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Journal of Geophysical Research - Atmospheres
Supporting Information for
Changes in atmospheric carbonyl sulfide over the last 54000 years inferred from measurements in Antarctic ice cores
M. Aydin 1 , J. E. Campbell 2 , T. J. Fudge 3 , K. M. Cuffey 4 , M. R. Nicewonger 1 , K. R.
Verhulst 1,5 , E. S. Saltzman 1
1 Department of Earth System Science, University of California, Irvine, California USA
2
3
Environmental Engineering, University of California, Merced, California USA
Department of Earth and Space Sciences, University of Washington, Seattle,
4
Washington USA
Department of Geology, University of California, Berkeley, California USA
5 Jet Propulsion Laboratory, California Institute of Technology, Pasadena,
California USA
Text S1 to S3
Figure S1
Figure S2
Table S1
This document contains: 1) a description of the Taylor Dome chronology used in the manuscript, 2) a summary of previous findings on the COS hydrolysis in ice cores and a description of the corrections made to the ice core measurements, and 3) a description of how the ice core thermal histories used in the corrections were determined. Two figures and one table are also included in this supporting document.
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There is considerably more uncertainty in the Taylor Dome chronology compared with the WAIS Divide one primarily because of the lower accumulation rate at the Taylor Dome site, particularly during the last glacial period and the last glacial/interglacial transition [Steig et al., 1998 and 2000]. Additional issues regarding the dating of gas measurement are highlighted in publications regarding the four different gas chronologies based on methane (CH
4
) and carbon dioxide
(CO
2
) measurements, with only the original CH
4
-based chronology covering the entire core (Fig. 7). The previously published Taylor Dome COS measurements
[Aydin et al., 2014] were all on the CO
2
-based chronology by Monnin et al. [2004].
Some of the Taylor Dome measurements presented in this paper are from the last glacial/interglacial transition and the last glacial period that cannot be dated with the chronology by Monnin et al. [2014].
Instead of using the CH
4
-based chronology for the entire data set, we develop a hybrid chronology, using the most recent age scale when available (Fig. S1). As a result, the measurements younger than and equal to 11.2 kyr are on the chronology by Monnin et al. [2004] and the measurements older than and equal to 31.5 kyr are on the chronology by Ahn et al. [2007]. There are four measurements between 31.5 kyr and 11.2 kyr. They are dated to 12.0, 13.0, 13.8, and 20.4 kyr using the chronology by Brook et al. [2000]. The exact dates at the COS measurement depths are determined by linear interpolation between the closest depths available in the chronologies.
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Detailed descriptions of the ice core COS hydrolysis and the correction methods developed to recover atmospheric COS records from the measurements can be found in an earlier publication by Aydin et al. [2014]. Briefly, ice core measurements in bubbly ice from multiple Antarctic sites provide strong evidence for in situ reactive loss of COS in ice cores and depletion of COS levels in ice core air
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62 bubbles. The hydrolysis of COS is temperature-dependent. When contemporaneous measurements from different sites are compared, we find lower COS in warmer ice cores. The observations can be adequately described by a first order rate equation:
d [ COS ] dt
k
[ COS ] (1) where the rate constant k is a function of temperature only: k ( T )
A
e
-
E a
R
×
T
(2)
In equation (2), the A (pre-exponential constant) and the E a
(activation energy) are the Arrhenius parameters that control how the reaction rate varies with temperature. Provided the temperature histories of the ice core samples are known, it is possible to correct the ice core measurements using equations (1) and (2) for any given set of Arrhenius parameters. Each measurement is corrected separately by tracking the ice core sample from the depth it was recovered to an estimated bubble lock-in depth, integrating equation (1) at each time step. The temperature histories for the ice cores are obtained from 1-D ice flow and heat flux models constrained by borehole temperature measurements and the temperature changes at the surface of the ice sheet as indicated by ice core water isotope measurements
(see section 3 below).
