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Total and particulate ²³⁴Th activity and POC collected during MD166 BONUS-GoodHope (IPY-GEOTRACES)

Originator's Data Acquisition and Analysis

Eleven stations were sampled for total ²³⁴ Th activity. Among these, five stations were also sampled for particulate ²³⁴ Th (S1 to S5) in order to obtain POC/ ²³⁴ Th ratios of sinking particulate matter and derive POC export fluxes.

Analytical procedures

Total ²³⁴ Th activities were obtained from small volume (4 L) seawater samples collected from 12 L Niskin bottles. The samples for super stations (S1 to S5) were taken at 19-20 depths between the surface and 1000 m. For large stations L2, L4 and L6, samples were collected at 5, 9 and 8 depths between surface and 300, 200 and 250 m depth, respectively. For large stations L3, L5 and L7, 15-17 samples were collected between the surface and 1000 m depth.

Seawater samples were processed for total ²³⁴ Th activity measurement following the procedure developed by Pike et al. (2005). Samples were acidified to pH 2 using nitric acid and spiked with a known amount of ²³⁰ Th which serves as a yield monitor for ²³⁴ Th recovery. After 12 h equilibration the pH was raised to 8.5 using concentrated NH ₄ OH and Th was co-precipitated by adding 100 uL of MnCl ₂ (2.0 g L ^-1 ) and 100 uL of KMnO ₄ (7.5 g L ^-1 ). The samples were allowed to stand for 12 h before filtration on high-purity quartz microfiber filters (QMA, Sartorius; nominal pore size = 1 um;25 mm). Filtered samples were dried overnight, mounted on nylon filter holders and covered with Mylar film and Al foil. On board each sample was counted twice using a low level beta counter (RISO, Denmark). Beta counting was allowed to proceed till counting uncertainty was below 2% RSD. Residual beta activity was estimated for each sample after a delay of six ²³⁴ Th half-lifes (around 6 months) and was subtracted from the gross counts recorded on-board.

For details of Th recovery please see Planchon et al. (2012)

The parent ²³⁸ U activity was estimated with salinity measurements using the relationship of Pates and Muir (2007): 238U (dpm L ^-1 ) = (0.0713 ± 0.0012)* Salinity. The standard deviation for this calculation was the same for all samples 0.04 dpm/L ^-1 (0.71 Millibequerels per litre).

Measurements of particulate ²³⁴ Th

For particulate ²³⁴ Th and POC, suspended particulate matter was collected at five stations (S1, S2, S3, S4 and S5) via in situ large-volume filtration (150-2000 L) systems (Challenger Oceanics and McLane WTS6-1-142LV pumps) equipped with 142 mm diameter filter holders. Water was pumped through two successive filters: (i) a 53 um mesh nylon screen (filter SEFAR-PETEX; polyester) and (ii) a QMA quartz fiber filter (~1 um porosity, Pall Life). Prior to use, PETEX screens were conditioned by soaking in HCl 5 %, rinsed with Milli-Q grade water, dried at ambient temperature under a laminar flow hood and were stored in clean plastic zip-bags till use. QMA filters were conditioned by combustion at 450°C during 4 h and were stored in clean plastic zip-bags till use.

Filters were partitioned amongst the different end-users using sterile scalpels and a custom-build INOX steel support for the PETEX screens and a plexiglass punch of 25 mm diameter for the QMA filters. For large size particles (>53 um), particles on the PETEX screen parts dedicated to ²³⁴ Th were re-suspended in filtered seawater in a laminar flow hood, and collected on 25 mm diameter silver filters (1.0 um porosity). Silver and QMA filters were dried overnight, and once mounted on nylon holders and covered with Mylar and aluminum foil, were ready for beta counting. As for total ²³⁴ Th activity, particulate samples were counted twice on board until relative standard deviation was below 2 %. Residual beta activity was measured in the home-based laboratory after six ²³⁴ Th half-lives (round 6 months).

Particulate POC

Following beta counting, particulate samples (QMA and Ag filters) were processed for POC measurement by elemental analyser isotope ratio mass spectrometer (EA-IRMS). Size-fractionated samples were dismounted from filter holders and fumed under HCl vapor during 4 h inside a glass desiccator, to remove the carbonate phase. After overnight drying at 50 °C, samples were packed in silver cups and analysed with a Carlo Erba NA 2100 elemental analyser configured for C analysis and coupled on-line via a Con-Flo III interface to a Thermo-Finnigan Delta V isotope ratio mass spectrometer. Acetanilide standard was used for C concentration calibration and C blanks were 0.98 umol and 0.54 umol for QMA and silver filters, respectively.

References

Planchon F., A.-J. Cavagna, D. Cardinal, L. André, F. Dehairs, (2012). Late summer particulate organic carbon export and twilight zone remineralisation in the Atlantic sector of the Southern Ocean. Biogeosciences Discussion, 9, 3423-3477.

Pike, S. M., Buesseler, K. O., Andrews, J., and Savoye, N. (2005) Quantification of 234Th recovery in small volume sea water samples by inductively coupled plasma-mass spectrometry, J. Radioanal. Nucl. Ch., 263, 355-360.

Pates, J. M. and Muir, G. K. P.(2007). U-salinity relationships in the Mediterranean: Implications for 234Th:238U particle flux studies, Mar. Chem., 106, 530-545.

BODC Data Processing Procedures

Data were received by BODC in two Microsoft Excel spreadsheets. Each spreadsheet contained the following metadata fields: station, latitude, longitude, depth. Sample metadata was checked against information held in the database, no discrepancies were found

Total SPM concentrations for ²³⁴ Th _p and POC were not stored in the database since this parameter is calculated by the sum of both fractions i.e. 1-53 um and >53 um.

Parameter codes defined in the BODC parameter dictionary were mapped to the variables as follows:

Data Quality Report

No data quality issues to report.