A HIGH-RESOLUTION, SPELEOTHEM RECORD OF THE INDIAN MONSOON DURING THE GLACIAL/INTERGLACIAL TRANSITION FROM SOCOTRA ISLAND, YEMEN

A high-resolution speleothem record from the Indian Ocean yields oxygen and carbon isotope ratios that can be used to reconstruct variations in the East African-Indian Monsoon system. A stalagmite (M1-4) was taken from Moomi Cave on Socotra Island off the coast of Yemen. Annual rainfall on the island is convective activity associated with the migratory patterns of the inter-tropical convergence zone (ITCZ). Ages were determined using ²³⁴U/²³⁰Th dating techniques, and a preliminary age model indicates speleothem growth from approximately 10.5 to 35 ky BP. More than 1000 C and O stable isotope measurements allow a detailed reconstruction of climate for this period. Oxygen isotope values range from approximately -4 to +2 per mil and carbon isotope values range from about -10 to 0 per mil (both vs VPDB). The oxygen isotopes in this system express primarily the isotopic composition of rainfall and to a lesser extent cave temperature, with d¹⁸O values being inversely related to the amount of precipitation. The most negative values are found during the Holocene, with generally lower values indicating a drier climate during the last glacial. The oxygen isotopes show a millennial-scale pattern of variation that is similar, but not identical to the Greenland ice cores. D/O events are of lesser magnitude in the stalagmite. The carbon isotopes display several sharp rapid increases to values of near 0 per mil against a more steady background of much negative values of -8 to -10. Carbon isotope fluctuations are often thought to express vegetation changes from C₃ (forest) to C₄ (grassland) plants however, we interpret the least negative carbon isotope values to represent periods of extreme drought and near lack of vegetation when dissolved carbon input to the groundwater was primarily from the atmosphere.