Abstract:
Trace metal cycling is one of many processes that influence ocean ecosystem dynamics.
Cobalt, iron, and manganese are redox active trace metal micronutrients with oceanic
distributions that are influenced by both biological and abiotic sources and sinks. Their
open ocean concentrations range from picomolar to nanomolar, and their bioavailabilities
can impact primary production. Understanding the biogeochemical cycling of these
hybrid-type metals with an emphasis on cobalt was the focus of this thesis. This was
accomplished by determining the dissolved distributions of these metals in oceanic regions
that were characterized by different dominant biogeochemistries.
A large subsurface plume of dissolved cobalt, iron, and manganese was found in the
Eastern South Atlantic. The cause of this plume is a combination of reductive dissolution
in coastal sediments, wind-driven upwelling, advection, biological uptake, and
remineralization. Additional processes that are discussed as sources of metals to the
regions studied during this thesis include isopycnal uplift within cold-core eddies (Hawaii),
ice melt (McMurdo Sound, Antarctica), riverine input (Arctic Ocean), and winter mixing
(McMurdo Sound). The biological influence on surface ocean distributions of cobalt was
apparent by the observation of linear relationships between cobalt and phosphate in mid to
low latitudes. The cobalt:phosphate ratios derived from these correlations changed over
orders of magnitude, revealing dynamic variability in the utilization, demand, and sources
of this micronutrient. Speciation studies suggest that there may be two classes of cobalt
binding ligands, and that organic complexation plays an important role in preventing
scavenging of cobalt in the ocean.
These datasets provided a basis for comparing the biogeochemical cycles of cobalt,
iron, and manganese in three oceanic regimes (Hawaii, South Atlantic, McMurdo Sound).
The relative rates of scavenging for these metals show environmental variability: in the
South Atlantic, cobalt, iron, and manganese were scavenged at very different rates, but in
the Ross Sea, mixing and circulation over the shallow sea was fast, scavenging played a
minor role, and the cycles of all three metals were coupled. Studying the distributions of
these metals in biogeochemically distinct regions is a step toward a better understanding of
their oceanic cycles.
