Geoscience Reference
In-Depth Information
in the lake or constitute the shells of organisms that live in the lake. h ese
minerals and shells record the relative amounts of the two oxygen isotopes
(wh ich have sl ig ht ly d i f erent masses that can be measured in the laboratory
on a mass spectrometer) as they form and settle to the bottom of the lake,
where they accumulate and preserve this information. During dry periods,
more water evaporates of the lake surface than is replenished by river inl ow.
Evaporation causes the lake water to become more enriched in oxygen's
heavier isotope (oxygen-18). During wetter periods, rain and runof into the
lake are greater than evaporation, so the lake grows larger, and the lake water
becomes more enriched in the lighter oxygen isotope (oxygen-16).
Estuaries
Estuaries, where rivers draining the land meet the sea, also contain rich
records of past climatic and environmental change. For instance, the San
Francisco Bay estuary has formed where the rivers draining the Sierra Nevada
merge and l ow together, mixing with marine waters from the Pacii c Ocean.
h e volume of this freshwater inl ow, rel ecting precipitation and runof
in the bay's watershed, alter the bay's salinity. h e salinity changes in turn
alter the types of organisms living in the estuary—from single-celled plank-
ton and foraminifera to invertebrate organisms like clams and mussels. When
these organisms die, they settle to the bottom, their hard parts becoming
entombed in the sediments, later to be brought to the surface and examined
by geologists and paleontologists.
Paleontologists analyze the assemblages of fossil remains and how these
have changed over time to assess changes in the estuarine environment. h e
geochemistry of these fossil shells also rel ects the environmental condi-
tions of the bay water, particularly the salinity and temperature. As these
organisms grow and secrete a calcium carbonate shell, they incorporate
elements from the water surrounding them, including oxygen. Tiny varia-
tions in the mass of these oxygen atoms can be used to provide information
about past environments. River water entering San Francisco Bay through
the delta is lighter than Pacii c Ocean water that enters the bay through the
Golden Gate, meaning it has a higher proportion of oxygen-16 compared to
oxygen-18. By analyzing fossil shells separated from cores taken beneath the
bay from a range of ages, a time series of past salinity can be determined,
providing a long history of climate over the entire watershed draining into
the estuary.
