Publication Date:
2022-05-25
Description:
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy
at the
Massachusetts Institute of Technology
and the
Woods Hole Oceanographic Institution
September 2018
Description:
Many chemical constituents are removed from the ocean by attachment to settling particles, a
process referred to as “scavenging.” Radioisotopes of thorium, a highly particle-reactive element,
have been used extensively to study scavenging in the ocean. However, this process is complicated
by the highly variable chemical composition and concentration of particles in oceanic waters.
This thesis focuses on understanding the cycling of thorium as affected by particle concentration
and particle composition in the North Atlantic. This objective is addressed using (i) the distributions
228,230,234Th, their radioactive parents, particle composition, and bulk particle concentration, as
measured or estimated along the GEOTRACES North Atlantic Transect (GA03) and (ii) a model
for the reversible exchange of thorium with particles. Model parameters are either estimated by
inversion (chapter 2-4), or prescribed in order to simulate 230Th in a circulation model (chapter 5).
The major findings of this thesis follow. In chapters 2 and 3, I find that the rate parameters
of the reversible exchange model show systematic variations along GA03. In particular, 𝑘1, the
apparent first-order rate "constant" of Th adsorption onto particles, generally presents maxima
in the mesopelagic zone and minima below. A positive correlation between 𝑘1 and bulk particle
concentration is found, consistent with the notion that the specific rate at which a metal in solution
attaches to particles increases with the number of surface sites available for adsorption. In chapter
4, I show that Mn (oxyhydr)oxides and biogenic particles most strongly influence 𝑘1 west of the
Mauritanian upwelling, but that biogenic particles dominate 𝑘1 in this region. In chapter 5, I find
that dissolved 230Th data are best represented by a model that assumes enhanced values of 𝑘1 near
the seafloor. Collectively, my findings suggest that spatial variations in Th radioisotope activities
observed in the North Atlantic reflect at least partly variations in the rate at which Th is removed
from the water column.
Description:
This work was supported by the US National Science Foundation. Two US NSF grants have
supported the research in this thesis (OCE-1232578 and OCE-155644).
Keywords:
Thorium
;
Chemistry
Repository Name:
Woods Hole Open Access Server
Type:
Thesis
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