Aydin et al. [2014] used an objective search algorithm to identify the E a
and A pairs that minimize the difference between contemporaneous measurements from different sites when a hydrolysis correction is applied. They found that the range of
E a
values that best describe the COS measurements in bubbly ice cores from
Antarctica lie in 19 to 25 kcal mol -1 range, and the corresponding A values can be estimated from the relationship below: log A
=
0.864
E a
-
3.580
(3)
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This range of Arrhenius parameters imply that the lifetime (k -1 ) of COS at -30°C
(current annual mean surface temperature at the WAIS Divide) is on the order of
20000 years (Table S1).
In figures 2 and 6 of the manuscript, we correct the ice core measurements from the both the WDC-06A and the Taylor Dome M3C1 ice cores using E a
=22 kcal mol -1 and A=2.68x10
15 y -1 (from equation 3). The E a
of 22 kcal mol -1 was used because it is the mid-value of the 19 – 25 kcal mol -1 range. In figures 5 and 7, the corrections are shown for the full range of possible Arrhenius parameters, with E a
=
19, 22, and 25 kcal mol -1 and the corresponding A values calculated from equation
(3). The hydrolysis loss calculated using a range of Arrhenius parameters illustrate the uncertainty in the corrected COS records arising from the uncertainty in predicted reaction rates at a given temperature.
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The temperature histories of the ice core samples are essential for calculations of thermally-activated chemical processes such as the hydrolysis of COS.
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WDC-06A
The climatic temperature history at WAIS Divide has been reconstructed by coupled analysis of borehole temperatures, stable isotopes of ice, and nitrogen gas isotopes [Buizert et al., 2015]. In this analysis, the temperature history provides the surface boundary condition for calculations of temperature at depth in the ice sheet, using a 1-dimensional model that accounts for heat conduction and advection. Ice flow is constrained by the depth-age relationship, the history of accumulation rate, and the borehole temperatures. We have used this model to simulate the histories of depth and temperature for ice layers deposited at the ice sheet surface, from time of deposition to the present (Figure 4).
114 Taylor Dome M3C1
4

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The temperature histories for Taylor Dome are calculated similarly to those for WAIS Divide using a 1-dimensional model that accounts for heat conduction and advection. Because Taylor Dome is much thinner and receives less snowfall than
WAIS Divide, the surface temperature cannot be as well constrained by the borehole temperatures. As a result, the modern borehole temperatures can be matched by a range of combinations of isotope calibration factors and geothermal fluxes. To assess the uncertainty in the inferred thermal histories, we vary the isotope calibration factors within their estimated uncertainty and find the geothermal flux that yields the best match to borehole temperature profile. The recommended
Taylor Dome isotope calibration is 0.5‰ per ˚C with an uncertainty of 0.2‰ per ˚C
[Steig et al., 1998]. The geothermal flux that minimizes the misfit with the modern borehole temperature profiles for the different isotope calibrations of 0.3‰ per ˚C,
0.5‰ per ˚C, and 0.7‰ per ˚C are 63 mW m -2 , 65 mW m -2 , and 66 mW m -2 , respectively. We found that the corrections for COS loss to hydrolysis at Taylor
Dome are not sensitive to which set of these parameters are used (Fig. S2).
References
Ahn, J., and E. J. Brook (2007), Atmospheric CO
2
and climate from 65 to 30 ka B.P,
Geophys. Res. Lett., 34(10), L10703, doi:10.1029/2007GL029551.
Aydin, M., T. J. Fudge, K. R. Verhulst, M. R. Nicewonger, E. D. Waddington, and E. S.
Saltzman (2014), Carbonyl sulfide hydrolysis in Antarctic ice cores and an atmospheric history for the last 8000 years, J. Geophys. Res. Atmos., 119,  doi:10.1002/2014JD021618.
Buizert, C. et al. (2015), The WAIS Divide deep ice core WD2014 chronology - Part 1:
Methane synchronization (68-31 ka BP) and the gas age-ice age difference, Clim.
Past, 11, 153-173, doi:10.5194/cp-11-153-2015.
Brook, E. J., S. Harder, J. Severinghaus, E. J. Steig, and C. M. Sucher (2000), On the origin and timing of rapid changes in atmospheric methane during the last glacial period, Global Biogeochem. Cycles, 14(2), 559–572.
