Atmospheric CO2 Reconstructions Using Polar Ice Cores: Development of a New Dry Extraction Device and Insights from Highly Resolved Records

Bereiter, Bernhard (2012). Atmospheric CO2 Reconstructions Using Polar Ice Cores: Development of a New Dry Extraction Device and Insights from Highly Resolved Records (Unpublished). (Dissertation, Universität Bern, Philosophisch–naturwissenschaftliche Fakultät, Physikalisches Institut, Abteilung für Klima– und Umweltphysik)

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Without greenhouse gases the earth's mean surface temperature would be about −8 °C [Stauffer et al., 2005]. The reason for a roughly 32°C warmer planet with an annual mean surface temperature of currently 14.41 °C (2011)1 is the total concentration of 0.6% H2O, CO2, CH4, N2O and O3 in the atmosphere [IPCC, 2001; Trenberth et al., 2007]. The small concentrations of these natural greenhouse gases have a large effect which clearly shows the importance of the greenhouse gases for the climate of our planet.
Even though H2O, in the form of water vapor and clouds, is responsible for about 2/3 of the greenhouse effect, it is the concentration of CO2 − responsible for about 1/4 of this effect [Schmidt et al., 2010] − which is the most important factor for the anthropogenic climate change [IPCC, 2007], as well as the natural temperature changes associated with glacial-interglacial changes [Schilt et al., 2010; Shakun et al., 2012]. Even on geological times scales, for example back in the Cretaceous period (145-65 million years ago), estimates of the mean global temperature are in the range of 20-22 °C which is mainly attributed to an atmospheric CO2 concentration 4-6 times higher than what is observed today [Tajika, 1998; Crowley and Berner, 2001].
The reason for the dominant role of CO2 in the natural greenhouse gas effect is twofold: First, its atmospheric concentration is the second highest after H2O and it is at least 400 times higher than the concentration of the remaining greenhouse gases. This high concentration overcompensates a relatively weak IR-radiative effciency of the CO2 molecule being about 2500 times weaker compared to tropospheric O3, 214 times weaker compared to N2O and 26 times weaker compared to CH4, on a per molecule basis [IPCC, 2007]. Second, CO2 has no natural sinks in the atmosphere such as CH4, N2O and O3, and it does not condensate and fall out of the atmosphere such as H2O, for which reason a perturbation of the atmospheric concentration has the longest lifetime for CO2. It ranges from a few centuries up to several millennia whereas the other “long lived” natural greenhouse gases CH4 and N2O show perturbation lifetimes of 12 and 114 years, respectively [IPCC, 2007]. This long perturbation lifetime of CO2 allows emissions to get accumulated in the atmosphere over timescale of decades and more and to affect the energy balance of the earth system over millennia. Therefore, changing CO2 emissions can force the climate in a certain direction. In contrast to that, the “short lived” greenhouse gases H2O and tropospheric O3 primarily play a role as climate feedback rather than as climate forcing due to a perturbation lifetime of hours to a couple of days and – in the special case of H2O − since the atmospheric water content is mainly driven by the temperature dependent storage capacity of the atmosphere (Clausius-Clapeyron relation) [IPCC, 2001; Grewe and Stenke, 2008].
The perturbation lifetime of greenhouse gases in the atmosphere is determined by their sinks. In the case of CH4 and N2O, the main sink is the decomposition within the atmosphere by OH radicals. This single process follows an exponential decay which can be characterized by a single lifetime. In the case of CO2 many sinks are involved all working on different time scales, for which reason no specific perturbation lifetime can be given. The atmospheric CO2 sinks are associated with the exchange with the other carbon reservoirs: biosphere and the ocean (and also geological reservoirs). A process that works on millennial timescales is the carbonate chemical neutralization at the ocean surface which is responsible for the fact that 20-35% of the emitted CO2 stays in the atmosphere for many millennia [Archer et al., 2009]. In order to better determine these processes and to understand the interplay of the carbon reservoirs in a changing climate, their evolution in the past is studied. In the case of the atmosphere, the past changes can be precisely reconstructed using air bubbles enclosed in ice cores, which is the only paleoclimatic archive that contains samples of atmospheric air from before the 1950's, when direct measurements began. Such reconstructions showed that CO2 concentration changes associated with glacial-interglacial variations reach from roughly 180 ppmv in glacial up to 300 ppmv in interglacial times during the last 800,000 years [Lüthi et al., 2008], and that the accumulated emissions since the beginning of the industrial period (1750) caused an increase from about 280 ppmv [MacFarling Meure et al., 2006] up to 395.77 ppmv2 today. Since the current atmospheric CO2 concentration is 32% higher than at any other stage during the last 800,000 years and since the speed of this anthropogenic increase is also unprecedented for at least the last 22,000 years [Joos and Spahni, 2008], no historic analogue can be studied with ice cores. In order to draw conclusions for the recent changes, the dynamics of previous changes must be studied, for which reason the most accurate and detailed reconstructions of the past atmospheric CO2 concentrations are required.
