Guest Post by David Middleton
In my first guest post (CO2: Ice Cores vs. Plant Stomata), we discussed the merits of ice cores vs. plant stomata as paleo-CO2 measurements. One of the key stomata papers I cited was Thomas van Hoof’s “Atmospheric CO2 during the 13th century AD: reconciliation of data from ice core measurements and stomatal frequency analysis.” Van Hoof and his coauthors demonstrated that the Antarctic ice cores only reflected the low-frequency component of the CO2 “signal”…
It is well known that diffusion processes within the firn layer and the gradual enclosure of the air in the lock-in-zone of the ice lead to a reduced signal of the original atmospheric variability and may obscure high-frequency variations (e.g. Trudinger et al., 2003).
This “diffusion process” is primarily a function of snow accumulation rate. The higher the accumulation rate, the less diffusion and the higher frequency resolution. Compaction effects due to burial can also add to the diffusion process. Dr. Van Hoof’s paper presented strong correlative evidence that the plant stomata chronologies were capable of recording a much higher frequency CO2 signal than the ice cores. I was curious about whether or not any evidence of the diffusion effect on CO2 resolution could be identified in the ice cores through the correlation of CO2 mixing ratios and snow accumulation rates. NOAA’s paleoclimatology library does not include any accumulation rate data for Antarctic ice cores with published CO2 chronologies; but the accumulation rate can by approximated by calculating a sample rate.
I used data from two Antarctic ice cores (Law and Taylor Domes) over most of the Holocene (11 kya to the early 20th century) to compare the sample rate to the CO2 mixing ratio. The sample rates were calculated by simply dividing the sample depth interval by the ice age interval:
sr = [(zn – zn+1) / (tn – tn+1)]
Where sr = sample rate (m/yr), z = sample depth (m) and t = ice age (yr)
I then plotted the CO2 mixing ratio against the sample rate. Not surprisingly, there is an extremely strong correlation between the sample rate and the CO2 mixing ratio throughout the Holocene…
This makes it very clear that the low CO2 values in the Antarctic ice cores during the Holocene could easily be the result of diffusion and do not constitute valid evidence of a stable pre-industrial atmospheric CO2 level of ~275 ppmv.
Etheridge, D.M., L.P. Steele, R.L. Langenfelds, R.J. Francey, J.-M. Barnola, and V.I. Morgan. 1996. Natural and anthropogenic changes in atmospheric CO2 over the last 1000 years from air in Antarctic ice and firn. Journal of Geophysical Research 101:4115-4128.
Indermühle A., T.F. Stocker, F. Joos, H. Fischer, H.J. Smith, M. Wahlen, B. Deck, D. Mastroianni, J. Tschumi, T. Blunier, R. Meyer, B. Stauffer, 1999, Holocene carbon-cycle dynamics based on CO2 trapped in ice at Taylor Dome, Antarctica. Nature 398, 121-126.
Van Hoof, T.B., K.A. Kaspers, F. Wagner, R.S.W. van de Wal, W. Kürchner, H. Vissher, 2005. Atmospheric CO2 during the 13th century AD: reconciliation of data from ice core measurements and stomatal frequency analysis. Tellus (2005), 57B, 351–355.