Abrupt intensification of the SW Indian Ocean monsoon during the last deglaciation: Constraints from Th, Pa, and He isotopes

Sediments from western Arabian Sea core 74KL representing the last 23 ka were analyzed for helium, thorium, and protactinium isotopes. Assuming global average fluxes of extraterrestrial ³He and ²³⁰Th, the average ³He-derived sediment mass accumulation rate (MAR) is a factor of 1.8 higher than the average ²³⁰Th-derived MAR. ³He- and ²³⁰Th-derived MARs converge, however, during the Younger Dryas (YD) and during the peak of the early Holocene humid interval. These features, not seen anywhere else in the world, probably reflect a combination of climate-driven changes in the flux of ²³⁰Th and ³He. Ratios of xs²³¹ Pa/xs²³⁰Th, proxies of paleoproductivity, are lowest during the last glacial maximum (LGM), and increase abruptly during the Bolling-Allerod. Later, following a sudden decrease to near-LGM values during the YD, they rise abruptly to maximum values for the entire record in the early Holocene. We hypothesize that low xs²³¹ Pa/sx²³⁰Th ratios reflect low productivity due to the decreased intensity of the SW monsoon, whereas the opposite is true for high ratios. The correlation between Arabian Sea productivity and monsoonal upwelling, on the one hand, and North Atlantic climate variability, on the other, suggests a linkage between high- and low-latitude climates caused by changing patterns of atmospheric circulation.