4.4 Calibrating the clumped isotope-temperature relationship in
cold (<30°C) carbonates
Our lab-grown A. islandica shells offer more control on formation
temperature than naturally grown carbonates precipitated under variable
temperatures. Ideally, the temperature of these natural samples is
monitored so an average temperature can be calculated for the targeted
growth period (e.g. Kele et al., 2015; de Winter et al., 2020; 2021b;
Huyghe et al., 2021). However, formation temperatures are often
indirectly estimated through other proxies (e.g.
δ18Oc) and/or estimates of the living
environment (e.g. water depth) of the carbonate producer, accumulating
uncertainty (e.g. Peral et al., 2018; Piasecki et al., 2018; Meinicke et
al., 2020). These caveats obscure the full uncertainty on the formation
temperatures of natural carbonates as well as the effect of this unknown
uncertainty on the calibrations. Considering the methods by which the
“known” temperatures of natural carbonates are estimated in previous
studies, it seems possible that the ~1.5°C temperature
offset between Anderson et al. (2021) and Meinicke et al. (2020; 2021;
see Fig. 3 ) and the 2.71 ± 2.03°C offset between Anderson et
al. (2021) and our A. islandica data is partly caused by
uncertainty on the formation temperatures of the calibration dataset.
The similarity between Δ47 values of our data with
Meinicke et al. (2021; ΔΔ47 0.004 ± 0.007‰) and the
theoretical calcite temperature dependence by Guo et al. (2009;
ΔΔ47 0.002 ± 0.007‰; Fig. 1 , Fig. 3 ;S8 ) shows that precise control on formation temperatures leads
to tighter constraint on the clumped isotope temperature dependence of
low-temperature samples. Precisely temperature-controlled carbonates
thus better constrain the slope of the
∆47-\(\frac{1}{T^{2}}\) relationship for cold carbonates
(improving calibration accuracy) while reducing the uncertainty on the
calibration (improving calibration precision).
The ~1.5°C difference in reconstructed temperature
between the calibrations in the low temperature range (<30°C)
may seem trivial and requires the complete A. islandica dataset
(N = 278; see Fig. 4 ) to resolve. However, in paleoclimate
reconstructions (e.g. Petersen et al., 2016; de Winter et al., 2017;
2021a; Vickers et al., 2020b; Meckler et al., 2021; Agterhuis et al.,
2021), this temperature offset may have significant consequences. A
~1.5°C cold bias in temperature reconstructions may lead
to a significant underestimation of climate sensitivity to
CO2 forcing, biasing the physical science basis for
informing policymakers about future climate change (e.g. Dennis et al.,
2013; Modestou et al., 2020; Westerhold et al., 2020; Tierney et al.,
2020; IPCC, 2021). Accurate clumped isotope-based temperature
reconstructions therefore require calibration datasets with precisely
constrained formation temperatures tailored to the temperature range of
the samples.