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

Abstract

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 bioturbation6,7,10,11. 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 (75 kyr) limits its use for this purpose to the past 200–250 kyr. Here we explore the use of extraterrestrial 3He from interplanetary dust particles1–4 (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, respect-ively. 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.