Dataset: Dissolved iron concentrations from an iron incubation experiment using upwelled waters in the California Current System during the PUPCYCLE II cruise in May and June 2023

Data collection took place on the R/V Sally Ride from May 29th to June 10th, 2023. To simulate upwelling conditions under different iron treatments, an onboard incubation experiment was conducted. Seawater for the incubation experiment was collected within the northern CCS, off the southern coast of Oregon at 43°02'42.7"N, 124°33'07.2"W. The collected seawater was deemed as freshly upwelled water and taken from a depth of 55 m – which corresponds to a depth slightly below the euphotic zone receiving less than 1% irradiance – using trace-metal clean techniques on May 30th, 2023, 13:30 GMT. The seawater was pumped and homogenized using trace metal clean techniques, then transferred into a total of thirty 20 L low-density polyethylene cubitainers. Three cubitainers were immediately harvested for the initial timepoint (T0). The remaining twenty-seven cubitainers were assigned treatments: nine were unamended (labeled Ctrl), nine were amended with 5 nM FeCl2 (labeled +Fe), and nine were amended with 200 nM desferrioxamine B, a strong iron chelator that inhibits dissolved iron uptake (labeled DFB). The cubitainers were placed in an on-deck incubator covered with two layers of neutral density screening to simulate 26% of incidence irradiance supplied with flow-through surface seawater to maintain ambient surface temperature. Three cubitainers from each treatment were harvested for each of the three subsequent timepoints: 48 hours (T1), 168 hours (T2), and 264 hours (T3) after incubation.

Iron Incubation Experimental Set-Up. The seawater for the iron incubation experiment was pumped into a positive pressure trace metal clean laboratory and then into 50-gallon acid-washed high-density polyethylene (HDPE) drums. Seawater in these drums was homogenized using a Wilden air-operated double diaphragm pump made of polytetrafluoroethylene and acid-washed HDPE tubing. The seawater was then transferred into a total of thirty 20 L low-density polyethylene cubitainers. 

Dissolved Iron Sampling. Samples for measurement of dissolved iron were filtered directly from the cubitainer through an acid-cleaned Acropak supor membrane cartridge filter with 0.2 micrometer pore size. Filtered samples were collected in acid-cleaned LDPE bottles, which were rinsed three times with sample prior to filling. Samples were acidified at sea with Optima hydrochloric acid to a pH of ~1.8. 

Dissolved Iron Analysis. Samples were analyzed post-cruise in a trace metal clean room using the methods of Lohan et al. (2006) with modifications as described in Biller et al. (2013). This method involves pre-concentrating iron on a chelating resin; we followed Biller et al. (2013) and used Toyopearl Chelate-650 resin, which recovers >93% of Fe(III) at pH 2. We added a small amount of Optima ammonium hydroxide to each sample before analysis to increase the pH from 1.8 to 2. We also added hydrogen peroxide (10 micromolar) to each sample ten minutes before analysis to ensure quantitative oxidation of all Fe(II) to Fe(III), which Lohan et al (2005) found to be effective. After pre-concentration, the iron is eluted into a reaction stream with the colorimetric agent N,N-dimethyl-p-phenylenediaminedihydrochloride (DPD), which turns deeper pink the more iron is present. The signal is detected with a flow-through spectrophotometer. 

Standards were made by spiking low metal seawater with known amounts of added iron and creating a standard addition curve. Blanks were assessed by analyzing acidified MilliQ as a sample. Reference samples (Geotraces GSC and D1) measured in the same analytical runs as the samples were in line with consensus values. 

Note about the DFB treatments: high concentrations (>10nM) of dissolved Fe were measured in the treatments with added DFB. We think this is likely because the strong chelator solubilized Fe. It is also possible there was some Fe in the DFB spike. Regardless, the dissolved Fe present in the DFB treatments was almost certainly not biologically available, as it was chelated with DFB. 

Notes about methods papers cited:
* Lohan et al. (2005, doi: 10.1016/j.aca.2004.09.005) - This paper shows that 10 micromolar H2O2 and ten minutes is sufficient to quantitatively oxidize Fe(II) to Fe(III) in acidified seawater
* Lohan et al. (2006, doi: 10.4319/lom.2006.4.164) - The main methods paper, but we used some modifications as in Biller et al., 2013. 
* Biller et al. (2023, doi: 10.1016/j.csr.2013.07.003) - The paper that has the modifications we used to Lohan et al 2006's method.