In order to better forecast the response of the
coastal oceans to growing human
populations and changing climate,
uncertainties need to be reduced in our
understanding of biogeochemical cycles,
including the role of sediment dynamics in the
alteration of terrestrial and marine-derived
organic carbon. Surficial sediments and
sediment cores were collected from two
distinct depositional regimes of the York River
subestuary of Chesapeake Bay to document
seasonal inputs, spatial variability, and
longer-term (>40 years) fate of total organic
carbon (TOC), lipid biomarker compounds
and polycyclic aromatic hydrocarbons (PAHs).
Compounds were selected to represent a
range of chemical reactivities, biological and
anthropogenic sources, and modes of entry to
the environment. The depositional
environments were characterized with a suite
of analytical tools: x-radiographs, Eh, Pb-210
and Cs-137, total organic carbon, total
nitrogen, and their stable isotopes.

Each compound class was quantified in
extractable and bound phases. Distributions
of extractable lipid compounds and PAHs in
surface sediments from both study sites
showed a strong seasonal signal in organic
matter inputs driven by primary productivity in
the overlying water column, which remained
dominant throughout the year. PAHs were
largely derived from atmospheric deposition of
aerosols to the surrounding watershed. The
fate of gaseous PAH input to the estuary was
determined to be uptake and metabolism
within the aquatic food web.

The study sites were selected to represent
contrasting depositional regimes: biological
mixing in the lower York and episodic physical
mixing at the mid-river site. Episodic mixing at
the mid-river site resulted in stronger oxidizing
conditions throughout the upper 45 cm of the
core (Eh?150 mV). These conditions
promoted enhanced degradation of labile
organic matter (e.g. diatoms) vs. more
refractory material (e.g. higher plants) in
extractable sedimentary phases in sediments
<5 yrs old. However, while apparent rate
constants for bulk organic matter and total
lipid were higher in older sediments (<40
years) under physically mixed conditions,
degradation rates of fatty acid and sterol
biomarkers were similar at both study sites.
Fatty acids and sterols may be transformed or
incorporated into bound sedimentary phases
and/or microbial biomass similarly over
longer periods of time.

PAHs and lipid biomarkers isolated from
"bound" phases were better preserved over
time than corresponding "extractable"
compounds. However, stabilization in the
bound phase was not the same among
compound classes. Differences in compound
class fate were a function of inherent
compound class reactivity (fatty acids > sterols
and PAHs) rather than source or depositional
regime. While compounds in bound phases
may be formed over time during organic
matter diagenesis, organic compounds did
not increase in bound residues over time
regardless of depositional regime,
suggesting that bound phase compounds are
formed within the source organism or very
rapidly upon cell death and/or deposition to
the sediments.

The fate of organic carbon in coastal
sediments is dependent upon the source and
reactivity of organic carbon, the depositional
regime, and its association with the
underlying sediment/macromolecular matrix.
Models of coastal carbon dynamics that
consider these parameters and how they
change will yield more accurate forecasts of
coastal biogeochemical cycling.