Correcting 18O/16O vital effects in biogenic carbonate using 17O/16O and clumped isotope analysis.

古温度(T)通常是利用生物碳酸盐的氧同位素组成(δ18O)得出的。这样的温度估计需要水的同位素组成--通常是未知的。新的簇状同位素(Δ47)只允许从碳酸盐计算古温度。然而，这两种方法都需要碳酸盐-水平衡。不平衡常常是由所谓的“生命效应”引起的，这些效应被归因于几个过程，包括：(I)扩散；(Ii)二氧化碳的水化(CO2+H2O)和羟化(CO2+OH-)；(Iii)液/固界面的pH效应或不平衡。与这些过程中的每一个过程相关的同位素漂移都可能改变古T的估计。尤其是珊瑚衍生的δ18O-T，与已知的生长量-T相比，可以抵消高达20℃的误差。现在还观察到新型Δ47代理的明显偏移量高达9°C。因此，这两种方法都不能从这样的样品中约束准确的古T。在这里，我建议执行δ17O分析，以校正重要影响。如果样品与相应的水形成平衡，所有三个同位素比率(δ17O、δ18O和Δ47)必须指示相同的T。如果温度计得出不同的T，则生物碳酸盐一定是由于生命效应而不平衡形成的。在δ18O与δ17O空间中，每个底层物理过程都落在唯一且可很好解析的斜坡上。因此，具有可变生命效应的样本套件将落在三重氧同位素空间中特定于工艺的斜率上，从而能够区分扩散、水化、羟化和pH效应。线性相关可以外推到平衡点，在该平衡点上，δ17O和δ18O给出了相同的古T，这与碳酸盐的实际生长T相对应。如果水的同位素组成未知，则需要进行额外的成束同位素分析。这种方法还有望纠正在成壤碳酸盐、洞穴或热液碳酸盐等无机碳酸盐中观察到的动力学同位素效应。

Paleo temperatures (T) are routinely derived from biogenic carbonates using their oxygen isotopic composition (δ18O). Such T estimates require the – generally unknown - isotopic composition of the water. Novel clumped isotopes (Δ47) allow calculating paleo T only from carbonate. Both methods, however, require carbonate-water equilibrium. Disequilibrium is frequently induced by so called “vital effects” which have been attributed to several processes including: (i) diffusion; (ii) hydration (CO2 + H2O) and hydroxylation (CO2 + OH-) of CO2; and (iii) pH effects or disequilibrium at the liquid/solid interface. Isotopic shifts associated with each of these processes can alter paleo T estimates. Especially coral derived δ18O-T can be offset by as much as 20°C from known growth-T. Apparent offsets by up to 9°C are now also observed for the novel Δ47 proxy. Thus both methods fail at constraining accurate paleo T from such samples. Here I propose to perform δ17O analysis to correct for vital effects. If a sample forms in equilibrium with the respective water, all three isotope ratios (δ17O, δ18O and Δ47) must indicate identical T. If the thermometers yield different T, biogenic carbonate must have formed out of equilibrium due to vital effects. Each underlying physical process falls on a unique and well resolvable slope in δ18O vs. δ17O space. Hence, a sample suite with variable vital effect will fall on a process-specific slope in triple oxygen isotope space allowing to distinguish between diffusion, hydration, hydroxylation and a pH effect. The linear correlation can be extrapolated to the equilibrium point where both the δ17O and δ18O give the same paleo T, which corresponds to the real growth T of the carbonate. Additional clumped isotope analysis are required if the water isotopic composition is not known. This approach may also be promising to correct for kinetic isotope effects observed in inorganic carbonates such as pedogenic carbonates, speleothems or hydrothermal carbonates.