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Abstract Iron (Fe) in seawater is an essential micronutrient for marine phytoplankton, and Fe deficiency limits their growth in high-nutrient, low-chlorophyll areas. The bioavailability of Fe for phytoplankton largely depends on its chemical speciation in seawater. In surface water, the reduction of Fe(III) to Fe(II) is an important step in the uptake of Fe by phytoplankton. However, the marine biogeochemical cycle of Fe(II) in the open ocean has not been fully investigated. In oxic open-ocean waters, Fe(II) is rapidly oxidized and exists at sub-nanomolar levels, making it difficult to determine the Fe(II) concentration of seawater. In this study, we applied the flow analytical method of determining the Fe(II) concentration of seawater using luminol chemiluminescence in an in-situ analyzer (geochemical anomaly monitoring system, GAMOS). In the onboard laboratory, we successfully detected sub-nanomolar levels of Fe(II) in seawater using the GAMOS. In the central Indian Ocean, this analyzer was deployed at a depth of 1000 m to determine the Fe(II) concentration in the water column. During deployment, the detection limit (0.48 nM) was insufficient to determine the concentration. Therefore, we need to lower the blank values and enhance the stability of signal of the in-situ analytical method for application to open-ocean seawater samples. Graphical abstract 

Abstract. Tritium and helium isotope data provide key information on oceancirculation, ventilation, and mixing, as well as the rates of biogeochemicalprocesses and deep-ocean hydrothermal processes. We present here globaloceanic datasets of tritium and helium isotope measurements made by numerousresearchers and laboratories over a period exceeding 60 years. The dataset'sDOI is https://doi.org/10.25921/c1sn-9631, and the data are available athttps://www.nodc.noaa.gov/ocads/data/0176626.xml (last access: 15 March2019) or alternatelyhttp://odv.awi.de/data/ocean/jenkins-tritium-helium-data-compilation/(last access: 13 March 2019) and includes approximately 60 000 valid tritiummeasurements, 63 000 valid helium isotope determinations, 57 000 dissolvedhelium concentrations, and 34 000 dissolved neon concentrations. Somequality control has been applied in that questionable data have been flaggedand clearly compromised data excluded entirely. Appropriate metadata havebeen included, including geographic location, date, and sample depth. Whenavailable, we include water temperature, salinity, and dissolved oxygen. Dataquality flags and data originator information (including methodology) arealso included. This paper provides an introduction to the dataset along withsome discussion of its broader qualities and graphics.