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) 通常是利用生物碳酸盐的氧同位素组成 (δ18O) 得出的。这样的 T 估计需要通常未知的水的同位素组成。新型团块同位素 (Δ47) 允许仅根据碳酸盐计算古 T。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) 液/固界面的 pH 效应或不平衡。与这些过程中的每一个相关的同位素变化都会改变古温度估计。特别是珊瑚衍生的 δ18O-T 可以与已知的生长 T 抵消多达 20°C。对于新型 Δ47 代理，现在也观察到高达 9°C 的明显偏移。因此，这两种方法都无法从此类样本中限制准确的古温度。在这里，我建议进行 δ17O 分析以校正生命效应。如果样品与相应的水形成平衡，则所有三种同位素比（δ17O、δ18O 和 Δ47）必须指示相同的 T。如果温度计产生不同的 T，则由于生命效应，生物碳酸盐必定已形成不平衡。每个潜在的物理过程都落在 δ18O 与 δ17O 空间中独特且可很好解析的斜率上。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特定于过程的斜率上，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，这对应于碳酸盐的实际生长 T。如果水同位素组成未知，则需要进行额外的聚集同位素分析。这种方法也有望纠正在无机碳酸盐（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。