‘Organics in ice’: Novel organic compounds in ice cores for use in palaeoclimate reconstruction

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King, Amy Constance Faith  ORCID logo  https://orcid.org/0000-0002-1285-7568

The majority of current ice core studies focus on analysing the inorganic component of atmospheric aerosol, trapped and preserved in the ice as a record of past atmosphere. However, this does not fully represent the make-up of atmospheric aerosol, which can be up to 50% organic. This thesis aims to develop the understanding and quantification of a number of these organic compounds in ice core samples. A novel and promising area of ‘organics in ice’ research lies within the groups of primary and secondary compounds released from the terrestrial and marine biospheres; these compounds may help us to form a record of past biosphere emissions, with implications for biological productivity and atmospheric chemistry. A small selection of studies obtaining new records from these types of organic compounds in ice have demonstrated this concept, for example lipid compounds in snow layers dating back 450 years in Greenland, oxidation products of isoprene and monoterpenes in ice up to 350 years old in Alaska, and carboxylic acids and inorganic ions between 1942-1993 from Grenzgletscher (Monte Rosa Massif) in the southern Swiss Alps. Compound concentrations were related back to Northern Hemisphere temperature, atmospheric transport pathways and intensities, and biomass burning signals respectively. There are many terrestrial and marine biogenic compounds not yet investigated in ice core samples. Thus we are presented with an almost untapped reservoir of new climate information. Therefore, it is timely to produce a method of analysis for a long list of the most promising of these compounds (herein ‘target compounds’), namely fatty acids and secondary oxidation aerosol of terpenes (SOA), allowing quantification of these novel analytes in ice core samples to investigate the concept further. This project begins with an investigation in to the possible contamination sources of the target compounds in ice core samples. It attempts to quantify the threat of contamination throughout the drilling, storage and analyses processes. It finds there is substantial presence of organic compounds in media used during ice core processing, but the risk to target compounds is minor where clean-protocols are followed, and the threat is limited to outer surface ice of a solid ice core. Recommendations for ice core processing steps in preparation for organics analyses are outlined based on these results. A method of high performance liquid chromatography-mass spectrometry (HPLC-MS) including rotary evaporation preconcentration of samples is then optimised for detection of target compounds. The final method achieves good average recoveries of 80%, and reproducibility of 9% RSD. The method is reproducible on different instruments, based on an interlab comparison. An extension of this method, direct injection HPLC-MS analysis (where sample preconcentration is eliminated) is tested for the benefits of reducing sample volume and contamination introduced by preconcentration steps. The method is successful for SOA compounds (7% RSD) but not for fatty acids where background contamination is very high. The method of preconcentration HPLC-MS is then applied to samples from two ice cores; the marine-aerosol dominated Bouvet Island (sub-Antarctic) ice core, and the terrestrial-aerosol dominated Belukha Glacier (Russian Altai Mountains) ice core. Novel organic compounds are detected in both cores. Compound concentration time-series are investigated statistically by comparison to pre-existing inorganic compound records from the same cores and to historical climate data, and by application of back trajectory modelling using the Met Office's Numerical Atmospheric-dispersion Modelling Environment (NAME). In the case of Bouvet Island, the fatty acid oleic acid is found to have statistically significant correlations with the sea-ice marker methanesulfonic acid, and indeed with sea ice concentration during July to September in a geographical region extending westwards from the island along the maximum sea ice extent margin. The mechanism behind this correlation is suggested to be that of algal blooming during spring months, releasing oleic acid which is transported by strong westerly winds to the island. This highlights the prospects for a suite of marine biogenics in ice cores being used as sea ice markers. In the Belukha core, where sample records are available at sub-annual resolution, a suite of SOA compounds display summer-time peaks in concentrations. This seasonal variability is shown to be related to emission signals of these organic compounds, which is an exciting prospect for future work in improving our understanding of budgets of these aerosols in the atmosphere. The records detected demonstrate great promise for the use of organic compounds as environmental markers.

Wolff, Eric
Thomas, Elizabeth
Kalberer, Markus
Ice core, Biomarkers, Mass Spectrometry, Liquid Chromotography, Palaeoclimate, Bouvet, Sea Ice, Belukha
Doctor of Philosophy (PhD)
Awarding Institution
University of Cambridge