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core sediments; and (3) peaks in bulk density, magnetic sus-
ceptibility, and charcoal density found in both sets of cores.
One cubic centimeter of material was extracted from each
subsample from the primary set of cores (top core sediments
and deep core sediments) and processed to determine char-
coal abundance, magnetic susceptibility, and bulk density.
Drying preweighed sediments in a muf
quake that occurred sometime between 6474 and 6250 years
ago. This paleoearthquake would probably have occurred
along a still active fault located just to the east of Coburn
Lake within the Sierra Valley.
Visible layers of charcoal, organic detritus, and clay were
interspersed within a matrix of lake-derived, organic-rich
gyttja (Figure 2). The number of charcoal particles usually
peaked in association with peaks in organic detritus, inor-
ganic sediments (clay), sediment bulk density values, and
magnetic susceptibility values: all proxies for severe erosion.
As a result, charcoal from Coburn Lake had a very peaked
distribution suggesting episodic deposition over time. The
average lake sediment subsample over the entire 8500 year
span of the Coburn Lake charcoal record contained <10
macrocharcoal particles per cm
3
. However, long intervals of
time produced little or no charcoal, while relatively short
intervals of time produced very large amounts of charcoal
(Figure 2).
From 8400 to 6900 years ago, deposition was almost all
clay. A large and a small peak of charcoal were deposited
into the lake at 8120 and 7300 years ago, respectively. From
6900 to 6560 years ago, a lot of gyttja was deposited into
Coburn Lake. However, high rates of clay deposition oc-
curred from 6560 to 6000 years ago in association with a
small charcoal peak.
Clay deposition declined after 6000 years ago with depo-
sition being primarily gyttja from that time to the present.
Only four thin lenses of clay were visible between 6000 and
4600 years ago, two being associated with charcoal peaks.
No clay lenses were visible after 4600 years ago.
Charcoal was deposited continuously into Coburn Lake
from approximately 6000 to 5000 years ago, including the
deposition of four large charcoal peaks. Conversely, almost
no charcoal was deposited into Coburn Lake during the time
period from 5000 to 3100 years ago. Charcoal deposition
into Coburn Lake recommenced approximately 3100 years
ago, becoming massive over the past 1800 years. Four large
charcoal peaks from 3100 to 1100 years ago were associated
with visible lenses of detritus.
e oven overnight at
40°C, 450°C, and 900°C was used to determine percentages
of water, organic carbon, and calcium carbonate within sub-
samples, respectively, after accounting for the weight of
material lost.
Charcoal from Coburn Lake was processed using standard
procedures [Whitlock, 2001]. Samples were disaggregated in
5% KOH solution overnight and then washed through a
series of 250, 125, and 63
m sized sieves in order to
concentrate charcoal particles into progressively smaller sizes.
Charcoal particles of >250
μ
m size were counted within
every 1 cm
3
subsample using a 36
dissecting microscope.
I also counted 125
μ
m sized charcoal in a portion of
both the primary and secondary sets of core sediments to
check for differences in charcoal patterns within the two sets
of cores.
The Coburn Lake study was designed to develop a record
of local
-
250
μ
fires within the Coburn Lake watershed. Individual
charcoal particles of >125
m in size that can be seen with
the naked eye are referred to as macrocharcoal. Macrochar-
coal particles are thought to be heavy enough to travel only a
few hundred yards through the air before deposition [Clark,
1988; MacDonald et al.,1991;Clark and Royall,1996;
Clark et al., 2002]. I chose to focus on macrocharcoal parti-
cles of >250
μ
res in
order to further strengthen the relationship between particle
size and aerial distance traveled. However, stream or sheet
μ
m in size as my proxy for past local
flow can potentially carry charcoal of any size into the lake.
From this point onward, I will refer to macrocharcoal parti-
cles from Coburn Lake simply as charcoal. Charcoal num-
bers were not converted to charcoal accumulation rates
because the purpose of this study was to investigate individ-
ual charcoal peaks in relation to individual ACCEs.
5. DISCUSSION
4. RESULTS
The accuracy of the dating of the primary set of cores was
confirmed by comparing the two sets of cores as described in
section 3. Little difference in charcoal patterns resulted from
counting 125
5.1. Sierra Nevada Climate Records
5.1.1. Pyramid Lake drought record. The Coburn Lake
charcoal record was compared with the 7400 year long
Pyramid Lake drought record for the northern Sierra Nevada
[Mensing et al., 2004] (Figure 4). There was good agreement
between the timing of Coburn Lake charcoal peaks and the
beginnings of severe droughts in the Northern Sierra Nevada.
Charcoal deposition increased at the beginning of most of the
m sized charcoal parti-
cles within the same time intervals of both sets of cores.
A thrusting of older sediments over newer sediments was
visible in the cores in the
-
250
μ
m versus >250
μ
field. This overthrusting was
confirmed by radiocarbon dating (Figure 3, center right).
This overthrusting was potentially caused by a paleoearth-
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