Island weathering: river sources of rare earth elements to the Western Pacific Ocean

Abstract

This paper focuses on two of the largest rivers and estuaries of Papua New Guinea (PNG), the Fly and Sepik and explores the degree to which river input and estuarine reactions affect the rare earth element (REE) composition of surface sea water in the western tropical Pacific Ocean. The dissolved phases of the Fly and Sepik River waters have striking REE compositions in the form of large MREE-enrichments as defined by a gradual increase in their shale normalized ratios toward the middle of the REE series. This river/weathering signature is quite distinct from the REE composition of Pacific Ocean sea water. Large scale removal of dissolved river REE occurs in the low salinity regions of the Fly and Sepik River estuaries due to the coagulation of Fe-humic colloids. Laboratory experiments show that the reaction of Fly River particles with sea water leads to the preferential release of MREE to estuarine waters. Additional experiments suggest that the development of MREE-enrichments in PNG rivers is associated with phosphate minerals and their aquatic chemistry. The development of MREE-enrichments in the Fly and Sepik Rivers, through the combination of weathering and estuarine reactions, provides what is termed an island weathering signature to the ocean waters of the western tropical Pacific Ocean. MREE-enrichments appear in the high salinity estuarine waters surrounding PNG and in the subsurface waters of the equatorial (150°E) Pacific Ocean north of PNG (station SA-5 of Zhang and Nozaki [Zhang, J., Nozaki, Y., 1996. Rare earth elements and yttrium in seawater: ICP-MS determinations in the East Caroline, Coral Sea, and South Fiji basins of the western South Pacific Ocean. Geochim. Cosmochim. Acta 60, 4631–4644.]). We interpret MREE-enrichments in the Pacific Equatorial Undercurrent (P-EUC) at SA-5 to reflect the entrainment of an island weathering signature by the regional currents. The REE data in this paper support, but do not conclusively confirm, the proposition of Gordon et al. [Gordon, R.M., Coale, K.H., Johnson, K.S., 1997. Iron distributions in the equatorial Pacific: implications for new production. Limnol. Oceanogr. 42, 419–431.] that there is transport of Fe and other lithogenic elements across the Pacific Ocean within the Equatorial Undercurrent. While they suggested volcanism and hydrothermal activity in the region of PNG to be the source of the lithogenic elements, we favour the hypothesis of Milliman et al. [Milliman, J.D., Farnsworth, K.L., Albertin, C.S., 1999. Flux and fate of fluvial sediments leaving large islands in the East Indies. J. Sea Res. 41, 97–107.] that the massive weathering of islands in the East Indies is a quantitatively more important source. This process involves the entrainment of dissolved and particulate matter from island rivers into the Bismarck Archipelago Undercurrents, which originate in the Coral Sea and move to the P-EUC through the Solomon and Bismarck Seas. Specifically, our REE data are consistent with hydrographical and sedimentological studies which show that the rivers on New Guinea's north coast directly inject their dissolved and particulate matter into the New Guinea Coastal Undercurrent (NGCU), which feeds the Equatorial Undercurrent.