Geoscience Reference
In-Depth Information
the annual march of the seasons brings changes in overall salinity as well,
particularly in this region marked by wet winters and dry summers.
h ese annual cycles are superimposed on the year-to-year variability in
salinity, rel ecting variations in climate patterns. A drought in the water-
shed, if prolonged and severe, can cause higher salinity downstream in
the estuary as the inl ow of freshwater drops. In response, salt-tolerant
species in the marshes expand further inland toward the delta, and the fresh-
water species retreat. Conversely, unusually wet winters generate fresher
conditions in the estuary, leading to an expansion of freshwater-adapted
species.
On the basis of this relationship between rainfall and wetland species in
the estuary marshes, Roger Byrne, a Geography professor at the University
of California, Berkeley, reasoned that a history of marsh plant assemblages
could be used as a proxy record of past climate conditions, particularly for
droughts and wetter periods experienced over the whole watershed. Such
a history of vegetation could be produced from the evidence buried in the
marsh sediments, which have accumulated abundant organic plant remains
over thousands of years. Byrne invited the research group of this topic's
authors to collaborate on this ambitious project, and together we cored
marshes along the edges of San Francisco Bay across a range of salinities
between the delta and the Golden Gate, with the goal of assessing past
climate regimes over California as recorded by the marsh ecosystems (see
i gure 21 in the preceding chapter).
A number of proxy climate indicators were used in the project. For
instance, pollen from the marsh sediment cores was used to assess past veg-
etation change. h is pollen was extracted from the sediments, in addition to
seeds and other vascular plant remains that had been deposited on the marsh
surface over the millennia. Snapshots of the ancient plant assemblages were
created from each layer, providing clues about the climate.
Suisun Marsh, located midway between the delta and the Golden Gate,
provides a particularly sensitive record of climate change. By AD 1850, the
wetlands surrounding Suisun Bay represented the most extensive area of
brackish marsh on the west coast of North America. h ese marshes have
experienced rapid losses since that time, as over 90 percent of Suisun Marsh
has been i lled for urban development and agriculture. Our study site in the
northern part of Suisun Bay, called Rush Ranch, was a small, relatively pris-
tine relict of the Suisun tidal marsh. By assessing the pollen remains in the
sediment cores, we could determine the percentage of marsh plants adapted to
