The distribution of dissolved Fe and Al in the upper waters of the Eastern Equatorial Pacific

The concentrations of dissolved Fe and Al were determined in the upper waters of the eastern Equatorial Pacific (EEP) between 5°N and 4°S at 110°W and along the equator from 115°W to 140°W between 3 December 2004 and 2 January 2005. The most notable feature is a distinct Al maximum that appears as a 100-150 m thick layer that corresponds closely to the Equatorial Undercurrent (EUC). The origin of this enrichment is likely from sediment resuspension processes in the source water regions of the EUC in the western Pacific. There is no corresponding feature in dissolved Fe but vertical profiles of Fe show typical nutrient-like distributions. Concentrations of dissolved Al decline slightly within the EUC along its advective flow path, but concentrations of Fe decline much more significantly at all depths along the section as a result of particle scavenging. Upwelling of the upper part of the EUC is the main source of Fe to the surface waters of this region, since eolian deposition, calculated from surface water dissolved Al, is minimal. The continual depletion of Fe in the source waters feeding the upwelling results in a decline of the Fe supply to surface waters. At 140°W in the surface waters, the N:Fe ratio (∼15,000:1) implies the presence of sufficient Fe to allow full utilization of upwelled nitrate. By 110°W, the N:Fe ratio is >60,000:1, implying surface waters that are severely Fe-limited. The change in the status of the surface waters from Fe-sufficient to Fe-limited appears to be largely the result of the inorganic scavenging of Fe from the water column by the vertical particle rain rate, which itself is produced in surface waters by the upwelling of nutrients and Fe. Thus the progressive depletion of Fe from west to east appears to be directly related to production stimulated in the surface waters by the Fe-rich upwelling in the west. The large loss of Fe over this relatively short spatial scale in a rapidly moving water mass implies an extremely short residence time under these conditions and also suggests that the EUC might provide a natural laboratory for studying the relative scavenging rates for a variety of trace elements. These results imply that the persistent high-nutrient, low-chlorophyll conditions in the EEP are a result of Fe-limitation, but that changing climatologic conditions in the source regions of the EUC may result in temporal variation in the amount of that Fe supply.