Triple oxygen isotope fractionation in the DIC-H2O-CO2 system: A numerical framework and its implications
Triple oxygen isotope fractionation in the DIC-H2O-CO2 system: A numerical framework and its implications
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DOI：
10.1016/j.gca.2018.11.018
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发表时间：
2019-02
影响因子：
5
通讯作者：
Weifu Guo;Chen Zhou
中科院分区：
文献类型：
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作者：
Weifu Guo;Chen Zhou
碳酸盐矿物的三氧同位素组成反映了碳酸盐沉淀水的同位素组成，并正在成为约束过去地球水文气候和地表环境变化的有希望的代理。然而，当碳酸盐矿物在与水的同位素平衡下不形成时，这种代用物的定量解释并不直接。对于这些碳酸盐，沉淀溶液中溶解无机碳（DIC）的同位素组成通常对其同位素组成起主导作用。本文通过推导DIC- h2o - co2体系中基本平衡和动力学三氧同位素分馏因子，模拟DIC- h2o同位素交换、co2脱气和co2吸收三种常见同位素分馏过程中三氧同位素组成的演化，探讨了DIC- h2o - co2体系中三氧同位素分馏的分馏系统。我们发现，在热力学平衡下，溶解的HCO - 3和co32 -都表现出与方解石和文石相似的Δ ' 17 O（在25°C时每meg在10以内），温度依赖性约为每meg/°C 0.61。然而，在我们模拟的所有三个过程中，与CO 2水化和羟基化及其逆反应相关的动力学同位素分馏可以在DIC中产生一系列不平衡的三氧同位素效应。这些不平衡效应的大小随时间和溶液的物理化学条件（如温度、pH值和初始组成）而变化。特别是，我们预测DIC在CO 2脱气期间Δ ' 17 O和Δ 18 O的相关富集和CO 2吸收期间的耗尽。这些相关性的斜率主要作为溶液pH的函数而变化，而不是温度，DIC浓度或空气pCO 2的函数，在pH= 8和9时，DIC的产值分别为8.8和12.0每meg/‰，在25℃下。DIC中的这种不平衡同位素效应预计将由这些溶液形成的碳酸盐矿物（例如，洞穴石，珊瑚骨架和高pH石灰华）继承，如果没有解释，可能导致对母水的三氧同位素组成和碳酸盐地层温度的不准确估计。我们的数值模型为解释DIC三氧同位素组成和纠正天然碳酸盐中不平衡三氧同位素效应提供了定量框架。
Triple oxygen isotope composition of carbonate minerals reflects the isotope composition of the water from which carbonates precipitate, and is emerging as a promising proxy for constraining past changes in Earth hydroclimate and surface environment. However, quantitative interpretation of this proxy is not straightforward when carbonate minerals do not form under isotope equilibrium with water. For these carbonates, isotope composition of dissolved inorganic carbon (DIC) in the precipitating solution usually exerts the dominant control on their isotope composition. Here we examine the systematics of triple oxygen isotope fractionation in the DIC-H 2 O-CO 2 system by deriving the fundamental equilibrium and kinetic triple oxygen isotope fractionation factors in this system and simulating the evolution of triple oxygen isotope composition of DIC during three common isotope fractionation processes, ie, DIC-H 2 O isotope exchange, CO 2 degassing and CO 2 absorption. We show that under thermodynamic equilibrium dissolved HCO 3–and CO 3 2–both exhibit similar Δ′ 17 O as calcite and aragonite (within 10 per meg at 25° C), with temperature dependences around 0.61 per meg/° C. However, kinetic isotope fractionations associated with CO 2 hydration and hydroxylation and their reverse reactions can produce a range of disequilibrium triple oxygen isotope effects in DIC during all three processes we simulated. The magnitudes of these disequilibrium effects vary with both time and the physicochemical conditions of the solution, eg, temperature, pH and initial composition. Particularly, we predict correlated enrichments in Δ′ 17 O and δ 18 O of DIC during CO 2 degassing but depletions during CO 2 absorption. The slopes of these correlations vary mainly as a function of solution pH but not temperature, DIC concentration or air pCO 2, yielding values of 8.8 and 12.0 per meg/‰ at pH= 8 and 9, respectively, at 25° C. Such disequilibrium isotope effects in DIC are expected to be inherited by carbonate minerals that form from these solutions (eg, speleothem, coral skeleton, and high pH travertine) and, if not accounted for, could lead to inaccurate estimates of the triple oxygen isotope compositions of the parent water and carbonate formation temperatures. Our numerical model provides a quantitative framework for interpreting triple oxygen isotope composition of DIC and for correcting disequilibrium triple oxygen isotope effects in natural carbonates.