Cleaned sponge and diatom opal was dissolved via wet alkaline digestion (Cardinal et al., 2007; Ragueneau et al., 2005) in 0.2 M NaOH at 100 °C for 40 min (diatoms) or up to 1 week (sponge spicules). The samples were acidified to pH∼2 with 0.2 M thermally distilled HCl and separated from major ions using cation exchange resin (BioRad AG50W-X12, Georg et al., 2006). Silicon isotope analysis was carried out using a Nu Instruments Nu-Plasma HR multi-collector inductively coupled plasma mass spectrometer run in medium resolution mode (m/Δm∼3500 at 5% and 95%). Samples were introduced via a self-aspirating PFA microconcentric nebuliser (ESI) in a Cetac Aridus II desolvating unit. Measurements included six to eight standard-sample brackets (brackets where the rate of machine drift outstripped bracketing rate were disregarded), each composed of twenty eight-second integrations. Samples were measured relative to the NIST RM 8546 standard. The external diatomite standard (1.26±0.2‰, Reynolds et al., 2007) yielded a mean and 2SD of 1.23±0.25‰ (n=104). Error bars in the figures and text are this 2SD external reproducibility unless the internal reproducibility of the standard sample brackets was larger, in which case this is quoted instead.
wet alkaline digestion
NaOH
0.2 M
M
Cleaned sponge and diatom opal
dissolved via wet alkaline digestion
100 °C
°C
diatom opal
dissolved via wet alkaline digestion
40 min
min
Cleaned sponge
dissolved via wet alkaline digestion
up to 1 week
week
samples
acidified
pH∼2
pH
thermally distilled HCl
0.2 M
M
mass spectrometer
m/Δm∼3500
5% and 95%
%
standard-sample brackets
six to eight
integrations
twenty
integrations
eight-second
second
Samples
mean and 2SD of 1.23±0.25‰
‰
Samples
n
104
external diatomite standard
1.26±0.2‰
‰