4. DISCUSSION
As explained in the introduction, the consensus values for SLAP with
respect to VSMOW were established in 1976. The established δ2H value was based on the absolute abundance
measurements, however, the same for δ 18O was lacking
and thus the mean δ 18O value, -55.5‰, of an
interlaboratory calibration exercise performed at that time was chosen
by consensus. Among the representatives of the several participating
laboratories, there was already a discussion that possible memory
effects would contract the scale, so probably a more negativeδ 18O value would have been more appropriate.
In later years, thanks to improvements to both equipment and analysis
procedures such as correction for cross-contamination
(Meijer19, laboratories indeed determined more
negative values for SLAP. In our laboratory, we typically find values
around δ 18O = -55.85‰ by IRMS
(CO2-H2O equilibration) and, more
recently to our surprise, δ 18O = -55.7‰ using
the LGR-LWIA. We expected that by having a well-maintained IRMS and
using the appropriate corrections, our results for SLAP would be close
to the real values.
However, Kaiser (2009) already suggested a re-analysis of the data of an
intercomparison exercise of 7 expert laboratories described in
Verkouteren and Klinedinst (2004), resulting in a much more negativeδ 18O value for SLAP, i.e., -56.1 ± 0.2‰. On
the other hand, the δ 18O value of -55.11‰ for
SLAP measured by Barkan and Luz8, is puzzling. The
method Barkan and Luz used was also based on the isotopic exchange
equilibration between H2O and CO2 in
sealed ampoules, but followed by a fluorination of water using
CoF3 to produce O2. Although this
approach is different from the standard equilibration method, results
should be identical as long as the fluorination is complete. However,
their approach consistently points towards less negative values of
-55.11‰ (Hillaire-Marcel22).
For a robust locking of the second anchor of the VSMOW scale we
performed the work described in this paper. The reliability of our
method of quantitative 18O abundance determination of18O water using Quadrupole Mass Spectrometry is
crucial for our results. Taking various effects such as fragmentation
difference of H216O and
H218O into account, and by validating
the method with a dilution series, and considering the excellent
agreement of the fitted QMS signals and the measured ones, we are
confident that the method is reliable.
A systematic deviation of our 18O abundance result of
0.1% higher/lower values would lead to a more/less negative result for
SLAP of 0.05‰. However, such a deviation is highly unlikely: It is good
to realize that, as we use very highly-enriched 18O
water (batches of 98% and 99% 18O), in fact we do
not measure this high 18O abundance, but rather
quantify the remaining part of 16O exactly by QMS.
Since there is only room for 1 to 2% 16O, it is in
fact this amount that has to be measured with an accuracy of ≤ 0.1%,
which is not a high relative accuracy. Furthermore, if we would still
suffer from some systematic deviation, one can expect this deviation to
be larger for the water portions with 2% 16O
remaining (the Rotem waters) than those with 1% (Cortec). We see no
such effect in our results (Figure 6 and in the supplementary material
Table 4). The portion of 17O only plays a minor role
in the 18O/16O ratio, and this
abundance can be determined using a dilution method.
The uncertainty in 18RVSMOW leads to a
systematic uncertainty in our final answer of ± 0.013‰, small compared
to our final uncertainty.
So, the result of this study is δ 18O -56.33 ±
0.03‰, thus a very negative δ 18O value for
SLAP, which was an unanticipated finding.
The implication of this is that apparently complete understanding of all
IRMS effects (not to mention those in optical spectroscopy) is still
lacking. Measuring cross-contamination-effects
(Meijer19) obviously is not enough for correcting the
isotope measurement such that the measured delta values are very close
to the real delta values.
One of the issues emerging from this lack of complete understanding of
all IRMS effects, relates specifically to second order measurements such
as 17O excess (Δ 17O) in water. For
these measurements, in which the small deviation of the measured
δ17O from the natural relation between
δ18O and δ17O is determined (Meijer
and Li20, Aron21), the question
raises, how well these very small deviations (around 0.02‰ or less) can
be defined, if there are such large discrepancies between measured
δ18O and real δ18O values. The
assumption that 17O and 18O will
fully obey mass dependent fractionation in the ion source of the IRMS
may not be completely true. To put it another way: if the measured scale
for δ18O is already so much contracted, who can
guarantee that the δ17O scale contracts exactly
according to the equilibrium relation between δ17O and
δ18O.
Also clumped isotope measurements, which determine the minute deviations
from stochastic distribution of the delta values for multiply
substituted isotopologues, can probably not rely on the fully
mass-dependent scale contraction of their machines. Also, here, full
understanding of IRMS effects is key.
The oxygen isotope compositions are typically reported on the VSMOW
scale, not only for water samples, but also for other types of samples,
such as oxides and silicates. The VPDB scale is mostly used for
reporting the stable isotope (carbon and oxygen) results of carbonate
minerals and also for oxygen isotope measurements in atmospheric
CO2. These two coexisting stable isotope scales for
reporting 18O/16O ratios or δ18O values, can be converted into each other
(Hillaire-Marcel22). For both scales an extra
conversion step to CO2 is necessary, because the
measurand in the IRMS is CO2. This extra reaction step
is for the VSMOW-CO2 scale, water equilibration of VSMOW
with CO2 under standard conditions (first described by
Epstein and Mayeda23, Meijer17 and
for the VPDB-CO2 scale, acidification of IAEA-603
(formerly NBS-19) with phosphoric acid (McCrea24,
Meijer17, Hillaire-Marcel22. The
difference between VSMOW-CO2 and
VPDB-CO2 on the two δ 18O
scales is 0.28-0.29‰ (Hillaire-Marcel22). In our
laboratory, we have the habit to realize the two scales (water and
carbonate) independently and use this scale difference as a quality
check. When using two-point calibration scales, the result of a more
negative δ18O value for SLAP, (the second anchor of
the VSMOW scale), could give potential discrepancies in the transfer ofδ 18O from and to the VPDB scale. Considering
the fact that the water equilibration reaction is more robust and easier
to control (and therefore more reliable and accurate) than the
carbonate-acid reaction, we propose the VSMOW-CO2δ18O scale be defined as the primary
δ18O scale. The definition of the
VPDB-CO2 scale could then simply be expressed in terms
of the VSMOW-CO2 scale. Final decisions about these
isotopic scales are under the auspices of the commission on isotopic
abundances and atomic weights (CIAAW).
Identical treatment of sample and references, the frequent use of
international reference materials and clear guidelines about how to
express the results on the international scale(s) is key to provide
normalized interlaboratory-comparable stable isotope measurements. This
study does not affect those measurements; the VMSOW-SLAP scale can be
taken as is. However, knowing the absolute ratios and/or abundances of
all scale determining references would give us clear insight how large
the scale contraction processes really are. In fields where
VSMOW-SLAP-scaled δ18O values are converted into
absolute abundances and vice versa, our new δ18O value
for SLAP does matter. An example of such a field is energy expenditure
measurements using doubly-labelled water, in which the used enriched
reference waters will change their delta value
(Faghihi15).