Trace Metal Enrichments and Their Sources in the Equatorial Undercurrent of the Central and Eastern Pacific

Elevated concentrations of dissolved Al were found associated with the Equatorial Undercurrent (EUC) during two cruises to the central and eastern Pacific. At 140?W dissolved Al levels of up to 10 nM were found exactly associated with the high velocity core of the EUC. These elevated Al values were observed continuously across the equatorial Pacific associated with the core of the EUC with values of 7 nM found at 115?W. Dissolved Fe levels were also observed to be elevated in these sections but the pattern of enrichment was not as closely correlated with the high velocity core of the EUC as in the case of Al. Dissolved Fe concentrations at the depth of the EUC core at 140?W were up to 0.7 nM, during both cruises, but values dropped dramatically along the EUC flow path to less than 0.1 nM in the upper waters at 120?W. The relative enrichment of the two trace elements suggests that the source is more likely pluvial than hydrothermal. It seems likely that the high-energy sediment remobilisation regimes off the mouths of the large tropical rivers that drain the tropical highlands of Papua New Guinea, and that are close to the pathways of the EUC source waters, are the main sources of this enrichment. The extensive penetration of this continental signal into the open ocean provides an unexpected link between climatic signals and oceanic biogeochemistry. The relative depletion of the dissolved Fe compared to that of the dissolved Al provides a clear example of geochemical separation of trace elements along an advective flow path and thus may provide a useful natural laboratory for calibrating the relative removal rates of trace elements from the upper waters of the open ocean.