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Dissolved hafnium (Hf), neodymium (Nd), rare earth elements (REE) concentrations and isotope in surface water measured on the Polarstern IPY/GEOTRACES cruise ANTXXIV/3

Originator's Protocol for Data Acquisition and Analysis

Sample Collection

Samples were collected during the International Polar Year (IPY) cruise ANTXXIV/3, this cruise was also part of the international GEOTRACES program. Between 100 and 130 L of seawater were taken for each sample mostly when the ship was underway. The collection was carried out in three different ways: (1) using a towed stainless steel fish, (2) using the ship's own seawater intake system, (3) Niskin bottles on a CTD rosette collecting water from 25 to 480 m water depth. All samples were filtered through a 142 mm MilliPore. filter disc with a pore size of 0.45 um into acid cleaned LDPE cubitainers. Filtration was performed within 12 h after collection or directly during collection when possible. Samples were then acidified to pH 2 with double distilled, concentrated HNO ₃ . From each sample an aliquot of 2 L was kept for concentration analysis. About 500-600 mg iron (Fe), in the form of previously cleaned dissolved Fe-chloride, was added to the large volume samples and left for 24 h for equilibration. In a second step, ammonia solution (25%, Merck Suprapur.) was added to bring the pH back to 8 in order to co-precipitate the dissolved trace metals (i.e. Hf and Nd) with Fe hydroxide (FeOOH). The supernatant was discarded and the residual FeOOH precipitate was transferred into pre-cleaned 2 L LDPE-bottles.

For sample preparation and Hf/Nd/REE extraction refer to originators published method in Stichel et al 2012. All methodology details provided have been taken from Stichel et al 2012.

Hf, Nd and REE concentration measurements

The Hf and Nd concentrations were obtained by isotope dilution (ID) following Rickli et al. (2009). Weighed spike solutions of ¹⁷⁸ Hf and a mixture of ¹⁵⁰ Nd/ ¹⁴⁹ Sm were added to an acidified 0.5 L aliquot of each sample. The samples were left for 4 to 5 days for complete equilibration. FeCl ₃ solution was added to the samples and Hf and Nd were co-precipitated with iron hydroxides by adding ammonia to raise the pH to 8. For mass spectrometric analysis, purification of Hf and Nd using a single cation chromatographic separation step (1.4 ml resin bed, Biorad AG50W-X8, 200-400 um mesh-size) was sufficient. Hafnium was loaded and collected in a mixture of 1 M HCl and 0.05 MHF (0.5 ml loading solution, 2 additional ml for elution), whereas Nd was collected in 5 ml 6M HNO ₃ , following the elution of barium in 12 ml of 2 M HNO ₃ . The respective cuts containing Nd and Hf were evaporated to dryness and then oxidized by adding 200 ul of a 1:1 mixture of 0.5 M HNO ₃ and H ₂ O ₂ (30 wt.%) to destroy remaining organic compounds originating from the resin before measurement on the MC-ICPMS. Replicates for each element were processed and yielded an external reproducibility of better than 1% for Nd and between 3% and 10% for Hf depending on concentration. Laboratory blanks were quantified by processing 0.5 L of MQ-water in the same way as the samples and corresponded to less than 1% for Nd, where no blank corrections were applied. The Hf content of the samples being very low was closer to the blank level. The blank amounted to 5 ±0.7 pg (n = 11, corresponding to 50% of the smallest sample), which was subtracted from all samples to achieve a correct seawater concentration.

The concentrations of all the naturally occurring rare earth elements (REE) concentrations were determined using a seaFAST system (Elemental Scientific Inc.) connected to an Agilent 7500ce ICP-MS. The seaFAST system performs preconcentration, matrix removal and online elution into the ICP-MS enabling direct analysis of trace metals from 4 ml of undiluted seawater (Hathorne et al., 2012). The overall reproducibility for all REE concentrations was better than 8% (1 SD), estimated from repeated measurements of a seawater sample (n = 10). The agreement for Nd concentrations between the seaFast measurements and isotope dilution is better than 10%.

