Abstract
Marine Biogeochemical Cycling of Carbon and Cadmium
by
Hannah Bourne
Doctor of Philosophy in Earth and Planetary Science
University of California, Berkeley
Professor James Bishop, Chair
The actions of single celled organisms living in the surface ocean drive the distribution
of many elements in the ocean - both vertically and horizontally across the ocean surface,
determining global chemical signatures. Autotrophs fix CO2 and dissolved nutrients into
their cells, building biomass that forms the base of the food chain, providing the energy
source for all higher trophic levels. The subsequent export of fecal pellets produced by
zooplankton and small particles aggregated into larger particles drive the biological pump.
The magnitude and processes of biomass export from the surface determines distributions
of elements and the partial pressure of CO2 near the surface, which ultimately affects global
climate.While the spatial distribution of near-surface biomass is well quantified through high
frequency satellite observations, observations of the vertical distribution and the processes
determining these distributions are relatively few.
In the first section, I present a model predicting the horizontal global distribution of Cd,
a toxic trace metal, in particles in the surface ocean. Particulate Cd concentrations is higher
in HNLC environments with cold temperatures and low dissolved silica levels than in warm,
oligotrophic regions. I hypothesize that this is due to inadvertent uptake of cadmium by
transporter cells outside of cells targeting other divalent trace metals such as iron and zinc.
In regions with low concentrations of bioavailable necessary divalent trace metals, Cd is more
likely to be accidentally taken up by cells. Another finding in this study is that particulate
P occurs in two forms with different labilities. One phase, such as phosphorus in DNA, is
more labile than Cd while the other phase, such as P bound in phospholipids, is less labile
than Cd. These differences in remineralization lead to subsurface Cd:P peaks occurring near
the euphotic zone depth.
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In the second part, I present the development of a novel in-situ sampling system which
was integrated with robotic profiling floats called Carbon Flux Explorers. We built two of
these integrated CFEs, which are called CFE-Cals. The CFE-Cals are capable of collecting
particles in-situ while drifting with currents at depth. During a 30 day research expedition
in June 2017 off the coast of California, I deployed the CFE-Cals 15 times. With samples
collected during the expedition, I successfully calibrated the CFEs optical measurements of
particle flux in terms of carbon and nitrogen. This translates sensor images into biogeochemical
terms, and is an important step forward in the advancement of autonomous optical
carbon export observations for quantifying processes in the carbon pump. Samples were
collected in varied environments - from very productive near shore regions where export
was dominated by large anchovy fecal pellets to low nutrient, low flux offshore waters dominated
by small particles. Regression slopes for the optical properties measured by the CFE,
Volume Attenuance (VA), to concentrations of particulate organic carbon and particulate
nitrogen (units mATN-cm2: mmol; R2, n, p-value in parentheses) were 10:07103 (0.86, 12,
5:7 10-8), 10:05 104 (0.87, 15, 5:2 10-10) respectively.
In the last section, I examine carbon export beneath a highly productive filament of
nutrient rich upwelled water in dynamic waters off the coast of California. In this study,
export is relatively high and increases with depth, contrary to typical observations of export
magnitude rapidly attenuating with depth. These observations indicate that in coastal regions,
efficient export by filtering organisms and lateral transport of material facilitate high
levels of carbon export.