Geoscience Reference
In-Depth Information
lake remained relatively small until 3,600 years ago, when moister climate
conditions returned to the region.
Clearly, a large area in the American West was af ected by a hotter, drier
climate during the mid-Holocene. h is climate also altered the vegetation:
fossilized pollen preserved in marsh and lake sediments at many locations
throughout southern Oregon and extending into the Great Basin and California
show shit ing plant assemblages. In the mountains, drought-tolerant species,
including grasses and shrubs, replaced forests and other densely vegetated areas,
and oaks moved to elevations once dominated by pines.
Vegetation cha nge wa s a lso seen in coa sta l reg ions. In the va st Sacra mento-
San Joaquin Delta, a freshwater marsh plant community gave way to more
salt-tolerant species (saltgrass, pickleweed, and certain types of rushes) as
river l ows diminished. Along the Southern California coast, including Santa
Barbara and the Santa Barbara Channel Islands, drought-adapted marsh
plants came to dominate the tidal wetlands.
the long (and warm) drought
h is period of prolonged mid-Holocene dryness was coined the “Long
Drought” in the late 1930s by Swedish sedimentologist Ernst Antevs, who
documented that Lake Summer in south-central Oregon (see location in i g-
ure 18) had completely desiccated between 7,500 and 4,000 years ago. h is
drought in the American West occurred during what is now called the mid-
Holocene “climatic optimum”—a period of unusual global warmth. In the
early twentieth century, when Antevs was assembling paleoclimatic data,
he named this period the “Altithermal.” Antevs's research showed that rela-
tively high temperatures, estimated to be 2-9°C (3.5-16°F) warmer than they
are today, prevailed during this time in the high latitudes of the Northern
Hemisphere. Later studies showed that temperatures at lower latitudes rose
less dramatically, to perhaps 1°C (1.8°F) warmer than at present. Antevs's
research also suggested that this mid-Holocene period of warm and dry
climate was followed by a cool, wet period from about 4,000 to 2,000 years
ago, a period we now refer to as the Neoglacial (discussed in the next chapter).
More compelling evidence for increased warmth during the mid-Holocene
comes from studies of bristlecone pines in California's White Mountains
at their upper limits in the mountains (the tree line, which represents the
elevation above which trees cannot survive the cold, inhospitable conditions).
