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accumulation-precipitation record from central Greenland
[Meese et al., 1994; NSIDC and WDC-A, 1997]. Boucher et
al. [2006] developed a logistics regression model to test for
signi
cant correlation between the timing of the deposition
of the seven largest charcoal peaks deposited into the Coburn
Lake over the past 8500 years and the timing of peak drought
conditions in central Greenland (Greenland Ice Sheet Project
2 (GISP2) ice cores) over that same time period.
Figure 9. Distribution of charcoal peaks from Coburn Lake com-
pared with a precipitation record from Greenland for the past 2000
years. (a) Abundance of charcoal particles (>250 μ m in size) per
cm 3 of Coburn Lake sediment. The two largest Coburn Lake char-
coal peaks (>1000 charcoal g 3 ) were truncated to fit the column
size. Dashed horizontal lines and arrows in column represent the
centers of major charcoal peaks. (b) A reconstructed Holocene
North Atlantic snow accumulation record from the central Green-
land GISP2 ice core [Meese et al., 1994]. Arrows with numbers
represent peak drought conditions in Greenland.
A statistical model was created where (1) x was the pre-
dictor variable, ice accumulation; (2) π was the probability
that a value of the charcoal time series will be categorized as
a peak; and (3) β o and β 1 were regression coefficients:
Figure 8. Statistical analysis of the relationship between the timing
of the deposition of charcoal peaks from Coburn Lake and peak
drought conditions in central Greenland. (a) A reconstructed Holo-
cene North Atlantic snow accumulation record from the central
Greenland GISP2 ice core [Meese et al., 1994]. (b) The number of
charcoal particles >250
¼ β 0 þ β 1 x :
π
1 π
log
m in size per sediment sample from
Coburn Lake. (c) Mean precipitation values within 200 year-wide
windows around the seven largest charcoal peaks from Coburn Lake
(black horizontal bars) and around 21 randomly chosen dates (gray
horizontal bars). (e) Magnitude of precipitation differences within
the same 200 year-wide windows. Bold bars refer to values for the
seven largest charcoal peaks from Coburn Lake. Faint bars refer to
values for 21 randomly chosen points within the time series. Statis-
tical results from testing the null hypotheses that there are no statis-
tical differences between the sampling windows aligned with
Coburn Lake charcoal peaks and randomly placed windows, in terms
of either (d) mean precipitation values within windows or (f ) the
magnitudes of variation within windows, adapted from Boucher
et al. [2006]. Time is displayed on the x axes. The 14 C-AMS dates
are converted to calibrated years before A.D. 1950 (cal years B.P.).
μ
A 200 year window was centered on each of the dates of
the seven charcoal peaks and superimposed onto the
Greenland ice accumulation time series, such that the win-
dows would not overlap with other windows (Figure 8).
Two hundred year windows allowed for 100 years of
variability between the timing of charcoal peaks in Coburn
Lake and the timing of peak drought conditions in central
Greenland.
Using this model, the seven largest charcoal peaks were
compared with 21 randomly chosen noncharcoal peak years.
Probabilities were calculated such that the values Pr(>z value)
= 0.1 and 0.05 refer to the statistical correlation between
charcoal peaks fromCoburn Lake and peak drought conditions
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