Geoscience Reference
In-Depth Information
millennia, in the low spots on Earth—such as at the bottom of lakes, bogs,
marshes, estuaries, and the ocean—and these layers create what can be read
as the earth's history topics. Fossilized remains of plants and animals can give
us clues about the prevailing environmental conditions and climate during
the time the organisms were living. h e nature of the sediments themselves,
including the size, shape, and mineral composition of the particles, how they
are layered, their color, even their odor, provide important clues that help
to determine past environmental and climatic conditions. Earth scientists
have used these methods for at least two centuries. More recently, over the
past few decades, new methods have been developed using the chemistry
and isotopic composition of the sediments to extract increasingly detailed
environmental and climatic information from them.
Sediment cores have been retrieved from up and down the Pacii c Coast
of North America—beneath the coastal ocean, from estuaries and tidal
marshes, and inland from lakes, marshes, and other settings. One of the most
closely examined coastal regions in the world is the Santa Barbara Basin, just
of the coast of Santa Barbara in Southern California. Researchers l ock to
this place to study cores of sediments that have accumulated on the sea l oor
over many millennia.
Initially, relatively short sediment cores, only a few meters, were taken
from the basin, spanning hundreds to a couple of thousand years. h ese
early studies showed that the sediments were laid down season at er season
in distinct annual layers, or “varves.” h is basin is ideal for preserving the
bottom sediments, partly because the oxygen content of the ocean water
in the bottom waters of the Santa Barbara Channel, unlike most marine
settings, is extremely low. h e living conditions there are inhospitable to
marine life, so bottom-dwelling marine organisms cannot survive. Normally,
these animals—worms, clams, and snails—churn and burrow through
bottom sediments in search of food and shelter, stirring up the sediments and
destroying detailed sedimentary layering.
In the Santa Barbara Basin, however, these animals are absent, and the
sediment layers accumulate undisturbed, year at er year, for many thousands
of years. Each year, two layers of sediment are deposited: a light layer during
the spring/summer, and a dark layer in the winter. Sediments that erode of
the land are washed into the basin by the winter rains and form the dark
layer. h e light layer is composed of the remains of diatoms—microscopic
ocean plants composed of silica—produced during periods of high primary
productivity, or blooms, during the spring and early summer. Reminiscent
