(Table 1) Helium concentration and flux in DSDP Hole 94-607 samples

Most of the helium-3 in oceanic sediments conies from interplanetary dust particles (IDPs), and can therefore be used to infer the accretion rate of dust to the Earth through time (Ozima et al., 1984, doi:10.1038/311448a0; Takayanagi and Ozima, 1987, doi:10.1029/JB092iB12p12531; Farley, 1995, doi:10.1038/376153a0). 3He records from slowly accumulating pelagic clays indicate that the accretion rate varies considerably over millions of years, probably owing to cometary and asteroidal break-up events3. Muller and MacDonald have proposed (Muller and MacDonald, 1995, doi:10.1038/377107b0) that periodic changes in this accretion rate due to a previously unrecognized 100-kyr periodicity in the Earth's orbital inclination might account for the prominence of this frequency in climate records of the past million years (Imbrie et al., 1993, doi:10.1029/93PA02751). Here we report variations in the 3He flux to the sea floor that support this idea. We find that the flux recorded in rapidly accumulating Quaternary sediments from the Mid-Atlantic Ridge oscillates with a period of about 100 kyr. We cannot yet say, however, whether the 100-kyr climate cycle is a consequence of, a cause of, or an effect independent of these periodic changes in the rate of delivery of interplanetary dust to the sea floor.

Benthic foraminiferal communities at DSDP Sites 94-609 to 94-611

Benthic foraminiferal assemblages from northeast Atlantic DSDP Sites 609, 610, and 611 have been interpreted with reference to modern assemblages known to be linked with the overlying bottom-water masses. It is shown that the water masses in the late Miocene to Pleistocene were similar to those of today. The distribution of the water masses changed with time, however. Antarctic Bottom Water ("AABW"), which at present is restricted to the area south of the Azores, reached as far north as the Gibbs Fracture Zone in the early Pliocene. Increased production of North Atlantic Deep Water in the late Pliocene displaced the AABW to the south

Oxygen and carbon isotope data for benthic foraminifera from DSDP Site 94-608 and ODP Site 208-1264

Small biserial foraminifera were abundant in the early Miocene (ca. 18.9-17.2 Ma) in the eastern Atlantic and western Indian Oceans, but absent in the western equatorial Atlantic Ocean, Weddell Sea, eastern Indian Ocean, and equatorial Pacific Ocean. They have been assigned to the benthic genus Bolivina, but their high abundances in sediments without evidence for dysoxia could not be explained. Apertural morphology, accumulation rates, and isotopic composition show that they were planktic (genus Streptochilus). Living Streptochilus are common in productive waters with intermittent upwelling. The widespread early Miocene high Streptochilus abundances may reflect vigorous but intermittent upwelling, inducing high phytoplankton growth rates. However, export production (estimated from benthic foraminiferal accumulation rates) was low, possibly due to high regeneration rates in a deep thermocline. The upwelled waters may have been an analog to Subantarctic Mode Waters, carrying nutrients into the eastern Atlantic and western Indian Oceans as the result of the initiation of a deep-reaching Antarctic Circumpolar Current, active Agulhas Leakage, and vigorous vertical mixing in the Southern Oceans.