Geoscience Reference
In-Depth Information
large shell mound close to present-day Richmond, and a small mound located
on a tiny nearby island (Brooks Island).
A graduate student working in our group, Peter Schweikhardt (now
a professor at the College of Alameda), developed a novel way of using
fossilized shell fragments to determine the season of harvest. h e results
showed that shells from the Ellis Landing mound were harvested mainly
during the spring and fall seasons. Some appeared to have been harvested
during the summer, but none were harvested during the winter. h is suggests
that these mounds were used seasonally during the spring and fall but were
largely abandoned during the summer and winter. h e smaller mound from
Brooks Island also showed seasonal occupation during the fall to early win-
ter, though this mound appeared to have been used more during the summer
as well, starting in July. h e native populations apparently followed the water
and other resources in the region throughout the year as the weather shit ed
between the cold, wet winter season and the warm, dry summer season. As
we shall see in later chapters, this mobility served them well when the climate
again shit ed to drier, warmer conditions.
a more variable climate
h e oxygen isotopic measurements from the San Francisco Bay sediment
cores also show that the abundance of freshwater was part of an overall pat-
tern of climate l uctuations between wetter and drier conditions occurring
about every 1,500 years. h is climate cycle has appeared elsewhere in the
West. For instance, researchers studying Tulare Lake have determined that
the lake experienced seven or eight high stands over the past 11,500 years,
spaced approximately 1,500 years apart, and coastal sea surface temperatures
reconstructed from oxygen isotopic measurements of foraminifera from
Santa Barbara Basin sediment cores show a 1,500-year cycle, as we discussed
near the end of chapter 6.
In the bay sediment cores, we also detected l uctuations that were smaller
and more frequent, with 200-, 90-, and 55-year periods. We will return to
these cycles in later chapters, since they appeared in a number of paleoclimate
records in the West, as well as globally, and may be related to sunspot cycles
and associated changes in solar radiation.
Relative to the early and mid-Holocene, the climate of the late Holocene
period—beginning about 3,500 years ago—exhibited larger and more rapid
