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A large amount of charcoal was deposited into Coburn Lake
approximately 3000 years ago (Figure 4). Though charcoal
deposition into Coburn Lake was often associated with the
beginning of severe droughts in the Sierra Nevada, I am not
aware of evidence for a severe drought in the Sierra Nevada
region beginning at approximately 3000 years ago [Benson et
al.,2002;Barron et al., 2003]. Unfortunately, the Pyramid
Lake record [Mensing et al., 2004] is missing information on
precipitation in the northern Sierra Nevada covering the period
from approximately 3400 to 2750 years ago (Figure 4). Still,
3000 years ago was a time of great climatic change. A severe
drought ended in Greenland (see Figure 5 and section 5.1.3,
below) then, and one of the three largest reductions in North
Atlantic MOC occurred 3000 years ago [Oppo et al., 2003].
On the other hand, the Coburn Lake charcoal record was
strongly correlated with the onset of severe droughts in the
Sierra Nevada over the past 2200 years. Increases in charcoal
into Coburn Lake coincided with the beginnings of 12 of the
12 severe droughts that occurred in the Northern Sierra
Nevada over the past approximately 2200 years, and in-
creases in charcoal did not occur except at the beginnings of
those 12 severe droughts (Figure 6) [Wathen, 2011].
Figure 7. Environmental history of the Sierra Nevada over the past
2000 years based upon the integration of three high-resolution
paleorecords. (a) A northern Sierra Nevada drought record from
Pyramid Lake, Nevada [Mensing et al., 2004], based upon the ratio
of Artemisia (sagebrush) to Chenopodiaceae (shadscale) (A/C) pol-
len types. Horizontal dashed lines denote the beginnings of
droughts, which are numbered from top to bottom, from youngest
to oldest. The bold vertical solid line denotes the median A/C ratio
value for the past 7400 years. (b) A southern Sierra Nevada tem-
perature record based upon the analyses of foxtail pine tree ring
chronologies that grew near tree line on Cirque Peak, California.
Colder temperature anomalies are to the left, and warmer tempera-
ture anomalies are to the right of zero; the mean is from A.D. 1 to
1980 [Scuderi, 1993]. (c) The abundance of charcoal particles of
>250 μ m in size per cm 3 within Coburn Lake sediments. The ver-
tical dashed line designates charcoal abundances of >50 particles
cm 3 . The two largest Coburn Lake charcoal peaks with >1000
charcoal particles g 3 were truncated to fit the column size. Values
to the right of Figure 7c denote temperatures, or abrupt changes in
temperature, at tree line on Cirque Peak at the beginning of severe
droughts in the northern Sierra Nevada. C is cold; H is hot.
5.1.2. Cirque Peak temperature record. The beginnings of
severe droughts in the northern Sierra Nevada and the
deposition of charcoal peaks into Coburn Lake coincided
with extreme high temperatures, or an abrupt transition to
higher temperatures, in the southern Sierra Nevada over the
past 2000 years [Scuderi, 1993; Wathen, 2011] (Figure 7).
As a rule, either high temperatures or an abrupt transition to
higher temperatures occurred at the beginnings of severe
droughts, while cold temperatures or abrupt changes in
temperature running in either direction occurred at the
beginnings of pluvial periods. Of the beginnings of 13 of
the most severe droughts over the past 1800 years, only a
drought that began approximately 1600 years ago was not
associated with high temperatures. In addition, severe
res
appear to have occurred at Coburn Lake at the beginning of
every severe drought-hot temperature event over the past
1800 years.
Taking into consideration all of these relationships, I pro-
pose an abrupt climate change-severe fire and erosion hy-
pothesis. This hypothesis argues that the abrupt onset of
severe droughts and extreme temperatures during the late
Holocene caused vegetation and mountain slopes in some
ecosystems to be out of balance with new, abruptly changed,
climates. This severe imbalance would have resulted in
widespread forest die-off, followed by stand-replacing fires
and severe soil erosion [Wathen, 2011].
5.1.3. Abrupt climate change-severe fire and erosion
hypothesis. High temperatures at the beginning of severe
droughts would have put additional stress on vegetation adapt-
ed to predrought climatic conditions. As mentioned above in
section 4, charcoal peaks from Coburn Lake were usually
associated with indicators of severe soil erosion: peaks in orga-
nic detritus, inorganic sediments (clay), sediment bulk density
values, and magnetic susceptibility (Figure 2). This close asso-
ciation with severe soil erosion suggests that charcoal peaks
from Coburn Lake represent severe stand-replacing
5.2. Greenland Precipitation and Temperature Records
Some colleagues and I statistically compared the entire
8500 year long Coburn Lake charcoal record with an ice
res.
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