My dissertation is comprised of three distinct but
complementary projects that contribute to the current
understanding of two major themes. The first theme is defining
links between the carbon cycle, marine primary productivity and
climate change. The second theme is characterizing
sedimentary carbon and nutrient dynamics at the land-sea
interface.

In the first project, I used a multi-proxy approach to investigate
paleoproductivity, nutrient burial and carbon cycling across the
Eocene/Oligocene (E/O) boundary (study interval: 36.9 - 32.7
Ma in the western equatorial Atlantic. Export production
appears to have been externally forced by orbital parameters at
eccentricity frequencies during the study interval, based on
spectral analysis of the biogenic barium and reactive
phosphorus records. A shift in relative abundance of siliceous
vs. carbonate productivity may have resulted in a change in
relative organic carbon burial and may have contributed to the
positive excursion in global oceanic delta C13 subsequent to the
E/O boundary.

For the second project, I measured multiple tracers of export
production and nutrient burial in a laminated core from the
Santa Barbara Basin off California, containing approximately 120
years of recent environmental history (study range:
~1880-2001). Productivity (as reflected by the sedimentary
proxies) decreased throughout the study interval toward the
present, consistent with hypothesized ‘spin-down’ of the
California Current System. Superimposed on this longer-term
trend are several interdecadal-scale cycles. Two sedimentary
transitions occur at ~1922 and ~1970, both roughly coincident
with climatic shifts to a warm Pacific Decadal Oscillation (PDO)
regime. Productivity appears to have decreased after ~1970,
consistent with expectation for the California Current after the
~1977 switch to warm PDO regime, and consistent with the
hypothesis that decreased organic carbon rain may be
responsible for a return to bioturbated sediments at the edges
of the southern California borderland basins. Primary
productivity appears to have responded to decadal scale
changes in the mean state of climate at this northeastern Pacific
site.

In the final project I used sedimentary techniques to characterize
carbon and nutrient phosphorus geochemistry in surface
sediments from five sites on an east-west sample transect in the
Sacramento-San Joaquin Delta, CA. Sedimentary organic carbon
concentrations and carbon to phosphorus ratios decreased,
while reactive phosphorus concentrations increased moving
inland in the Delta. The fraction of total phosphorus
represented by organic phosphorus increased inland, while that
of authigenic phosphorus was higher bayward than inland
reflecting increased diagenetic alteration of organic matter
bayward. The distribution of phosphorus fractions and carbon
to phosphorus ratios may reflect the presence of labile,
phytoplankton-derived organic matter in upstream surface and
near-surface sediments. Sediment carbon and phosphorus
geochemistry is influenced by site-specific particulate organic
matter sources, the sorptive power of the sedimentary material
present, physical forcing and by early diagenetic transformations
presumably driven by organic carbon oxidation.

These projects are unified by the fact that phytoplankton
production plays a key role in the global carbon cycle, both from
a climate change perspective and a health and function of
ecosystem perspective. This holds true through the range of
projects: from an open ocean site in the ancient past, to the
coastal ocean during the recent historical past, to a delta
ecosystem in the present day. The sedimentary record provides
a valuable tool both for studying important periods in Earth’s
history and for achieving temporal resolution in the modern era
that would not be practical for water column measurements.