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higher salinity—such as saltgrass (
Distichlis spicata
) and cordgrass (
Spartina
foliosa
), the so-called Chenopodiaceae—relative to those adapted to fresher
conditions, like tules, cattails, and rushes, to assess the salinity conditions in
past periods when the plants were growing. We knew that the marsh vegeta-
tion responds quickly to the changing salinity of freshwater inl ow.
Our research project also involved the measurement of carbon isotopes
from partially decomposed plant remains in the marsh cores as another way
of deciphering changes in marsh vegetation. Nearly all carbon atoms have
an atomic mass of 12 (carbon-12), but a small number of them have a mass of
13 (carbon-13). During times of drought, Suisun Bay would have been a saltier
place, because less freshwater would have entered from the river. With higher
salinity, the two salt-hardy marsh species, saltgrass and cordgrass, increased
in dominance. Both of these species have higher proportions of carbon-13
relative to carbon-12, so their remains in the sediments could be detected
using a mass spectrometer to measure the relative proportions of carbon
isotopes.
Another important element of the research project included a third proxy
tool: diatoms. Diatoms are microscopic phytoplankton that live in all aquatic
environments, including marshes, and secrete a minuscule silica frustrule.
Each diatom species produces a uniquely shaped frustrule, and these accu-
mulate in the marsh sediments over time. Some species of diatoms prefer a
more freshwater environment, whereas others prefer a more saline environ-
ment. By looking at the relative proportions of these diatoms in the sediment
cores, our collaborator, paleontologist Scott Starratt at the U.S. Geological
Survey, was able to provide an independent measure of water salinity in the
marsh over time.
We dated the Suisun marsh core sediments using radiocarbon (carbon-14)
measurements on the seeds of marsh plants separated from various levels of
the core. h is allowed us to establish a chronology delineating the vegetation
changes detected through the pollen, diatom, and isotope analyses.
h e results were compelling. h ey revealed that the salinity of the marsh
increased (meaning the average freshwater inl ow to San Francisco Bay was
reduced) by close to 40 percent above today's levels for a thousand years,
between 1,750 and 750 years ago. h e peak of this salty, low-inl ow interval
occurred approximately 1,200 to 900 years ago, during the early part of the
Medieval drought. Only ver y low precipitation and runof over San Francisco
Bay's vast watershed, covering almost half the area of California, could
explain these results (see i gure 23).
