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 we re analyzed for helium, thorium, and protactinium isotopes. Assuming global average fluxes of extraterrestrial He-3 and Th-230, th, average He-3-deriv ed sediment mass accumulation rate (MAR) is a factor of 1.8 higher than the average Th-230-derived MAR. He-3- and Th-230- derived MARs converge, howev er, during the Younger Dryas (YD) and during the peak of the early Holocene humid interval. These features, not seen anywhere else in the world, proba bly reflect a combination of climate-driven changes in the flux of Th-230 a nd He-3. Ratios of xs(231)Pa/xs(230)Th, proxies of paleoproductivity, are l owest during the last glacial maximum (LGM), and increase abruptly during t he Bolling-Allerod. Later, following a sudden decrease to near-LGM values d uring the YD, they rise abruptly to maximum values for the entire record in the early Holocene. We hypothesize that low xs(231)Pa/xs(230)Th ratios ref lect low productivity due to the decreased intensity of the SW monsoon, whe reas the opposite is true for high ratios. The correlation between Arabian Sea productivity and monsoonal upwelling, on the one hand, and North Atlant ic climate variability, on the other, suggests a linkage between high- and low-latitude climates caused by changing patterns of atmospheric circulatio n. (C) 2001 Elsevier Science B.V. All rights reserved.