The tracer, 76 kg of trifluoromethyl sulphur pentafluoride (CF3SF5), was released from DIMES cruise RR0901 (US1) on the 27.906 kg m-3 neutral density surface (in the Upper Circumpolar Deep Water near 1500 m depth) west of the Drake passage in the Antarctic Circumpolar Current near 58° S, 107° W, in early February 2009 (Ledwell et al., 2011). The tracer patch was surveyed one year later during the DIMES cruise TN246 (US2) West of Drake passage in January 2010.
During JC054 the tracer was surveyed from 58° W to 79° W in the region of Drake passage 22 months after the release. A total of 1350 seawater samples including approximately 10% of duplicates were measured. Water samples were collected from 10 litre Niskin bottles from a 24-rosette sampling system mounted on a Sea-Bird 9/11 plus CTD. The Niskin nitrile 'O' rings were first washed in isopropanol and baked in a vacuum oven for 24 hours to remove susceptible contamination before installation in individual Niskin bottles. Water samples were collected in 2 litre ground-glass stoppered bottles that were filled from the bottom using Tygon tubing and overflowed one time to expel all water exposed to the air. Immediately after sampling, the glass bottles were immersed in a cool box of cold deep seawater in the sampling hangar until the analysis. Ice packs were added as necessary to maintain a temperature below 5 °C and prevent degassing.
Sample analysis was performed as soon as possible within six hours of the sampling. Samples were introduced to the system by applying nitrogen (N2) pressure to the top of the sample bottles, forcing the water to flow through and fill a 1135 cm3 calibrated volume. The measured volumes of seawater were then transferred to a purge and trap system, entering the sparge tower under vacuum. The water was sparged with a N2 flow at 270 ml min-1 for three minutes and trapped at -110°C on a Unibeads 3S trap (two inches of 1/8 inch tubing) immersed in the headspace of liquid nitrogen. The traps were heated to 110°C and injected into a gas chromatograph. The CF3SF5, CFC-13 and CFC-12 separation was achieved using a 1 m Porasil B packed pre-column and a 1.5 m carbograph AC main column. A six inch molecular sieve post column was used to remove N2O. The three columns were kept in the oven at 75°C. The carrier gas, N2, was cleaned by a series of purifying traps (VICI nitrogen purifier and oxygen trap). The running time per sample was approximately 13 minutes.
The CF3SF5, CFC-13 and CFC-12 concentrations in air and water were calculated using external gaseous standards. The working standard was supplied by National Oceanic and Atmospheric Administration (NOAA) (Brad Hall, March 2010). It corresponds to clean dry air enriched in CF3SF5 inside a 29 litre Aculife-treated aluminum cylinders. The standard was intercalibrated for the tracer CF3SF5with Ledwell 5B tank during the cruise with the instrument, which has been calibrated by Busenberg (pers comm.). The tracer was found to give a very nearly linear response over a large range and so a linear calibration was used for all the levels that we encountered The routine calibration curves were made by multiple injections of 9 different volumes (0.1, 0.25, 0.3, 0.5, 1, 2, 3, 5, 8 ml) of standard that span the range of tracers measured in the water for CF3SF5 and SF6). Multiple injections of large loops of standard, up to 136 ml were made to calibrate CFC-12 and CFC-13 in surface waters as the large volume of seawater required for the tracer was inappropriate for surface seawater measurements of CFC-12 and CFC-13. Routine calibration curves were made when time permitted, approximately once a day. The changes in the sensitivity of the system were tracked by measuring a fixed volume of standard gas approximately every two hours and used to adjust the calibration curves respectively. The calibration precision was better than 1% for the tracer CF3SF5 and SF6, and for CFC-13, CFC-12 levels at the target density range. For high surface values of CFC-12 and CFC-13 the calibration precision was estimated to be only 5%.
The other compound blanks were assessed in more detail and were accounted for in the final data. Sparging efficiency was determined by successive resparge of a single sample until complete no further compound could be detected. The final data set was corrected for sparging efficiency accordingly (see table below).
| Tracer | Sparge efficiency |
|---|---|
| CFC-13 | 91.7% |
| CF3SF5 | 96.1 % |
| CFC-12 | 88% |
| CF6 | 90.2% |
The precision (or reproducibility) for the water samples measurements were determined from replicate samples drawn on the same Niskin. In total, 100 duplicate samples were drawn from the rosette, spanning the all range of concentrations encountered along the cruise. The average standard deviation for duplicates was 0.005 fmol l-1 (or 1% if greater) for CF3SF5 0.02 fmol l-1 for CFC-13 and 0.007 pmol l-1 for CFC-12 and 0.005 fmol l-1 for SF6
An unusually large volume of water (1135 ml) was analysed in order to increase the detection limit of the CF3SF5 in water as less tracer was released than originally planned. The detection limit for CF3SF5 was 0.003 fmol l-1.
The instrumentation was built and developed at the University of East Anglia (UEA) from the Lamont Doherty Earth Observatory (LDEO) design (Smethie et al., 2000). The system was set up in the UEA laboratory container which was installed on the after deck of the RRS James Clark Ross. The purge and trap system was interfaced to an Agilent 6890N gas chromatograph with electron capture detector (MicroECD at 320°C).
Ledwell, J. R., L. C. St. Laurent, J. B. Girton, and J. M. Toole, 2011. Diapycnal mixing in the Antarctic Circumpolar Current, Journal of Physical Oceanography, 41, 241-246.
Smethie, W. M, Schlosser, P., BöNisch, G., Hopkins, T. S., 2000. Renewal and circulation of intermediate waters in the Canadian Basin observed on the SCICEX 96 cruise, 105(C1), 1105-1121.
Data were received in one Matlab file containing all CTD stations. Data received were loaded into the BODC database using established BODC data banking procedures. CTD salinity and CTD temperature were provided in the Matlab file but were not loaded. These are available on request. All quality control flags provided by the originator were converted into BODC standard flags, 2 (valid) = no flag, 3 (questionable) and 4 (not valid) = 'L'. In addition all CF3SF5 values identified below the instrument detection limits were flagged as such and all absent data values were removed. Instrument detection limits were not provided for the other variables therefore zero values where flagged as improbable. The data were screened in-house and units were converted from mole l-1 to either pmol l-1 or fmol l-1 prior to loading. The following table shows how the variables were mapped to appropriate BODC parameter codes:
| Originator's Parameter | Unit | Description | BODC Parameter Code | BODC Unit | Comments |
|---|---|---|---|---|---|
| SF6 | mole l-1 | Sulphur hexafluoride (SF6) | DSF6GCDX | fmol l-1 | Unit conversion from mole l-1 to fmol l-1 by * 1015 |
| F13 | mole l-1 | Trifluoro chloromethane (CFC-13) | CFC13GCD | fmol l-1 | Unit conversion from mole l-1 to fmol l-1 by * 1015 |
| SF5CF3 | mole l-1 | Trifluoromethyl sulphur pentafluoride (CF3SF5) | CF3SF5CD | fmol l-1 | Unit conversion from mole l-1 to fmol l-1 by * 1015 |
| F12 | mole l-1 | Dichlorodifluoromethane (CFC-12) | FR12GCTX | pmol l-1 | Unit conversion from mole l-1 to fmol l-1 by * 1012 |
None (BODC assessment).
None (BODC assessment).