Abstract:
Zinc (Zn) stable isotopes can 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. By measuring Zn isotopes in a diverse array of marine samples, we hope to
understand how Zn isotopes are fractionated in the oceans and how Zn isotopes may be
used as tracers of marine biogeochemical processes. Common forms of anthropogenic Zn
had δ66Zn from +0.08‰ to +0.32‰, a range similar to Zn ores and terrigenous
materials. Larger variations were discovered in hydrothermal fluids and minerals, with
hydrothermal fluids ranging in δ66Zn from 0.02‰ to +0.93‰, and chimney minerals
ranging from -0.09‰ to +1.17‰. Lower-temperature vent systems had higher δ66Zn
values, suggesting that precipitation of isotopically light Zn sulfides drives much of the
Zn isotope fractionation 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
δ66Zn = -0.2‰ for high-affinity uptake and δ66Zn = -0.8‰ for low-affinity uptake. This
work is the first to describe the molecular basis for biological fractionation of transition
metals. 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. Seawater dissolved Zn isotopes also reflect the chemical and biological
cycling of Zn. The δ66Zn of deep seawater in the North Pacific and North Atlantic is
about 0.5‰, and the dissolved δ66Zn gets lighter in the upper water column. This is
unexpected based our observations of a biological preference for uptake of light Zn
isotopes, and suggests that Zn transport to deep waters may occur by Zn adsorption to
sinking particles rather than as primary biological Zn. 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.
