Geoscience Reference
In-Depth Information
Estimated Salinity (%)
AMS Dates
(cal B.P.)
12 16 20 24
79
110
157
580
235
970
Modern
Salinity
313
Depth
(inches)
2980
391
469
figure 24. Oxygen isotope
record from foraminifers from San
Francisco Bay sediments, show-
ing a shit to higher bay salinity (in
parts per thousand) starting about
1,800 years ago and peaking 600
years ago. (Data from Schweikhardt,
Sloan, and Ingram, 2008.)
3170
6510
547
6700
625
-6 -5 -4 -3 -2
δ
18
O
(‰ PDB)
foram
Further evidence from San Francisco Bay sediment cores has been found
for the major shit to a drier climate in California. h e oxygen isotopic ratios
(oxygen-18/oxygen-16) of calcium carbonate shells of microscopic, single-
celled organisms (foraminifera) vary with salinity in the bay, as described in
chapters 5 and 8. A research group that included Peter Schweikhardt and one
of this topic's authors, B. Lynn Ingram, retrieved sediment cores from the
central part of San Francisco Bay near Point Richmond. Along with micro-
paleontologist Doris Sloan, we chose one specii c species of foraminifera that
faithfully recorded salinity of the bay waters and was abundant over the past
several thousand years,
Elphidium excavatum.
We carefully separated twenty
to thirty of the tiny fossilized shells from successive layers in the core for
oxygen isotope analysis on the mass spectrometer in our laboratory. When
plotted against the shells' depths and ages in the core, our measurements
revealed that salinity in the bay began to shit from less saline waters to more
saline conditions about 1,800 years ago, peaking in salinity about 600 years
ago. h ese results corroborate those from Suisun Marsh—that dry condi-
tions in California persisted for at least a thousand years (see i gure 24).
