187Os-enriched domain in an Archean mantle plume: evidence from 2.8 Ga komatiites of the Kostomuksha greenstone belt, NW Baltic Shield

The Nd isotopic composition (IC) and Rare Earth patterns of hydrodynamic structures of the Equatorial Pacific Ocean were characterized along 140°W. The Nd IC of Antarctic Intermediate Water (AAIW) and of the lower layer of the Equatorial Undercurrent (EUC) at 140°W (13°C Water) are much more radiogenic at the equator than at their origin in the South Equatorial Current (12°S), revealing that these water masses have been in contact with the highly radiogenic Papua New Guinea (PNG) slope. In both cases, only a small fraction (less than 9%) of the sediment deposited on the PNG slope is required to be exchanged or dissolved to explain these Nd IC variations, whereas the hydrographic properties of the same water masses remain unchanged. This confirms the usefulness of this tracer to identify pathways of water masses. These results emphasize the importance of jets in transporting lithogenic material into the subsurface layers of remote areas, where aeolian inputs are particularly weak and corroborate the previous results on Fe and Al maximum in this area [M.L. Wells, G.K. Vallis, E.A. Silver, Nature 398 (1999) 601¯604]. The Nd IC of the upper layer of the EUC contrasts strongly to that of the subpycnocline layer, indicating that the equatorial upwelling only affects the surface waters and is not effective between 120 and 150 m. We calculate that the Nd imprint of the PNG input is likely to vanish from this surface layer as it traverses the basin, due to the replacement of upwelled waters by non-radiogenic ones.