Dissertation Abstract
continuous chemistry in ice cores: Phosphorus, pH and the photolysis of humic like substances
Kjær, Helle Astrid 2014
Niels Bohr Institute, University of Copenhagen (Denmark), 342 pp.
Ice cores provide high resolution records of past climate and environment. In recent years the use of continuous flow analysis (CFA) systems has increased the measurement throughput, while simultaneously decreasing the risk of contaminating the ice samples. CFA measurements of high temporal resolution increase our knowledge on fast climate variations and cover a wide range of proxies informing on a variety of components such as atmospheric transport, volcanic eruptions, forest fires and many more. New CFA methods for the determination of dissolved reactive phosphorus (DRP) and pH are presented as part of this thesis.
Phosphorus is an essential nutrient for the biosphere. The phosphorus is cycled mainly via hydrology, but some biological systems, such as the remote oceans and old forests are dependent on atmospheric deposition of phosphorus. The flux of phosphorus to the ocean is suggested to have increased during the glacial and in recent times there is clear evidence of increased phosphorus load in waters as a result of human influences. To investigate if atmospheric deposition of P, too has changed over time a CFA method was developed for the continuous determination of dissolved reactive phosphorus (DRP) by means of a reaction with molybdenum blue. The concentration of DRP in polar ice is low and thus the method relies on enhancing the limit of detection by increasing the absorption length by means of a 2.5 metre Liquid Waveguide Capillary Cell. The method was applied to a firn core from the North East Greenland Ice
Stream (NEGIS) and to glacial sections of the Greenland NEEM ice core. In the NEGIS firn core concentrations were about 2.7 nM phosphate and there was no evidence of any recent anthropogenic impact during the past 300 years. Sources of DRP to the NEGIS site were found to be dust as well as a secondary source-likely of biological origin. The DRP deposited at the NEEM site during the last glacial maximum
was 14 times higher than the DRP deposited at present at the NEGIS site. The study of the NEEM ice core sections also included determination of total P. The ratio between DRP and total P reveal large changes in the soluble fraction of P over time, with low solubility in cold glacial periods.
A high resolution optical method was also developed using the dyes chlorophenol red and bromophenol blue for the determination of pH in ice cores.
pH controls many chemical processes and equilibria. Hence pH data are important for the proper interpretation of data for the various proxies found in the ice. Further, determination of the acidity can be used to identify layers originating from volcanic eruptions, crucial for cross-dating ice cores and relevant for climate interpretations.
The method includes a heat bath to minimize the acidifying effect of CO2 both from the laboratory and from the ice itself. While for acidic ice the method finds similar
concentrations of H+ as are deduced from the ionic budget, for more alkaline ice there seems to be an acidifying effect of the CO2, causing the indicator method to error towards lower pH. The pH method was applied on the NEGIS firn core where it proved an improvement over both conventional ECM and DEP, which require density corrections. Further the method successfully identified volcanic eruptions
as well as the underlying anthropogenic signal related to the industrial pollution. The method was also successfully applied to Holocene ice from the Antractice ice core RICE.
Phosphorus is an essential nutrient for the biosphere. The phosphorus is cycled mainly via hydrology, but some biological systems, such as the remote oceans and old forests are dependent on atmospheric deposition of phosphorus. The flux of phosphorus to the ocean is suggested to have increased during the glacial and in recent times there is clear evidence of increased phosphorus load in waters as a result of human influences. To investigate if atmospheric deposition of P, too has changed over time a CFA method was developed for the continuous determination of dissolved reactive phosphorus (DRP) by means of a reaction with molybdenum blue. The concentration of DRP in polar ice is low and thus the method relies on enhancing the limit of detection by increasing the absorption length by means of a 2.5 metre Liquid Waveguide Capillary Cell. The method was applied to a firn core from the North East Greenland Ice
Stream (NEGIS) and to glacial sections of the Greenland NEEM ice core. In the NEGIS firn core concentrations were about 2.7 nM phosphate and there was no evidence of any recent anthropogenic impact during the past 300 years. Sources of DRP to the NEGIS site were found to be dust as well as a secondary source-likely of biological origin. The DRP deposited at the NEEM site during the last glacial maximum
was 14 times higher than the DRP deposited at present at the NEGIS site. The study of the NEEM ice core sections also included determination of total P. The ratio between DRP and total P reveal large changes in the soluble fraction of P over time, with low solubility in cold glacial periods.
A high resolution optical method was also developed using the dyes chlorophenol red and bromophenol blue for the determination of pH in ice cores.
pH controls many chemical processes and equilibria. Hence pH data are important for the proper interpretation of data for the various proxies found in the ice. Further, determination of the acidity can be used to identify layers originating from volcanic eruptions, crucial for cross-dating ice cores and relevant for climate interpretations.
The method includes a heat bath to minimize the acidifying effect of CO2 both from the laboratory and from the ice itself. While for acidic ice the method finds similar
concentrations of H+ as are deduced from the ionic budget, for more alkaline ice there seems to be an acidifying effect of the CO2, causing the indicator method to error towards lower pH. The pH method was applied on the NEGIS firn core where it proved an improvement over both conventional ECM and DEP, which require density corrections. Further the method successfully identified volcanic eruptions
as well as the underlying anthropogenic signal related to the industrial pollution. The method was also successfully applied to Holocene ice from the Antractice ice core RICE.