Terrigenous helium in deep-sea sediments

We have measured the isotope ratios of helium contained in various terrigenous materials that contribute to deep-sea sedments. These materials include ice-rafted debris from the North Atlantic, Chinese Loess, and sediment collected at or near the mouths of three large rivers: the Amazon, the Ganges, and the Yangtze. We observe terrigenous ³He/⁴He ratios that vary from 1.8 × 10^-9 to 4.6 × 10^-7, i.e., values that are higher than the theoretically-derived range of 10^-9 to 10^-8. Atlantic and Pacific deep-sea sediment ³He/⁴He ratios can be explained by mixing of helium from similar extraterrestrial but different terrigenous sources. Terrigenous sources for North Atlantic and North Pacific sediments are characterized by ³He and ⁴He contents that are higher, and ³He/⁴He ratios that are lower, than those for central and eastern equatorial Pacific sediments. This is consistent with the supply to the North Atlantic and North Pacific oceans of older cratonic continental material that contains high amounts of in situ-produced nucleogenic and radiogenic helium. Terrigenous material transported to central and eastern equatorial Pacific sediments contains lower amounts of ³He and ⁴He and higher ³He/⁴He ratios, indicative of supply from a more juvenile Andean source. In the equatorial Atlantic (core V31-135), we have used previously-published ²³⁰Th data to determine an extraterrestrial ³He flux of (1.16 ± 0.15) × 10^-12 cm³STP.cm^-2.ka^-1, within error of that previously determined in sediments from the equatorial Pacific Ocean ([0.78 ± 0.29] × 10^-13 cm³STP.cm^-2.ka^-1; Marcantonio et al., 1996).