigned to particular highstand events on the basis of their measured U-Th compositions and the consideration of diagenesis above. The sequence of highstands preserved at each site is now discussed briefly, working from the basin toward the platform edge.
This site has marine oxygen isotope Stages 1, 5, 9, and 11 but is missing Stage 7. This has been ascribed to a coring gap in Hole 1006A; the same coring gap does not exist in Hole 1006B (Kroon et al., Chap. 2, this volume). Sedimentation has, therefore, occurred and been preserved for each of the major highstand events back to Stage 11 for this basin site.
The uppermost sample in this core yields a U-Th age which is older than Holocene (24 ka). This probably reflects mixing of small amounts of older material upward in the core as this sample was taken from the transition from high- to low-aragonite-content sediment. The second sample in this core is too old to date by U/Th methods. This site has, therefore, suffered either a significant hiatus or considerable sediment erosion so that the highstands associated with Stages 5, 7, 9, 11, and possibly others are missing. This conclusion confirms the initial interpretation of the Western Seismic Line and the observations made by the Leg 166 shipboard party, both of which indicate a significant hiatus near the top of Site 1007 (Eberli, Swart, Malone, et al., 1997).
This
site is also missing several highstand events, a fact hinted at by the abrupt
change in 18O
between the uppermost sediment package and deeper packages, indicating a change
in sediment diagenesis (Rendle et al., Chap.
6, this volume). Stage 11 has been preserved at this site, but Stages 5,
7, and 9 are missing. The four deeper sediment packages measured in this study
are older than the U/Th limit of ~450 ka.
This site, closest to the platform, appears to have a complete sequence of the major highstands, yielding ages from Stages 1, 5, 7, 9, and 11. The relative spacing of these ages is somewhat problematic, however. Stage 5 is very close to Stage 1, possibly suggesting sediment erosion of much of the Stage-5 sediment. Stage 7 features two distinct aragonite peaks separated by some distance. This might be explained if each package was from a different substage of Stage 7, but the U-Th dating presented here cannot distinguish these events.
Samples analyzed in this study were selected from horizons in the cores with the highest aragonite content. Such high aragonite contents are only produced when the banks are flooded and large areas of shallow water become available for aragonite production. None of these peak aragonite intervals has been dated as Stage 3, or as Substage 5a or 5c. It is possible that the absence of high-aragonite sediment of these ages reflects erosion of any such material originally deposited. Erosion of sediment is, after all, indicated by missing highstands in the sediments of three of the four sites investigated here. It is, however, more difficult to allow for removal of such sediment from Site 1006, which is situated on horizontal sea floor, some way from the platform. The lack of erosion from this site is supported by the complete sequence of major highstands recovered there.
Site 1006 features a rather monotonic decrease in aragonite concentration upward from the Substage 5e aragonite peak (Fig. 1). There is nothing approaching a return to the high values seen in Substage 5e. This pattern is similar to that reported for a core from the Tongue of the Ocean by Droxler et al. (1983). The lack of a return to high aragonite values in Substages 5c and 5a suggests that the banks were not flooded during these intervals.
The
position of sea level during Substage 5c and particularly 5a has been the
subject of significant controversy. Estimates based on U-Th dated corals have
placed it anywhere from +2 m to -19 m compared to today's sea level (see Ludwig
et al., 1996, for summary). Estimates from 18O
and Mg/Ca measurements from deep-sea cores put Substage 5a sea level
significantly lower (
-60
m) (Dwyer et al., 1995). The banks in the vicinity of the Leg 166 Sites range
from 6 to 10 m in water depth, and the Bahamas are tectonically reasonably
stable. The lack of a return to high aragonite fractions in Bahamas sediments
during these periods suggests that the banks were not flooded and that sea level
therefore did not come within
10
m of the modern value. At this stage, such a conclusion is not robust due to the
possibility of erosion of important sections of the few cores studied. However,
if continuing work on Bahamas slope sediments fails to provide evidence for
high-aragonite sedimentation during Substages 5a and 5c, it will increasingly
suggest that sea level during these periods was lower than today.