3.3 Isotope measurement
Water samples were filtered (0.2 µm cellulose acetate) prior to analysis
of stable isotopes (δ18O and δ2H
values) in the water isotope lab at IGB Berlin, using a Picarro (Santa
Clara, CA, USA) L2130-i cavity ring-down spectrometer (CRDS).
Measurements were routinely checked for organic contamination using the
Picarro ChemCorrect software.
Isotope values and standard deviations are based on six replicate
measurements of each sample, discarding the first three measurements to
account for memory effects. In house criteria excluded all injections
with a water SD higher than 400 ppm or water amount deviation greater
than 3000 ppm to furthermore improve precision.
For instrument calibration we used 3 laboratory standards for each group
of 24 samples: L (δ18O -17.86 ‰ and
δ2H -109.91 ‰), DEL (δ18O -10.03 ‰
and δ2H -72.81 ‰), H (δ18O 2.95 ‰
and δ2H 0.29 ‰). A fourth lab standard, M
(δ18O -7.68 ‰ and δ2H -56.70 ‰), was
used as quality and drift control after every 6 samples. All lab
standards were referenced against primary measurement standards: VSMOW2
(Vienna Standard Mean Ocean Water 2), GRESP (Greenland Summit
Precipitation), and SLAP2 (Standard Light Antarctic Precipitation 2)
from the IAEA (International Atomic Energy Agency, Vienna International
Centre, A-1400 Vienna, Austria).
The measurement uncertainty was quantified by error propagation,
including the parameters a) uncertainties of lab standards; b) errors of
standard calibration; c) average standard deviation of replicate
measurements. Based on this, measurement uncertainty was estimated to
account 0.16 ‰ for δ18O and 0.57 ‰ for
δ2H.