The accretion rate of extraterrestrial He-3 based on oceanic Th-230 flux and the relation to Os isotope variation over the past 200,000 years in an Indian Ocean core

In the eastern equatorial Indian Ocean, the flux of extraterrestrial He-3, a proxy of interplanetary dust particles (IDPs), has been relatively constant over the past 200 ka. The flux is equal to (1.1 +/- 0.4) x 10(-12) cm(3) STP cm(-2) ka(-1), a value obtained using the xs(230)Th profiling method. Variations in mass accumulation rates (MARs) derived assuming a constant extraterrestrial 3He flux have a 40-ka periodicity similar to that observed in the delta(18)O-derived MARs. This frequency is similar to that of the Earth's obliquity. Measured Os-187/Os-188 ratios are less radiogenic than present-day seawater (0.49-0.98), reflecting the mixing of Os derived from extratenestrial, terrigenous and hydrogenous sources. When coupled with He data measured on the same samples, Os isotope data yield important information about the terrigenous component supplied to the eastern equatorial Indian Ocean. The amount of Os in the sample derived from the extratenestrial component can be deduced with the help of the helium systematics. Once corrected for the extraterrestrial component of Os, Os isotope signatures, in conjunction with the He-4 concentrations, suggest a supply of terrigenous material from Indonesian ultramafic and Himalayan crustal sources that clearly varies through time. (C) 1999 Elsevier Science B.V. All rights reserved.