Samples for in-situ dissolved Fe(II) (dFe(II)) detection (see Supplemental Files: Table 1 and Figure 1):
157 samples were as soon as possible analysed from the Geotraces carousel from Stn 38-66;
5 ice samples from Stn43 /46 (operated by Ana Aguilar, Rob Rember);
3 small boat surface samples from Stan60,61,66 (operated by Ana Aguilar, Rob Rember);
1 Multicorer niskin sample from Stn66 (operated by Greg Cutter).

Shipboard measurements were performed by Dr. Maija I. Heller for Fe(II) following a method by King et al. The photomultiplier (PMT) for analysis was borrowed from the FS Polarstern (Germany) at the North Pole from scientist Micha Rijkenberg, NIOZ, (Netherlands) since the PMT owned by UCSC (USA) was damaged at the beginning of the cruise, likely due to a bad power supply.

Dissolved Fe(II) concentrations were determined using an automated flow injection analysis system (FeLume II Waterville Analytical) employing a luminol chemiluminescence based detection system (King et al, 1995). Luminol, prepared in 18.2 MO Milli-Q water and buffered to pH 10.3 with ultra pure ammonia (here used quartz distilled at UCSC), reacts with an Fe(II)- containing solution, resulting in luminol oxidation with concurrent chemiluminescent emission (Rose & Waite, 2001; Croot & Laan, 2002). The FeLume was fitted with a standard quartz flow cell and a Hamamatsu HC135 photon counter configured with the following settings: flow rate: 2.5 mL per minute; photon counter integration time: 200 ms; load time: 20–40 s. The mixing and reaction occur in a spiral flow cell positioned in front of a photomultiplier tube. The sample and luminol reagent were directly continually mixed in the flow cell by omitting the injection valve (Rose & Waite, 2001; Hopkinson & Barbeau, 2007; Roy et al, 2008). Once the signal was in steady-state, the mean of the last 50 data points was used to determine the signal. Fe(II) was quantified by 6-8 standard additions of Fe(II) (typically 0.025, 0.05, 0.1, 0.2, 0.4 and 0.8 nM) to samples of seawater which were stored in the dark for several days and showed low initial signals for Fe(II). A 0.01 M Fe(II) stock solution was prepared with ferrous sulfate (Sigma) in 0.2 M HCl. A working Fe(II) standard solution (100 nM) was prepared daily. Detection limits were determined for dark corrected aged seawater samples where ferrous Fe was negligible based on a standard 3r evaluation of the baseline signal (Moffett, et al. 2007; Kondo & Moffett, 2013; Heller et al., 2017).

Instrumentation:
The chemiluminescence was detected with a Hamamatsu HC135 Photon counter build into a flow injection system (FeLume, Waterville Analytical, US) borrowed from NIOZ (Netherlands) as described above (Rijkenberg et al., 2014).

Problem Report:
The required PMT for analysis was borrowed from the FS Polarstern (Germany) at the North Pole from NIOZ scientist Micha Rijkenberg, (Netherlands), since the PMT owned by UCSC (USA) was damaged at the very beginning of the cruise, most likely by a faulty power supply.