COMPARATIVE-STUDY OF ACCUMULATION RATES DERIVED BY HE AND TH ISOTOPE ANALYSIS OF MARINE-SEDIMENTS

We present a detailed down-core analysis of helium isotope ratios and concentrations for bulk sediments from the central Equatorial Pacific that span the last two glacial-interglacial cycles. Measured He-3/He-4 ratios range from 1.0 X 10(-5) to 2.1 x 10(-4), or 7.4 to 149 times t he atmospheric ratio. The He-3 from interplanetary dust particles (IDP s) constitutes virtually all of the He-3 measured within the sediment. Because carbonate accumulation rates are high in the Equatorial Pacif ic, the measured He-3 concentrations are lower than have been measured elsewhere, and range from 4.7 X 10(-13) to 3.0 X 10(-12) cm(3)STP . g (-1). If the cosmic dust He-3-flux is constant with time, sediment mas s accumulation rates can be determined from the 3He concentration in s ediments. The excess Th-230 technique is an entirely independent metho d for calculating sediment mass accumulation rates because its source is in-situ decay of U-234 in seawater. To first order, initial excess Th-230 activities correlate with 3He concentrations within this core. Based on the Th-230 results, we estimate the 3He-flux to the Earth's s urface as (9.6 +/- 2.0) X 10(-16) cm(3)STP . cm(-2). a(-1). If this fl ux has remained constant over extended periods of time, it can be used to determine sediment accumulation rates beyond the Th-230 range (300 ,000 yr).