Iron isotopes in seawater:method development and results from the Atlantic ocean

• Main Author: Klar, Jessica K.
• Format: Thesis
• Language: English
• Published: 2014
• Subjects: Southern Ocean
• Online Access: https://eprints.soton.ac.uk/374829/; https://eprints.soton.ac.uk/374829/1/Klar%252C%2520Jessica_Feb_15_PhD.pdf

The analysis of the iron (Fe) isotopic composition of seawater can provide unique information about Fe sources to seawater, and Fe cycling within the oceans, which are important for understanding global climate because of the links between the marine carbon and iron cycles. The low dissolved Fe (dFe) concentrations found in seawater mean that analyses of the iron isotopic composition of seawater is an analytical challenge. This thesis describes the development methods for accurate and precise analysis of Fe isotopes in seawater with concentrations as low as ~0.4 nM Fe, and the results of iron isotope analysis of seawater samples from within the oxygen minimum zone (OMZ) of the tropical Atlantic Ocean, and the dissolved phase of hydrothermal plumes in the Southern Ocean. Briefly, Fe is preconcentrated from seawater using NTA resin and is then purified by anion exchange chromatography. Iron isotope ratios are determined by high resolution multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS, Thermo Fisher Neptune) and mass bias effects are corrected using a double-spike technique. The Fe isotope spike consists of 47 % of 57 Fe, 53 % of 58 Fe, and a small amount (< 0.5 %) of 54 Fe, which allows precise measurement of a wide range of sample to spike mixing ratios. Isotope ratios are expressed in delta notation (?56Fe), relative to 54 Fe and relative to IRMM-14. The precision of the MC-ICPMS measurements is δ 56 Fe ~ 0.06 ‰ (2 SD), based on replicate analyses of two different iron isotope standards. The iron isotopic composition of dissolved Fe (δ 56 Fe dFe ) was measured in seawater samples collected along an E-W transect along ~12 °N in the tropical Atlantic Ocean (GEOTRACES section GA06), extending from the Senegalese shelf towards the open ocean in order to assess the behaviour of dFe in the OMZ. Bottom waters within the OMZ have elevated dFe concentrations and low δ 56 Fe signatures, as low as ~ -0.3 ‰, which suggests that dFe is principally derived from sediment pore waters that have ...