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) 通常是使用其氧同位素组成 (δ18O) 从生物碳酸盐中得出的，这种 T 估算需要（通常未知的）水的同位素组成 (Δ47)，从而只能根据碳酸盐来计算古温度。然而，这些方法需要碳酸盐-水平衡，这种不平衡常常是由所谓的“生命效应”引起的，这归因于几个过程，包括：（i）扩散；（ii） CO2 的水化（CO2 + H2O）和羟基化（CO2 + OH-）；以及 (iii) 与这些过程相关的 pH 效应或同位素变化会改变古 T 估计值，尤其是珊瑚衍生的 δ18O。 -T 可以从已知的生长-T 偏移多达 20°C，现在对于新型 Δ47 代理也观察到了高达 9°C 的表观偏移。在此，我建议执行 δ17O 分析来校正生命效应，如果样品与相应的水处于平衡状态，则所有三种同位素比率（δ17O、δ18O 和 Δ47）必须指示相同的 T。如果温度计产生不同的 T，则由于重要的影响，生物碳酸盐的形成一定是不平衡的。因此，具有可变生命效应的样品套件将落在三氧同位素空间中的特定过程斜率上，从而可以区分扩散、水合、羟基化和 pH 效应。线性相关性可以外推到平衡点，其中δ17O 和 δ18O 都给出相同的古 T 值，这对应于碳酸盐的真实生长 T 值，如果水同位素组成不是这样，则需要进行额外的聚集同位素分析。已知这种方法也可能有望纠正无机碳酸盐（例如成土碳酸盐、洞穴或热液碳酸盐）中观察到的动力学同位素效应。