AMT14 POC and 234Thorium measurements from CTD bottle samples
Originator's Protocol for Data Acquisition and Analysis
Seawater samples for 234 Th analysis were collected from a designated CTD cast every 3 days. Twenty litre water samples were collected from 9 depths on each cast. The sampling distribution was concentrated in the surface 300 m, with greatest sampling resolution in the upper 100 m.
In addition, for each of the sample depths a further 2 litre sample was filtered onto GF/F filters for particulate organic carbon (POC).These samples were frozen at -20°C and stored in a dark room at Southampton Oceanography Centre.
The analytical methods are described in Thomalla et al. (2006) have been summarised here. The total 234 Th in each sample was measured by adding potassium permanganate (KMnO 6 ), manganese dichloride (MnCl 2 ) and concentrated ammonia (NH 3 ) to each 20L sample. MnO 2 precipitate was allowed to accumulate for 8 hours and was then filtered onto 142 mm 0.8 µm polycarbonate filters that were folded and wrapped in mylar foil. Analysis for total 234 Th activity was carried out onboard using non-destructive beta counting on a RIS0 National Laboratory low background gas flow counter, operated in anti-coincidence mode. Additionally samples were analysed at multiple times over the following months and the data were decay corrected to the point of sample collection.
POC was measured after inorganic carbonates were removed from the filters by acidification with fuming concentrated hydrochloric acid. The filters were dried in a 60°C oven for 24 hrs, packaged in pre-combusted aluminium foil (Hilton et al., 1986) and analysed on a Thermo Finnegan flash EA122 elemental analyser using acetanilide as calibration standard.
References Cited
Hilton, J., Lishman, J.P., Mackness, S. and Heaney, S.I., 1986. An automated method for the analysis of particulate carbon and nitrogen in natural waters. Hydrobiologica, 141, 269-271.
Thomalla, S., Turnewitsch, R., Lucas, M. and Poulton, A., 2006. Particulate organic carbon export from the North and South Atlantic gyres: The 234Th/238U disequilibrium approach. Deep-Sea Res. II, 53, 1629-1648.
Instrumentation Description
Not relevant to this data set.
BODC Data Processing Procedures
The data from AMT14 were submitted to BODC in Microsoft Excel spreadsheet format and archived with accession number UCT060503. The metadata matched the records for the cruise already held at BODC. The 234 Th data units were changed from disintegrations per minute per litre (dpm l -1 ) to milli-Bequerels per litre (mBq l -1 ) to be consistent with other 234 Th measurements from other cruises. New parameter codes were setup for the Thorium methodology and analysis.
A parameter mapping table is provided below;
| Originator's Parameter | Units | Description | BODC Parameter Code | Units | Comments |
|---|---|---|---|---|---|
| POC | µmol l -1 | Concentration of carbon (organic) {POC} per unit volume of the water body [particulate >GF/F phase] by filtration, acidification and elemental analysis | CORGCAP1 | µmol l -1 | - |
| 234Thorium | mBq l -1 | Activity of thorium-234 {234Th} per unit volume of the water body [dissolved plus reactive particulate] by beta spectroscopy | T234MNTX | mBq l -1 | - |
| 234Thorium SD | mBq l -1 | Activity standard deviation of thorium-234 {234Th} per unit volume of the water body [dissolved plus reactive particulate] by beta spectroscopy | T234SDTX | mBq l -1 | - |
Data Quality Report
BODC were not advised of specific quality checks carried out by the data originator but were advised that the second batch of filters on the cruise had unusually high background counts. The propagated errors for total 234 Th activities were therefore higher on these CTD samples (CTDs 43, 52, 62, 70, 79 and 88) than the CTD samples analysed with the first filter batch (CTDs 5, 14, 25 and 35).
Problem Report
Not relevant to this data set.
