Dataset: GP16 Thorium-234 Particulate
Deployment: TN303

Detection Limits: Limits of detection are not reported because they are not applicable to the Th-234 beta counting method. A 'non-detect' for Th-234 or a case where there is no Th-234 present (initially or after 6 months of decay) will still result in a measurable amount of background radioactivity due to the beta decay of long-lived natural radionuclides that are also collected on the pump filters. These background values are utilized and therefore, they are not reported as a non-detection of Th-234.

Uncertainty: At the start and conclusion of each cruise, high activity U-238 standards and background counts (empty detectors) were measured to confirm correct operation of the RISØ detectors and to determine detector to detector variability. The reported uncertainty on each particulate Th-234 measurement represents the propagated counting uncertainty and detector to detector normalization. Counting uncertainty is generally the largest source of uncertainty so whenever possible samples were counted until errors were below 5%.

Inter-calibration Efforts: Results from the GEOTRACES Th-234 inter-calibration efforts are published in Maiti et al. (2012). Fifteen labs participated in two cruises that centered on particulate, total and dissolved Th-234 inter-calibration. Particulate Th-234 inter-calibration was assessed for small particles collected on QMAs and SUPORs, as well as for large particles collected on 51 um screens. Additional laboratory experiments were performed to understand the effects of filter type, flow rate, particle loading, and other parameters on particulate Th-234 activity. Pertinent excerpts from Maiti et al. (2012) are included below:

'It is important to understand the inherent variability associated with initial sample collection and processing. The variability…may arise due to inter-filter variability, i.e., the variability associated with subsamples taken from the same filter, and inter-pump variability, i.e, the variability between pumps even when they are operating simultaneously for a fixed volume with similar flow-rates…Given the inter-pump and inter-filter variability, the inter-laboratory variability for both small and large particles appears to be reasonable. Overall, results from no laboratory was found to be consistently different from the mean 234Th activity reported for the inter-calibration exercise conducted on samples from stations with different particle types from the Atlantic (2008) and the Pacific (2009).'

'Particulate inter-calibration results indicate good agreement between all the participating labs with data from all labs falling within the 95% confidence interval around the mean for most instances. Filter type experiments indicate no significant differences in 234Th activities between filter types and pore sizes (0.2–0.8 µm). The only exception are the quartz filters, which are associated with 10% to 20% higher 234Th activities attributed to sorption of dissolved 234Th.'

'Overall, results from the current inter-calibration effort indicate that maximum variability can be expected in a high particle environment…Thus, for a station like SBB [Santa Barbara Basin], where fluxes can be measured from a typical six point profile in the upper 150 m of the water column, we can expect a maximum of 15% variability in the estimates between different laboratories based on our inter-calibration results. In comparison, we can expect good agreement in flux measurements amongst laboratories in an open ocean setting with lower 234Th deficiency.'

Because of the short half-life of Th-234 (~24 days), the nature of radionuclide analyses, and the high demand for particulate subsamples (the large size fraction in particular), inter-calibration between multiple labs was not possible during the 59-day GP16 cruise. However, the shallow, intermediate, and deep pump casts were made to overlap to provide some means of assessing cast to cast differences and reproducibility. While conditions can change during the duration of a pump cast (~4 hours) as well as from one cast to the next, noticeable differences (larger than expected for natural variability) in overlapping casts were only observed at one station during GP16. In addition, the same internal uranium standards were used by the Buesseler lab during the NAZT and EPZT GEOTRACES cruises and will continue to be used on future cruises. These standards provide a consistent means for calibration and comparison.

Problem reporting:
Empty fields denote that there were pump or filterhead issues that resulted in a compromised sample or no sample at this depth for either the QMA or screen or both. Data were flagged using the SeaDataNet quality flag scheme. For more information on SeaDataNet flags, see: https://www.geotraces.org/geotraces-quality-flag-policy/ and https://www.seadatanet.org/Standards/Data-Quality-Control

SeaDataNet quality flag definitions:
0 = No quality control;
1 = Good value;
2 = Probably good value;
3 = Probably bad value;
4 = Bad value;
5 = Changed value;
6 = Value below detection;
7 = Value in excess;
8 = Interpolated value;
9 = Missing value;
A = Value phenomenon uncertain.

BCO-DMO Processing:
- renamed fields;
- added date/time fields in ISO8601 format;
- 11 June 2020: replaced with GEOTRACES DOoR-formatted/IDP version.