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Fig. 1.
Dome C CO2 (yellow triangles), CH4 (blue diamonds), N2O (red circles) and deuterium
(top trace) [Jouzel et al., 2001] records for the Holocene time period.
The CO2, N2O and, CH4 measurements are plotted together with their analytical
reproducibility (1s). Smoothed splines were calculated according to Enting [1987]
with cut off periods of 3000 yr for CO2, 3000 yr (solid line) and 1500 yr (dashed line)
for N2O and 1500 yr for CH4, highlighting long-term trends of the three greenhouse gases.
For CO2 and N2O it is believed that the splines with cut off periods of 3000 yr represent
the long time trends of the atmospheric concentration. The CH4 spline was calculated with
a smaller cut off period (1500 yr) due to a much shorter lifetime of CH4 compared to
CO2 and N2O. N2O variations as indicated by the spline with a cut off period of 1500
yr are at the limit of significance given the analytical uncertainty of the data.
The time scale for the ice and the enclosed air (which is younger than the surrounding ice
because it is enclosed at the bottom of the firn layer) in years before 1950,
is based on the time scale by Schwander et al. [2001]. The uncertainty of the
absolute time scale for the ice is estimated to ±200 yr. The gas-ice age difference
is about 2000 yr with an estimated uncertainty of about 10%.
As all three gas records were measured on the air bubbles of the same core,
there is no uncertainty in the relative timing of the gas records.
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Fig. 2.
Comparison of the Dome C long term trends of CO2 (top trace, yellow line),
CH4 (middle trace, blue line), and N2O (bottom trace, red line) from Fig. 1
with previously published CO2 and CH4 data and published source distribution
calculations for CH4. The disagreement between the CO2 data measured along
the Taylor Dome ice core, Antarctica (black diamonds, plotted on the original
Taylor Dome time scale) [Indermühle et al., 1999] and the Dome C CO2
trend in the time period 4 to 8 kyr BP can probably be attributed to differences
in the time scales of the two ice cores [Stauffer et al., submitted, 2001].
For CH4 the Dome C trend agrees well with CH4 results from the Antarctic
ice cores D47 (black triangles) and Byrd (black circles), as well as with results
from the GRIP ice core, Greenland (grey circles) when taking the interpolar
difference into account [Chappellaz et al., 1997]. However, the used
timescales are not synchronized. The GRIP, D47, and Byrd results are
shown on a synchronized GRIP time scale [Chappellaz et al., 1997;
Schwander et al., 1997], while the Dome C data are plotted on the
original Dome C time scale [Schwander et al., 2001]. Additionally to the
long term trends of CO2, CH4 and N2O, previously published source
distribution calculations deduced from the interpolar gradient of CH4 are
shown for the periods 2.5-5 kyr BP, 5-7 kyr BP, and 9.5-11.5 kyr BP
[Chappellaz et al., 1997]. Black bars represent the tropical source
(30°S to 30°N), grey bars the source of the mid to high northern latitudes
(30°N to 90°N). The source of the mid to high southern latitudes
(30°S to 90°S) was assumed to be constant over the Holocene
time period (15 Tgyr-1) and is, therefore, not shown in the figure.
Comparison of the long term trends of CH4 and N2O to the CH4
source distributions for the above mentioned time periods show that
CH4 was mainly controlled by drying of the tropics while N2O seems
more related to the mid to high northern latitudes. Not shown are the
source distribution calculations for the time period 0.25-1 kyr BP because
tentative results deduced from the gradient between the Dome C and the
GRIP CH4 measurements indicate a different source distribution
than found by Chappellaz et al. [1997].
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Fig. 3.
Dome C N2O data compared to non-sea-salt calcium (nss-Ca2+) and nitrate (NO3-)
measured along the same core for two depth interval of rapid shifts in the N2O
concentration. The two depth intervals 165 to 210 m (a) and 355 to 400 m (b),
centered around 3 and 10 kyr BP, respectively, on the gas age scale are marked
as grey shaded areas in the N2O overview (c). nss-Ca2+ is shown in (a) and (b) as
2 cm mean (middle trace, grey line) and 55 cm mean (middle trace, green line).
NO3- was only measured in the deeper interval (b) and is plotted as 10 cm mean
(top trace, grey line) and 55 cm mean (top trace, blue line). The N2O results
(red circles) are complemented by the mean values over the corresponding
depth interval (grey dashed line). Neither non-sea-salt calcium (nss-Ca2+) nor
NO3- show an abrupt shift in the same depth interval as the N2O record.
It is, therefore, unlikely that the shifts in the N2O record are due to an
artefact produced by chemical compounds.
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