Reconstructions of atmospheric CO2 and 13C from Antarctic ice cores over the last glacial cycle

Eggleston, Sarah (2015). Reconstructions of atmospheric CO2 and 13C from Antarctic ice cores over the last glacial cycle (Unpublished). (Dissertation, Universität Bern, Philosophisch–naturwissenschaftliche Fakultät, Physikalisches Institut, Abteilung für Klima– und Umweltphysik)

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As the second most important greenhouse gas in the atmosphere after water vapor, carbon dioxide plays a major role in the Earth’s climate system. This is manifested both in direct effects, e.g. by altering the radiative balance in the atmosphere, as well as indirectly, e.g. through ocean acidification, and through complex feedbacks: for example, rising CO2 concentrations and the associated warming can lead to fertilization of the terrestrial biosphere, which in turn causes an increased drawdown of atmospheric CO2; on longer timescales, the carbonate system in the ocean interacts with atmospheric CO2 through the feedback process known as carbonate compensation. In order to better understand the natural variations of climate as well as to be able to more accurately predict the impact of future CO2 emissions scenarios on various aspects of the Earth system, paleoclimatologists turn to the past to investigate climatic changes that have occurred on millennial to glacial/interglacial timescales and beyond. The only direct archive of past CO2 concentrations is the air from past atmospheres trapped in bubbles and clathrates in ice sheets. As the result of many previous studies from various Antarctic ice cores, we now have a complete [CO2] record extending from 800 kyrBP to the present. The atmospheric concentration itself, however, only tells a part of the story; further constraints are needed to understand the observed natural variations and to explain which processes and reservoirs are important as sources and sinks of CO2 on various timescales.
The central goal of this thesis is accordingly to place such a constraint on the system through measurements of the stable carbon isotope of CO2. Due to isotopic fractionation associated with processes inducing CO2 fluxes between or within the major reservoirs (atmosphere, ocean, and terrestrial biosphere), the knowledge of the δ13CO2 evolution coupled with the [CO2] record allows us to evaluate possible driving mechanisms active on different timescales. These include the upwelling of deep, carbon-rich and isotopically depleted water, changes in the marine biological export from the surface ocean, growth and decay of the terrestrial biosphere, and changes in oceanic uptake of CO2 driven by variations in sea surface temperature. In this thesis, the first complete record of δ13CO2 over the past 155 kyr is presented, measured using a unique sublimation technique and greatly extending the work of previous studies focused on Terminations I and II. In this study, we were able to constrain the role of the marine biological pump in driving the drop and subsequent rise in [CO2] at the beginning and end of Marine Isotope Stage (MIS) 4, respectively. Furthermore, based on the evidence of an increased upwelling event observed in the Southern Ocean opal flux record and corresponding to a drop in δ13CO2 during the MIS 4-3 transition, we concluded that a pool of carbon-rich and isotopically depleted water must have been transferred to the deep ocean before MIS 4, in agreement with previous studies.
An important correction to account for in δ13CO2 measurements is due to gravitational fractionation in the firn column; this effect is on the order of the largest variations seen over the entire record. Because the magnitude of the gravitational fractionation varies both spatially and temporally, it is necessary to have an accurate estimate of this effect for each sample measured in each individual ice core. Previous studies have relied largely on models or the roughly linear relationship between δ15N2 and stable water isotopes, but these assumptions add to the uncertainty related to the δ13CO2 measurement. Thus, in order to ensure the accuracy and highest possible precision of our measurements, a new method was developed to measure δ15N2 on the same sample, thus providing us directly with the correction for gravitational fractionation.
In producing such high-precision measurements, it is also crucial to understand the caveats and limitations of the available analytical techniques. A secondary focus of this thesis was the re-evaluation of CO2 concentrations measured in the deepest part of the oldest ice core drilled at Dome Concordia, Antarctica. Using a combination of different gas extraction techniques with varying degrees of extraction efficiency and ice samples with different storage histories, the results of this study indicate that in ice containing both bubbles and clathrates, CO2 is preferentially enclosed in clathrates; this is also the case when ice originally containing only clathrates relaxes during storage. This was the proposed explanation for the higher, and more accurate, CO2 concentrations measured in this study using ice stored at -50◦C that had not relaxed and therefore contained only clathrates, as well as by employing high-efficiency gas extraction techniques on ice containing only clathrates or both bubbles and clathrates. Thus, to accurately measure [CO2] in regions of the ice core containing both bubbles and clathrates, it is crucial to extract the same relative amount of gas from these two physical structures in the ice or to be able to correct for this effect.
The analysis of δ18O(CO2) also has the potential to shed light on this subject, as this stable isotope equilibrates with that of the ice in the presence of water or liquid-like layers forming on the surface of the ice crystalline structure. The rate and extent of this equilibration is therefore a function of temperature and possibly the ice structure (bubbles or clathrates). In the final chapter of this thesis, new data are added to a compilation of previously measured δ18O(CO2) datasets, and the stage is set for further investigation of this topic. We show that isotopic exchange timescales between CO2 and ice are on the order of years to decades, which is in line with studies in the firn column, thus suggesting that a similar exchange mechanism is at play both in ice containing bubbles and clathrates. While we are not yet able to precisely constrain the relationship between the exchange timescale and the physical structure of the ice, it appears that the fastest exchange occurs in the Bubble-Clathrate Transition Zone, with a timescale of approximately 3 years.

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