2.2.1 Mass, total POC/PON, BSi, CaCO3.
For particulate organic carbon (POC) and particulate organic nitrogen
(PON) analyses, 3 to 5 mg of the freeze-dried powder was weighed
directly into pre-combusted (450°C, 24 h) silver cups. Samples were
decarbonated by adding 20 μL of 2M analytical-grade HCl (Sigma-Aldrich).
Samples were dried overnight at 50°C. POC and PON were measured with a
CHN analyser (Perkin Elmer 2400 Series II CHNS/O elemental analyser)
calibrated with glycine. Samples were analysed in triplicate with an
analytical precision of less than 0.7 %.
For BSi analysis, 2 to 8 mg of material was used. For BSi sample
digestion we followed the protocol from (Ragueneau et al. 2005) and the
silicic acid concentrations in the solutions were determined manually
following Aminot and Kérouel (2007). The precision of BSi measurement
was 10% (Ragueneau et al. 2005).
For bulk CaCO3 analyses, 5 mg of freeze-dried material
was weighed into Teflon vials for the mineralization. One mL of 65%
(v/v) HNO3 (Sigma analytical grade) was added and
samples were placed in an ultrasonication bath for 20min. Samples were
then dried overnight at 130 °C, then 0.5 mL of 40% (v/v) HF (Sigma
analytical grade) and 5 mL of 65% HNO3 were added. The
samples were ultra-sonicated a second time and dried overnight. The
resulting residue was dissolved in 10 mL of 0.1N HNO3and the calcium (Ca) content was analyzed by inductively coupled plasma
– optical emission spectrometry (ICP-OES, Perkin-ElmerOptima2000). The
efficiency of the mineralization procedure was estimated using the
reference material GBW-07314. The efficiency was 96% and the precision
of the Ca measurement was 2% (Rembauville et al. 2016). Based on a
Ca/Ti ratio in basalt of Kerguelen (3.3 mol/mol) we estimate that the
contribution of Ca of lithogenic origin to the total Ca flux was low. We
therefore equalled the Ca flux to the biological CaCO3flux.