Hf and Nd isotope measurements

The Hf isotope compositions were measured on a Nu Plasma HR MC-ICPMS at IFM-GEOMAR in manual time resolved mode. The samples were dissolved in 250 to 500 ul 0.5 M HNO ₃ /0.1 M HF to obtain Hf concentrations of approximately 20 ppb corresponding at total beam of at least 4 V. The measured isotope ratios were integrated over the time of analysis (3-5 min) and corrected for instrumental mass fraction to ¹⁷⁹ Hf/ ¹⁷⁷ Hf = 0.7325 applying an exponential mass fractionation law. All ¹⁷⁶ Hf/ ¹⁷⁷ Hf ratios were normalized to the accepted literature value of 0.28216 (Nowell et al., 1998). The external reproducibility for these samples was ±0.8 epsilon Hf and was estimated by repeated measurements (4 to 6 times each session) of the JMC 475 standard.

The Nd isotope composition was measured on the Nu Plasma instrument. The measured isotopic composition was corrected for instrumental mass bias to ¹⁴⁶ Nd/ ¹⁴⁴ Nd = 0.7219 applying an exponential mass fractionation law. All ¹⁴³ Nd/ ¹⁴⁴ Nd ratios were normalized to the accepted value for the JNdi-1 standard of 0.512115 (Tanaka et al., 2000). The external reproducibility was between ±0.3 and ±0.39 epsilon Nd units (2 S.D.) estimated by repeatedly measuring JNdi-1 and an internal laboratory standard.

References Cited

Chu, N.C., Taylor, R.N., Chavagnac, V., Nesbitt, R.W., Boella, R.M., Milton, J.A., German, C.R., Bayon, G., Burton, K., 2002. Hf isotope ratio analysis using multi-collector inductively coupled plasma mass spectrometry: an evaluation of isobaric interference corrections. J. Anal. At. Spectrom. 17, 1567-1574.

Rickli, J., Frank, M., Halliday, A.N., 2009. The hafnium-neodymium isotopic composition of Atlantic seawater. Earth and Planetary Science Letters 280, 118-127.

Stichel T., Frank M., Rickli J., Hathorne E. A., Haley B. A., Jeandel C., Pradoux C., 2012. Sources and input mechanisms of hafnium and neodymium in surface waters of the Atlantic sector of the Southern Ocean. Geochimica et Cosmochimica Acta 94, 22-37

Nowell G. M., Kempton P. D., Noble S. R., Fitton J. G., Saunders A. D., Mahoney J. J. and Taylor R. N., 1998. High precision Hf isotope measurements of MORB and OIB by thermal ionisation mass spectrometry: insights into the depleted mantle. Chem. Geol. 149, 211-233.

Tanaka T., Togashi S., Kamioka H., Amakawa H., Kagami H., Hamamoto T., Yuhara M., Orihashi Y., Yoneda S., Shimizu H., Kunimaru T., Takahashi K., Yanagi T., Nakano T., Fujimaki H., Shinjo R., Asahara Y., Tanimizu M. and Dragusanu C., 2000. JNdi-1: a neodymium isotopic reference in consistency with LaJolla neodymium. Chem. Geol. 168, 279-281.

BODC Data Processing Procedures

Data were submitted to BODC in a text format. Sample metadata provided (Station, CTD cast, lat, lon, depth and date) were checked against information held in the database and the cruise report. Bottle numbers have been assigned to the data by BODC based on information provided, however to gain the required sample size for Hf/Nd/REE analysis water from multiple Niskin bottles was combined. BODC's database structure only allows the assignment of one Niskin bottle and one CTD cast to each sample. Bottle number for these samples is thus arbitrator and was assigned at random from the bottles fired at the correct depth and CTD cast.

Station number and CTD numbers submitted to BODC did not always match the final event log. The following errors were found in the naming of events submitted. Files were renamed and corrected to match the final Event log and bottle file;

Originators label 116-1 was changed to 117-1 as 116-1 was an UCCTD event not an iron fish event
Original name 241_7 (Hand net) changed to 241_8 (CTD)
Original name 104-3 no samples collected at 75 m changed to 104-6
Original name 161-2 no samples collected at 100 m changed to 161-4
Original name 244-8 no samples collected at 25 and 50 m changed to 244-9 50 m and 244-10 25 m.

Data were loaded into BODC's database from the text file but depths from the seabird files were used over the originators round up/down depth.

The mapping between the originator's variable and BODC parameter codes is detailed in the table below;

Data Quality Report

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