Continental Delivery of Iron to Ocean Station Papa in the Subarctic Pacific

The phytoplankton assemblage at Ocean Station Papa (OSP) in the Subarctic Pacific in February 1996 was dominated by large chain forming diatoms and abundant coccolithophores. This phytoplankton assemblage is atypical of an HNLC region and strongly suggests a recent supply of bioavailable iron. Using synchrotron X-Ray Fluorescence (XRF) and Scanning Transmission X-Ray Microscopy (STXM), we show that marine aggregates collected at this time display micron sized discrete hotspots of iron. The hotspots are observed in aggregates collected from surface to deep, and are also prevalent in the small size fraction (1-53μm m). X-ray absorption spectroscopy (XAS) of several hotspots indicates that this iron is not organically bound and is predominantly oxide in nature. Aerosol monitoring stations do not show evidence of any dust delivery to the subarctic Pacific at this time, and the Ti:Fe ratios of the observed hotspots ( ∼0.02) are well below that of Asian dust ( ∼0.1). We conclude that the Fe hotspots at OSP originated from the continental margin. We ran an ocean general circulation model with an iron-like tracer source at the continental margin to confirm that micron-sized particulate iron can be transported to OSP on a reasonable time scale.