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toward the sun, and, during the Northern Hemisphere summer, the earth
was slightly closer to the sun, resulting in an increase in solar radiation dur-
ing that season. h ese changes in the earth's orbit over time are known as
the Milankovitch Cycles. We describe these cycles and their inl uence on the
earth's climate in more depth in chapter 11.
Pollen evidence of vegetation change suggests that the transition from the
late Pleistocene to the early Holocene brought warming and drying to the
American West, concurring with the Great Basin lake-level evidence. Just as
the massive lakes of the Great Basin were shrinking, Sierra lakes were form-
ing in California: the massive alpine glaciers had scoured out the bedrock,
and their moraines later dammed these carved valleys, forming chains of
mountain lakes. Freshwater, supplied by glaciers melting in the higher eleva-
tions, fed the mountain lakes even as overall conditions were drying.
Forests usually colonize the mountainsides in the wake of retreating gla-
ciers. Natural vegetation zones exist along the steep mountain slopes, tied
to elevation and climate. In the higher elevations within the forested slopes,
pine trees dominate, even throughout major climate shit s. Paleoclimate
researchers look for particularly sensitive locations at mid and low elevations,
where meadows, marshes, and lakes are found. h ese sensitive environments
contain sedimentary archives that record the regional environmental history.
Relatively small changes in climate—such as a slight warming or a change
in ef ective moisture availability—af ect the local plant assemblages as the
vegetation shit s in response to the new conditions. h e forests surrounding
small mountain lakes or meadows produce pollen and other macrofossils that
are blown or washed into the lakes, to be incorporated into the slowly accu-
mulating sediments on the lake bottom. Researchers can derive a series of
snapshots from the sediments that rel ect changes in the local l ora over time.
One such lake, called Lake Moran, has provided a wealth of paleoclimatic
information about the transition from the late Pleistocene to the Holocene.
Moran is a small lake set in a mid-elevation forest, a bit over 6,600 feet, on
the western slopes of the Sierra Nevada. It is the type of setting desired by
paleoclimate researchers: set well into the forest away from any main roads,
the lake is like a jewel surrounded by sugar pines, ponderosa pines, and i r
trees. h e lake itself drains a relatively small area (about thirty acres) with no
inlet stream, so that climate researchers infer that the pollen, charcoal, and
other fossil evidence found in the sediments are derived locally. Since the late
1980s, Roger Byrne, a paleoclimatologist and pollen expert in the Geography
Department at the University of California, Berkeley, has used fossilized
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