Beyond temperature: Clumped isotope signatures in dissolved inorganic carbon species and the influence of solution chemistry on carbonate mineral composition
Beyond temperature: Clumped isotope signatures in dissolved inorganic carbon species and the influence of solution chemistry on carbonate mineral composition
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DOI：
10.1016/j.gca.2015.06.021
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发表时间：
2015-10
影响因子：
5
通讯作者：
A. Tripati;P. Hill;R. Eagle;J. Mosenfelder;Jianwu Tang;E. Schauble;J. Eiler;R. Zeebe;J. Uchikawa;T. Coplen;J. Ries;D. Henry
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文献类型：
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作者：
A. Tripati;P. Hill;R. Eagle;J. Mosenfelder;Jianwu Tang;E. Schauble;J. Eiler;R. Zeebe;J. Uchikawa;T. Coplen;J. Ries;D. Henry
“块状同位素”测温法是一种新兴的探测地表和地下环境温度历史的工具，其基础是测量13C18O16O22−组碳酸盐矿物中13C和18O同位素相互结合的比例(重同位素“块状”)。虽然大多数块状同位素地质测温隐含地假设碳酸盐晶体已达到晶格平衡(即矿物的热力学平衡，这与溶液化学无关)，但温度以外的其他几个因素，包括溶解无机碳(DIC)形态可能会影响矿物的同位素特征。因此，我们采用多种方法相结合的方法来了解不同变量对矿物聚集同位素(和氧同位素)组成的潜在影响。我们在单一温度和不同pH下进行毒重石沉淀实验，经验地测定了25℃平衡时CO32-Δ和HCO3-δ分子的13C-18O键序(Δ47)和δ18O。基于密度泛函理论的从头计算模型预测了不同DIC物种和矿物的13C-18O键丰度和DIC18O平衡随温度的变化。实验和理论表明，Δ离子的δ_(47)和−_(18)O组分明显不同于CO_3·−和CO_3。实验限制了Δ47-δ18O斜率的pH效应(0.011±0.001；12⩾-pH和⩾=7)。快速生长的温带珊瑚表现出不平衡的矿物同位素特征，Δ47-δ18O斜率为0.011±0.003，与pH效应一致。我们对碳酸盐矿物的理论计算表明，Δ47和δ18O的平衡晶格方解石的值介于HCO3−和CO32−之间。我们分析了在0.5至50摄氏度的温度下生长的合成方解石，其中存在和不存在碳酸氢酶存在的情况。这种酶催化DIC物种之间的氧同位素交换，存在于许多自然系统中。这两种类型的实验产生了统计上难以区分的结果，而这些测量产生的校准与我们对方解石平衡状态的理论预测重叠。缓慢生长的魔鬼洞方解石的Δ47和δ18O值符合晶格平衡。影响DIC物种形成(pH、盐度)和DIC平衡时间的因素，以及矿物-溶液界面上的反应，都可能影响碳酸盐矿物的块状同位素特征和δ18O。在快速生长的碳酸盐矿物中，溶液化学可能是一个重要因素，特别是在极端的pH和盐度条件下。如果晶体生长太快而不能达到内部平衡(即达到依赖温度的矿物晶格平衡值)，它可能记录DIC物种的团簇同位素特征(例如，HCO3−的温度依赖平衡)或DIC物种的混合物，从而记录不平衡的矿物组成。对于生长极慢的晶体，以及在平衡时HCO3DIC池占主导地位的pH值下生长的快速样品，溶液化学的影响可能相对较小或可以忽略不计。总而言之，生长环境、溶液化学、表面平衡和沉淀速率都可能在决定晶体是否达到平衡或不平衡的块状同位素特征中发挥作用。
“Clumped-isotope” thermometry is an emerging tool to probe the temperature history of surface and subsurface environments based on measurements of the proportion of13C and18O isotopes bound to each other within carbonate minerals in13C18O16O22−groups (heavy isotope “clumps”). Although most clumped isotope geothermometry implicitly presumes carbonate crystals have attained lattice equilibrium (i.e., thermodynamic equilibrium for a mineral, which is independent of solution chemistry), several factors other than temperature, including dissolved inorganic carbon (DIC) speciation may influence mineral isotopic signatures. Therefore we used a combination of approaches to understand the potential influence of different variables on the clumped isotope (and oxygen isotope) composition of minerals.We conducted witherite precipitation experiments at a single temperature and at varied pH to empirically determine13C–18O bond ordering (Δ47) and δ18O of CO32−and HCO3−molecules at a 25 °C equilibrium.Ab initiocluster models based on density functional theory were used to predict equilibrium13C–18O bond abundances and δ18O of different DIC species and minerals as a function of temperature. Experiments and theory indicate Δ47and δ18O compositions of CO32−and HCO3−ions are significantly different from each other. Experiments constrain the Δ47–δ18O slope for a pH effect (0.011 ± 0.001; 12 ⩾ pH ⩾ 7). Rapidly-growing temperate corals exhibit disequilibrium mineral isotopic signatures with a Δ47–δ18O slope of 0.011 ± 0.003, consistent with a pH effect.Our theoretical calculations for carbonate minerals indicate equilibrium lattice calcite values for Δ47and δ18O are intermediate between HCO3−and CO32−. We analyzed synthetic calcites grown at temperatures ranging from 0.5 to 50 °C with and without the enzyme carbonic anhydrase present. This enzyme catalyzes oxygen isotopic exchange between DIC species and is present in many natural systems. The two types of experiments yielded statistically indistinguishable results, and these measurements yield a calibration that overlaps with our theoretical predictions for calcite at equilibrium. The slow-growing Devils Hole calcite exhibits Δ47and δ18O values consistent with lattice equilibrium.Factors influencing DIC speciation (pH, salinity) and the timescale for DIC equilibration, as well as reactions at the mineral–solution interface, have the potential to influence clumped-isotope signatures and the δ18O of carbonate minerals. In fast-growing carbonate minerals, solution chemistry may be an important factor, particularly over extremes of pH and salinity. If a crystal grows too rapidly to reach an internal equilibrium (i.e., achieve the value for the temperature-dependent mineral lattice equilibrium), it may record the clumped-isotope signature of a DIC species (e.g., the temperature-dependent equilibrium of HCO3−) or a mixture of DIC species, and hence record a disequilibrium mineral composition. For extremely slow-growing crystals, and for rapidly-grown samples grown at a pH where HCO3−dominates the DIC pool at equilibrium, effects of solution chemistry are likely to be relatively small or negligible. In summary, growth environment, solution chemistry, surface equilibria, and precipitation rate may all play a role in dictating whether a crystal achieves equilibrium or disequilibrium clumped-isotope signatures.