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 consta nt 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 ex traterrestrial 3He flux have a 40-ka periodicity similar to that observed i n the delta(18)O-derived MARs. This frequency is similar to that of the Ear th's obliquity. Measured Os-187/Os-188 ratios are less radiogenic than pres ent-day seawater (0.49-0.98), reflecting the mixing of Os derived from extr atenestrial, 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 O cean. The amount of Os in the sample derived from the extratenestrial compo nent can be deduced with the help of the helium systematics. Once corrected for the extraterrestrial component of Os, Os isotope signatures, in conjun ction with the He-4 concentrations, suggest a supply of terrigenous materia l from Indonesian ultramafic and Himalayan crustal sources that clearly var ies through time. (C) 1999 Elsevier Science B.V. All rights reserved.