Master thesis defence by Magnus Kristensen – Niels Bohr Institute - University of Copenhagen

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Master thesis defence by Magnus Kristensen

First Continuous Phosphate and pH records from a Greenland shallow ice core


Abstract

Ice cores has long been used as a reliable and precise archive of past climate conditions, and the analysis of themprovide information on a range of processes fromglobal air temperatures and the atmospheric composition to the extent of wildfires in North America and biological activity levels in the oceans. In the present study new techniques was applied to a shallow ice core in order to measure the phosphate content and the pH level of the ice. Neither of these measurements are standard procedures for the continuous flow analysis (CFA) setup, and the present study is the first continuous phosphate and pH record from a Greenland ice core.

Phosphate is an important and possibly limiting nutrient for primary production in the
oceans. Because of human activities such as widespread use of fertilizers and conversion of
forest and grasslands into farmland, many changes to the phosphate cycle has occurred over the last centuries, the extent of which is not known exactly. Analysing phosphate concentrations in ice coresmay help gain important knowledge about the extent of those processes. pH on the other hand is a master variable that strongly influences as varied processes as the solubility and hence the weathering of minerals, the speciation of aerosols and the equilibrium concentration of any chemical reaction that involves the hydrogen ion. The pH is however usually estimated fromthe conductivity of ice and melt water rather than measuring the actual pH. It is however believed that there is a strong relation between these measures, and conductivity measurements has the advantage of being both faster than pH measurements as well as being non destructive.

A continuous and highly sensitive absorption method developed by Kjær [2010] (for phosphate) and Raghuraman et al. [2006] (for pH) was applied to the detection of both of these species in ice cores. The ice core analysed was the NEGIS shallow ice core from the Greenland ice sheet, which covers the past 400 years. Results showed that the average level of phosphate in the NEGIS core was 0.32 ppb, and no clear indications of anthropogenic changes was found. A high correlation between phosphate and dust as well as between phosphate and pH suggests that dust is either a source or a transport mechanism for phosphate, and that the acidity level may alter the solubility of atmospheric dust to release more phosphorus.
The relation between the pH measurements and the electrical conductivity measurements
was investigated, and all previously observed trends fromthe literature regarding these
relations was confirmed. The relation, though, has been developed for use with ice cores, and the application on the much less dense firn core requires some alterations to the relation.

Supervisors:
Anders Svensson, Paul T. Vallelonga og Helle A. Kjær
Centre for Ice and Climate