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Indermühle, A., E. Monnin, B. Stauffer, and T. F. Stocker (2000), Atmospheric CO
2 concentration from 60 to 20 kyr BP from the Taylor Dome ice core, Antarctica,
Geophys. Res. Lett., 27, 5, 735-738.
Monnin, E., E. J. Steig, U. Siegenthaler, K. Kawamura, J. Schwander, B. Stauffer, T. F.
Stocker, D. L. Morse, J.-M. Barnola, B. Bellier, D. Raynaud, and H. Fischer (2004),
Evidence for substantial accumulation rate variability in Antarctica during the
Holocene, through synchronization of CO
2
in the Taylor Dome, Dome C and DML ice cores, Earth Planet. Sci. Lett., 224, 45–54, doi:10.1016/j.epsl.2004.05.007.
Steig, E. J., D. L. Morse, E. D. Waddington, M. Stuiver, P. M. Grootes, P. A. Mayewski, M.
S. Twickler, and S. I. Whitlow (2000), Wisconsinan and Holocene climate history form and ice core at Taylor Dome, Western Ross Embayment, Antarctica,
Geografiska, 82 A, 2-3.
Steig, E. J., E. J. Brook, J. W. C. White, C. M. Sucher, M. L. Bender, S. J. Lehman, D. L.
Morse, E. D. Waddington, and G. D. Clow (1998), Synchronous climate changes in
Antarctica and the North Atlantic, Science, 282, 92-95.
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340 360
Depth (m)
380
Brook et al. [2000]
Monnin et al. [2004]
Ahn et al. [2007]
Indermuhle et al. [2000]
COS data
400
0
50 100 150 200 250 300 350 400 450
Figure S1. The different Taylor Dome gas chronologies and the chronology used for the COS measurements are shown for the last 50 kyr and (inset) for 35 – 7 kyr. The
CH
4
-based gas-age chronology (black) [Brook et al., 2000] has been updated by
Indermühle et al. [2000] (blue), by Monnin et al. [2004] (orange), and by Ahn et al.
[2007] (red), using CO
2
measurements-based synchronization with other Antarctic cores. For the Taylor Dome COS measurements (green), a hybrid age scale was used
(see supporting information). Most of the COS data are on the chronologies by Ahn
et al. [2007] or Monnin et al. [2004]. The only exceptions are the four measurements from 367, 372, 374, and 381 m that are dated to 12.0 kyr, 13.0 kyr, 13.8 kyr, and
20.4 kyr (inset) using the chronology by Brook et al. [2000].
7

Taylor Dome
Hydrolysis loss corrected (E a
=22 kcal mol
-1
)
0.5
o
C per mil, Q=65 mW m
-2
(shown in the manuscript)
0.3
o
C per mil, Q=63 mW m
-2
0.7
o
C per mil, Q=66 mW m
-2
500
400
300
200
100
162
0
55 50 45 40 35 30 25 20 15 10 5 0
Figure S2. The corrected Taylor Dome COS records (E a
=22 kcal mol -1 ) for different isotope calibrations factors and geothermal fluxes: 0.5 °C per ‰ with 65 mW m -2
(blue line), 0.3 °C per ‰ with 63 mW m -2 (red line), and 0.7 °C per ‰ with 66 mW m -2 (green line). Taylor Dome measurements are also shown (blue squares). All corrected Taylor Dome records shown in the main text of the manuscript are based on an isotope calibration factor of 0.5 °C per ‰ following Steig et al. [1998].
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Table S1. COS Lifetime (k -1 ) Estimates for Ice Core Hydrolysis at Selected
Temperatures (adapted from Aydin et al. [2014])
Temperature (K) Lifetime a (y) Lifetime b (y)
223.15
233.15
243.15
253.15
263.15
273.15 a E a
=19 kcal mol -1 and A=6.90x10
12 y -1 b E a
=25 kcal mol -1 and A=1.06x10
18 y -1
603,109
95,896
17,738
3,749
892
236
2,988,084
265,834
28,858
3,733
564
98
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