Geoscience Reference
In-Depth Information
Sediments that have accumulated beneath San Francisco Bay also contain
evidence for prolonged wetness and occasional extreme events during that
period. Variations in freshwater inl ow alter the bay's salinity over periods
of decades to centuries and af ect the organisms living in the estuary—from
single-celled plankton and foraminifera to mollusks (oysters, clams, and
mussels). h e assemblages of species living in the bay adjust in response to
changes in river inl ow: during higher river inl ow, species that prefer lower
salinity predominate, whereas during periods of decreased inl ow, such as
would occur during drought, species that prefer higher salinity conditions
predominate. When these organisms die, their hard shell remains settle to
the l oor of the bay and are entombed in the sediments brought in by creeks
and rivers. h ese accumulated sediments provide a source of environmental
information for paleoclimatologists.
In addition to the identii cation and analyses of bay fossils, the geochem-
istry of these shells rel ects the environmental conditions of the bay water,
particularly the salinity and temperature. Such factors in turn rel ect changes
in the precipitation regime over the bay's vast watershed that covers about
half of the state of California today (see i gure 21). h is means that the chem-
istry of the bay water can inform us about climate over a very large region.
As shelli sh grow and secrete their calcium carbonate shell, they incorporate
elements from the water surrounding them, including oxygen. Not all water
is the same. Tiny variations in the mass of oxygen atoms (actually, isotopes)
have been exploited by geochemists to provide information about past envi-
ronments. River water entering San Francisco Bay through the delta is lighter
than the Pacii c Ocean water that enters the bay through the Golden Gate in
that it has a greater proportion of oxygen-16. h e dif erences in the amounts
of fresh- and saltwater in the bay determine its salinity. h e amount of river
water l owing into the bay varies with the seasons, with more river water
l owing during the wetter winter months. During years and decades marked
by higher rainfall, the average salinity in the bay is lower than it is during
years and decades of drought.
During the late 1980s and early 1990s, a research group including this
topic's authors, in collaboration with other scientists researching San
Francisco Bay at that time, collected and analyzed samples of bay waters dur-
ing dif erent seasons over several years, measuring the salinity and oxygen iso-
topes of these waters. h e samples provided evidence that both the seasonal
and the annual average salinity l uctuations were rel ected in the propor-
tions of oxygen-16 and oxygen-18 in the bay water. We then collected modern
