Potential reasons for discrepant calibrations at 90 °C and 25 °C digestion temperatures

4 Empirical calibration of the clumped isotope paleothermometer using calcites of various origins

4.4.2 Potential reasons for discrepant calibrations at 90 °C and 25 °C digestion

4 Empirical calibration of the clumped isotope paleothermometer using calcites of various origins

regression line in the absolute reference frame that is shifted by −0.02 to −0.03‰ relative to its true position as reflected by applying the correct Δ47value of 0.404‰. In this respect, relatively small uncertainties in assigned Δ47values for NBS 19 can induce relatively large, but rather constant biases in secondarily projected Δ47values.

Additional interlaboratory comparisons are necessary to identify homogeneous isotopic standards that guarantee accurate secondary projections of data. However, the isotopic heterogeneity of NBS 19 cannot be responsible for the discrepant set of calibration slopes resulting from 25 °C and 90 °C reactions.

4.4.2.2 Phosphoric acid fractionation: sensitive to the bulk or clumped isotopic composition of carbonates?

For most calibrations of the clumped isotope thermometer, carbonates were digested either at 25 °C using sealed vessels or at 90 °C using a common acid bath. The only exception is the study performed at ETH Zurich where they reacted modern foraminifera at 70 °C using a Kiel device (Grauel et al., 2013).

Notably, in a 1/T² vs. Δ47plot, two distinct sets of calibration regression lines were reported where analytical conditions within each set were similar. For example, a steeper slope of ca.

0.060 was determined when carbonates were digested at 25 °C (e.g., Ghosh et al., 2006a;

Tripati et al., 2010; Thiagarajan et al., 2011; Zaarur et al., 2013). In contrast, shallower regression lines with slopes of ~0.034, which are comparable to the theoretically predicted temperature dependence of Δ47 of carbonate-liberated CO2, were obtained at reaction temperatures of 90 °C (Dennis and Schrag, 2010; Henkes et al., 2013; Eagle et al., 2013; this study). Therefore, it is possible that the distinct sets of regression lines may be related to the two digestion temperatures used in the different studies. As a consequence, it is necessary to investigate whether the clumped isotope fractionation Δ47* between the carbonate crystal and the evolved CO2 depends only on the digestion temperature, or if it is also a function of the bulk and/or clumped isotopic composition (Δ63) of the carbonate.

Based on transition-state-theory models, Guo et al. (2009) calculated a non-ideality in clumped isotope fractionations of phosphoric acid digestions of carbonates. Their calculations suggest that the bulk isotopic composition of the carbonate has only a very small effect on Δ47* (e.g., 0.002‰ per 50‰ increase in δ¹³C values), i.e. smaller than the precision of Δ47 analyses defined by counting statistics alone. The same calculations imply that the effect of Δ63on Δ47*

seems to be small, causing an increase of ~0.035‰ for every 1‰ increase in Δ63. Δ63 of

4 Empirical calibration of the clumped isotope paleothermometer using calcites of various origins

naturally occurring carbonates should only vary over a range of 0.5‰ provided that their clumped isotopic composition is primarily controlled by temperature (Schauble et al., 2006).

As a consequence, one may expect that the effect of Δ63on Δ47* would remain negligible in Δ47

analyses as well.

Further support for the theoretical prediction that Δ47* is relatively insensitive to both the bulk and clumped isotopic composition of the reacted carbonate comes from calibration data. For their original calibration, Ghosh et al. (2006a) reacted two calcites (HA12 and HA4) that were precipitated at 50 °C, but exhibited a ~4‰ difference in δ47 values. However, the mean Δ47

values of these samples were identical. The same picture arises when comparing Δ47values measured for D. wyvillei and the cold seep carbonate in this study. Both samples precipitated at very similar temperatures of 10 and 9 °C, respectively. Though their δ47 values differ by more than 30‰, mean Δ47values are within errors indistinguishable from each other (Table 4.2).

On the other hand, if Δ47* depends on Δ63, the slope of the 1/T² vs. Δ47relationship obtained from acid digestions at temperatures between 25 and 90 °C, should exhibit an intermediate value between 0.060, the slope characteristic for “25 °C calibrations” (Ghosh et al., 2006a;

Tripati et al., 2010; Thiagarajan et al., 2011; Zaarur et al., 2013), and 0.034, the slope characteristic for “90 °C calibrations” (Dennis and Schrag, 2010; Henkes et al., 2013; Eagle et al., 2013; this study). Grauel et al. (2013) report an empirical calibration of the 1/T² vs. Δ47

relationship using foraminifera with known growth temperatures and an acid digestion temperature of 70 °C. Their results are in agreement with the experimental calibration of Ghosh et al. (2006a) and with the empirical calibration for foraminifera of Tripati et al. (2010).

Therefore, we deem it unlikely that the discrepant calibration lines are caused by variations in fractionation factors depending on the bulk isotopic composition of the carbonates and/or the degree of excess clumping between ¹³C-¹⁸O in the carbonates.

