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 NO3.

• Fe, Zn, and Cd further purified by anion exchange chromatography• δ56 Fe, δ66 Zn, δ114 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 δ56 Fe values are observed throughout the eastern basin near the continental margin, and within

the hydrothermal plume. Low δ56Fe 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 ‰) δ56Fe observed in surface waters in the eastern portion of the basin[1].

• Low δ66Zn (~-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].

• δ114Cd 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

References1. 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

Dep

th (m

) [Si] µM

-152 -143 -134 -125 -116 -107 -98 -89 -80

Longitude (degrees)

Dep

th (m

) [P] µM

-152 -143 -134 -125 -116 -107 -98 -89 -80

Longitude (degrees)

Dep

th (m

)

Biological Uptake

CdS Precipitation

Dissolved δ

114 Cd (‰IRM

M-014 )

-152 -143 -134 -125 -116 -107 -98 -89 -80 Longitude (degrees)

Dissolved δ

66 Zn (‰IRM

M-014 )

-152 -143 -134 -125 -116 -107 -98 -89 -80

Dep

th (m

)

Zinc Scavenging

Longitude (degrees)

Hydrothermal Input

Hydrothermal Vents

-152 -143 -134 -125 -116 -107 -98 -89 -80

Dep

th (m

) [Fe] nM

Longitude (degrees)

Dep

th (m

)

[Cd] pM

-152 -143 -134 -125 -116 -107 -98 -89 -80 Longitude (degrees)

H

i[Zn] nM

-152 -143 -134 -125 -116 -107 -98 -89 -80

Possible Hydrothermal Input

Dep

th (m

)

Longitude (degrees)

Longitude (degrees)

[O] µM

Dep

th (m

)

-152 -143 -134 -125 -116 -107 -98 -89 -80

Dissolved δ

56 Fe (‰IRM

M-014 )

Dep

th (m

)

Reduced Fe Input

-152 -143 -134 -125 -116 -107 -98 -89 -80 Longitude (degrees)

Dust Input

Dissolution of Fe from

Continental Shelf