Continuous high-resolution aerosol record from the East Greenland Ice Core Project covering the entire Holocene

Jensen, Camilla Marie (2021). Continuous high-resolution aerosol record from the East Greenland Ice Core Project covering the entire Holocene (Submitted). (Dissertation, Institute for Climate and Environmental Physics and Oeschger Centre for Climate Change Research, University of Bern, Faculty of Science)

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Impurities preserved in the Greenland ice sheet are great sources of information about past atmospheric processes along with long-range transport, and atmospheric composition. The information from the impurities can be unraveled through the analysis of ice cores. The particulate and soluble impurities in ice cores, originating from a wide variety of sources and processes, are measured using the continuous flow analysis (CFA) system, specifically developed for simultaneously melting and measurements of ice cores with very high, often seasonal, resolution.

Chemical analysis of several deep Greenland ice cores has been obtained, using the CFA system. However, due to the brittle ice zone causing poor sample quality over several hundred metres of the ice cores, a gap of several thousand years during the mid-Holocene remained unaccounted for in the records. This motivated the strive for a new continuous high-resolution record to improve our understanding of the mechanisms of climate variations during the Holocene period. As the framework for this study, chemical analysis of the upper 1393 m (from 14 m depth) of the East Greenland Ice-core Project (EGRIP) core, covering the last 15k years, was performed using the Bern CFA system. The analysis took place over three measurement campaigns and resulted in the first continuous high-resolution multi-proxy record from interior Greenland covering the entire Holocene. One of the major species preserved in the ice is ammonium.

The ammonium record is composed of a background signal reflecting the seasonal cycle in nitrogen turnover in soils mainly in North America (NA), and occasional high summer peak
caused by biomass burning events that again mainly derive from NA fire sources. This study focuses on the development of a simple yet refined approach to determine NA fire events frequency in the EGRIP ammonium record. The fire peaks were detected by defining the background concentrations and trends, subtracting those from the initial time series to determine the residuals. This was carried out using an asymmetric least squares approach, and identifying the years of fire events, from the natural interannual variability using a statistically defined threshold above the background of the residuals. The approach was used to determine the fire peak frequency from the EGRIP ammonium record, and statistical modeling was used to estimate fire probabilities for the entire 15k years. In order to examine the consistency of the reconstruction of trends in background and fire probabilities of Greenland ammonium records, additional fire probabilities and background trends covering the last 15k years were determined from the preexisting high-resolution ammonium records from the Greenland Ice Core Project (GRIP), the North Greenland Ice sheet Project (NGRIP), and the North Greenland Eemian ice Drilling (NEEM). The results from the comparison of the four records show good coherence between the long-term trends in both ammonium background trends and fire probabilities over the last 15k years. However, differences in the phasing of the trends comparing the sites suggest that particularly the differences in the fire probability records appear possibly due to the differences in the seasonal precipitation patterns and atmospheric transport to the sites.

The thesis additionally contains a discussion of the first high-resolution multi-proxy
ecord obtained from the EGRIP ice core covering the entire Holocene, focussing on the ongoing and planned investigations of upstream effects, and possible corrections for those prior to the interpretation of the climatic signals preserved in the ice. Moreover, the theses comprises a peer reviewed co-authored publication (Erhardt et al., 2020) presenting the first geochemical characterisation of single mineral dust particles in ice cores using a Inductively Coupled Plasma Time-Of-Flight Mass Spectrometer (ICP-TOFMS) that was published in the course of this thesis and an Appendix with three additional co-authored publications (Burgay et al., 2019, Schrod et al., 2020, Stoll et al, in preparation), in which the data obtained in this
thesis were used.

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