Good age control is essential to make full use of the sedimentological and paleoceanographic information preserved in marine slope sediments such as those recovered from the Bahamas during Leg 166. In such slope environments, however, sedimentation rates are highly responsive to sea-level change, which makes the development of age models difficult. During highstands, the flooded banks produce large amounts of aragonite, which forms high-sedimentation-rate aragonite-rich packages of sediment on the slopes (Heath and Mullins, 1984; Neumann and Land, 1975; Wilber et al., 1990; Droxler and Schlager, 1985, Glaser and Droxler, 1991). During lowstands, aragonite production is dramatically reduced and sedimentation rates on the slopes are significantly slower (e.g., Kier and Pilkey, 1971; Droxler et al., 1983; Boardman et al., 1986). This changing sedimentation rate makes linear extrapolation between biostratigraphic marker horizons of only limited use.
On the other hand, the changeable sediment composition can be an aid to developing age models. Aragonite-rich sediment packages can simply be counted off from the top of the core and matched with the known sequence of sea-level highstands. But two major problems exist with this approach. The first is that it's necessary to know the sea-level height required to produce a distinct sediment package. For instance, do the highstand events of marine isotope Substages 5a, 5c, and 5e result in discrete sediment packages? Or is all of Stage 5 recorded as one event? Or does only Substage 5e cause sufficient flooding of the banks to produce an aragonite-rich sediment package? The second problem is that a low-aragonite interval in the core may represent more than a single lowstand, possibly including a hiatus of sedimentation or a period of sediment erosion due to down-slope or lateral transport.
In the Pleistocene portions of the Leg 166 cores, recognition of biostratigraphic reference markers such as the first occurrence of Emiliania huxleyi is made difficult by the large dilution of pelagic material by bank-derived sediment. Biostratigraphic age control is therefore not sufficiently good to unambiguously assign ages to sediment packages and independent dating is required. C-14 can be used to test whether the uppermost sediment packet is Holocene, but other dating tools are required to assess the age of older sediment packages.
In this study, we make use of the high U content of the aragonite that makes up much of the highstand sediment on the Bahamas slopes (Slowey et al., 1996). This enables us to pursue U/Th techniques to derive age information for the Leg 166 cores. We have used recently developed techniques for U/Th dating of aragonite-rich sediments (Slowey et al., 1996; G.M. Henderson, N.C. Slowey, and M.Q. Fleisher, unpubl. data) to investigate key sediment horizons from four of the five sites of the northerly Leg 166 transect. These U/Th ages enable refinement of the Pleistocene age model for each of the cores. In addition, these data contribute to our understanding of sediment diagenesis and fluid flow in the Bahamas banks and provide clues about sea-level history.