Environmental context. Iron is arguably the most important trace element for the growth of marine organisms and is the limiting micronutrient for primary production in many parts of the world’s oceans. The concentration of dissolved iron in seawater therefore influences the global carbon cycle and consequently Earth’s climate. Hence, it is important to understand the marine biogeochemistry of iron and quantify its spatial and temporal distribution. In order to achieve this objective, it is essential that reported open-ocean concentrations of dissolved iron are accurate.

Abstract. Results from a 3-laboratory blind intercomparison exercise with two widely used analytical methods for the determination of iron in seawater are presented. The two methods used are coprecipitation followed by isotope dilution inductively coupled plasma mass spectrometry (ICP-MS) and chemical reduction to iron(ii) followed by flow injection with chemiluminescence detection (FI-CL). The five samples used were collected from the South Atlantic Ocean as part of the IRONAGES intercomparison exercise. To avoid any inter-bottle variability, the same sample bottles were sent sequentially to three laboratories in England, Australia and the United States over a 12-month period. The results show that there is no statistical difference (P = 0.05) between the shipboard FI-CL method and the directly traceable, low blank, isotope dilution ICP-MS method for the determination of iron in surface South Atlantic seawater. There was also excellent agreement between the overall mean of the three laboratories (0.54 ± 0.03 nM) and the consensus value from an earlier community-wide separate bottle intercomparison using the same IRONAGES sample water (0.59 ± 0.21 nM).