Iron in the western Pacific: a riverine or hydrothermal source for iron in the Equatorial Undercurrent?

Upwelling of the (comparatively) iron-rich waters of the Equatorial Undercurrent (EUC) may control new production in the central and eastern equatorial Pacific. In an attempt to determine the source of this iron, we measured total dissolvable iron (FeTD) concentrations in the Bismarck Sea on three cruises of R.V. Franklin as part of the Australian Joint Global Ocean Flux Study (JGOFS) program in 1993 and the Tropical River Ocean Processes In Coastal Settings (TROPICS) program in 1997 and 2000. Data in the Bismarck Sea were collected during the SE tradewind season at a time of a strong El Niٌo and severe drought in Papua New Guinea (PNG) (FR07/97), and during the NW monsoon season (FR01/00). Our results show that the New Guinea Coastal Undercurrent (NGCU) and the EUC in the western equatorial Pacific are enriched in iron that does not seem to be supplied directly from the Sepik River nor from known deep hydrothermal sources in the Bismarck Sea. The concentration of FeTD in the NGCU does increase as it flows along the north coast of PNG and we propose that the Fe is mainly supplied by sediments deposited on the narrow continental shelf by the Sepik and other rivers. There is indirect evidence for an additional supply of iron via the New Ireland Coastal Undercurrent.