Radiocarbon ages determined by the AMS method on hand-picked foraminifera shells are reported. The results allow estimates of the ventilation rate of the deep Atlantic and Pacific Oceans during glacial time. They also extend our knowledge of the chronology of events associated with the transition from full glacial conditions ca 15,000 years ago to full interglacial conditions ca 8000 years ago. This and the previous lists (Broecker et al 1988c) contain all the results obtained as part of this program through the fall of 1989.

Concentrations of U, Th, 231Pa and 10Be were measured in Holocene sediments from two cores collected off the west coast of South America, two cores from the East Pacific Rise, two from the equatorial Pacific and one from the south Pacific central gyre. Our results, together with data from 5 cores reported in the literature, show that boundary scavenging plays a major role in the removal of 10Be from the Pacific Ocean. Deposition rates of 10Be at three margin sites are more than an order of magnitude greater than at sites of red clay accumulation in the deep central Pacific. Deposition of 231Pa is 4 to 5-fold greater at the margin sites. The residence time of 10Be with respect to chemical scavenging, defined as its inventory in the water column divided by its rate of removal to the sediments, varies regionally from > 1000 years at the red-clay sites in the deep central Pacific to ∼ 100 years at the margin sites. Different factors control boundary scavenging of Pa and Be. For example, scavenging of 231Pa is enhanced by metal-oxide coatings of particles, whereas this seems to have little influence on the scavenging of 10Be. © 1990.

BERYLLIUM-10 (half-life 1.5 Myr) is produced by spallation of nitrogen and oxygen atoms by cosmic rays in the upper atmosphere. Its production rate is proportional to the flux of cosmic rays, which is modulated by solar activity and the strength of the Earth's magnetic field1,2. Weakening of the magnetic field allows more cosmic rays to impinge on the Earth's atmosphere, thereby increasing 10Be production. Here we report that the ocean-wide average accumulation rate of 10Be in Pacific sediments, which reflects the global average production rate of 10Be (ref. 3), was at least 25% greater during the height of the most recent ice age (∼ 24,000-16,000 yr ago) than during the Holocene (the past 10,000 yr). The higher production rate of 10Be records the lower intensity of the geomagnetic field during that period and is consistent with the hypothesis developed to explain the younger 14C ages of fossil corals compared with ages obtained by U-Th dating4. These results also point to a more general need to consider variations in production rate in geochronological studies using other cosmogenic nuclides. © 1992 Nature Publishing Group.