Marine sedimentary ikaite is the parent mineral to glendonite, stellate pseudomorphs found throughout the geological record which are most usually composed of calcite. Ikaite is known to be metastable at earth surface temperatures and pressures, readily breaking down to more stable carbonate polymorphs when exposed to warm (ambient) conditions. Yet the process of transformation of ikaite to calcite is not well understood, and there is an ongoing debate as to the palaeoclimatic significance of glendonites in the geological record. This study uses a combination of techniques to examine the breakdown of ikaite to calcite, outside of the ikaite growth medium, and to assess the palaeoclimatic and palaeoenvironmental significance of stable and clumped isotope compositions of ikaite-derived calcite. Powder X-ray diffraction shows that ikaite undergoes a quasi- solid-state transformation to calcite during heating of samples in air, yet when ikaite transforms under a high temperature differential, minor dissolution-recrystallisation may also occur with the ikaite structural waters. No significant isotopic equilibration to transformation temperature is observed in the resulting calcite. Therefore, in cases of transformation of ikaite in air, clumped and stable isotope thermometry can be used to reconstruct ikaite growth temperatures. In the case of ancient glendonites, where transformation of the ikaite occurred in contact with the interstitial waters of the host sediments over unknown timescales, it is uncertain whether the reconstructed clumped isotope temperatures reflect ikaite crystallisation or its transformation temperatures. Yet clumped and stable isotope thermometry may still be used conservatively to estimate an upper limit for bottom water temperatures.

Furthermore, stable isotope along with element/Ca ratios shed light on the chemical environment of ikaite growth. Our data indicate that a range of (bio)geochemical processes may act to promote ikaite formation at different marine sedimentary sites, including bacterial sulphate reduction and anaerobic oxidation of methane. The colours of the ikaites, from light brown to dark brown, indicate a high organic matter content, favouring high rates of bacterial sulphate reduction as the main driver of ikaite precipitation. Highest Mg/Ca ratios are found in the most unstable ikaites, indicating that Mg acts to destabilise ikaite structure.

海洋沉积 ikaite 是钙辉石的母体矿物，在整个地质记录中发现的星状假形体通常由方解石组成。众所周知，Ikaite 在地球表面温度和压力下是亚稳态的，当暴露于温暖（环境）条件时，很容易分解成更稳定的碳酸盐多晶型物。然而，ikaite 转化为方解石的过程尚不清楚，关于地质记录中菱镁石的古气候意义一直存在争议。本研究使用多种技术在 ikaite 生长培养基之外检查 ikaite 分解为方解石，并评估 ikaite 衍生方解石的稳定和聚集同位素组成的古气候和古环境意义。粉末X射线衍射表明，在空气中加热样品过程中，ikaite经历了准固态转变为方解石，但是当ikaite在高温差下转变时，ikaite结构水也可能发生轻微的溶解-重结晶。在所得方解石中没有观察到与转变温度的显着同位素平衡。因此，在 ikaite 在空气中发生转变的情况下，可以使用聚集稳定同位素测温法来重建 ikaite 的生长温度。在古代格伦顿岩的情况下，在未知的时间尺度上，ikaite 的转变发生在与宿主沉积物的间隙水接触时，不确定重建的团块同位素温度是否反映了 ikaite 结晶或其转变温度。

此外，稳定同位素以及元素/Ca 比率揭示了 ikaite 生长的化学环境。我们的数据表明，一系列（生物）地球化学过程可能会促进不同海洋沉积地点的 ikaite 形成，包括细菌硫酸盐还原和甲烷厌氧氧化。ikaite 的颜色，从浅棕色到深棕色，表明有机物含量高，有利于细菌硫酸盐还原率高，这是 ikaite 沉淀的主要驱动力。在最不稳定的 ikaite 中发现了最高的 Mg/Ca 比率，这表明 Mg 起到了使 ikaite 结构不稳定的作用。