David A. Richards 1,2, Gina E. Moseley 1,2, Dirk L. Hoffmann 1,2, Peter L. Smart 1, R. Lawrence Edwards 3
1Geographical Sciences, University of Bristol, Bristol, BS8 1SS, UK (David.Richards@bristol.ac.uk)
2Bristol Isotope Group, Wills Memorial Building, Queens Rd, Bristol, BS8 1RJ
3Geology and Geophysics, University of Minnesota, Minneapolis, MN55455, USA
We present a compilation of U-series ages for flowstone sequences (-18.1 to +0.5 m above present mean sea level) from caves of the Turks and Caicos Islands and Bahamas that provide elevation and timing constraints for sea level highstands associated with marine isotope stages 7 and 9. The presence of submerged speleothems from these stable carbonate platforms compliments the existing coral and δ18O-model estimates by providing robust ages for maximum constraints on sea level elevation. A specific advantage of these continuous phases of secondary calcite growth is the preservation potential, in spite of submergence in brackish and saline water for many thousands of years. This can be tested using the unambiguous stratigraphy and trends in initial 234U/238U ratios.
Conch Bar and Sagittarius cave systems were probably formed during the Middle Pleistocene on the Caicos and Little Bahama Bank carbonate platforms. The subsidence history of the carbonate platforms that host these caves is constrained by magnetostratigraphy. The U-Th chronology of multi-phase flowstone records from these caves is presented, where hiatuses in growth correspond to periods of sea level submergence. In some cases, there are dissolution surfaces that are likely to be the result of mixing-zone corrosion, thus providing an additional elevation constraint.
While coralline material can be used to estimate elevations of interglacial sea levels approaching the limit of U-series methodology, the likelihood that is has remained unaffected by post-depositional alteration is low. We consider, here, the potential of using the past record of initial 234U/238U in secondary calcite precipitating in caves to reveal signs of alteration in speleothems, and also capture some of the temporal trends in the diagenetic fluids that might be affect the corals records. In so doing, we will provide evidence that can be compared with open-system models of U and Th behaviour in the carbonate rocks.