4.4.2.3 Partial re-equilibration of CO2 at either 25 or 90 °C

As stated previously, two different sets of slopes of calibration regression lines were obtained that largely seem to depend on the carbonate digestion technique. Therefore, it might be reasonable to suspect that these two different temperature sensitivities of Δ47reported in the corresponding studies are caused by systematic analytical artifacts, preferentially occurring at either 25 or 90 °C. It is well known that oxygen isotope exchange between traces of water and evolved CO2 or intermediate products can occur during phosphoric acid digestion of carbonates (Wendeberg et al., 2011). Heterogeneous oxygen isotope exchange would also

4 Empirical calibration of the clumped isotope paleothermometer using calcites of various origins

accompany modification of the clumped isotope signature. The excess of ¹³C-¹⁸O bonds in the exchanged CO2 will depend on reaction temperature and reaction degree. If secondary exchange proceeds to equilibrium, the evolved CO2 will exhibit Δ47 values of 0.925‰ and 0.651‰ for reaction temperatures of 25 and 90 °C, respectively (Wang et al., 2004; Dennis et al., 2011). In the following, we will investigate whether re-equilibration of analyte CO2

occurring during acid digestions either at 25 °C or at 90 °C can explain the discrepant calibration lines.

Wacker et al. (2013) observed that sample size can have an effect on Δ47values if carbonates are digested in sealed vessels at 25 °C. Higher Δ47mean values and a larger data scatter for samples <7 mg were found compared to larger carbonate aliquots. This possibly arises from partial secondary re-equilibration of CO2 or reaction intermediates with trace amounts of water.

On the other hand, they did not observe such a relationship for digestions at 90 °C.

Conspicuously, only the Δ47 average of the low-temperature carbonate precipitated at 0 °C analyzed for the “25 °C calibration line” of Ghosh et al. (2006a) deviates significantly from the theoretical calculation of Guo et al. (2009) and from the 1/T²–Δ47relationships for which a digestion temperature of 90 °C was used (Dennis and Schrag, 2010; Henkes et al., 2013; Eagle et al., 2013; this study). Ghosh et al. (2006a) reacted ~5 mg aliquots of calcite at 25 °C in sealed vessels. This is exactly the sample size where a bias towards higher mean Δ47values was observed to occur in the experiments of Wacker et al. (2013). Provided that Ghosh et al.

(2006a) preferentially used small aliquots for the analyses of their 0 °C precipitate, but larger aliquots for all other samples, the steeper slope obtained by Ghosh et al. (2006a) could be the result of an unnoticed sample-size effect. However, the Ghosh et al. (2006a) calibration line has been confirmed by other studies. These investigations include reactions at 25 °C (Came et al., 2007; Tripati et al., 2010; Thiagarajan et al., 2011; Saenger et al., 2012; Zaarur et al., 2013), as well as digestions at 70 °C (Grauel et al., 2013). Therefore, it seems unlikely that the discrepant calibrations obtained from the different digestion techniques are caused by an analytical artifact exclusively occurring at digestion temperatures of 25 °C. On the other hand, re-equilibration of CO2 might preferentially occur at 90 °C. If so, the calibration line would

pivot around a value of 0.651‰ with its final slope depending on the degree of re-equilibration. However, interlaboratory comparisons do not indicate that significant re-equilibration occurs at 90 °C. Calcites with different Δ47values (NBS 19 and DSC-45923) were measured in several laboratories using different digestion temperatures, and no systematic difference in Δ47 values was observed (Dennis et al., 2011). For example, samples were digested at 25 °C in sealed vessels at Yale University, whereas carbonate reactions were

4 Empirical calibration of the clumped isotope paleothermometer using calcites of various origins

performed in a common acid bath at 90 °C at Johns Hopkins University. Applying a Δ47* 25–90

value of 0.081‰ both laboratories reported indistinguishable Δ47 values for NBS 19 that averaged to 0.399‰ (Yale) and 0.404‰ (Johns Hopkins), respectively (Dennis et al., 2011).

Considering the same Δ47*

25–90 value, our long term mean Δ47 of NBS 19 is 0.376‰ and,

therefore, even lower than the corresponding Yale average. Provided secondary re-equilibration would exclusively occur at the elevated reaction temperature of 90 °C, a trend

in the opposite direction, i.e., to higher Δ47values should be observed for reactions carried out at 90 °C relative to those performed at 25 °C. Likewise, carbonate samples with a Δ47value

>0.7‰ should exhibit a bias towards a value of 0.651‰ if re-equilibration at 90 °C is significant. However, within errors, Δ47 values reported for DSC-45923 analyzed at Johns Hopkins at 90 °C (0.775‰) and at Yale at 25 °C (0.781‰) were indistinguishable from each other (Dennis et al., 2011). In conclusion, we exclude that partial re-equilibration at 90 °C is responsible for the different slopes of published calibration regression lines.