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The geological calcium cycle is linked to the geological carbon cycle through the weathering and burial of carbonate rocks. As a result, calcium (Ca) isotope ratios (⁴⁴Ca/⁴⁰Ca, expressed as δ^44/40Ca) can help to constrain ancient carbon cycle dynamics if Ca cycle behavior can be reconstructed. However, the δ^44/40Ca of carbonate rocks is influenced not only by the δ^44/40Ca of seawater but also by diagenetic processes and fractionation associated with carbonate precipitation. In this study, we investigate the dominant controls on carbonate δ^44/40Ca in Upper Permian to Middle Triassic limestones (ca. 253 to 244 Ma) from south China and Turkey. This time interval is ideal for assessing controls on Ca isotope ratios in carbonate rocks because fluctuations in seawater δ^44/40Ca may be expected based on several large carbon isotope (δ¹³C) excursions ranging from − 2 to + 8‰. Parallel negative δ¹³C and δ⁴⁴ …