Leier et al.
Methods –
Clumped-isotope measurements were made over the course of two sessions: one
measurement each of AL 6-46, 36a and 1-128 were made in September 2009; all other
analyses were made in September of 2010. Samples were reacted with phosphoric acid
in a common acid bath at 90 ˚C and product CO2 purified by cryogenic and gas
chromatographic methods using the automated device described by Passey et al. (2010).
All purified gases were analyzed for δ13C, δ18O and ∆47 using a Thermo IRMS 253
configured for clumped isotope measurements. Measurements of δ13C and δ18O were
standardized based on accepted values for NBS-19 reacted at 25 ˚C. Measurements of
∆47 were standardized by comparison with CO2 heated for at least 2 hours at 1000 ˚C,
using methods described by Huntington et al., (2009).
Analyses of unknown samples were accompanied by periodic analyses of intralab
consistency standards, Carmel Chalk, Carrera Marble, and 102-GC-AZ01. Measured
values of 6 of these standards in the 2010 session in which most of our data was
generated yielded an average offset from accepted values of 0.00±0.007 (1se). These data
are reported without any secondary corrections (i.e., beyond normalization to the heated
gas reference frame of Huntington et al., (2009)). The two standards analyzed
concurrently with unknowns in the 2009 session yielded measured values 0.036±0.001
(1se) greater than accepted values; these three measurements have been corrected by this
difference (note all three of these measurements were replicated in the 2010 session with
an average external reproducibility of 0.013 ‰ in ∆47, thus the two data sets appear to
have been accurately standardized with respect to one another.
The average standard error of each analysis based on mass spectrometric
reproducibility alone was ±0.010 (1se). All samples were analyzed in duplicate or (in the
case of those analyzed in both 2009 and 2010) triplicate, with average reproducibility of
±0.011 (1sd) and average external error of ±0.007 (1se). These average errors are
generally consistent with expected counting statistics limits and suggest no significant
additional experimental errors. The average standard error for the average of each
unknown sample is equivalent to a temperature error of approximately ±1.6 ˚ (though
varying slightly with absolute temperatures).
The measurements reported in this study were conducted prior to the creation of
the absolute reference frame of Dennis et al. (2011) and so we report them relative to the
Caltech intralab reference frame described in Huntington et al. (2009) (which is
consistent with the initial calibration data of Ghosh et al. (2006) and other calibrations
published prior to 2011). We convert ∆47 values into equivalent temperatures using the
function given by Ghosh et al. (2006). At the time this paper is published there exists
some uncertainty regarding the consistency of this calibration across different
laboratories, materials and methods of acid digestion. However, those uncertainties are
generally similar to nominal analytical errors in the temperature range recorded by our
samples, and these discrepancies appear to arise principally from differences in methods
and materials used in separate laboratories; thus we believe the best practice at present is
to use calibrations based on measurements in the Caltech lab. For convenience, we also
present equivalent values of ∆47 in the absolute reference frame, based on the transfer
function: ∆absolute = ∆CIT x 1.0369 + 0.027. These should be regarded as approximate,
as they are based on accepted values for these standards as measured in the Caltech
intralab reference frame of Huntington et al., rather than on direct calibration of the
absolute reference frame during the sessions in question. Nevertheless, they provide a
reasonable estimate of expected values for any future attempts to reproduce the data we
present or to compare our data with independently generated data sets.
Dennis, KJ, Affek HP, Passey BH, Schrag DP, and Eiler JM, 2011, Defining an absolute
reference frame for ‘clumped’ isotope studies of CO2: Geochimica et
Cosmochimica Acta, 75, 7117-7131.
Ghosh, P., Adkins, J., Affek, H., Balta, B., Guo, W.F., Schauble, E.A., Schrag, D., Eller,
J.M., 2006. C-13-O-18 bonds in carbonate minerals: A new kind of
paleothermometer. Geochimica Et Cosmochimica Acta 70, 1439-1456.
Huntington, K.W., Eiler, J.M., Affek, H.P., Guo, W., Bonifacie, M., Yeung, L.Y.,
Thiagarajan, N., Passey, B., Tripati, A., Daëron, M., Came, R., 2009. Methods and
limitations of ‘clumped’ CO2 isotope (Δ47) analysis by gas-source isotope ratio
mass spectrometry. Journal of Mass Spectrometry 44, 1318-1329.
Passey, B.H., Levin, N.E., Cerling, T.E., Brown, F.H., and Eiler, J.M., 2010, Hightemperature environments of human evolution in East Africa based on bond
ordering in paleosol carbonates: Proceedings of the National Academy of Sciences,
107, 11245-11249.