1. Fe, Zn, and Cd stable isotopes from the eastern tropical South Pacific from GEOTRACES cruise GP16 – Methods and data Josh Helgoe, Emily Townsend, & Seth John Department of Earth and Ocean Science, University of South Carolina Fe Zn Cd Background Methods Conclusion Knowledge of Fe, Zn, and Cd concentration and isotopic distributions is critical to understanding the biogeochemical cycling of these biologically important trace metals. This study presents the first large-scale isotopic analysis of these elements in the Eastern Tropical Pacific, based on the analyses of samples from US GEOTRACES cruise GP16. Preliminary Concentration Analysis Seawater samples are acidified to pH 2. Nobias resin with EDTA/IDA functional groups is added to 10mL of sample. Fe binds to the resin at low pH, after ~2h pH is raised to 5.0-6.5 to make Zn and Cd to bind. All subsequent chemistry is automated on the prepFAST including removal of seawater, resin rinsing, and elution of resin into acid. Concentrations determined by isotope dilution on an Element ICPMS. Concentration Data Fe, Zn, and Cd are all depleted in surface waters due to biological uptake. [Fe] is elevated near the continental shelf and west of the East Pacific Rise hydrothermal vent site. [Zn] has a similar concentration distribution to that of silica. The data suggest possible input of Zn from EPR hydrothermal vents. Input of hydrothermal Zn has not previously been observed. [Cd] has a similar concentration distribution to that of phosphate. Isotope Analysis Nobias resin with EDTA/IDA functional groups is added to ~1L samples. Fe binds to the resin at low pH, after ~2H pH is raised to 5.5-6.5 to make Cd and Zn bind. Resin is rinsed with UPW. Metals eluted from Nobias resin with 3N NO 3. Fe, Zn, and Cd further purified by anion exchange chromatography δ 56 Fe, δ 66 Zn, δ 114 Cd are analyzed on a Neptune multi-collector ICPMS. Double Spike Addition Based on preliminary concentration data, large-column samples were amended with an isotope double-spike before isotope analysis. Isotope Data Low δ 56 Fe values are observed throughout the eastern basin near the continental margin, and within the hydrothermal plume. Low δ 56 Fe values near the continental margin are most likely a result of reductive sedimentary dissolution in anoxic porewaters [1]. Terrestrial dust input is thought to be the primary cause of the high (+0.4 to +1 ‰) δ 56 Fe observed in surface waters in the eastern portion of the basin [1]. Low δ 66 Zn (~-0.2 ‰) are observed in surface waters, with higher values observed (~+0.5 ‰) in the deep ocean. We propose that this is due to scavenging of heavy Zn isotopes onto sinking organic particles [2]. δ 114 Cd increases in surface waters due to the preferential uptake of lighter Cd isotopes by marine microorganisms [2]. The heavier Cd isotope ratios observed in the upper 1000 m of the eastern basin may be due to precipitation of isotopically light Cd sulfides in low-oxygen waters [3]. Acknowledgments and Funding Concentration and Isotope Data Fe concentration Zn concentration Cd concentration Fe isotope ratio Cd isotope ratio Zn isotope ratio Thank you to Chris Marsay at the University of South Carolina for help working with the prepFAST, Beth Blair for help with analysis in the CEMS, and to the captain, crew, and sampling team aboard US GEOTRACES cruise GP16. Side view of PrepFAST sampling robot layout ICP Mass Spectrometer at USC Phosphate concentration Silicate concentration References 1.Conway, Tim M., and Seth G. John. "Quantification of dissolved iron sources to the North Atlantic Ocean." Nature 511.7508 (2014): 212-215. 2.John, Seth G., and Tim M. Conway. "A role for scavenging in the marine biogeochemical cycling of zinc and zinc isotopes." Earth and Planetary Science Letters 394 (2014): 159-167. 3.Janssen, David J., et al. "Undocumented water column sink for cadmium in open ocean oxygen-deficient zones." Proceedings of the National Academy of Sciences 111.19 (2014): 6888-6893. Oxygen concentration Depth (m) [Si] µM -152 -143 -134 -125 -116 -107 -98 -89 -80 Longitude (degrees) Depth (m) [P] µM -152 -143 -134 -125 -116 -107 -98 -89 -80 Longitude (degrees) Depth (m) Biological Uptake CdS Precipitation Dissolved δ 114 Cd (‰ IRMM-014 ) -152 -143 -134 -125 -116 -107 -98 -89 -80 Longitude (degrees) Dissolved δ 66 Zn (‰ IRMM-014 ) -152 -143 -134 -125 -116 -107 -98 -89 -80 Depth (m) Zinc Scavenging Longitude (degrees) Hydrothermal Input Hydrothermal Vents -152 -143 -134 -125 -116 -107 -98 -89 -80 Depth (m) [Fe] nM Longitude (degrees) Depth (m) [Cd] pM -152 -143 -134 -125 -116 -107 -98 -89 -80 Longitude (degrees) Hi [Zn] nM -152 -143 -134 -125 -116 -107 -98 -89 -80 Possible Hydrothermal Input Depth (m) Longitude (degrees) [O] µM Depth (m) -152 -143 -134 -125 -116 -107 -98 -89 -80 Dissolved δ 56 Fe (‰ IRMM-014 ) Depth (m) Reduced Fe Input -152 -143 -134 -125 -116 -107 -98 -89 -80 Longitude (degrees) Dust Input Dissolution of Fe from Continental Shelf