Extraterrestrial ³He as a tracer of marine sediment transport and accumulation

THE deposition rate of deep-sea sediments, and their focused redeposition by deep-sea currents, can be evaluated from analyses of sedimentary ²³⁰Th with a temporal resolution limited only by bioturbation. ²³⁰Th is produced uniformly throughout the ocean by radioactive decay of dissolved ²³⁴U and is removed sufficiently fast by sorption onto sinking particles to act as a 'constant-flux' tracer of sedimentation rates. But the half-life of ²³⁰Th (75kyr) limits its use for this purpose to the past 200250 kyr. Here we explore the use of extraterrestrial ³He from interplanetary dust particles (IDPs) as a constant-flux proxy that is free from this limitation. A comparison of ³He with ²³⁰Th in two cores from the equatorial Pacific Ocean indicates that the variability in the mean flux of IDPs over the past 200 kyr is less than 75%. But in contrast to this relatively constant rate of supply of ³He to the deep sea, the local burial rates of ³He and ²³⁰Th have varied by a factor of five over the past 450 and 200 kyr, respectively. We interpret this variability as reflecting sediment focusing, with a temporal pattern that suggests regular cycles of climate-driven reorganization of near-bottom currents in the deep Pacific Ocean.