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

Abstract

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 ⁵⁷Fe, 53 % of ⁵⁸Fe, and a small amount (< 0.5 %) of ⁵⁴Fe, which allows precise measurement of a wide range of sample to spike mixing ratios. Isotope ratios are expressed in delta notation (?56Fe), relative to ⁵⁴Fe and relative to IRMM-14. The precision of the MC-ICPMS measurements is δ⁵⁶Fe ~ 0.06 ‰ (2 SD), based on replicate analyses of two different iron isotope standards.
The iron isotopic composition of dissolved Fe (δ⁵⁶Fe_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 δ⁵⁶Fe signatures, as low as ~ -0.3 ‰, which suggests that dFe is principally derived from sediment pore waters that have undergone dissimilatory iron reduction. Towards the open ocean, δ⁵⁶Fe_dFe values within the OMZ increase, due to formation of iron ligands and/or mixing with adjacent water masses. Dissolved aluminium concentrations in surface waters at the open ocean stations are very high (up to 27 nM), indicating significant supply from atmospheric dust, and the surface waters have high δ⁵⁶Fe values (up to +0.5 ‰), much higher than bulk silicate Earth, indicating that Fe isotopes are fractionated during dust dissolution.
Analyses of the Fe isotopic composition of dFe in hydrothermal plumes from 3 sites in the East Scotia Sea reveal that during the early stages of mixing between hydrothermal fluids and seawater, Fe isotopes are significantly fractionated from the vent fluid; the δ⁵⁶Fe_dFe value in the plume is as low as -1.2 ‰, compared to vent fluid values of -0.64 to +0.28 ‰. As the plume continues to mix with seawater, δ⁵⁶Fe_dFe values increase, converging to values of between -0.6 to -0.3 ‰. This strongly suggests that dFe is stabilised by the formation of colloidal Fe-(oxy)hydroxides and Fe-L in the distal parts of the plume. This stabilised dFe is likely to be transported long distances away from its source, contributing to the deep ocean dFe budget.

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