Calcareous nannofossil biostratigraphy of the central East Pacific Rise

During Leg 92 of the Deep Sea Drilling Project, sediments containing calcareous nannofossils of latest Oligocene to Holocene age were recovered from 14 holes at six sites (597 to 602) along the East Pacific Rise. The combined sections yield a virtually complete record for the region, with a compressed upper Miocene to Pleistocene interval. The nannofossil content of 14 U.S.N.S. Eltanin piston cores from the study area were also examined in order to supplement data generated during Leg 92. Two taxonomically new combinations are presented: Sphenolithus umbellus and Pontosphaera segmenta. Assemblages of calcareous nannofossils juxtaposed in reversed stratigraphic order within the upper Miocene provide strong evidence for downslope transport of sediments along the East Pacific Rise during the Messinian. Narrow bands of dark metalliferous sediment of coccolith Zone CN8b alternate with normal light-colored, in situ, pelagic sequences of Zone CN9b. This may indicate more vigorous bottom current activity between 5.40 and 6.70 Ma.

Contents and accumulation rates of the rare earth and some other elements in metalliferous sediments from DSDP Hole 92-598

The sediments recovered during DSDP Leg 92 (Site 598) include a complete 16 m.y. record of hydrothermal sedimentation along the western flank of the East Pacific Rise at 19°S. Fifty samples from this sediment column were analyzed to test the hypothesis that the REE composition of the hydrothermal component is primarily acquired via scavenging from seawater. Site 598 provides an ideal sample suite for this purpose: the sediments are lithologically "simple," primarily consisting of a mixture of hydrothermal materials and biogenous carbonates; the composition of the hydrothermal component is essentially constant through space and time; and the sediments have undergone minimal diagenetic alteration. The following observations suggest the above-stated hypothesis is true. The Ce anomaly as well as key indices of light and heavy REE behavior all show that the REE pattern of hydrothermal sediments approaches that of seawater with increasing paleodistance from the rise crest. Moreover, shale-normalized REE patterns are similar to that of seawater, varying only in absolute REE content: the REE content increases with distance from the paleo-rise crest and exhibits a pronounced increase in sediments deposited below the paleolysocline. Based on significant correlative relationships between paleodistance from the rise crest and both the concentration and mass accumulation rates (MARs) of REEs and Fe, we conclude the REEs in the hydrothermal component are derived from the interaction of seawater and Fe in the hydrothermal plume.