Chapter 2
58
This chapter gives an overview of the different steps undertaken on samples collected during
the GEOTRACES GA01 (GEOVIDE) cruise in the North Atlantic, including sample
collection,
chemical processing, analytical protocols and mathematical determination of the fluxes.
I have participated to the GEOVIDE cruise during which I was in charge
of the excess barium
sampling and where I substantially contributed to the
in-situ pump sampling (preparation and
recovery of the pumps, filter handling in the clean bubble). Back in the lab, I have determined
the
234
Th recovery (MRAC, Tervuren), the POC and PN concentrations (VUB, Brussels), the
trace element concentrations (IUEM, Brest) and the excess barium concentrations (MRAC,
Tervuren). The sampling, preparation and Beta counting of the samples dedicated to
234
Th
were made by F. Planchon at sea; the Beta counting of the
234
Th residual activities were
determined by A. Plante, E. Le Roy (Master students, VUB), F. Planchon and myself (VUB and
IUEM); the BSi concentrations were determined by S. Roig (Master student, IUEM); and
replicates of POC and PN concentrations were determined by C. Mariez (Master student,
IUEM).
2.1. Sample collection, chemical processing and
analyses
2.1.1. Total Thorium-234:
234
Th
2.1.1.1.
Sampling
Total
234
Th activities were determined on 4L seawater samples collected with 12L Niskin
bottles (Pike et al., 2005) triggered at 18 depths in the upper 1000m. Using a silicone tube,
seawater was transferred from the 12L Niskin bottle to a 4L polypropylene bottle (Nalgene).
Before the sampling, the 4L-bottles were cleaned with a solution of 7M HNO
3
/1M H
2
O
2
(suprapur grade, Merck) at ambient temperature, to avoid memory effects on
234
Th and to
remove MnO
2
impurities, they were then rinsed three times with Milli-Q water and finally with
the sampled seawater.
Immediately after sampling, samples were acidified to pH 2 using concentrated HNO
3
(suprapur grade, Merck) and spiked with 1 mL of
230
Th (5.2 ng.g
-1
) in order to monitor the