This thesis presents new technical developments, new data and modeling work that provide a more detailed and robust knowledge of the historical atmospheric CO2 concentrations found within ice cores. The main part of this thesis forms the development of a new extraction device for the trapped air in ice core samples. Chapter 1 reviews the history of the techniques applied to analyze the trapped air in natural ice starting at the end of the 19th century. Chapter 2 discusses the advantages and disadvantages of the different techniques and explains the motivation and scientific reasons for an investment into the development of a completely new approach.
The new extraction device is described in chapter 3 and presents the draft version of a technical publication. The extraction device − called Centrifugal Ice Microtome (CIM) − shows clear and quantifiable improvements in the amount of air released from the ice and the contamination of the released air. In parallel to the extraction device, a gas analysis component has also been developed. This component is described in the work of Siegrist [2011] and also in the technical paper in chapter 3. When coupled, these developments build a new and stand-alone system for atmospheric ice-core based CO2 reconstructions which achieves highest precision and sample throughput requirements.
Chapter 4 provides a publication on CO2 data derived using the previous system within our lab, and forms the basis for the second part of this thesis. Two Antarctic ice cores have been analyzed and the resulting data combined for this publication, in order to derive a highly time resolved CO2 record between 115,000 years BP and 38,000 years BP. The data revealed a new detail of the atmospheric CO2 evolution during the millennial-scale climate variations associated with D-O events, and showed that the carbon cycle responded differently to these events in the early part of the record compared to the later part. Based on these measurements we hypothesize a carbon seesaw affecting the later stages due to a change in deep ocean carbon content which could explain the observation. The publication is one example of how detailed atmospheric CO2 reconstructions can provide new insights into the natural carbon cycle.
The third part of this thesis is presented in chapters 5 and 6. They contain two publications and a continuative section which examine − among others −issues of gas diffusive processes in the ice and their influence on the trapped air composition in the ice. Chapter 5 starts with a publication which describes a model to calculate outgassing of ice cores due to molecular diffusion of gas species through the ice and the associated changes in the trapped gas composition. The model allowed us to provide guidelines (depending on the storage duration and temperature of the ice) on how much ice should be removed from the sample surface in order to avoid contaminations associated with outgassing. It further demonstrates the improvement of the trapped gas quality as a result of reduction of the storage temperature from about −25°C down to −40°C or below. The publication is a continuation of the work presented in Bereiter [2007] and forms the basis of the third part of this thesis. In a second part of this chapter, a combination of this gas diffusion model with a one-dimensional ice flow and temperature model is described in order to investigate a possible dampening of trapped gas compositions in a 1.5 million years old ice core. This part aims on the IPICS (International Partnerships in Ice Core Sciences) target to retrieve such an old ice core in Antarctica. The results suggest that a signal dampening is to be expected for CO2 concentrations, and O2/N2 ratio variations in the ice older than 1 million year, however, the magnitude of the dampening, in particular for the O2/N2 ratio, is weakly constrained.
Chapter 6 contains a publication first-authored by Dieter Lüthi in which CO2 variations on centimeter-scale just below the Bubble to Clathrate Transformation Zone (BCTZ) are described. These variations are non-climatic and have been found in three different ice cores. Their behavior suggests that they are an artifact of gas fractionation in the BCTZ. A strong fractionation of O2 and N2 due to gas diffusion processes between the coexisting bubbles and clathrates in the BCTZ is known, for which reason a similar process is suspected to be behind these CO2 variations. It is hypothesized that the transformation of bubbles into clathrates takes place preferentially in layers, with axial extent in the range of centimeters explaining the observed CO2 variations. The hypothesis is backed up with calculations using the diffusion model of chapter 5.
The thesis is concluded by an outlook, that discusses first how the new dryfextraction device could be further improved and for what other applications it could be of interest. The proposed improvements focus on the way how finer ice powder could be produced with the device, since the results in chapter 3 suggest that this has a significant influence on the extraction effciency and the quality of the results in the BCTZ. Second, further development steps of the gas analysis component and future measurement campaigns are discussed. Potential for improving the gas analysis is een in particular in the usage of a new optical gas analyzer technology which would enable the analysis of several greenhouse gases in parallel and also derive their isotopic composition at the same time. The proposed future measurements focus on providing higher quality records for the period between 115,000 and 450,000 year BP and the Holocene.

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