Paleoclimate Implications from a Geochronologic and Sr Isotope Study of a Bahamian Speleothem from San Salvador, Bahamas

Strontium and U-Th isotope data from a stalagmite from Lighthouse Cave on San Salvador Island, Bahamas were analyzed in order to assess the utility of Sr as a proxy to interpret Late Pleistocene to Holocene rainfall variations in the Bahamas. The soil above the cave, the limestone host rock, and the cave drip-waters have distinctive ⁸⁷Sr/⁸⁶Sr values. Drip-water compositions are intermediate between relatively high soil values and low limestone values, reflecting derivation of dissolved Sr in vadose waters from a balance between ion-exchange processes in soils and carbonate mineral-solution reactions in the limestone. Changes in this balance over time, as recorded in the speleothem Sr isotopes, may yield insight into changes in flow paths and residence time in the limestone. Speleothem Sr samples were assigned ages based on ²³⁸U-²³⁰Th geochronometry. Between 32.8–31.9 ka the speleothem ⁸⁷Sr/⁸⁶Sr values are slightly higher and resemble modern soil ⁸⁷Sr/⁸⁶Sr values, and at 5.3 ka the ⁸⁷Sr/⁸⁶Sr values for the speleothem are slightly lower and more closely resemble the limestone ⁸⁷Sr/⁸⁶Sr values. The temporal changes in the ⁸⁷Sr/⁸⁶Sr values may reflect shifts to more diffuse vadose water flow paths around 5 ka, which in turn may be a function of lower effective precipitation. The speleothem exhibits fast growth (0.1 mm yr^-1) during 32.8-31.9 ka, followed by a hiatus in growth after 31.9 ka, and slower growth (0.001 mm yr^-1) after 5.3 ka. These changes in the growth rate, coupled with the ⁸⁷Sr/⁸⁶Sr values of the speleothem, are interpreted to reflect climate shifts between wetter conditions in the Bahamas during 32.8-31.9 ka and drier conditions from 31.9 ka to the present day.