Geoscience Reference
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mussel shells growing in parts of the bay of known salinity and were able to
show that the oxygen isotopes of these shells rel ected the salinity of the bay.
h ese calibration studies were done to support the analyses of sediment
cores taken by our research group from the central bay (see i gure 21). h e
cores provide a record that extends back in time more than 5,000 years.
Once collected from the bay, the cores were brought to the laboratory, where
fossilized shells were separated in sequence from the sediments every two
centimeters (three-quarters of an inch). h e shells were processed, including
cleaning to avoid contamination from the surrounding sediment and pow-
dering to homogenize the samples. A tiny amount of this powder was then
analyzed using a mass spectrometer to measure the proportions of oxygen-18
relative to oxygen-16.
h e results of these measurements showed that the San Francisco Bay
estuary was much fresher during the Neoglacial period. h e decrease in salin-
ity suggests that the average amount of river inl ow was twice as high as the
modern average inl ow. h e l ows peaked between about 4,300 and 3,800
years ago, and average river l ows remained relatively high for the following
2,000 years.
We were able to apply newly developed geochemical methods for assessing
river inl ows and salinity in San Francisco Bay to the shell mound collec-
tions. Our collaborators, archaeologists Kent Lightfoot at the University
of California, Berkeley, and Ed Luby of San Francisco State University,
have spent much of their careers researching the purpose of these mounds,
including when and why they were occupied. h ey provided access to shell
mound materials housed in the Hearst Museum of Anthropology on the UC
Berkeley campus. h e animal remains in some Bay Area mounds had been
investigated to assess the season of occupation by analyzing the proportion of
migratory species that were hunted by the native populations. For instance,
more than half the bird remains in a mound from southern Marin County
were migratory species that were present only during the fall and winter.
Two large mounds—the West Berkeley and Emeryville shell mounds on the
eastern shore—contained abundant chinook salmon remains, suggesting an
occupation during the spring and fall migratory periods.
Our geochemical methods would allow us to assess the season of occupa-
tion in other mounds that did not contain these types of faunal remains and
thus could potentially be applied to a larger number of mounds throughout
the Bay Area. We decided to test our methods on two mounds located along
the eastern shore of the central part of San Francisco Bay: Ellis Landing, a
