3.2 Aragonite clumped isotope temperature dependence
Clumped isotope values of low-temperature (<30°C) aragonites are indistinguishable from the calibration of Anderson et al. (2021) within the 95% confidence level (Fig. 2B ). A linear regression through these low-temperature datapoints is similar to calcite calibrations by Anderson et al. (2021), Meinicke et al., (2021) and the Guo et al. (2009) theoretical temperature relationships:
\begin{equation} \Delta_{47}\left(I-CDES\right)=0.0425\pm 0.0009*\frac{10^{6}}{T^{2}}+0.1178\pm 0.0106\ (T\ in\ K,\ \pm 1\sigma;\sigma_{\text{res}}=0.041\%0)\nonumber \\ \end{equation}
Including higher temperature (>30°C) datapoints in the linear regression increases the slope and decreases the intercept:
\begin{equation} \Delta_{47}\left(I-CDES\right)=0.0457\pm 0.0024*\frac{10^{6}}{T^{2}}+0.0791\pm 0.0286\ (T\ in\ K,\ \pm 1\sigma;\sigma_{\text{res}}=0.040\%0)\nonumber \\ \end{equation}
(see Fig. 2 ). The low-temperature (<30°C) linear regression has higher Δ47 values for the same temperatures than the full dataset at 0°C (0.009 ± 0.008‰; 95% CL) and lower values at 30°C (-0.005 ± 0.006‰; 95% CL), equivalent to a temperature difference of −1.99°C (−1.96°C / +2.00°C; 95% CL) at 0°C and +1.79°C (−1.98°C / +2.14°C; 95% CL) at 30°C. Both regressions yield the same temperature for samples precipitated at ±18°C (Δ47 of 0.619‰). Including high-temperature (>30°C) data in the linear regression thus leads to overestimation of the temperature of warmer (>18°C) datapoints while underestimating colder datapoints (<18°C;S7 ). Point-by-point offsets of all data from the calibration lines are provided in S8 .
There is no consistent offset between the Anderson et al. (2021) calibration and the combined aragonite dataset both with (ΔΔ47 = −0.002 ± 0.004‰) and without the high-temperature data (ΔΔ47 = +0.002 ± 0.004‰; seeS7 ). However, the formation temperatures of our A. islandica data on the very cold end of the calibration domain are significantly underestimated by Anderson et al. (2021; ΔΔ47 = +0.009 ± 0.007‰; −2.71 ± 2.03°C; 95% CL;Fig. 3 ). The theoretical aragonite clumped isotope-temperature relationship (Guo et al., 2009) severely overestimates A. islandica temperatures (−0.016 ± 0.007‰; +4.35 ± 1.88°C; 95% CL;Fig. 3 ). Contrarily, the Meinicke et al. (2020; 2021) calibration (ΔΔ47 = +0.004 ± 0.007‰; −1.17 ± 2.00°C; 95% CL; Fig. 3 ) and the theoretical calcite temperature relationship (Guo et al., 2009; ΔΔ47 = +0.002 ± 0.007‰; −0.47 ± 1.98°C; 95% CL; Fig. 4 ) do not significantly over- or underestimate the formation temperature of our A. islandicashells.