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

使用 17O/16O 和聚集同位素分析校正生物碳酸盐中 18O/16O 的重要影响。

• 批准号: 415866908
• 负责人: Privatdozent Dr. Daniel Herwartz
• 金额: --
• 依托单位: Institut für Geologie und Mineralogie
• 依托单位国家: 德国
• 项目类别: Research Grants
• 财政年份: 2019
• 资助国家: 德国
• 起止时间: 2018-12-31 至 2021-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/415866908?language=en
• 关键词: Correcting; 18O; 16O; vital; effects

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.

古温度（t）通常使用其氧同位素组成（Δ18O）从生物碳酸盐量衍生而来。这种T估计需要 - 通常未知的 - 同位素组成。新型聚类的同位素（Δ47）仅允许从碳酸盐计算古T。但是，两种方法都需要等效碳酸盐。不平衡通常是由所谓的“重要效应”引起的，这些效应归因于几个过程，包括：（i）扩散； （ii）CO2的水合（CO2 + H2O）和羟基化（CO2 + OH-）； （iii）在液体/实心界面处的pH效应或二动效果。与这些过程中的每一个相关的同位素移位可以改变古估计。尤其是衍生的Δ18O-T可以与已知生长-T相抵消多达20°C。现在，对于新型Δ47代理，也观察到了最高9°C的明显偏移。这两种方法都无法限制此类样品的准确古t。在这里，我建议执行Δ17O分析以纠正重要效应。如果样品与相对水的平衡形式形成，则所有三个同位素比（Δ17O，Δ18O和Δ47）必须表示相同的T。如果温度计产生不同的T，则由于重要效应而产生的生物碳酸盐必须从平衡中形成。每个潜在的物理过程都属于Δ18O与Δ17O空间的独特且可分解的斜率。因此，具有可变效应的样品套件将落在三氧同位素空间中的过程特异性斜率上，从而可以区分扩散，水合，羟基化和pH效应。线性相关性可以推断到等效点，其中Δ17O和Δ18O给出了相同的古t，这对应于碳酸盐的实际生长t。如果不知道水同位素组成，则需要其他聚类的同位素分析。这种方法也可能有望纠正在无机碳酸盐（例如成源性碳酸盐，脾或水热碳酸盐）中观察到的动力学同位素效应。