4.3 Non-linear temperature dependence of clumped isotopes in
aragonites
Current clumped isotope calibrations (Meinicke et al., 2020; 2021;
Anderson et al., 2021) show subtle differences in the low-temperature
end of the calibration (<30°C) that would result in
~1.5°C colder temperatures when applying Anderson et al.
(2021) compared to Meinicke et al. (2020). This has led some authors to
suggest that forcing of the linear
∆47-\(\frac{1}{T^{2}}\) regression through warm
(>30°C) datapoints in the Anderson et al. (2021)
calibration creates a “cold bias” in this calibration on the cold end
of the calibration domain (e.g. Meckler et al., 2022). In addition, the
cold-water (<30°C) carbonate based Meinicke et al. (2020)
calibration more closely resembles the modelled temperature relationship
for calcites in Guo et al. (2009). The difference between
∆47-\(\frac{1}{T^{2}}\) regressions through the
low-temperature (<30°C) and the full dataset (seesection 3.2 ; Fig 2 ) likely highlights non-linear
behavior of the ∆47-\(\frac{1}{T^{2}}\) relationship in
aragonites. In fact, previous studies based on both clumped isotope
analyses and ab initio modelling have suggested a non-linear
∆47-\(\frac{1}{T^{2}}\) relationship to be a better fit
for both calcites (Guo et al., 2009; Jautzy et al., 2020) and dolomites
(Guo et al., 2009; Müller et al., 2019) precipitated on a large range of
known temperatures. Non-linear behavior is also observed in the Anderson
et al. (2021) dataset, where Δ47 values of calcites
precipitated between 100°C and 1000°C are underestimated by the linear
relationship, while the hottest datapoints (calcites heated to 1100°C)
fall on the linear regression, mimicking the reduced
∆47-\(\frac{1}{T^{2}}\) slope at the high temperature
end of the polynomial regressions through calcite and dolomite data (Guo
et al., 2009; Jautzy et al., 2020; Müller et al., 2019). A linear
∆47-\(\frac{1}{T^{2}}\) relationship through a
calibration dataset with a large temperature range will thus
overestimate temperatures for samples with ∆47 values
between 0.2‰ and 0.4‰ (temperatures of 100°C‒1000°C; see residuals in
Anderson et al., 2021) and underestimate temperatures of cold
(<30°C) samples, as confirmed by regressions through our
low-temperature datapoints (see Fig. 2-3 and section
4.4 ). Therefore, more high-temperature aragonite datapoints are needed
to constrain the clumped isotope-temperature relationship for
temperatures >100°C.