The Marine Biogeochemistry of Zinc Isotopes

Seth G. John, Ph.D., 2007
Edward Boyle, Advisor

Zinc (Zn) stable isotopes record information about important oceanographic processes. This thesis presents data on Zn isotopes in anthropogenic materials, hydrothermal fluids and minerals, cultured marine phytoplankton, natural plankton, and seawater. Common forms of anthropogenic Zn have d66Zn from +0.08 ‰ to +0.32 ‰, a range similar to terrigenous materials. Hydrothermal fluids ranged in d66Zn from 0.02 ‰ to +0.93 ‰, and chimney minerals ranged from -0.09 ‰ to +1.17 ‰. Our results suggest that precipitation of isotopically light Zn sulfides is very important in hydrothermal systems. In cultured diatoms, a relationship was discovered between Zn transport by either high-affinity or low-affinity uptake pathways, and the magnitude of Zn isotope fractionation. We established isotope effects of d66Zn = ‑0.2 ‰ for high-affinity uptake and d66Zn = ‑0.8 ‰ for low-affinity uptake. Biological fractionation of Zn isotopes under natural conditions was investigated by measuring Zn isotopes in plankton collected in the Peru Upwelling Region and around the world. The d66Zn of deep seawater is about 0.5‰, and the dissolved d66Zn gets lighter in the upper water column. The thesis, by presenting data on several important aspects of Zn isotope cycling in the oceans, lays the groundwork for further use of Zn isotopes as a marine biogeochemical tracer.