TERRIGENOUS HELIUM IN DEEP-SEA SEDIMENTS

We have measured the isotope ratios of helium contained in various ter rigenous materials that contribute to deep-sea sediments. These materi als include ice-rafted debris from the North Atlantic, Chinese Loess, and sediment collected at or near the mouths of three large rivers: th e Amazon, the Ganges, and the Yangtze. We observe terrigenous He-3/He- 4 ratios that vary from 1.8 x 10(-9) to 4.6 x 10(-7), i.e., values tha t are higher than the theoretically-derived range of 10(-9) to 10(-8). Atlantic and Pacific deep-sea sediment 3He/4He ratios can be explaine d by mixing of helium from similar extraterrestrial but different terr igenous sources. Terrigenous sources for North Atlantic and North Paci fic sediments are characterized by He-3 and He-4 contents that are hig her, and 3He/4He ratios that are lower, than those for central and eas tern equatorial Pacific sediments. This is consistent with the supply to the North Atlantic and North Pacific oceans of older cratonic conti nental material that contains high amounts of in situ-produced nucleog enic and radiogenic helium. Terrigenous material transported to centra l and eastern equatorial Pacific sediments contains lower amounts of 3 He and 4He and higher He-3/He-4 ratios, indicative of supply from a mo re juvenile Andean source. In the equatorial Atlantic (core V31-135), we have used previously-published Th-230 data to determine an extrater restrial He-3 flux of (1.16 +/- 0.15) x 10(-12) cm(3)STP.cm(-2).ka(-1) , within error of that previously determined in sediments from the equ atorial Pacific Ocean ([0.78 +/- 0.29] x 10(-13) cm(3)STP.cm(-2).ka(-1 ); Marcantonio et al., 1996). Copyright (C) 1998 Elsevier Science Ltd.