Description: We report new 187Os/186Os data and Re and Os concentrations in metalliferous sediments from the Pacific to construct a composite Os isotope seawater evolution curve over the past 80 m.y. Analyses of four samples of upper Cretaceous age yield 187Os/186Os values of between 3 and 6.5 and 187Re/186Os values below 55. Mass balance calculations indicate that the pronounced minimum of about 2 in the Os isotope ratio of seawater at the K-T boundary probably reflects the enormous input of cosmogenic material into the oceans by the K-T impactor(s). Following a rapid recovery to 187Os/186Os of 3.5 at 63 Ma, data for the early and middle part of the Cenozoic show an increase in 187Os/186Os to about 6 at 15 Ma. Variations in the isotopic composition of leachable Os from slowly accumulating metalliferous sediments show large fluctuations over short time spans. In contrast, analyses of rapidly accumulating metalliferous carbonates do not exhibit the large oscillations observed in the pelagic clay leach data. These results together with sediment leaching experiments indicate that dissolution of non-hydrogenous Os can occur during the hydrogen peroxide leach and demonstrate that Os data from pelagic clay leachates do not always reflect the Os isotopic composition of seawater. New data for the late Cenozoic further substantiate the rapid increase in the 187Os/186Os of seawater during the past 15 Ma. We interpret the correlation between the marine Sr and Os isotope records during this time period as evidence that weathering within the drainage basin of the Ganges-Brahmaputra river system is responsible for driving seawater Sr and Os toward more radiogenic isotopic compositions. The positive correlation between 87Sr/86Sr and U concentration, the covariation of U and Re concentrations, and the high dissolved Re, U and Sr concentrations found in the Ganges-Brahmaputra river waters supports this interpretation. Accelerating uplift of many orogens worldwide over the past 15 Ma, especially during the last 5 Ma, could have contributed to the rapid increase in 187Os/186Os from 6 to 8.5 over the past 15 Ma. Prior to 15 Ma the marine Sr and Os record are not tightly coupled. The heterogeneous distribution of different lithologies within eroding terrains may play an important role in decoupling the supplies of radiogenic Os and Sr to the oceans and account for the periods of decoupling of the marine Sr and Os isotope records.

Description: The SO-84 cruise from Las Palmas to Cape Town studied the underwater volcanism associated with the St Helena hotspot. Wide-spaced grid mapping of a region south and west of St Helena island identified three previously unknown hotspot volcanoes (which we named Josephine 16°27'S-9°W, Jade 16°13'S-7°46'W and Benjamin 16°12'S-8°31'W) and several older, probably near-ridge-generated seamounts. The newly discovered hotspot seamounts, and the previously-known seamounts of Bagration, Bonaparte and Kutuzov were mapped and sampled. Onboard geochemical studies confirm that they all show trace-element (Rb, Nb, Y, Zr, Sr) enriched, hotspot magma-type compositions. Morphological studies and the state of the samples collected from the seamounts (all have at least 1 mm-thick Mn crusts) suggest that none of the volcanoes is presently active, and that the St Helena hotspot is probably extinct at the present time. Josephine appears to be the youngest volcano, and one dredge here yielded fossil hydrothermal material (Fe-hydroxides). To test the hypothesis that the extinction of the hotspot is due to its magmas being channelled to the Mid-Atlantic Ridge, several dredges were taken on the ridge opposite St Helena. Furthermore, in the proposed influence zone of the St Helena plume head, Grattan seamount (9°44'S-12°48'W) was also sampled to examine whether its composition has been affected by the St Helena plume. During the transit from Las Palmas to St Helena, several sediment cores were taken in 3000 m water depth close to the ridge axis for palaeoclimatological work.