Oxygen-isotope measurements of fossil carbonates remain the most common method for paleoclimatic temperature reconstructions. A well-known limitation of this approach is the influence of the oxygen isotope composition of water in which mineralization occurs, which may vary significantly through space and time, and is often difficult to constrain precisely. Carbonate clumped-isotope thermometry is an alternative approach applicable to many carbonates. It is based on measurements of Δ₄₇ (a tracer of small statistical anomalies in the abundance of rare, doubly-substituted carbonate isotopologues), and requires no independent information on the oxygen-isotope composition of parent waters. Here, we report new calibration observations of clumped isotopes in four species of calcitic marine bivalves (A. colbecki, N. cochlear, S. cucullata, M. gigas) from various ecosystems including coastal and deep-sea environments, with calcification temperatures ranging from −2 °C to 27 °C and very different amplitudes of seasonal temperature variability. At two localities with large seasonal temperature variability, calcification time intervals were constrained using a sclerochronological approach to test whether seasonal gradients of temperature can be accurately quantified based on Δ₄₇ measurements.

Our results indicate that the mature bivalves we analyzed have clumped-isotope compositions entirely consistent with earlier calibration studies processed in the I-CDES reference frame and based on biogenic/abiotic/synthetic materials. By contrast, juvenile M. gigas oysters yield substantially lower Δ₄₇ values than expected based on their calcification environments, suggesting that their early growth phase is associated with yet poorly understood isotopic biases affecting both δ¹⁸Ο and Δ₄₇ values. The link between seawater temperatures and bivalve Δ₄₇ values is thus potentially applicable to seasonal reconstructions, but only if shell sections formed in cold seasons are precisely identified and precisely sampled, and taking into account that winter calcification is likely to be biased due to reduced growth rate. Moreover, the excellent agreement between our observations and the existing I-CDES calibrations further demonstrates the efficacy of the I-CDES standardization approach, and adds to the evidence that many different types of carbonates conform to statistically indistinguishable relationships between Δ₄₇ and crystallization temperature.

化石碳酸盐的氧同位素测量仍然是古气候温度重建的最常用方法。这种方法的一个众所周知的限制是发生矿化的水的氧同位素组成的影响，它可能随空间和时间而显着变化，并且通常难以精确约束。碳酸盐团块同位素测温法是一种适用于许多碳酸盐的替代方法。它基于对 Δ ₄₇（稀有双取代碳酸盐同位素丰度中的小统计异常的示踪剂）的测量，不需要有关母水氧同位素组成的独立信息。在这里，我们报告了四种方解石海洋双壳类动物中聚集同位素的新校准观察结果（A. colbecki、N. cochlear、S. cucullata、M. gigas）来自包括沿海和深海环境在内的各种生态系统，钙化温度范围从 -2 °C 到 27 °C，季节性温度变化幅度非常不同。在具有较大季节性温度变化的两个地点，钙化时间间隔使用硬化年代学方法进行限制，以测试是否可以根据 Δ ₄₇测量值准确量化季节性温度梯度。

我们的结果表明，我们分析的成熟双壳类动物的团块同位素组成与在 I-CDES 参考框架中处理并基于生物/非生物/合成材料的早期校准研究完全一致。相比之下，幼年M. gigas牡蛎产生的 Δ ₄₇值大大低于基于其钙化环境的预期值，这表明它们的早期生长阶段与影响 δ ¹⁸ Ο 和 Δ ₄₇值的同位素偏差有关，但仍知之甚少。海水温度与双壳类动物之间的联系 Δ ₄₇因此，这些值可能适用于季节性重建，但前提是精确识别和精确采样寒冷季节形成的壳截面，并考虑到冬季钙化可能因增长率降低而产生偏差。此外，我们的观察结果与现有 I-CDES 校准之间的极好一致性进一步证明了 I-CDES 标准化方法的有效性，并进一步证明了许多不同类型的碳酸盐符合 Δ ₄₇与结晶温度之间在统计上无法区分的关系。