Paper No. 11
Presentation Time: 1:30 PM-5:30 PM
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.