Sampling strategy and techniques

Radium sampling was performed during six expeditions to the Atlantic sector of the Southern Ocean in 1998, 1999 and 2000. in the following, expeditions with RV POLARSTERN are uniformly labelled "AMT" and one cruise with the US-research vessel NATHANIEL B. PALMER is given the abbreviation "NPB" (Fig. 11).

Fig. 11: Map of the Atlantic sector of the Southern Ocean with the sampling tracks for radium during expeditions with RV POLARSTERN (labelled ANT) and the US research vessel NATHANIEL B. PALMER (labelled NBP) from 1998 to 2000. The shaded box indicates a grid survey at the Polar Front at 20' E and dots refer to deep water stations during expedition ANT XVIl3.

Material and methods

A common method for the concentration of radionuclides from a large volume of sea water was followed, using Mn02-coated cartridge filters. The adsorbing power of Mn02 had already been observed in the context of trace element abundances in manganese ores (Ljunggren 1955). Radionuclides and a variety of other metals get adsorbed on the Mn02-coating of the cartridges. Prewound polypropylene filter cartridges (CUNO Micro Wind or Hytrex 11) were used with an outer diameter of 65 mm, a hole centered lengthwise and in variable lengths. They had been prepared before the cruises by Immersion overnight at 70" C in a bath of a saturated KMn04 solution. A detailed description of the coating technique is given in Rutgers van der Loeff and Moore (1999). The cartridges were sealed in plastic bags to keep them wet until they came into use On board the ship.

4.1.7 Surface water sampling for 228Radium

In order to test whether '''Ra can indeed originate from either the Argentinean shelf, the shelf regions along the Antarctic Peninsula or the Weddell Sea, surface water sampling for the analysis of ' ~ has been done in these possible source regions. Five a N-S-transects have been sampled through the Antarctic Circumpolar Current (ACC) along different longitudes. As the oceanographic fronts with their high geostrophic velocities seem particularly promising for a rapid transport of shelf signals, a dense sampling of the region between 46O and 52" S along the 20' E meridian was performed during a grid survey of the Polar Front during expedition ANT XVIl3 (Fig. 11).

All surface water samples except those of the expedition NBP 00-03 were taken parallel with a 20 l subsample for the quantitative determination of ^Ra. This was necessary for the calculation of absolute ' ~ activities (see chapter 4.3), but provided a at the Same time valuable information On the distribution and geochemical behaviour of

" R a in southern circumpolar waters (see chapter 3.2).

As one of the objectives of this study is the investigation of possible iron transport routes into the South Atlantic, samples of suspended particulate matter have been taken on most " ' ~ a surface water locations for the analysis of aluminium and the isotopic composition of neodymium (Hegner et al. in prep.). These tracers should help to illuminate the role and origin of terrigenous input of iron into the area of investigation.

R a sampling during expedition ANT XVIl3 was done in conjunction with iron measurements carried out by the NIOZ (Netherlands Institute for Sea Research). The results for ' ~ will be discussed in the context of these accompanying measurements a and will contribute to a better understanding of the dynamics and processes regulating the iron supply into this Part of the Southern Ocean.

The sampling was performed with a filter system connected to the ship's sea water supply with a water intake depth at approximately 8 m. The pumps were active constantly after leaving the harbour and the pipe system was flushed thoroughly before taking the first sample.

Material and rnethods

A cartridge length of 13 cm has been chosen for surface water sampling as it has proven to be a good compromise between sampling efficiency and further handling of the samples. The water sample was run through an uncoated cartridge used as a prefilter (1 pm) for removing particulate matter, two Mn02-coated cartridges put in series to concentrate radium and a flowmeter for recording the sample volume. It was tried to filter at least 2000 l of sea water for a good recovery of radium. Except for expedition NBP 00-03, this volume could be attained or, in the case of expedition ANT XVIl3, was largely exceeded. After finishing the sampling, the prefilters were discarded and the Mn02-coated cartridges either directly sealed or first rinsed with deionized water, dried and then sealed.

As most of the sampling was done on a sailing vessel, the results represent values integrated over as much as 120 km.

4.1.2 Profile sampling for ^Radium

During expedition ANT XVIl3, a transect of eight deep water stations down to 1000- 1800 m was carried out at 20' E in order to get a two-dimensional picture of the distribution of the desired radionuclides. Sampling locations are given in Fig. 11. At each station, a CTD profile? was run first to determine water mass properties. The sampling was done with four time-programmed pumping units that were loaded with two MnOz-coated cartridges each. For structural reasons, the length of the cartridges was 25 Cm. An integrated flowmeter recorded the sample volume.

No prefilter was used as the first sampling depth was at the bottom of the mixed layer and the suspended particle content of the water very low (Usbeck et al. in press). Any additional filter would increase the resistance of and lower the flow through the pumping System. Under oxic conditions in sea water, radium exists mostly in the dissolved form. By analyzing surface water samples with a generally higher particle loading than deep water samples, it could be shown that the radium activity of the particulate matter is less than 1% of the respective dissolved activity. '*'Ra and ^Ra are not expected to differ in their behaviour regarding the partitioning between the solid and the fluid phase, however, depending On their source region, particles could carry activity ratios different to the surrounding sea water with them (Legeleux and Reyss 1996). But taking the low particle content in deeper water layers and the weak particulate activities into consideration, the missing prefilter would not affect the results in any significant way.

A surface water sample for ' ' ' ~ a was taken from the ship's sea water supply at every deep water station. Apart from station 156, the sampling was done in conjunction with measurements of iron depth profiling done by the NIOZ, but the depth resolution for radium is coarser due to the limited number of pumping units.

Instrument for the rneasurernent of temperature, electrical conductivity, and under water pressure. Depth and salinity are deduced frorn these pararneters.

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Material and rnethods

4.1.3 Sarnpling for ^Radium

For a quantitative determination of the ^Ra surface water concentration, 20 I subsamples were taken in conjunction with the large volume MnOz-filtering. The water was filtered through an uncoated 1 j-im filter cartridge to remove the particulate fraction.

If taken during steaming of the ship, the subsample was either collected about midway of the 2 2 8 ~ a - s a m p l i n g (ANT XV/2+3) or split in three bottling times of each 6-7 l (ANT XVIl3): given an average sampling duration of 6 hours, the three parts of the subsample were taken one, three and five hours after having started the sampling. This procedure was adopted to level out possible local variations in the a concentrations. Samples from expedition ANT XVl4 and ANT XVIIl4 were taken solely during station time. All samples were weighed before further processing. No subsamples are available from expedition NBP 00-03.

A restricted number of 20 l subsamples from intermediate and deeper water layers has been taken with a rosette cast at stations 156, 169, 182, 190 and 207. The samples were not filtered before further treatment because the particle content in these subsurface waters was negligible.