Southern Hemisphere atmospheric history of carbon monoxide over the late Holocene reconstructed from multiple Antarctic ice archives
Abstract. Carbon monoxide (CO) is a naturally occurring atmospheric trace gas, a regulated pollutant and one of the main components determining the oxidative capacity of the atmosphere. Evaluating climate-chemical models under different conditions than today and constraining past CO sources requires a reliable record of atmospheric CO mixing ratios ([CO]) since pre-industrial times. Here, we report the first continuous record of atmospheric [CO] for Southern Hemisphere (SH) high latitudes over the past three millennia. Our continuous record is a composite of three high-resolution Antarctic ice core gas records and firn air measurements from seven Antarctic locations. The ice core gas [CO] records were measured by continuous flow analysis (CFA) using an optical-feedback cavity-enhanced absorption spectrometer (OF-CEAS), achieving excellent external precision (2.8–8.8 ppbv, 2σ), and consistently low blanks (ranging from 4.1 ± 1.2 to 7.4 ± 1.4 ppbv), enabling paleo-atmospheric interpretations. Six new firn air [CO] Antarctic datasets collected between 1993 and 2016 CE at the DE08-2, DSSW19K, DSSW20K, South Pole, ABN, and Lock-In sites (and one previously published firn CO dataset at Berkner) were used to reconstruct the atmospheric history of CO from ~1897 CE using inverse modeling that incorporates the influence of gas transport in firn. Excellent consistency was observed between the youngest ice core gas [CO] and the [CO] from the base of the firn, and between the recent firn [CO] and atmospheric [CO] measurements at Mawson station (East Antarctica), yielding a consistent and contiguous record of CO across these different archives. Our Antarctic [CO] record is relatively stable from −835 to 1500 CE with mixing ratios within a 30–45 ppbv range (2σ). There is a ~5 ppbv decrease in [CO] to a minimum at around 1700 CE, during the Little Ice Age. CO mixing ratios then increase over time to reach a maximum of ~54 ppbv by ~1985 CE. Most of the industrial period [CO] growth occurred between about 1940 to 1985 CE, after which there was an overall [CO] decrease, as observed at atmospheric monitoring sites around the world and in Greenland firn air. Our Antarctic ice core gas CO observations differ from previously published records in two key aspects. First, our mixing ratios are significantly lower than reported previously, suggesting previous studies underestimated blank contributions. Second, our new CO record does not show a maximum in the late 1800s. The absence of CO peak around the turn of the century argues against there being a peak in Southern Hemisphere biomass burning at this time, which is in agreement with (i) other paleofire proxies such as ethane or acetylene and (ii) conclusions reached by paleofire modeling. The combined ice core and firn air CO history, spanning −835–1992 CE, extended to the present day by the Mawson atmospheric record, provides a useful benchmark for future atmospheric chemistry modeling studies.
Xavier Faïn et al.
Status: open (until 16 Jun 2023)
- RC1: 'Comment on cp-2023-9', Vasilii Petrenko, 12 May 2023 reply
- RC2: 'Comment on cp-2023-9', Murat Aydin, 01 Jun 2023 reply
Xavier Faïn et al.
Xavier Faïn et al.
Viewed (geographical distribution)
The authors present a compilation of Antarctic paleoatmospheric CO measurements from 7 firn air campaigns and three ice cores measured at high resolution using continuous flow analysis. To the best of my ability to tell, none of these data (with the exception of Berkner Island firn air) have previously been published. These new records span the time range from -835 CE to the 2000s and link with modern atmospheric observations in Antarctica. Two firn air models are used to reconstruct the CO history for the part of the record covered by firn air.
The records appear to be of very high quality, with careful attention given to calibration and corrections for analytical effects. The robustness of the reconstructions is confirmed by good agreement among the records, as well as by agreement between firn air measurements and direct atmospheric observations for overlapping time intervals. The Antarctic ice core records also appear to be free of significant amounts of in situ – produced CO that has previously been seen in Greenland ice cores.
In my opinion this study represents a large advance in our understanding of atmospheric CO history. CO is a key player in global atmospheric chemistry, and a reliable CO history is required for a full understanding of natural variations in atmospheric chemistry as well as impacts of the industrial transition. This study provides this much-needed history.
The interpretation included in the paper is qualitative only, but I think this is OK as the reconstruction itself is a very significant contribution that merits publication in CP.
One of the most significant findings of the study is that prior Antarctic ice core CO measurements (presented in Wang et al., 2010) appear to have been biased high by 0 – 20 ppb (depending on ice core and time interval). I would trust the results of this new work over the older measurements due to improved measurement techniques, careful attention to procedural effects, agreement among multiple records as well as the fact that the continuous flow technique used here has been verified against at least one discrete-sample technique (Fain et al., 2022). I think this new paper should go somewhat further in their discussion of the prior Wang et al results in the sense of alerting the readers that the isotopic measurements in Wang et al should now be interpreted with more caution – extra 10 – 20 ppb represents an additional 25 – 50% of CO, and this extraneous CO could certainly have a large impact on CO isotopic values. I still think that the Wang et al conclusion that the LIA CO minimum was mainly driven by reduced biomass burning is likely correct however.
Figure 5: Instead of just showing charcoal index, I think it would be more useful here to have a multi-panel figure showing the full new CO record together with acetylene and ethane (Nicewonger et al papers cited in this paper) in addition to charcoal.
Line 103: Because Greenland ice core [CO] is so strongly affected by in situ production, absolute NH paleoatmospheric values are difficult to estimate. I think it would be more accurate to say that the Greenland ice core records allow for the reconstruction of “atmospheric trends” rather than “atmospheric history”
Table 1: should be “-44” for T at ABN
Line 161: air stored in electropolished stainless steel tanks is affected by slow [CO] growth in my experience. If measurements from these tanks are used for [CO] reconstructions, it would be useful to see results of tests of [CO] stability in the tanks that were used.
Line 200: The Wang et al 2012 and Petrenko et al., 2013 references cited here are missing in the references list at the end of the manuscript
Line 340: correct this sentence for grammar
Figure 1: middle panels (c and d) are too busy in my opinion – too many lines to be useful for the reader. I would recommend showing the GFs for just 2 – 3 depths per site – perhaps just the deepest sampled depth and another depth close to the lock-in depth.
Line 434: “filtered to remove lab air infiltrations”