In this study, we report chromium (Cr) isotopes for Cr(III), Cr(VI), and total Cr from three stations within the eastern tropical North Pacific (ETNP) oxygen deficient zone (ODZ) (see Figs. 1 and 2 of Huang, et al. 2021). Two of these stations (P1 and P2) were collected on cruises Roger Revelle 1804-5 (April to May 2018) and KM1920 (September 2019). The third (2T) was collected on cruise New Horizon 1410 (May 2014) whose primary data are reported in Moos, et al. (2020). Water samples were collected using Niskin Bottles on a CTD rosette or Niskin-X bottles on a TE clean rosette.

To separate Cr species and analyze their isotopic compositions, we used a Mg(OH)2 coprecipitation method similar to that reported in Janssen, et al. (2020) and Davidson, et al. (2019) with some differences in the details:

(1) A 50Cr–54Cr double spike was added to each sample before collecting Mg(OH)2. The double-spike was allowed to equilibrate with samples for ∼6 hours. Tests of equilibration time between 0.5 and 12 hours show no difference in isotopic values.

(2) No iron coprecipitation was done to separate Cr(III) from the Mg matrix. The Mg(OH)2 pellets were dissolved in HCl, pH-adjusted, and ready for column chromatography as described in Moos and Boyle (2019).

(3) To determine Cr(VI) isotopic composition, we filtered the supernatant after the Mg(OH)2 coprecipitation and acidified it to pH 1.9 with HCl for an extended time afterward.

The Cr(VI) that was left in solution converts to Cr(III) which we then analyze by a second double-spike Mg(OH)2 coprecipitation and column purification. Some Cr(VI) isotopic compositions were calculated using the concentration and isotopic composition of total Cr and Cr(III) by mass balance.

Additionally, we analyzed Cr(III) concentrations and isotope ratios by Cr(III) double-spike isotope dilution either at sea or immediately following thawing of frozen samples. Three samples were processed in both ways and all give the same results. The method for total Cr concentration and isotopic composition is described in detail in Moos and Boyle (2019). All of our samples have a within-run error of 0.02 to 0.19‰ (2 SE) on δ53Cr. The average [Cr] and δ53Cr values of a long-term in-house seawater standard are 3.20 ± 0.12 nmol/kg (SD, n = 20) and 1.02 ± 0.13‰ (2 SD, n = 20), respectively.

Instruments:
All Cr isotope measurements were made on an IsoProbe MC-ICP-MS. A 'peak-jump' mode was applied in the Cr isotope analysis on IsoProbe MC-ICP-MS. The plasma mass spectrometer was tuned on an 100 mM NH4S2O8 solution with 1 µM Cr to minimize polyatomic sulfur interferences (i.e. 32S16O1H+ on mass 49, 34S16O+ on mass 50, and 34S16O1H+ on mass 51) by lowering the signal of mass 49 (polyatomic) relative to mass 52 (Cr). Each sample was bracketed by two SRM-DS mixtures with the same sample Cr to DS ratio and similar signal level (within 10%). The δ53Cr data was calculated by iteration on each isotope correction and instrumental mass fractionation (β) in an Excel spreadsheet (Moos and Boyle, 2019). The [Cr] concentration was calculated by averaging the results from the single isotope dilution formula using corrected 50Cr and 54Cr.