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Clumped isotope studies on CO₂, Δ₄₇, that is the excess in the isotopologue containing both¹³C and¹⁸O at mass 47, can be very useful since they can give temperature estimates independent of the bulk isotopic composition. The measurement itself can be affected by a number of items. Here we develop a data processing model to examine the effects different interferences might have on the final calculated value. It incorporates known issues, for example, pressure baseline,¹⁷O excess, and shifts in absolute ratios for primary reference materials and parameters used for¹⁷O correction. We also included linearity effects as well as differences in isotopologue absolute abundances at a givenm/z. What normally would be considered acceptable mass spectrometer⁴⁵Rand⁴⁶Rlinearity can skew Δ₄₇results. That is 0.04‰/V and −0.04‰/V linearity on⁴⁵Rand⁴⁶Rrespectively would also cause an apparent shift in the parameters used for¹⁷O corrections. Measurements were made on CO₂(g) equilibrated with water, and we were able to match up the effects seen with model results. Linearity and small differences in amplitude between sample and working reference gas affected Δ₄₇determination, as did apparent shifts in isotopologue abundances under different conditions. This may (partially) account for discrepancies seen in Δ₄₇‐temperature calibrations curves between laboratories. We also developed an easy way to precisely calculate the δ¹³C and δ¹⁸O that works well in spreadsheets without the need for multiple iterations.

Research on coralline algal responses to ocean acidification and other environmental stressors has increased in recent years as coralline algae is thought to stand a higher chance of being affected by acidification stress than other macroalgae. To provide context and enhance the existing eco‐physiological framework for climate change studies, it is important to understand the effects of non‐extreme stressors experienced regularly by inter‐tidal coralline algae. In this study, we tested the potentially interacting effects of diurnal and tidal treatments on calcification in the geniculate coralline algaeCorallina frondescensandCorallina vancouveriensisusing¹³C‐labeled bicarbonate. Both species deposited more calcium carbonate during the day than at night, and also when submerged (high tide) compared with when emerged (low tide) in their apical and mature segments (intergenicula). These results indicate that inter‐tidal coralline algae do in fact pay a cost for living inter‐tidally at the edge of an adaptive zone.