Dataset: Chromium (Cr) concentration and isotope data of chelex-100 extraction method from samples collected on R/V New Horizon cruise NH1410 in May-June 2014

ValidatedFinal no updates expectedDOI: 10.26008/1912/bco-dmo.925670.1Version 1 (2024-04-23)Dataset Type:Cruise Results

Principal Investigator: Edward A. Boyle (Massachusetts Institute of Technology)

Student: Tianyi Huang (Massachusetts Institute of Technology)

Student: Simone B. Moos (Massachusetts Institute of Technology)

BCO-DMO Data Manager: Shannon Rauch (Woods Hole Oceanographic Institution)


Project: Cr Isotope Oceanography of the Eastern Tropical North Pacific Ocean (ETNP Cr Isotopes)


Abstract

Changes in chromium (Cr) isotope ratios due to fractionation between trivalent (Cr(III)) and hexavalent (Cr(VI)) are being utilized by geologists to infer oxygen conditions in past environments. But there is little information available on Cr in the modern ocean to ground-truth these inferences. Transformations between the two chromium redox species are important processes in oceanic Cr cycling. Here we present profiles of hexavalent and trivalent Cr concentrations and stable isotope ratios from...

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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.


Related Datasets

IsRelatedTo

Dataset: Cr concentration and isotope data using dissolved Cr and different Cr species from RR1805
Boyle, E. A. (2024) Chromium (Cr) concentration and isotope data determined using dissolved Cr and different Cr species from samples collected on R/V Roger Revelle cruise RR1805 in April-May 2018. Biological and Chemical Oceanography Data Management Office (BCO-DMO). (Version 1) Version Date 2024-04-23 doi:10.26008/1912/bco-dmo.925569.1

Related Publications

Results

Huang, T., Moos, S. B., Boyle, E. A. (2021). Trivalent chromium isotopes in the eastern tropical North Pacific oxygen-deficient zone. Proceedings of the National Academy of Sciences, 118(8). https://doi.org/10.1073/pnas.1918605118
Methods

Davidson, A. B., Semeniuk, D. M., Koh, J., Holmden, C., Jaccard, S. L., Francois, R., & Crowe, S. A. (2019). A Mg(OH)2 coprecipitation method for determining chromium speciation and isotopic composition in seawater. Limnology and Oceanography: Methods, 18(1), 8–19. Portico. https://doi.org/10.1002/lom3.10342
Methods

Janssen, D. J., Rickli, J., Quay, P. D., White, A. E., Nasemann, P., & Jaccard, S. L. (2020). Biological Control of Chromium Redox and Stable Isotope Composition in the Surface Ocean. Global Biogeochemical Cycles, 34(1). Portico. https://doi.org/10.1029/2019gb006397
Methods

Moos, S. B., and Boyle, E. A. (2019). Determination of accurate and precise chromium isotope ratios in seawater samples by MC-ICP-MS illustrated by analysis of SAFe Station in the North Pacific Ocean. Chemical Geology, 511, 481–493. https://doi.org/10.1016/j.chemgeo.2018.07.027
Related Research

Moos, S. B., Boyle, E. A., Altabet, M. A., & Bourbonnais, A. (2020). Investigating the cycling of chromium in the oxygen deficient waters of the Eastern Tropical North Pacific Ocean and the Santa Barbara Basin using stable isotopes. Marine Chemistry, 221, 103756. https://doi.org/10.1016/j.marchem.2020.103756