This subproject is one of many research subprojects utilizing theresources provided by a Center grant funded by NIH/NCRR. The subproject andinvestigator (PI) may have received primary funding from another NIH source,and thus could be represented in other CRISP entries. The institution listed isfor the Center, which is not necessarily the institution for the investigator.Iron is the most important micronutrient in the ocean, and limits productivity in large areas of the ocean. Of the total iron in the open ocean, up to 90% is found in the particulate form. Little is known about the chemical speciation of particulate iron, however, hampering understanding of its bioavailability and of its role in the global cycling of iron. Synchrotron XAS is an ideal technique to probe the speciation of particulate iron. I have already collected an extensive dataset (>200 XANES spectra) on particulate Fe speciation in the open ocean using a microprobe synchrotron beamline at the ALS (beamline 10.3.2). It is already obvious that there is enormous heterogeneity in particulate iron speciation in my dataset, and this has never before been reported. The u-XANES data suggest that the iron at ~150m, or the depth of the continental shelf, may be more reduced than iron at the surface or deeper down. This reduced iron coincides with a maximum in Fe concentrations, suggested a plume of reduced iron supplied to this depth. While u-XAS is in some ways an easier measurement, since the incident beam can be preferentially directed at high-Fe regions in the sample, the high heterogeneity between spots of each sample hampers our ability to make conclusions about overall differences in speciation between depths and between size fractions. Sample to sample differences are better addressed with bulk XAS analyses, since they average over a much larger area than microprobe beamlines do. Because of the higher spot size, however, the effective overall concentration will be far lower, and questions of detection limit become an issue. I therefore seek exploratory beamtime at SSRL to supplement my microprobe dataset with bulk Fe K-edge XANES and EXAFS data to see whether a bulk XAS measurement is feasible on these samples. If the Fe levels of my samples are too low for EXAFS, XANES will still be very useful for determining sample to sample difference in Fe valence.
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