A 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 ³He ⁴He ratios range from 1.0 × 10^-5 to 2.1 × 10^-4, or 7.4 to 149 times the atmospheric ratio. The ³He from interplanetary dust particles (IDPs) constitutes virtually all of the ³He measured within the sediment. Because carbonate accumulation rates are high in the Equatorial Pacific, the measured ³He concentrations are lower than have been measured elsewhere, and range from 4.7 × 10^-13 to 3.0 × 10^-12 cm³STP · g^-1. If the cosmic dust ³He-flux is constant with time, sediment mass accumulation rates can be determined from the ³He concentration in sediments. The excess ²³⁰Th technique is an entirely independent method for calculating sediment mass accumulation rates because its source is in-situ decay of ²³⁴U in seawater. To first order, initial excess ²³⁰Th activities correlate with ³He concentrations within this core. Based on the ²³⁰Th results, we estimate the ³He-flux to the Earth's surface as (9.6 ± 2.0) × 10^-16 cm³STP · cm^-2 · a^-1. If this flux has remained constant over extended periods of time, it can be used to determine sediment accumulation rates beyond the ²³⁰Th range (300,000 yr).