Isotopic constraints on cycling of dissolved organic carbon in the ocean
by Laodong Guo
Carbon is one of the most abundant elements in the universe and is the basis
for the existence of life on Earth. Dissolved organic carbon (DOC) is one
of the largest organic carbon reservoirs and is a key component in the interplay
between the biosphere, hydrosphere, and geosphere. Knowledge of the cycling
of DOC in the ocean is important not only for the understanding of the biogeochemistry
of a variety of elements, but also for the global carbon cycle and thus
climate changes of human concern.
My dissertation research contains both laboratory and field experiments.
Laboratory experiments were required to develop a reliable and accurate
procedure for measuring low concentrations of oceanic DOC and for sampling
marine colloids (including macromolecules and microparticles), or colloidal
organic carbon (COC). Field studies were conducted in the Gulf of Mexico
off Texas and the Middle Atlantic Bight off Cape Hatteras, as well as in
estuarine waters of Galveston Bay and Chesapeake Bay. Objectives of the
field studies were to investigate the abundance, distribution, and fluxes
of DOC in both oceanic and estuarine environments, examine the molecular
weight distribution of DOC, and gain a better understanding of the cycling
of dissolved and colloidal organic carbon in the ocean using a multiple-tracer
approach. In addition, I studied the proportions of thorium (Th) isotopes
in dissolved, colloidal, and particulate phases, and the interaction of
thorium isotopes with COC in order to better use them as tracers for the
cycling of organic carbon in the ocean.
I found that a considerable portion of traditionally defined "dissolved"
organic carbon is in a colloidal form (1 kilo-Dalton­p;0.2 microns),
from about 60% of the bulk DOC in estuarine waters to about 30-40% in oceanic
waters. Radiocarbon measurements showed that high-molecular-weight COC contains
contemporary 14C ages which are considerably younger than the apparent 14C
ages of the bulk DOC.
This research provides direct evidence that bulk DOC is a mixture of different
components with varying molecular weights and apparent 14C ages. High-molecular-weight
COC is more reactive than low-molecular-weight DOC and thus is a active
component in the marine carbon cycle and the biogeochemistry of other trace
elements in the ocean. Three types of colloids with different origins were
identified in both Gulf of Mexico and Middle Atlantic Bight waters: estuarine
colloids, offshore surface water (pelagic) colloids, and deep water colloids.
Sources of COC are distinguished by their radiocarbon signatures and concentrations
of carbon and nitrogen.
Using secondary ion mass spectro-metry, the first detailed profiles of 230Th
and 232Th in the Gulf of Mexico were measured. Results showed that the distribution
of 234Th was similar to that of organic carbon among dissolved, colloidal,
and particulate phases. Residence times of macromolecular COC calculated
from 234Th measurements were consistently short (1-60 days) regardless of
apparent 14C ages, indicating that high-molecular-weight colloids are turning
over more rapidly than the bulk DOC pool.
Values of distribution coefficients of thorium were negatively correlated
with those of particle or colloidal concentrations in seawater not only
for the long-lived thorium isotope but also for short-lived thorium isotope.
The significance of my research lies mainly in providing an improved understanding
of fluxes, turnover times, and sources of DOC in the ocean.
My research was supported by the National Science Foundation, the Department
of Energy and the Texas Institute of Oceanography. My advisor, Dr. Peter
Santschi, and other advisory committee members, Drs. John Morse, Luis Cifuentes,
Ethan Grossman, and Bruce Herbert guided me through this project.
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Oceanography, Texas A&M University
Updated May 27, 1996