Climatic interpretation of high-resolution chemistry and dust data from East Antarctica

Lambert, Fabrice (2007). Climatic interpretation of high-resolution chemistry and dust data from East Antarctica (Unpublished). (Dissertation, Universität Bern, Philosophisch–naturwissenschaftliche Fakultät, Physikalisches Institut, Abteilung für Klima– und Umweltphysik)

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Climate change has become real. Extreme weather events like floods, draughts and hurricanes have gripped peoples and the media’s attention and have been put in relation with rising greenhouse gas concentrations and deforestation. Desertification and extinction of species are now monitored and alternative energy production has become a profitablemarket. These signs show that people all over the world have accepted climate changeas a reality and are prepared to adapt. The question of how to adapt and what to do hasnaturally arisen. This year’s Intergovernmental Panel on Climate Change (IPCC) report predicts a global mean temperature rise of up to 4◦C until the end of the century [IPCC, 2007b] and describes the possible effects on human population, industry and geosphere [IPCC, 2007a]. These predictions are based on complex climate models that simulate the natural cycles on Earth. The phenomenal increase in computer power during these last twenty years has allowed more and more details and complexity in the simulation of the global climate. However, every climate model needs to be tested against observational data to validate its results. To this aim data from past climatic conditions need to be measured in various archives.
The reconstruction of past climatic conditions is a primary objective of climate sciences for two reasons. For one, to feed the climate models with data to check if the simulated results recreate the past climate evolution. Then we need to understand the mechanics of interaction between the different parts of the climate system, and to asses the effects of external forcing and internal feedback processes to improve these climate models and obtain more accurate results.
The positive radiative forcing feedback suggested by the IPCC [2007b] report is a sum of different factors, most prominently the greenhouse effect due to natural and anthropogenic emissions of greenhouse gases (fig. 1.1). Another factor are the atmospheric aerosols. These have a direct effect in the sense that they absorb and emit radiation, as well as an indirect effect as condensation nuclei that encourage cloud formation. Both aerosol effects have been found to be a net negative feedback forcing [IPCC, 2007b].
Thus, the reconstruction of past atmospheric aerosol concentration and finding the sources and sinks is an important part in the pursuit to improve our knowledge of the climate system. This work will focus on just one kind of aerosol, albeit arguably the most important: atmospheric dust.
Climate change is recorded in a variety of archives, amongst them ice cores. Dust in polar ice cores is an important source of information on atmospheric aspects of climate because the dust load in polar areas is exclusively of aeolian origin, and thus does not suffer from local disturbances. Microparticles that are deposited on the polar ice sheets originate from continental sources and travel polewards via the troposphere. They reveal information on source region climate and atmospheric conditions on their transport route.
The first introductory chapter provides an overview on the conditions that accompanied the collection of the data used in this work. The ice core that provided this dataset was drilled in the scope of the European Project for Ice Coring in Antarctica (EPICA) at Dome Concordia Station (Dome C) on the East Antarctic Plateau. Measurements were conducted by the means of a Continuous Flow Analysis (CFA) system [Röthlisberger et al., 2000].
The 3200 m of the Dome C ice core depict the climatic variations of the past 800 thousand years (800 ka). The high resolution dust record is presented in the two studies of chapter 3 and 4, along with the non-sea-salt soluble calcium (nssCa2+) in chapter 3, 4, and 6. The description of these datasets provides valuable information in unprecedented detail about past atmospheric climate in the Southern Hemisphere.
During the past 800 ka, glacial/interglacial cycles succeeded each other with an approximate 100 ka periodicity. In the earlier half of the record interglacials were cooler but lasted longer than in recent times. Dust flux to Antarctica and Antarctic temperature are increasingly correlated as climate becomes colder. This relationship is being interpreted as a progressive coupling between the Antarctic and the lower latitudes. Dust source strength and atmospheric particle life time played probably both a major role in the 25-fold glacial dust flux increase.
The high-resolution nssCa2+ and dust records are compared to asses the validity of nssCa2+ as a dust proxy in Antarctica. Results show that this assumption is valid for a first order approach. However, detailed analysis show that specific conclusions on the dust cycle based on nssCa2+ data should be avoided as the conversion is found to be more complex.
High-resolution Ca2+ and Na+ data from Dome C are examined to empirically estimate the marine and terrestrial Ca2+/Na+-ratio. Results diverge from literature values and the sea-salt and non-sea-salt contribution to both Ca2+ and Na+ is reassessed.
The sea-salt and mineral dust aerosol records of the two EPICA ice cores from two different parts of Antarctica are jointly used to estimate dust transport and emission efficiency, as well as to identify regional differences in sea-salt aerosol. A simple conceptual dust transport model is proposed that quantifies the glacial source strength and particle life-time increase. The higher glacial sea-salt aerosol fluxes are ascribed to larger sea-ice extent, perennial sea-ice being assumed to be the main source for sea-salt aerosols.
The interpretation of the two EPICA ice-cores can be understood as one big common task. The studies in chapters 3 to 6 are a contribution to the understanding of the big picture. To get a better overview, more related studies, that were the result of fruitful international collaborations, are detailed in the appendix.

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