Dissolved organic Fe-binding ligand data from the FRidge (GA13) expedition on RRS James Cook (cruise JC156) from December 2017 to February 2018

Samples were collected as part of the 2017-2018 U.K. GEOTRACES GA13 section cruise along the Mid-Atlantic Ridge (Cruise JC156 on RRS James Cook). Water samples from 11 venting and near venting locations were collected using a Seabird 911 conductivity, temperature, and depth (CTD) titanium rosette using conducting Kevlar wire with an oxidation-reduction potential (ORP) sensor to detect plumes. Teflon coated OTE (Ocean Test Equipment) bottles were pressurized to approximately 7 psi with 0.2 μm filtered air using an oil-free compressor. A Sartobran 300 (Sartorius) filter capsule (0.2 μm) was used to collect filtered seawater samples into clean 250 mL LDPE sample bottles. Bottles and caps were rinsed 3 times with the filtered sample before being filled. Samples were stored frozen at -20°C for organic iron-binding ligand characterization by voltammetry using competitive ligand exchange adsorptive cathodic stripping voltammetry. All dissolved organic iron-binding ligand samples were measured with a BASi controlled growth mercury electrode with an Ag/AgCl¬- reference electrode and platinum auxiliary electrode (Bioanalytical Systems Incorporated) using previously established methods for forward titrations (Buck et al., 2015, 2018; Bundy et al., 2018; Abualhaija and van den Berg, 2014; Hawkes et al., 2013 (Planet. Sci. Lett.)). Reverse titrations (Hawkes et al., 2013 (Anal. Chim. Acta)) were completed on 10 samples from Broken Spur, and TAG hydrothermal vent fields with dissolved iron concentrations between 19.01-90.25 nM.