Readme file for: Seawater dissolved chromium concentration, redox speciation, and stable isotope composition in the North Pacific Ocean

***** Dataset abstract *****
Dissolved seawater chromium concentrations, redox speciation, and stable isotope composition were measured on samples collected in the North Pacific Ocean. Samples were collected over diel cycles (2 for stations 1-5, 1 for station 6) on board the RV Kilo Moana cruise KM1713 from Seward Alaska to Honolulu Hawai’i. Sampling stations spanned the subarctic North Pacific (stations 1 & 2), the dynamic subarctic-subtropical convergence zone (stations 3 and 4) and the subtropical North Pacific (stations 5 and 6). Chromium was enriched from filtered samples by Mg(OH)2 co-precipitation and analyzed by MC-ICP-MS using either isotope dilution (Cr redox speciation) or double spike methodology.

***** Original data collection *****
Seawater samples were collected with a conventional CTD rosette. Samples were gravity filtered from rosette Niskin bottles through 0.2 µm Acropak filters. Chromium redox speciation samples were processed at sea by Mg(OH)2 co-precipitation followed by centrifugation and decanting of the supernatant. Sample pellets were dissolved in 1% HNO3 and stored until analysis. Chromium concentration and stable isotope samples were acidified in Bern, Switzerland with 2 mL L-1 concentrated HCl and stored until processing. Sample chromium was removed from the sample matrix and purified by chromatographic separation in one step (Cr redox speciation) or two steps (Cr concentration and stable isotope composition).

Samples were analyzed by Multi Collector ICP-MS using a Neptune Plus instrument (ThermoFisher). The reference material NIST SRM 979 was analyzed with each set of samples, as well as standard seawater samples. Sample purification and analytical procedures are described fully in Rickli et al. (2019). Nitrate + NO2 data were determined by autoanalyzer. Depth, temperature and salinity data were determined by CTD, and potential temperature and neutral density were calculated from CTD-measured parameters using Ocean Data View (Schlitzer, 2018).

***** Data processing *****
Raw data were processed using an R script following Rudge et al. (2009), including corrections for isobaric interferences on Cr from Ti, V and Fe.

***** Quality checking *****
The following quality checking procedures were used:
-Blanks were regularly measured, at a minimum at a rate of 1 blank per 11 samples (one batch of 12 columns for chromatography). Concentration samples were blank corrected. Blank contributions were negligible relative to sample Cr for stable isotope samples (approximately 1% or less) and no blank corrections were made to stable isotope samples.
-Outliers were present in chromium redox samples. These were identified by a modified Thompson τ test. Given the small percentage of Cr total present as Cr(III), it is believed that these outlier samples are due to insufficient removal of the seawater supernatant, which contains approximately 90% or more of the total Cr.
-The NIST SRM 979 reference material was measured with each set of isotope analyses in both pure form (N ≥ 6) and spiked form (N ≥ 11) to monitor and confirm instrument accuracy and stability.
-Reference seawater standard were included with each set of analyses.
-Full replicates as well as replicates samples from stations and depths but collected at different times were processed and are presented here, confirming external reproducibility

External reproducibility was determined for Cr concentration and stable isotope samples following Kenney & Keeping (1951) and determined to be 0.033‰ (2 SD) for δ53Cr and 0.82% (1 RSD) for Cr concentration samples.

***** Standards *****
Isotopic data are reported in delta units relative to the NIST SRM 979 standard.

***** Further information for interpreting the data and using the dataset *****
Samples were collected as part of a pseudo-Lagrangian study from Seward, AK USA to Honolulu HI USA. For stations 2-5, a profiling float was deployed and followed for at least 2 diel cycles. For station 6, this lasted only 1 diel cycle. For station 1, no float was deployed and the diel sampling was carried out at a fixed position.

***** Dataset contents *****
-Janssen_etal_KM1713_Cr_Data.xlsx
-Janssen_etal_KM1713_Cr_Data_header.txt
-Janssen_etal_KM1713_CrIII_Data.xlsx
-Janssen_etal_KM1713_CrIII_Data_header.txt
-Janssen_etal_KM1713_Readme.txt

***** Dataset contact *****
Janssen, David J. University of Bern, Switzerland. ORCID: 0000-0002-9091-8936
Email: david.janssen@geo.unibe.ch; janssen.davej@gmail.com

***** Dataset citation *****
Janssen, D.J., Rickli, J., Quay, P.D., White, A.E., Nasemann, P., Jaccard, S.L. (2019) Seawater dissolved chromium concentration, redox speciation, and stable isotope composition in the North Pacific Ocean. (Version 1.0) [Data set]. Zenodo. http://doi.org/10.5281/zenodo.3560082